ABSTRACT

The development of new technologies has introduced more new materials in the field of prosthesis manufacturing as described in chapter one. Prostheses themselves have improved immensely as designers try to come as close as possible to natural limbs, both functionally and visually. Better quality and lower-cost substitutes of currently used materials were examined for the artificial hu-man limbs and an attempt has been made to cover a sufficiently wide scope of world practice in the examined field. Chapter two presents a review for Shape Memory Alloys (SMAs). The metal al-loys, which can undergo substantial plastic deformation, and then be triggered into returning to its original shape by heat application. Particular consideration is given to a proliferation of diverse applications of SMAs in a variety of industries. Further research directions for SMAs and their application areas are also identi-fied. In chapter three, suitable acid resistant materials have been selected for the making of storage tanks. Additionally we have described the factors that might affect tank efficiency and utilization. We have also presented methods providing additional protection of metals. By analyzing the scientific investigations carried out in the field of electrochemical corrosion and anodic protection of metals, we have presented the experimental results of anodic protection of mild steel in di-luted sulphuric acid. Composite materials made by bonding individual long known and used or new materials have been examined in the chapter four. The manufacturing proc-esses used for the production of this constantly expanding and significant to modern industry material group, involve both previously known and new produc-tion techniques. The basic components of composite materials as the Thermo-plastics and the Thermosets are involved in the application of materials belong-ing to this vast group. Finally, bridge structures which are widely spread in engineering practice, with a variety of construction shapes and static arrangements defined by the purpose, usage features, nature of applied loads and the building materials have been analyzed in the fifth chapter. The materials used in practice to build such model constructions and the design solutions suitability have been carefully examined in this study. In many cases it is possible to consider them as a group of struc-tures incorporating common calculation and design considerations, set up on common technological grounds.

Table of Contents

CHAPTER 1: NEW MATERIALS IN ARTIFICIAL LIMBS: CONTEMPORARY MATERIALS IN ARTIFICIAL LIMBS, IN ORDER TO INCREASE LIVING ADAPTABILITY FOR PEOPLE WITH PROSTHESES.___ 1

INTRODUCTION ________________________________________________ 1 CRITICAL LITERATURE SEARCH _________________________________ 2 DATA ANALYSIS _______________________________________________ 6 CRITICAL DISCUSSION_________________________________________ 16

Polyester Hydrate_____________________________________________ 17 Carbon Fibres________________________________________________ 18 Titanium ____________________________________________________ 19 Bioactive glass _______________________________________________ 19 Biomaterials _________________________________________________ 19 Genetics and Biotechnology_____________________________________ 20

CONCLUSIONS _______________________________________________ 21 UNDERSTANDING COMMONLY USED TERMS _____________________ 23

CHAPTER 2: SHAPE MEMORY ALLOYS & THEIR APPLICATIONS________ 24

INTRODUCTION _______________________________________________ 24 REVIEW OF THE SMAS _________________________________________ 25

Definition of a Shape Memory Alloys ______________________________ 25 History _____________________________________________________ 26 Shape Memory Alloy Types _____________________________________ 26 Crystallography of the SMAs ___________________________________ 27 Thermomechanical Characteristics _______________________________ 28

Austenite and Martensite Phases_______________________________ 28 Shape Memory Effect (SME) __________________________________ 29 Pseudo-elasticity____________________________________________ 30

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SMA’s Applications____________________________________________ 31 THE SMAS AND THEIR APPLICATIONS ___________________________ 34 FURTHER WORK ______________________________________________ 36 CONCLUSIONS _______________________________________________ 37 REFERENCES ________________________________________________ 38

CHAPTER 3: SELECTING ACID-RESISTANT MATERIALS_______________ 40

INTRODUCTION TO THE ANODIC PASSIVATION ___________________ 40 MECHANICS OF ANODIC PASSIVATION __________________________ 41 EXPERIMENTAL STUDIES – ANODIC PROTECTION OF MILD STEEL IN SULPHURIC ACID SOLUTION____________________________________ 42 SELECTING MATERIALS FOR THE TANK _________________________ 44 FACTORS INFLUENCING THE EFFICIENCY OF THE TANK AND ITS USAGE ______________________________________________________ 47 USING THE TANK FOR STORAGE OF 10% HYDROCHLORIC ACID. ____ 48 ADDITIONAL TANK PROTECTION TECHNIQUES ___________________ 48 SELECTING THE METAL MATERIAL FOR MAKING A STORAGE TANK FOR 10% SULPHURIC ACID _____________________________________ 50

For a cost limit of 3 Pounds per kilogram___________________________ 50 Selecting a material for the design of the tank _______________________ 51 For a cost limit of 1 Pound per kilogram____________________________ 52 Selecting the steel for designing the storage tank ____________________ 53

FACTORS AFFECTING TANK EFFICIENCY AND USAGE _____________ 56 USING THE TANK FOR THE STORAGE OF 10% HYDROCHLORIC ACID 57 REFERENCES ________________________________________________ 57

CHAPTER 4: FIBRE REINFORCED COMPOSITE MATERIALS____________ 59

INTRODUCTION _______________________________________________ 59 COMPOSITE MATERIALS AND MANUFACTURING TECHNIQUES. _____ 59

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Plasticisers. _________________________________________________ 60 Fillers ______________________________________________________ 61 Stabilisers. __________________________________________________ 62 Antistatic agents______________________________________________ 63 Some composite manufacturing techniques ________________________ 63

GENERAL PROPERTIES OF POLYMERIC MATERIALS_______________ 64 INFLUENCE OF THE MATRIX (BASIC ELASTOMER) AND ADDITIVES ON THE QUALITY OF THE COMPOSITE MATERIAL. ____________________ 64

Influence of the type of elastomer on the quality of the composite material. 64 Influence of additive (reinforcement) material _______________________ 65 Influence of fibre arrangement and quantity_________________________ 67

RESIN-FIBRE INTERFACE (COUPLING AGENTS) ___________________ 69 CONCLUSION_________________________________________________ 69 REFERENCE__________________________________________________ 70

CHAPTER 5: MATERIALS & DESIGN FOR BRIDGE MODELLING ________ 71

INTRODUCTION _______________________________________________ 71 INITIAL DATA _________________________________________________ 71

Construction geometry _________________________________________ 72 Testing jig diagram____________________________________________ 72

DESIGN REQUIREMENTS _______________________________________ 73 RAW MATERIALS & CONSTRUCTION ELEMENTS __________________ 74

Characteristics _______________________________________________ 74 Material tests ________________________________________________ 74

CONSTRUCTIONS _____________________________________________ 76 STRUCTURE SELECTION & MANUFACTURE_______________________ 88 ALTERNATIVE CONSTRUCTIONS ________________________________ 88 STRUCTURE SELECTION & MANUFACTURE_______________________ 97 REFERENCES ________________________________________________ 97

CHAPTER 1 NEW MATERIALS IN ARTIFICIAL LIMBS: CONTEMPORARY

MATERIALS IN ARTIFICIAL LIMBS, IN ORDER TO IN-CREASE LIVING ADAPTABILITY FOR PEOPLE WITH

PROSTHESES.

INTRODUCTION

There is a strong interest in the use of new materials in artificial limbs that would help increase possibilities for adaptability and higher-quality life standard of one specific section of modern society - physically disabled people.

The development of material technology has allowed the designer to make radical improvements to the performance of products designed for disabled people. The development of plastics and alloyed metals has meant that designs are becoming lighter, more comfortable and more like the real function of human limbs than ever before.

The objectives of this work were to provide information about the intro-duction of new materials in the field of manufacturing prostheses, various ways of application of different types of materials as well as the effects these have on the technical characteristics and cost of artificial limbs.

The development of products for the physically disabled has greatly im-proved the quality of life for many, particularly those who need an artificial limb. The loss of a limb can impose dramatic barriers to interacting within our world but with the sophisticated products available, artificial limbs are able to break through these barriers with remarkable impact on the lives of their users.

In 484 BC, Herodotus recorded history's first mention of an amputation. He reported that Hegesistratus, a Persian soldier confined in stocks, freed himself by cutting off his foot and replacing it later with a wooden one. In the 4th century BC, Hippocrates reported the use of ligatures but the technique was lost during the Dark Ages only to be reintroduced by Am-broise Pare, a French military surgeon in 1529. Pare even designed artifi-cial limbs for his patients and supervised their fabrication by a locksmith. In 1674, Morel introduced the tourniquet and amputations became com-monplace in Europe.

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Prior to the mid-19th century there was little development in prosthetic equipment, as surgical technique was crude and medical knowledge to combat infection was limited. Materials used were generally either wood or, for the more wealthy, metal. Prostheses were designed to mimic as closely as possible the behavior of biological limbs. Historically, those who could afford to sought to have prosthetic legs to ensure minimal dis-ruption to their physical compatibility, while the poor had to resort to using crutches or propelling themselves on moveable benches.

The artificial leg developed initially in the world has been an exoskeleton, which was more of a cosmetic replacement than a functional one. Though these appear like natural limbs, they cannot provide normal gait to a per-son and also comfort for usability.

The world has already directed its efforts to fabricate the endoskeleton type of artificial limbs. The endoskeleton replicates the functionality of bones for load bearing and involves proper mechanical joints for normal gait.

The development of new technologies has introduced more new materials in the field of prosthesis manufacturing. Prostheses themselves have im-proved immensely as designers try to come as close as possible to natu-ral limbs, both functionally and visually. This is an additional motivation to look for better quality and lower-cost substitutes of currently used materi-als. The Literature search and review method was used to prepare the present chapter discussing the issue of involving new materials in artificial human limbs and an attempt has been made to cover sufficiently wide scope of world practice in the field being examined.

CRITICAL LITERATURE SEARCH

A literature search enables us to make the best use of previous work about new materials in artificial limbs, and hence to learn from the experi-ences, findings and mistakes of those who have previously carried out similar or related work. A literature search can provide invaluable insight into the program area being evaluated and search is also useful as a source of new hypotheses, to identify potential methodological difficulties, to draw or solidify conclusions, and as input to other data collection tech-niques.

The Literature search and review method was used in the present report utilizing literature sources classified in the following types:

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• BOOKS

Information books referring to the process of developing and usage of Po-rous Plastic to Prosthetic Sockets:

[1] Tetsuya, Watanabe, "Choice of Porous Socket Materials," Na-kamura Brace Co., Ltd., 1998, Japan.

[2] Kiyoyuki, Oji, et al., "Polyester Hydrate," Plastic Material, 4(40), 1972.

Information books dealing with the use of new biomaterials for artificial limbs manufacturing:

[3] Profio, A.E. Biomedical Engineering Wiley, 1993.

• JOURNALS

Information articles dealing with the use of new materials for artificial limbs manufacturing such as carbon fiber, titanium:

[4] Padula, Patricia A. and Friedmann, Lawrence W., "Acquired Amputation and Prostheses Before the Sixteenth Century." The Journal of Vascular Disease. February, 1987.

[5] Romm, Sharon, "Arms by Design: From Antiquity to the Ren-aissance," Plastic and Reconstructive Surgery. July, 1988.

[6] Sanders, Gloria T., “Amputation Prosthetics”. F.A. Davis Com-pany, Philadelphia, Pa., 1986.

[7] Wilson, A. Bennet, jr., “Limb Prosthetics”. Sixth Edition, Demos Publications, New York, N.Y., 1978.

[8] "Life without Limitations“; “Flex-foot: active living”; 10.2000;

Information articles dealing with the process of developing and use of Po-rous Plastic to Prosthetic Sockets:

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[9] Wu, Y., et al., "Scotchcast P.V.C. Interim Prosthesis for Below-Knee Amputees," Bull. Prosthet. Res., 10(40), 1981.

[10] Scheinhaus, Arthur, et al., "A Modification of the Porous Below-Knee Soft Socket Insert," Orthotics and Prosthetics, 32 (1), March, 1978.

[11] Rubin, C. and J.L. Byers, A Porous Flexible Insert for the Be-low-Knee Prosthesis," Orthotics and Prosthetics, 27(3), 1993.

An article providing information on the application of fibre-optic materials in manufacturing prostheses

[12] Dr Edouard Jallot and colleagues. “Bioactive glass gives new hope for bone operations”. “Journal of Physics D: Applied Physics”. 18 October 2000.

Information article on the use of some NASA aerospace materials - foam in manufacturing prostheses:

[13] SPINOFF 1995 is published by the National Aeronautics and Space Administration, Office of Space Access and Technology, Commercial Development and Technology Transfer Division, NASA Headquarters, Washington D.C. 20546. The document is designated as NASA Publication NP-217

• WEB SITES

[14] Atul Mittal, Sangeeta Nangia & Soumitra Biswas. “Artificial Limbs for the handicapped”; http://www.tifac.org.in/news/limbs.htm

[15] Toshiro Nakamura, O.A. Eiji Hatano, M.D. “Process of Devel-opment and Application of Porous Plastic to Prosthetic Sockets” http://www.oandp.org/jpo/14/14202.asp

[16] “Back to nature with biomimickry” http://azaz.essortment.com/biomimickry_rcur.htm

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[17] “Thermoplastic composite for artificial limbs” http://www.andhratoday.com/medicine/fro.htm

[18] GE Polymershapes Cadillac & Commercial; Proflex Product Page http://www.commercialplastics.com/proflex.asp

[19] Design for disability – a story of social and technological change http://www.brunel.ac.uk/research/exploring/Learningfromthepast/Disability.html

[20] Anthropology and Prosthetics http://www.ossur.com/

[21] Science news from MSNBC http://www.msnbc.com/news/271534.asp?cp1=1

[22] Prosthetics http://www.livingskin.com/products.htm

[23] ZDNet; Smart Business; Artificial Strongman by Shelby G. Spires, November 2000. “Plastic muscles may be the key to the factories - and people - of the future”. http://www.zdnet.com/smartbusinessmag/stories/all/0,6605,2635533,00.html

[24] V A Lakeside Medical Center (Valmc) 8/96 Northwestern University Prosthetics Research Laboratory (Nuprl). Joshua S. (Rovick) Rolock, Ph.D., Dudley S. Childress, Ph.D., Kerice Tucker http://www.repoc.northwestern.edu/progress/jrrd.dva.9608.SqrtShpe.html

[25] Alan Johnson MP; MOD Defence Industrial Policy Seminar; Wednesday, April 05, 2000; 6. Dual-use, technology transfer and defence diversification http://www.dti.gov.uk/ministers/speeches/johnson050400.html

[26] The Wall Street Journal, August 14, 1998; “Hydraulics and Computers Help Artificial Limbs Get 'Smarter'”. http://www.limbsforlife.org/media-wsj.htm

[27] BIOENGENEERING http://www.comptons.com/encyclopedia/ARTICLES/0000/00225792_A.

[28] Otto Bock's Home Page http://www.ottobockus.com/info/index.htm

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Figure 1

[29] NASA Builds Muscles; Tiny tools for space hold promise for the disabled; Dan Cray; March 22, 1999 Vol. 153 No. 11 http://www.time.com/time/magazine/printout/0,8816,21477,00.html

[30] The Story of Atlas Limbs by Paul Jamieson, http://www.dpa.org.sg/DPA/publication/dpipub/fall98/dpi30.htm

DATA ANALYSIS Inventors have tried to replicate what nature cannot replace. Prosthetics have been used since at least 300 BC, when crude devices 'containing metal plates hammered over a wooden ore, were attached to an ampu-tated limb. For centuries, wood and leather were the only materials for prosthetics for artificial limbs but today's physical therapist has a much wider range encompassing advanced plastics and carbon fibre, which are much stronger, lighter and more durable. [14] (Atul Mittal, Sangeeta Nan-gia & Soumitra Biswas, 2000)

Introduction of new technologies and materials is nowadays an over-whelming process. This is especially true for medical science including manufacturing prostheses for artificial human limbs. New materials are involved in various component parts of prostheses

The majority of materials used today for artificial limbs and accessories are non-porous plastics. The prescribers and manufacturers of artificial limbs are primarily concerned with the functional and structural characteristics of prostheses, whereas the majority of users express a decided preference for improvement in the feeling of compatibility and comfort when wearing these artificial limbs. Perspiration and body odor emanating from the socket in contact with the skin present an acute problem. [1] (Tetsuya, Watanabe, Nakamura Brace Co., Ltd., 1998, Japan)

The insides of sockets made from non-porous plastic material become quite similar to a steam bath, because there is no ventilation for the heat and perspi-ration generated.

In the past, prosthetic sockets were made of porous materials such as wood and Tuflite. In recent years, however, the majority of sockets are

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made of non-porous materials. [15] (Toshiro Nakamura, O.A. Eiji Hatano, M.D.)

Tetsuya Watanabe and Masayoshi Hatano, et al., recognized that polyes-ter hydrates approximated the porosity and moisture absorbing property of wood while still retaining the heat hardening property of plastics and, despite many repetitive trials and errors, progress was achieved toward producing a socket made from porous material (Fig. 1).

Initial consideration of selected materials referred to the following possible materials.

1. Sintering molding of polyethylene and polypropylene: Sintering molded olefin resin products are widely employed as filtration mate-rials; and depending upon the requirements, various materials can be produced with differing degrees of porosity. These materials are advantageous due to the differing degrees of porosity and ventila-tion, which can be achieved over a wide area. However, for correct molding, metal molds and special engineering knowledge of tem-perature control during the molding process are required. The sin-tering molding of sockets thus incurs considerable cost.

2. Secondary processing of sintering molded olefin resin products: Secondary processing experiments were attempted using the molded products in sheet and tubular form.

• Heat was applied to sheet material and it was drawn over socket forms. However, as the sheet was elongated, the po-rosity was likewise deformed and extended beyond a practi-cal degree.

• Forming tests were tried with finer porosity material, but sin-tered molded finer material was not as easily molded. Draw-ing into socket shapes invariably caused the material to rup-ture.

Therefore, the material was judged inappropriate for sockets.

3. Ceramics: Costs incurred for the necessary metal molds and the specialized engineering for molding were prohibitive.

4. Polyester hydrates: Ready availability of the material and the re-quired basic engineering knowledge made this material particularly appropriate for socket production. [2] (Kiyoyuki, Oji, et al., "Polyes-ter Hydrate," 4(40), 1972).

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Figure 2

The production of artificial limbs using porous material demands that po-rous material also be employed for the inner socket. To satisfy this re-quirement, the following materials were considered:

1. Polyethylene foam: Non-porous, separate cellular polyethylene foam has been widely used for the inner sockets, but when formed into continuous cellular foam, its rigidity is weakened (inferior creeping characteristic) and is liable to readily collapse.

2. Polyurethane foam: This material, which recently became available on the market, can be heat.

When this socket is dried at temperatures of 60° - 80°C, the water con-tained in the plastic evaporates and the areas where water was contained become cavities, resulting in the formation of a porous and permeable socket.

Characteristics, which can be enumerated are: (1) moisture absorbency and permeability; (2) breathes moisture; (3) has the texture of wood; and (4) apparent specific gravity is light. Also it can be adjusted by changing the water/plastic mix ratio.

Porosity can be determined by the apparent changes in weight caused by changes in moisture content during processing.

The relationship between the amount of water added and the strength of the material can be judged according to the increasing amount of water, which reduces the strength of the material and results in a relative reduc-tion in weight.

Some others new materials Proflex and Proflex with Silicone are extruded Ethyl Vinyl Acetate (EVA) sheets. They are an inherently flexible, cush-ioning material that is thermoformed to produce elastic suction sockets in artificial limbs (Fig. 2). Proflex is semi-transparent. The 2% silicone additive in Proflex with Silicone improves performance characteristics and gives the material a whitish color.

Proflex is pure Ethyl Vinyl Acetate (EVA) with a vinyl acetate concentration approaching 30%. This gives Proflex the right combination of stiffness and yield that Prosthetists have favored for years. A semi-transparent material can aid in viewing socket fit for increased user comfort. [18] (GE Polymershapes Cadillac & Commercial)

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Proflex with Silicone is nearly pure Ethyl Vinyl Acetate (EVA) sheet with 2% silicone (in an ethylene carrier) added. The addition of silicone has the benefit of less surface tack for increased user comfort. Proflex with Silicone also has a vinyl acetate concentration approaching 30% for the preferred combination of stiffness and yield.

Proflex and Proflex with Silicone are available in three thicknesses [18] (GE Polymershapes Cadillac & Commercial):

Stock Shape Size Gauge

Sheet 48' x 96" 1/4" (6mm), 3/8" (9mm), 1/2" (12mm)

Composites, the wonder material, after proving its worth in aerospace now serves mankind with advanced rehabilitation aids. The industry is really moving towards composite material because they are lighter in weight, easier to work with and more durable.

The Department of Aeronautical Engineering, Madras Institute of Tech-nology (MIT) has been extending technology support in terms of design, prototype development and complete testing of composite limbs.

In India, commonly used artificial legs are of exoskeleton type made of high-density polyethylene. Though the imported endoskeleton types of limbs are available in India, they are very expensive.

This below-the-knee endoskeleton limb consists of Five parts: a FRP tu-bular structure fabricated by filament winding of glass fibre in epoxy ma-trix top & bottom connectors made by injection molding of glass filled ny-lon, a polyurethane foot with composite keel embedded in it and a poly-propylene socket to accommodate the amputee stump.

The socket made of polypropylene is patient specific and does not create any problems like pressure sores even for diabetic patients. The FRP tube connects the socket to the foot. The connectors between the socket and tube and tube and the foot can be adjusted for angular alignment of the limb. [14] (Atul Mittal, Sangeeta Nangia & Soumitra Biswas, 2000).

Carbon-fiber is a material which has recently found exceptionally suc-cessful application in the manufacture of artificial limbs. [4] (Padula, Patricia A. and Friedmann, Lawrence W. February, 1987).

Charles A. Blatchford&Sons Ltd. of Hampshire, southern England is a world-renowned manufacturer of artificial limb components and assem-blies. The company has a reputation for innovation, having pioneered the

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use of carbon-fibre reinforced plastics and microprocessor controls in ad-vanced artificial lower limbs. The artificial limb named “Atlas” concept de-veloped by Blatchford applies innovative materials and design to best use the scarcest resource, that of skilled manpower. The company used a technology, which is totally new in its field to produce highly functional ar-tificial limbs quickly and at a reasonable cost.

The design of the socket connection allows different socket materials to be considered. Ideally a thermoplastic draped socket over a cast would be chosen but laminated sockets can also be accommodated. [30] (Paul Jamieson; http://www.dpa.org.sg/DPA/publication/dpipub/fall98/dpi30.htm).

Now the future of the industry will also be dependent on the effective use technology. There has always been a certain amount of benefit in the application of defence technology to civil markets. This has been particularly strong in civil aerospace, such as in propulsion or materials. There are also many examples of spin-off from defence technology to industry more widely. The medical sector benefits from titanium alloys for artificial joints and lightweight carbon-fibre material for artificial limbs. [25] (Alan Johnson, 2000).

Flex-Foot, Inc. designs and markets innovative lower limb prosthetic de-vices for amputees of all ages and activity levels. All premium Flex-Foot products are made from 100% carbon fiber, a material used extensively in the aerospace in-dustry for its superior strength and flexibility. [8] ("Life without Limitations“; “Flex-foot: active living”; 10.2000). Mr. Van Phillips, who founded Flex-Foot Inc., now based in Aliso Viejo, Calif, produced a model artificial limbs named Flex-Foot from the strong, but lightweight carbon-fiber composite, used by the area's aerospace manufacturers. Flex-Foot is the J-shaped, which uses the energy from a person's step to propel the next step. The Flex-Foot also has a flat spring at the toe and heel, which acts like a diving board to put bounce into a person's walk. [26] (The Wall Street Journal, August 14, 1998).

Figure 3 ICELOCK 100 Fabrica-

tion Kit Puck with Carbon Braid, designed for lami-

nation

Figure 4 ICELOCK Ratchet

TP 122 with Titanium Pyramid

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The company ÖSSUR has been also using carbon (Fig. 4) and titanium (Fig. 5) as materials for manufacturing various prostheses component parts. [20] (Anthropology and Prosthetics; http://www.ossur.com/). Based in Reykjavik, Iceland, ÖSSUR has been at the forefront of re-search and development in the field of prosthetics and techniques since 1971. Founded by Össur Kristinsson, the company operates through a global network of sales outlets and agencies, offering a broad range of customized products specifically designed to suit individual prosthetic needs.

Otto Bock Orthopedic Industry, Inc. is one of the most famous companies in the world, which deals with not just the development and production of artificial limbs and their accessories but also makes research and implementation of new materials. Otto Bock manufacture C-Leg® System, the world's first completely computer-controlled artificial leg for amputees. (Fig. 5). The product is made of lightweight carbon fiber material. It is powered by a rechargeable lithium ion battery with 25-30 hours of functional capacity. The C-Leg incorporates an on-board microprocessor-controlled hydraulic. [28] (http://www. ottobockus.com /info/index.htm).

Otto Bock's 1C40 C-Walk (Fig. 6) is a unique carbon fiber foot that combines comfort with dynamic response and multi-axial rotation. First, the C-element cushions the foot for a

comfortable heel strike. Next, the stored energy in the C-element is released to facilitate a smooth roll-over to mid-stance. As the forefoot loading increases, the C-spring element loads again along with the base spring.

Given underneath is a list of some of the materials used and offered by Otto Bock:

Figure 5 C-Leg® System is ma-de of lightweight carbon

fiber material

Figure 6 1C40 C-Walk is a unique carbon

fiber foot

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Description Category Bonding Agent for Silicone Elastomers Catalyst for Silicone Gel Elastomers Cleaner for UltraPro™ Sprayer Cleaners C-Orthocryl® Resin Lamination Resins Cyamet Fast Set Adhesive Adhesives Fast Curing Putty - Akemi Fillers Finish Concentrate - Part A Lacquers Hardener for Pedilen® Soft Foams Foam Resins Isopropylalcohol Cleaners Orthocryl® Extra Flexible Resin Lamination Resins Orthocryl® Flexible Resin Lamination Resins Orthocryl® Lacquer - Clear Lacquers Orthocryl® Sealing Resin Adhesives/Lacquers/Lam. Resins Special Adhesive for Textiles Adhesives Otto Bock Epoxy Adhesive for Use with 636W19 Adhesives Otto Bock Light Putty Fillers Parting Agent for Orthocryl® Resins Parting Agents Parting Agent HS Parting Agents Pedilen Flexible Foam 150 Flexible Foams/Foam Resins Pedilen Rigid Foam 200 Foam Resins/Rigid Foams Pedilen Rigid Foam 300 Foam Resins/Rigid Foams Pedilen® Duplicating Plastic Foam Resins Pedilen® Flexible Foam 300 Foam Resins Pedilen® Rigid Foam 450 Foam Resins Pigment Paste - Tan Lamination Resins/Pigments Pigment Paste - White Lamination Resins/Pigments Pigment Paste - Yellow Lamination Resins/Pigments Plastic Wood Fillers Polyester Resin Resins Sealing Resin Adhesive Gel Adhesives/Fillers Silicone Heat Conduction Paste Adhesives Special Adhesive for OrthoGel™ Liner Adhesives/Lacquers Silicone Gel Elastomers Socket Lacquer - Clear Lacquers Solvent C Thinners Thinner for Acrylic Resins Thinners Special Cleaner for Cosmetic Gloves Cleaners Special Lacquer - Colorless Lacquers Spray Lacquer - Caucasian Lacquers Spray Lacquer - Dark Brown Lacquers Stabilizing Agent for Silicone Gel Elastomers Teflon® Spray Lubricants/Parting Agents

Today's prosthetic devices utilize hydraulics, sophisticated knee joints, flexible carbon-fiber feet, silicone, plastics, and other high-tech products

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Figure 7 This photomicrograph shows the tangle of carbon nanotubes

within a thin sheet of "Bucky paper."

that enable many people to walk and move more naturally and comforta-bly than was ever dreamed possible. Advances in microelectronics allow artificial arms and hands to be manipulated more naturally. Prostheses have also improved in appearance. Modern artificial limbs incorporate fin-gers and toes, and some even appear to have veins. [3] (Profio, A.E. Biomedical Engineering Wiley, 1993).

Prosthetic devices now in use range from artificial limbs used to replace diseased, missing, or malfunctioning limbs, to systems that duplicate the function of critical body organs. Patients unable to walk due to disabling diseases or injuries can very often return to normal when their hip and knee joints are replaced with artificial joints. Most artificial joints are made from metals such as titanium, which have high strength, low weight, and good compatibility with body tissues. These devices are mechanically in-serted into the adjacent, healthy bone and fixed in place using an ortho-pedic bone cement called polymethylmethacrylate, or PMMA. Because metals that rub against other metals eventually wear down and produce hazardous splintered particles that may migrate throughout the body, bio-engineers have developed polymers, such as polyethylene, that are less likely to wear down. Metal surfaces are designed to articulate, or move, against these polymer surfaces. Joints that bear little force, such as the fingers, can be made entirely of polymeric materials. [27] (BIOENGENEERING; http://www.comptons.com/encyclopedia/ARTICLES/0000/00225792_A.).

Electroactive polymers (EAPs) use a layered matrix of carbon and plastics to mimic human muscle. Ron Pelrine of SRI International and Yoseph Bar-Cohen of the Jet Propulsion Laboratory are developing the emerging technology. In a process similar to the workings of human muscles, EAPs can expand and contract with electrical current; this movement, in turn, can move mechanical arms and legs, and close robotic hands. The artificial muscle is already more than just a lab experiment. NASA has chosen it to play a minor role in space exploration with the launch of an asteroid probe in 2002. Plastic muscles will work as wipers on the Mu Space En-gineering nanorover's camera. [23] (ZDNet; Shelby G. Spires, 2000).

An international team of researchers created thin sheets of tangled nanotubes - dubbed “Bucky paper” in honor of the

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late scientist-philosopher R. Buckminster Fuller. The nanotubes are long, thin straws of carbon measured in billionths of a meter, possessing un-usual strength as well as unusual electrical properties. When a charge is introduced, the nanotubes’ molecular structure. They applied strips of the Bucky paper to both sides of a length of double-sided adhesive tape, hooked up electrical leads to each sheet and placed the laminated “mus-cle” in a saline solution. Two sheets of carbon nanotubes are separated by double-sided adhesive tape, then hooked up to a battery. The intro-duction of a negative charge causes one side to expand more than the other - and as a result the strip curls. That's how the "muscle" flexes.

When current was applied, both pieces of Bucky paper expanded - but the negatively charged paper expanded more. As a result, the artificial muscle flexed.

The nanotube system is by no means the first type of artificial muscle to be developed. Scientists at NASA and elsewhere have developed light-weight polymers that move in response to electrical charges - and in fact could be used as “windshield wipers” for future interplanetary rovers.

The new material, however, has the advantage of incredible strength: “Carbon nanotubes are said to be among the toughest, strongest materi-als known. They have diamondlike mechanical properties,” Baughman said. [21] (Science news from MSNBC).

Yoseph Bar-Cohen, a researcher at NASA’s Jet Propulsion Laboratory who is an expert on artificial muscles made with polymers, said the newly announced technology “has great potential.”

Could nanotube muscles serve as replacements for human parts? “There’s a lot of hard work ahead, compared to what we’ve done so far,” Baughman said. But eventually, the technology just might be used for arti-ficial hearts, artificial limbs and replacement valves, he said.

Bar-Cohen said he and his colleagues were working toward the day when artificial muscles could win an arm-wrestling match against a flesh-and-blood bicep - an age when people could buy muscles as easily as they could buy bicycles or skateboards.

The research received funding from the Pentagon’s Defense Advanced Research Projects Agency, and also involved scientists from the Univer-sity of Florida, the University of Wollongong in Australia, the University of Pisa in Italy and the Max Planck Institute for Solid State Research in Germany. [21] (Science news from MSNBC).

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A more specific but very significant in its designed purpose is the applica-tion of another new type of material in manufacturing prostheses for artifi-cial limbs - bioactive glass, which has been studied by Dr Edouard Jallot. [12] (Dr. Edouard Jallot and colleagues; “Journal of Physics D: Applied Physics”. 18 October 2000).

The work, carried out by Dr Edouard Jallot and colleagues in France, looks at how a material called bioactive glass, used to bond artificial limbs to bone, reacts when in the body for a considerable length of time.

Life expectancy increases with the health of the population, but this means many people outlive their bones, teeth and mineralized tissue. Replacement synthetic materials are sought after, but finding something suitable is not straightforward. Tissue in the body can reject any foreign material that is placed in contact with it, so extensive tests have to be done before a material can be used in operations. In bony tissue, im-plants need to be stable for a long time and firmly fixed to the bone, but most implants used today suffer problems at the interface between the material and the tissue.

Bioactive glass has been developed to promote an intimate contact be-tween bone and any foreign material or implant. Many critical and com-plex reactions which determine the acceptance of the material take place at the bone/implant interface so an understanding of biocompatibility as well as physico-chemical reactions of bioactive glass at the interface is essential. For bioactive glass to bond to living bone, these materials must produce a layer of what is known as biologically-active apatite. This layer chemically bridges the host tissue and the implant material. This layer is an important step in the bonding mechanism between bone and bioactive glass. [12] (Dr. Edouard Jallot and colleagues; “Journal of Physics D: Ap-plied Physics”. 18 October 2000).

Results from this work give a much better understanding of bioactive glass and show that, although bioactive glass is already used in some small operations, it could have many new and important applications in the future development of artificial limbs.

Silicone skin Livingskin® (Fig. 8) is an other new material. Livingskin® has been recommended to finger amputees, hand and arm amputees, toe and foot amputees, and leg amputees by rehabilitation experts, such as hand surgeons, prosthetists, hand therapists, case managers and anaplastologists.[22](Prosthetics; http://www.livingskin.com/products.htm).

Livingskin® is the trademark name for Aesthetic Concerns Prosthetics, Inc. unique lifelike prosthetic skin. As illustrated below, LIVINGSKIN® is

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made to resemble human skin by simulating the three dermal layers of natural human skin.

The top or surface skin layer, the epidermis, holds the pores, wrinkles and fingerprints. This layer also contains melanin, the pigment that creates suntan and darker skin types. Like human skin, LIVINGSKIN®’s epider-mal layer also contains pores and skin detail. The natural melanin is simulated through custom applied pigments saturated within this silicone skin layer.

The dermis is the middle layer of skin. This translucent layer consists of a cloudy mesh of specialized skin cells. LIVINGSKIN®’s dermal layer is also a mesh of translucent (silicone) cells that closely resemble nature’s original.

The final and deepest skin layer, the subcutaneous layer, contains the fat cells and blood which allow for much of the visible skin color. LIVING-SKIN®’s subcutaneous layer is a custom pigmented silicone matrix of varying colors painstakingly applied to match each individual.

CRITICAL DISCUSSION

Society's demand for a rapid and diverse succession of new, specialized materials has resulted in a concentrated, systematic approach to materi-als research and education. In the past, specialized materials were de-veloped through a trial-and-error process. Today, the tools and expertise of scientists are being combined with those of engineers, resulting in pro-ductive cooperation in both applied and theoretical areas.

Figure 8

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The search for new materials and the need to make better use of old ones continues to broaden the field of materials science. The ability to create the next generation of advanced materials--polymers, ceramics, metals, semiconductors or superconductors - and advanced devices - such as lasers, micromotors, or artificial limbs - requires the control of materials and interfaces with atomic to macroscopic level understanding. This is the domain of materials science.

The accelerating pace of technology is having an immense effect on the lives of disabled people. The artificial limbs and orthopedic braces that prosthetics industry developed and fabricated are lighter in weight and better in comfort and appearance than ever before.

As a result of the data analysis carried out so far on the basis of the data collected using the Literature search and review method we can now pro-ceed with the discussion on the use of some specific new materials and their application in the manufacture of prostheses for artificial limbs.

Polyester Hydrate Polyester hydrate sockets are constructed of a fine, lightweight material, which simulates the texture of wood. These sockets provide the amputated residual limb with a totally clean, more comfortable environment. The exterior appearance, however, may be slightly inferior and re-quires considerable care in controlling the hardening time. In some cases, the socket may require special reinforcement. The development of polyester hydrate sockets now makes it possible for amputees to over-come some of the discomforts associated with wearing pros-theses. Another benefit is the low production cost which is possible using conventional lamination methods.

The unsaturated polyester resin (commercially branded Nakaresin®), a

Figure 9

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product developed by Kayaku Nouilly Co., Ltd., permits the absorption of water into the resin.

During production of the socket, first the unsaturated polyester is sub-jected to high-speed agitation and water is gradually added for an emulsi-fied distribution. A prescribed amount of hardening agent is then added to this emulsion and is injected into the molds where it gels in approximately 10-15 minutes. An open cell socket is formed from a plastic which con-tains water distributed throughout its structure (Fig. 9).

Carbon Fibres The Industrial Revolution grew out of the development and use of steel, the revolutionary material of the past. However, steel has many limitations - weight, corrosion, fatigue, etc. Composite Materials Engineering is over-coming many of these limitations.

A composite material is any material made up from two or more other ma-terials, eg. carbon fibres in a plastic matrix or aramid fibres in rubber.

The carbon fibres is known for its high specific stiffness and strength, given by carbon fibres. The material has an advantageous combination of good mechanical properties and low weight.

The properties of the material vary depending on the content and orienta-tion of the fibres.

It is used for very stiff and light structures within sport equipment, aero-space, medical equipment (protheses) and prototyping.

The carbon fibres can be woven into mats, which can be pressed into 3-dimensional shapes, and then plastic is added. The most used processes are manual fibre molding, RTM, filament winding, pultrusion, and auto-clave injection.

Technical properties of carbon fibres are excellent. They have a modulus of elasticity similar to steel, they do not corrode, they have a very high tensile strength and they are chemically inert. Due to the latter property they are not influenced by the aggressive alkaline environment of the sur-rounding concrete and environmental boundary conditions (e.g. chlo-rides). The tensile strength of carbon wires depends on the type of wire and equals about 4500 MPa.

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Titanium With efforts to reduce basic problems such as friction between the pros-thesis and the body; development of energy-storing parts that will receive an impact and rebound; and use of titanium implants, which were pio-neered in dentistry and will grow directly to bone, providing a stronger prosthesis, the world of rehabilitation medicine is offering patients new limbs that are not only lifelike, but perhaps will be an improvement on what nature originally provided

Titanium is a light and strong metal.

From a materials science viewpoint, titanium has three types: α - titanium, α+β - titanium and β - titanium. The α -titanium is high purity commercially pure unalloyed titanium;

α+β - titanium is represented by the Ti-6Al-4V alloy; and -titanium in-cludes the Ti-15V-3Cr-3Sn-3Al alloy. titanium α+β and β - titanium are generally called titanium alloys. As unalloyed titanium is softer than stainless steel. It is also an indispensable material when sophisticated fabrications is required. Titanium alloys, materials with other metals added to titanium, exceed stainless steel in strength. There are various titanium alloys, each with its own characteristics.

Bioactive glass Material called bioactive glass has been developed to promote an inti-mate contact between bone and any foreign material or implant. Bioactive glass, used to bond artificial limbs to bone, reacts when in the body for a considerable length of time.

This material definitely has its future and will find wider application and presence in the field of manufacturing prostheses for artificial limbs.

Biomaterials Today's engineers are not only developing new polymers for the covering of the limb. The use of computers has greatly advanced the field of pros-thetics and orthotics. Sensors, bioelectrodes, and computers within these devices are being used to develop new systems that will provide the user with a feedback mechanism which tells the patient the type of surface un-derfoot and the amount of pressure needed to sustain balance. Elec-trodes are placed on the sole of the device with feedback wires running up through the trunk to the residual limb. Sensors on the end of the limb

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are stimulated by the electrodes, giving the person feedback. Myoelec-trics involve the use of electromyogram signals (EMGs) from the brain for muscle and device control. This allows for the apparatus and residual limb to interface directly. When the muscles in the residual limb are con-tracted, the device responds.

These new advances are opening new doors for disabled people, but en-gineers have a greater goal ahead. The artificial limb does provide new opportunities to those who have lost limbs. The next step in development involves a completely bioartificial limb. Scientists and engineers are work-ing together to develop new biomaterials that will work together as a bioartificial limb. Artificial skin has is already being used in surgery and in the treatment of burn victims. Scientists hope to use this concept as a base for the protective covering of artificial limbs. Several graduate stu-dents at MIT are working together on the development of artificial muscle. Once the actual muscle is constructed, the students must create a detec-tion and response system that mimics the nervous system within the muscle. In addition, bioartificial bone is another aspect that is being con-sidered. Once attached to the patient, scientists hope that the body will accept the prosthesis as an extension of itself. Researchers believe that the artificial limb will become a reality in the near future.

Genetics and Biotechnology Technology in the biomedical field usually involves a “systems” or “de-vice” used to carry, separate, shield, or otherwise assist in a biological process. Bioactive polymers, artificial organs, drug delivery devices, artifi-cial limbs, and orthopedic appliances are examples.

The fields of genetics and biotechnology represent an exploding area of technology, which includes “gene splicing” and combining genetic materi-als from various sources, such as viruses, bacteria, fungi, plants and animals. Genes synthesized in this way are used to direct cells to make new products, such as novel proteins, or to produce greatly augmented quantities of cellular products.

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CONCLUSIONS

We are constantly reminded that we live in a world that is both dependent on materials and limited by materials. Everything we see and use is made of materials derived from the earth - large buildings, supersonic aircraft, coffee cups, advanced computers, sports equipment, and biomedical de-vices such as artificial limbs, joints, and implants. All of these are made of materials having specific properties that are the result of carefully con-trolled processes to convert raw materials into useful engineering materi-als. Furthermore, we read about exciting new developments in high-speed transportation systems, superconductors, shape-memory material devices, and diamond coatings, which are all possible because of ad-vances in the science and engineering of materials.

New technologies developed through engineering science will continue to make startling changes in our lives in the 21st century, and metallurgical and materials engineers will continue to be key contributors to these changes and advances. Materials/metallurgical engineers deal with the science and technology of producing materials that have properties and shapes suitable for practical use. Materials include metals, ceramics, polymers (plastics), and their combinations called composites. The activi-ties of materials/metallurgical engineers range from materials production, including extraction from ores and recycling, to the design, development, and processing of these materials for use in aerospace, transportation, electronics, energy conversion, and biomedical systems.

Artificial limbs have been used for hundreds of years, but new materials and creative designs in the last few years have brought a surge of innova-tion.

Old materials have gradually stepped back leaving the new brands ahead.

• wood - ............................ plastic materials

• leather- ........................... epoxy and polyester resins

• metal- ............................. polymers like HDPP, HDPE, PU Silicon

• Re-inforcing materials- ... carbon fibre glass, fibre

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What is the future?

The future is for Bio-materials, Biomedical Engineering and Genetics.

Biomedical engineering applies engineering techniques to health-related problems. Biomedical engineers develop aids for the deaf and blind. They cooperate with physicians and surgeons to design artificial limbs and or-gans and other devices and machines that assist or replace diseased or damaged parts of the body. Biomedical engineers help provide a wide va-riety of medical tools, from instruments that measure blood pressure and pulse rate to surgical lasers, concentrated beams of light that can be used to perform delicate operations.

In choosing materials for artificial limbs, biomedical engineers must un-derstand the physical and chemical properties of the materials and how they interact with each other and with the body. One of the chief areas in biomedical engineering research focuses on the development of materials that the human body will not reject as foreign substances. In their work, biomedical engineers often use principles of biology, chemistry, genetics and medicine and of electrical, materials, and mechanical engineering.

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Understanding Commonly Used Terms

amputation = ..................... the loss or absence of all or part of a limb.

cosmetic cover = .............. a plastic foam or rubber material, laminate or stocking that gives a prosthetic device a more natural appearance.

pneumatic/hydraulic =...... provides controlled changes in the speed of walking.

myoelectric prosthesis = . uses electrodes mounted within the socket to receive signals from muscle contraction to control a motor in the terminal device, wrist rotator or elbow.

orthosis/brace =................ a plastic or metal device used to straighten and/or Support a body part, improve function, or aid recovery.

prosthesis =....................... an artificial replacement for a body part.

endoskeletal/pylon = ........ prosthesis that consists of a lightweight plas-tic or metal tube encased in a foam cover.

exoskeletal = ..................... prosthesis made of plastic over wood or rigid foam.

prosthetist = ...................... a patient-care practitioner who evaluates, designs, fabricates and fits artificial limbs.

residual limb = .................. portion of limb remaining after amputation, sometimes referred to as a stump.

socket = ............................. portion of prosthesis that fits around residual limb/stump and to which prosthetic compo-nents are attached.

hard socket = .................... a prosthetic socket made of rigid materials.

soft socket = ..................... inner socket liner of foam, rubber, leather, other material for cushioning the residual limb.

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CHAPTER 2 SHAPE MEMORY ALLOYS & THEIR APPLICATIONS

INTRODUCTION This chapter presents a review for Shape Memory Alloys (SMAs), metal alloys, which can undergo substantial plastic deformation, and then be triggered into returning to its original shape by heat application. Since its discovery, many researchers have been working on different aspects of these alloys. Based on their studies some basic structural aspects are emphasized in the present document that contribute to the complexity of shape memory. This specific class of alloys is defined and their crystallo-graphic structure and mechanisms causing its unique characteristics are discussed. Particular consideration is given to a proliferation of diverse applications of SMAs in a variety of industries. Finally, further research directions for SMAs and their application areas are also identified.

Literature relating to the nature & basic characteristics of Shape Memory Alloys and their applications will be reviewed in the present report. The aim of the assigned review defined the main objectives of the study. The review will be concerned particularly with the definition of SMAs, identify-ing their general characteristics, crystallography, thermomechanical be-havior and their most important applications. The research work consid-ered, includes studying the major SMAs properties and critical analysis of their microstructural & thermodynamical aspects.

The main learning objectives of this literature review are to generate and define the structure of the main topics forming the integrity of the study: carry out a literature review on the subject of the study; demonstrate abil-ity to acquire, cumulate and select required information and distribute this among the main topics; make a critical analysis and generate suggestions for further work; provide for permanent link between used quotes and studied theses, and relevant data sources used to acquire this informa-tion.

Hodgson (1999) states that “Smart materials' are an innovation that has revolutionized the world of engineering over the last few decades” (p.32) [1]. The subject of this study is one of the most widely used of them - Shape Memory Alloys that are able to undergo major deformation and then, when triggered by heat, snap back to their original shape.

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They are a unique class of metal alloys, which can be deformed severely and afterwards recover their original shape after a thermomechanical cy-cle (shape memory effect), or a stress cycle within some appropriate temperature regimes (pseudo-elasticity). They exhibit peculiar thermome-chanical, thermoelectrical, and thermochemical behavior under different mechanical, thermal, electrical, and chemical working and environmental conditions, according to a recent study (Khalil-Allafia et al., 2002, p.29) [2]. The fields of SMAs application are very diverse ranging from every-day consumer products to biomedical implants to space applications. Hodgson (1999, p.43) identified SMAs of particular practical interest to be the nickel-titanium alloys and copper-base alloys [1].

The extremely complex behavior along with the increased use of the SMAs in innovative applications in many engineering and science fields (Duerig et al., 1990 p.59) [3] results in a greater need for a better under-standing of these materials. Therefore, there is a particular interest for them aimed at further understanding the mechanisms that determine their unique characteristics as well as expanding their field of application (Graesser & Cozzarelli, 1994, p.95) [4].

The main aim and the objectives of the present study are hereby pursued applying the research method of Literature Search & Review. Some limi-tations to the scope of the study were required due to the rather wide field of the subject’s area and it was therefore directed mainly to SMAs appli-cations.

Finally, this work is based on the assumption that SMAs are an excep-tionally promising and dynamically developing class of materials with in-novative ideas for applications.

REVIEW OF THE SMAs

Definition of a Shape Memory Alloys As an introduction to the report’s field it is worth reviewing some of the SMAs definitions, which various researchers come up with. Shape mem-ory refers to the ability of certain materials to “remember” a shape, even after severe deformations: Once deformed at low temperatures, these materials will stay deformed until heated, whereupon they will spontane-ously return to their original, pre-deformed shape. Oksuta and Wayman (1998, p.38) give one of the shortest and most accurate definitions incor-porating into them the names of the basic SMA’s properties. They define Shape Memory Alloys as metals, which exhibit two very unique proper-ties, pseudo-elasticity, and the shape memory effect [5].

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According to Hodgson (1999, p.57) the term Shape Memory Alloys is ap-plied to that group of metallic materials that demonstrate the ability to re-turn to some previously defined shape or size when subjected to the ap-propriate thermal procedure. He reveals that these materials can be plas-tically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deforma-tion [1].

Alternatively, Lagoudas, D.C., Qidwai, M.A., Entchev, P.B., & DeGiorgi, V.G. (2001, p.42) described Shape Memory Alloys as a unique class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature [6].

History Discovered in the early 30's and industrially rediscovered in the late 60’s, Shape Memory Alloys exhibit interesting characteristics due to a solid-solid phase transition (Meunier, 1995) [7]. The first recorded observation of the shape memory transformation was by Chang and Read in 1932. They noted the reversibility of the transformation in AuCd by metal-lographic observations and resistivity changes, and in 1951 the shape memory effect (SME) was observed in a bent bar of AuCd. In 1938, the transformation was seen in brass (CuZn). However, it was not until 1962, when Buehler, a researcher at the Naval Ordnance Laboratory in White Oak, Maryland, and co-workers discovered the effect in equiatomic nickel-titanium (NiTi), that research into both the metallurgy and potential practical uses began in earnest Lagoudas (2001, pp 8667) [6]. Nickel-titanium alloys have been found to be the most useful of all SMAs. The generic name for the family of nickel-titanium alloys is Nitinol. (Kauffman and Mayo, 1993, p.5) [8]. Within 10 years, a number of commercial prod-ucts were on the market, and understanding of the effect was much ad-vanced.

Shape Memory Alloy Types Two types of SMAs can be distinguished based on the manner of causing the shape memory exhibition - one-way shape memory and two-way shape memory. Hodgson (1999, p.57) [1] classifies materials exhibiting shape memory only upon heating as having a one-way shape memory. According to him some materials also undergo a change in shape upon recooling. These materials have a two-way shape memory. SMAs could alternatively be classified based on the type of metals consti-tuting their specific alloys. Since the discovery of Ni-Ti, at least fifteen dif-ferent binary, ternary and quaternary alloy types have been discovered

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that exhibit shape changes and unusual elastic properties consequent to deformation. Some of these alloy types and variants are shown in table 1 (Anson, 1999, pp746) [9].

Table 1 Shape memory alloy types (Anson, 1999) [9]. Titanium-palladium-

nickel Uranium-niobium Copper-aluminum-

iron Nickel-titanium-copper Hafnium-titanium-

nickel Titanium-niobium

Gold-cadmium Iron-manganese-silicon

Zirconium-copper-zinc

Iron-zinc-copper-aluminum

Nickel-titanium Nickel-zirconium-titanium

Titanium-niobium-aluminum

Nickel-iron-zinc-aluminum

Of all these systems, the NiTi alloys and a few of the copper-base alloys have received the most development effort and commercial exploitation, underlined by Lagoudas (Lagoudas et al., 2001, 8665) [6].

Crystallography of the SMAs According to Hodgson (1999, p.56) [1] the thermoelastic martensites are characterised by their low energy and glis-sile interfaces. These interfaces can be driven by small tem-perature or stress changes. As a con-sequence of this, and of the constraint due to the loss of symme-try during transforma-tion, thermoelastic martensites are crys-tallographically re-versible.

The herringbone structure of athermal martensites essentially consists of twin-related, self-accommodating variants (Fig. 1b). The shape change among the variants tends to cause them to eliminate each other.

Figure 1. (a) Beta phase crystal. (b) Self-accommodating twin-

related variants, A, B, C, and D, after cooling and transformation to martensite. (c) Variant A becomes

dominant when stress is applied (Hodgson, 1999) [1].

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As a result, little macroscopic strain is generated. In the case of stress-induced martensites, or when stressing a self-accommodating structure, the variant that can transform and yield the greatest shape change in the direction of the applied stress is stabilized and becomes dominant in the configuration (Fig. 1c). This process creates a macroscopic strain, which is recoverable as the crystal structure reverts to austenite during reverse transformation. (Hodgson, 1999, p.57) [1].

Thermomechanical Characteristics

Austenite and Martensite Phases The SMAs have two stable phases - the high-temperature phase, called austenite and the low-temperature phase, called martensite. In addition, the martensite can be in one of two forms: twinned and detwinned, as shown in Figure 2. “A phase transformation which occurs between these two phases upon heating/cooling is the basis for the unique properties of the SMAs” (Lagoudas et al., 2001, pp 8658) [6].

Upon cooling in the ab-sence of applied load the material transforms from austenite into twinned (self-accommodated) martensite. As a result of this phase transformation no observable macro-scopic shape change oc-curs. Upon heating the material in the martensitic phase, a reverse phase transformation takes place and as a result the mate-rial transforms to austen-ite.

“The key effects of SMAs associated with the phase transformation are pseudo-elasticity and shape memory effect” as stated in Oksuka and Wayman (1998, p.56) [5].

Figure 2.

Different phases of an SMA (Lagoudas et al., 2001) [6].

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Shape Memory Effect (SME) Through SME, material has the ability to memorize a very specific con-figuration either in low-temperature range (martensite phase) or high-temperature range (austenite phase). (Oksuka and Wayman, 1998, p.58) [5].

Rodriguez and Brown (1975, p.27) describe it in their research that on cooling below the transformation temperature, the austenite transforms to a thermoelastic martensite whose structure has many variants, typically sheared platelets. Because the martensitic structure is self-accommodating, the deformation on transformation to martensite is zero. The martensite deforms by a twinning mechanism that transforms the dif-ferent variants to the variant that can accommodate the maximum elonga-tion in the direction of the applied force. The interfaces between platelets in the martensite phase slip very readily and the material is deformed at low applied stresses. The austenite phase has only one possible orienta-tion, thus when heated, all the possible deformed structures of the mart-ensite phase must revert to this one orientation of the austenite memory phase and the material recovers its original shape [10].

There are four characteristic temperatures defining a thermoelastic mart-ensitic transformation; the martensite start temperature, Ms , at which martensite first appears in the austenite. The transformation proceeds with further cooling and is complete at the martensite finish temperature, Mf . Below Mf , the entire body is in the martensite phase, and a speci-men typically consists of many regions each containing a different variant of martensite (Lagoudas et al, 2001, pp 8659) [6]. The boundaries be-tween the variants are mobile under small applied loads. With heating, the austenite start tem-perature, As , is the tem-perature at which austen-ite first appears in the martensite. With further heating, more and more of the body transforms back into austenite, and this re-verse transformation is complete at the austenite finish temperature, Af . Above Af , the specimen is in the original undis-torted state. The evolution of the volume fraction of the martensite with tem-

Figure 3.

Volume fraction verses temperature (Rodrigue and Brown, 1975) [10].

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perature change is shown schematically in Figure 3 (Rodriguez and Brown, 1975, p 27) [10].

In Figure 4 Hodgson (1988, p.87) shows schematically the shape memory effect: at a temperature above Ms, the specimen is entirely in the austenite phase. When cooling to below Mf transfor-mation, the specimen progresses entirely to the martensite phase. Nevertheless the mac-roscopic volume of the specimen has not changed - a condition known as self-accommodation. With small loads the specimen can be easily deformed, and the deformed shape remains after removing the loads. Heating to above Af causes the reverse transformation to occur and the specimen returns to its original undistorted state [11].

Pseudo-elasticity SMAs also display pseudo-elasticity that is a mechanical type of shape memory. This effect is observed when alloys are strained just above their transformation temperature. (Kauffman and Mayo, 1993, p7) [8].

The mechanical properties of SMAs vary over the temperature range spanning their transformation. At low temperatures, the material exists as martensite and is deformed by a relatively small applied force; it also ex-hibits shape memory on heating. At high temperatures, the material exists as austenite, which is not easily deformed, and, on heating, no shape memory occurs because there is no phase change (Hodgson, 1988, p92) [1].

However, if the material is tested just above its transformation tempera-ture to austenite, the applied stress transforms the austenite to martensite and the material exhibits increasing strain at constant applied stress, i.e. considerable deformation occurs for a relatively small applied stress. When the stress is removed, the martensite reverts to austenite and the material recovers its original shape. This effect, which makes the alloy

Figure 4

Shape Memory Effect (Hodgson, 1988) [11].

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appear extremely elastic, is known as pseudo-elasticity or super-elasticity.

SMAs are called after their pseudo-elasticity behavior reversible upon some paths through the stress-strain-temperature space. This pseudo-elasticity is one of many particular cases of the highly nonlinear ther-momechanical behaviour, featuring a hysteresis, of these alloys (Meunier, 1995) [7].

SMA’s Applications There are several thousand patents for devices utilizing the properties of SMAs. Only a small percentage of these inventions have become successful products but the fields of application are very diverse ranging from everyday consumer products to biomedical implants to space ap-plications. “Nevertheless, the first industrial application occurred in 1969 when SMA couplings joined hydraulic pipes in the F-14 aircraft” recall Otsuka and Wayman (1998, p.95) [5].

This application has been extended to the joining of many other types of pipe, sometimes using a liner that is squeezed onto the pipes to make a joint. Harrison and Hodgson (1975, p.517) describe one application, where the cylindrical couplings are cooled to cryogenic temperatures to produce the martensitic phase when they can be easily expanded to slide over the pipes. On warming above the transformation temperature, the coupling tries to contract to its original size but is constrained by the pipes within. The stresses that result from this constraint are sufficient to create a joint that can be superior to a weld [12].

Similar to above mentioned, the Betalloy coupling is a CuZnAl coupling also designed and marketed by Raychem Corporation for copper and aluminum tubing. In this application, the CuZnAl shape memory cylinder shrinks on heating and acts as a driver to squeeze a tubular liner onto the tubes being joined. The joint strength is enhanced by a sealant coating on the liner (Hodgson, 1999, p.127) [1].

Figure 5 Different sizes of NiTinol tubes

(Anson, 1999) [9].

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In some applications, the shape memory component is designed to exert force over a considerable range of motion, often for many cycles. Such an application is the circuit-board edge connector made by Beta Phase Inc, found by Krumme (1987, p.41) [13]. In this electrical connector system, the SMA component is used to force open a spring when the connector is heated. This allows force-free insertion or withdrawal of a circuit board in the connector. Upon cooling, the NiTi actuator becomes weaker and the spring easily deforms the actuator while it closes tightly on the circuit board and forms the connections.

One obvious field of application is in devices to protect against fire. Fire sprinkler systems can be activated by the shape change induced by the heating of an SMA in a fire. Similarly, a fire safety valve in-corporating an SMA activator shuts off the flow of a flammable or toxic gas if a fire occurs. Barnes (1999) describes a construction appli-cation using an SMA actuator to lock ceiling plates in place if the temperature rises above 60C protecting pipes, cables and the floor above from the effects of the fire [14].

SMAs can also be used as an improved bi-metallic strip to regulate water temperature. An anti-scald device in a showerhead introduces cold water if the water temperature becomes too high. This application uses an SMA com-pression spring and a biasing steel spring. At high temperatures, the SMA spring expands and opens a needle valve allowing cold water to enter the mixing chamber. “When the temperature is reduced, the SMA returns to martensite and the steel spring resets the shape memory spring and si-multaneously closes the needle valve” Barnes explains (1999) [14].

It is possible to use only a part of the shape recovery to accurately posi-tion a mechanism by using only a selected portion of the recovery be-cause the transformation occurs over a range of temperatures rather than at a single temperature. “A device has been developed by Beta Phase Inc. in which a valve controls the rate of fluid flow by carefully heating a shape-memory alloy component just enough to close the valve the de-sired amount. Repeatable positioning within 0.25 mm is possible with this technique, as described in Otsuka & Wayman (1998, p.167) [5].

A number of products have been brought to market that use the pseudo-elastic (or superelastic) property of these SMAs. Eyeglass frames that use super-elastic NiTi to absorb large deformations without damaging the

Figure 6

SMA’s flexible eyewear (Hodgson, 1999) [1].

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frames are now marketed, and guide wires for steering catheters into vessels in the body have been devel-oped using NiTi wire, which resists per-manent deformation if bent severely. Arch wires for orthodontic correction us-ing NiTi have been used for many years to give large rapid movement of teeth (Hodgson, 1999, p.151) [1].

Rogers (1995, p.156) displayed that Nit-inol is being used in robotics actuators and micromanipulators to simulate hu-man muscle motion. The main advan-tage of Nitinol is the smooth, controlled force it exerts upon activation [15].

Other miscellaneous applications of shape memory alloys include use in household appliances, in clothing, and in structures. A domestic deep fat fryer uses an SMA blade to prevent the basket being lowered into the oil until the correct temperature of 170°C has been attained. “This high tem-perature application favors the use of a CuAlNi alloy. Similarly, CuAlNi al-loys are preferred for circuit breakers to prevent overload electric currents heating wires and cables above 140°C, identified by Falcioni (1992, p.114) [16].

Nitinol actuators as engine mounts and suspensions can also control vi-bration. These actuators can be helpful in preventing the destruction of such structures as buildings and bridges, described in Rogers (1995, p.154) [15].

According to Stoeckel and Yu, (1997 : 11) 'The properties of the NiTi al-loys, particularly, indicate their probable greater use in biomedical appli-cations.' The material is extremely corrosion resistant, demonstrates ex-cellent biocompatibility, can be fabricated into the very small sizes often required, and has properties of elasticity and force delivery that allow uses not possible any other way. Many biomedical applications use su-per-elastic wires and tubes. These include catheters and guide wires for steering catheters. Super-elastic arch wires for orthodontic correction have proven particularly effective by producing large rapid movement of teeth [17].

Hodgson (1999, p.189) describes another fast growing field of application is its use as blood clot filter. This involves the use of titanium-nickel wires that are first trained to blood clot trapping coiled configuration prior to the insertion of the cooled straightened wire. The wire is inserted into the

Figure 7.

SMA’s dental wires (Anson, 1999) [9].

34

vena cava, where, due to the heat caused by blood flow, reverts to the original blood-clot filtering configuration. The constriction of vessel re-stricts blood flow, so that surgeons have to often resort to by-pass and this can be avoided by the smart implant [1].

THE SMAs AND THEIR APPLICATIONS The general understanding of the nature of SMAs which is common for Hodgson (1999) [1] and Lagoudas et al. (2001) [6] in their researches in-volves the definition of the Shape Memory Alloys as a unique class of metal alloys that can be plastically deformed at some relatively low tem-perature, and upon exposure to some higher temperature will return to their shape prior to the deformation. Oksuta and Wayman (1998) [5] de-fine SMAs directly based on their two unique properties - pseudo-elasticity, and the shape memory effect.

Some authors give deeper consideration to the two transformation phases - austenite and martensite phases and their significance for the thermomechanical characteristics of the SMAs. Hodgson (1999) [1] paid particular attention to the crystallographic structure of the SMAs and the changes it undergoes during temperature treatment and pinpointed the structural changes at the atomic level which contributed to the unique properties these metals have. As a result of the martensitic phase trans-formation, the stress-strain response of SMAs is strongly non-linear, hys-teretic, and a very large reversible strain is exhibited. This behavior is strongly temperature-dependent and very sensitive to the number and sequence of thermomechanical loading cycles. In polycrystals, the differ-ences in crystallographical orientation among grains produce different transformation conditions in each grain. The polycrystalline structure also requires the satisfaction of geometric compatibility conditions at grain boundaries, in addition to compatibility between austenite and the differ-ent martensitic variants. Thus, the martensitic transformation is progres-sively induced in the different grains and, as opposed to the single crystal case, no well-defined onset of the transformation is observed.

Lagoudas et al (2001) [6] give similar description of the two SMAs stable phases: austenite and martensite, as well as of the phase transformation which occurs between these two phases upon heating/cooling. Also dis-cussed in this specific study are the two forms of the martensite: twinned and detwinned.

Much of the work described in Literature Search and Review section draws the attention to the significance of the two major properties of the SMAs - pseudo-elasticity, and the shape memory effect. Rodriguez et al.

35

(1975) [10] and also Lagoudas et al. (2001) [6], describe the mechanism of the shape memory effect through austenite transformations to a ther-moelastic martensite whose structure has many variants, typically sheared platelets, while the austenite phase has only one possible orien-tation. Thus, when heated, all the possible deformed structures of the martensite phase must revert to this one orientation of the austenite memory phase and the material recovers its original shape. Above men-tioned researchers as well as Hodgson (1988) [11] use four characteristic temperatures to define a thermoelastic martensitic transformation: Ms, Mf , As and Af.

Kauffman & Mayo (1993) [8] and Hodgson (1988) [11] describe the other basic property - pseudo-elasticity (or super-elasticity) as a mechanical type of shape memory. It can be summerised from their research work and also Meunier’s (1995) [7] that slightly above its transformation tem-perature, martensite can be stress-induced. It then immediately strains and exhibits the increasing strain at constant stress behavior. Upon unloading, though, the material reverts to austenite at a lower stress, and shape recovery occurs, not upon the application of heat but upon a reduc-tion of stress and it is exactly this particular effect, which causes the ma-terial to be extremely elastic and is defined by researchers as pseudo-elasticity.

The only two alloy systems that have achieved any level of commercial exploitation are the NiTi alloys and the copper-base alloys. The unusual properties mentioned above are being applied to a wide variety of appli-cations in a number of different fields and this is mentioned by almost all authors considered in the literature review. Harrison and Hodgson (1975) [12], Otsuka & Wayman (1998) [5] and Hodgson (1999) [1] describe the uses of the constrained recovery event for joining and fastening purposes and the development of tube and pipe couplings for aircraft, marine and other applications.

However, from the literature review (Section 2.6) we must take into ac-count the fact that established applications for shape memory alloys in-clude domestic appliances: shower mixer valves, coffee makers, rice cookers, deep fat fryers (Falcioni, 1992) [16], eyeglass frames and cellu-lar phones (Hodgson, 1999) [1]; utility applications: safety shut off valves for fuel lines in the event of fire and air conditioning systems (Barnes, 1999) [14]. The shape memory alloys can also contribute to the miniaturi-zation of equipment and systems, decrease the number of parts required and extend the life expectancy too due to the favorable fatigue properties of the alloy.

36

Besides their super-elastic properties, SMAs have excellent fatigue and corrosion resistance. As Stoeckel and Yu (1997) [17] realize, the largest commercial successes of SMAs are in the field of bioengineering and biomedical applications. Most successful is the use of orthodontic arch wires (in contrast to similar stainless steel wires) which will gradually re-turn to their shape exerting a small and nearly constant force on the mis-aligned teeth. Hodgson (1999) [1] confirms the above and describes an-other fast growing field of application: SMAs use as blood clot filter.

FURTHER WORK The many uses and applications of shape memory alloys ensure a bright future for SMAs. Future applications are envisioned to include engines in cars and airplanes and electrical generators utilizing the mechanical en-ergy resulting from the shape transformations. Nitinol with its shape memory property is also envisioned for use as car frames, explained in (Kauffman & Mayo, 1993 : 3) [8].

Some other directions for SMAs’ future yet are obvious. The cost of these alloys has slowly decreased as use has increased, so uses that require lower-cost alloys to be viable are being explored, as identified in Hodgson (1999, p.234) [1]. Alloy development has yielded several ternary composi-tions with properties improved over those obtained with binary material, and alloys tailored to specific product needs are likely to multiply. The medical industry has developed a number of products using NiTi alloys because of their excellent biocompatibility and large pseudo-elasticity, and many more of these applications are likely. Finally,as classified in Falcioni (1992, p.114), the availability of small wire that is stable, is easily heated by a small electrical current, and gives a large repeatable stroke should lead to a new family of actuator devices. These devices can be in-expensive, are reliable for thousands of cycles, and are expected to move NiTi into the high-volume consumer marketplace [16].

Recent interest in the development of iron-base shape memory alloys has challenged the concept that long-range order and thermoelastic marten-sitic transformation are necessary conditions for shape memory effect. “The commercial potential of these alloys has yet to be determined, but the effort has opened up new classes of alloys for exploration as shape memory alloys. These new classes include Beta-Ti alloys and iron-base alloys”, as explained in Hodgson (1999, p.256) [1].

37

CONCLUSIONS This investigation has shown several important aspects that need to be considered when looking at the nature and thermomechanical properties of SMAs and their applications. Shape Memory Alloys undergo a phase transformation in their crystal

structure when cooled from the stronger, high temperature form (aus-tenite) to the weaker, low temperature form (martensite). This inherent phase transformation is the basis for the two unique properties of these alloys: - Shape Memory Effect - the unique ability of shape memory alloys

to be severely deformed and then returned to their original shape simply by heating them and

- Pseudo-elasticity - an almost rubber-like flexibility demonstrated by shape memory alloys.

The most effective and widely used alloys include NiTi (Nickel - Tita-nium), CuZnAl, and CuAlNi;

Some of the main advantages of SMAs include: bio-compatibility; di-verse fields of application; good mechanical properties (strong, corro-sion resistant);

SMAs are still relatively expensive to manufacture and machine com-pared to other materials such as steel and aluminum;

SMAs are being used in a variety of applications. They have been used for aerospace, medical, safety, domestic, and robotics applica-tions. The largest commercial successes of SMAs are in the field of bioengineering and biomedical applications;

The further investigations will be aimed at new fields of application of currently existing SMAs and at developing new classes of SMAs in-cluding Beta-Ti alloys and iron-base alloys.

38

REFERENCES 1. Hodgson, Darel (1999) Shape Memory Alloys. Harrison Alloys, Inc,

p.31-256.

2. Khalil-Allafia, J., Dlouhyb, A. and Eggeler, G. (2002). Ni4Ti3-precipitation during aging of NiTi shape memory alloys and its influ-ence on martensitic phase transformations. Institut für Werkstoffe, Ruhr-University, Bochum: p.89.

3. Duerig, T.W., Melton, K. N., Stokel, D. and Wayman, C.M. (1990). Engineering Aspects of Shape Memory Alloys. Butterworth-Heinemann, Stoneham, MA: p. 65.

4. Graesser, E. J. and Cozzarelli, F. A. (1994). A proposed three-dimensional constitutive model for shape memory alloys. J. Intell. Mater. Systems Struct, p 37.

5. Otsuka, K. and Wayman, C. M., (1998). Shape Memory Materials. Cambridge University Press, ISBN: 0-521-44487 X hardback.

6. Lagoudas, D.C., Qidwai, M.A., Entchev, P.B., and DeGiorgi, V.G. (2001). Modeling of the Thermomechanical Behavior of Porous Shape Memory Alloys. International Journal of Solids and Structures Vol. 38, pp. 8653-8671.

7. [Meunier, Marc-Antoine (1995) Experimental characterization, ther-momechanical modelisation and fatigue study of shape memory al-loy pieces (SMA). Retrieved April 15, 2003, from http://www.meca.polymtl.ca/SMA/publication_1/

8. Kauffman, G. and Mayo, I. (1993). Memory Metal. Chem Matters Oct. 1993: p. 4-7.

9. Anson, Tony (1999). Shape Memory Alloys and their Commercial Exploitation. Materials World, Vol. 7, No. 12, pp 745-747 December.

10. Rodriguez, C. and Brown, L. C. (1975). In Shape Memory Effect In Alloys. J. Perkins, ed., Plenum Press, p 27.

11. Hodgson, D.E. (1988). Proceedings of Engineering Aspects of Shape Memory Alloys. East Lansing, MI.

12. Harrison, J.D. and Hodgson, D.E. (1975). Shape Memory Effects in Alloys. J. Perkins, Ed., Plenum Press, p 517

13. Krumme, J. F. (1987). Shape Memory Alloys. Connect. Technol., Vol 3 (No. 4), April: p 41

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14. Barnes, Clive (1999). Shape Memory and Superelastic Alloys, Re-trieved April 15, 2003, from http://innovations.copper.org/1999/07/shape.html

15. Rogers, Craig (1995). "Intelligent Materials." Scientific American Sept.: p 154-157.

16. Falcioni, John G. (1992). "Shape Memory Alloys”. Mechanical Engi-neering Apr. 1992: p 114.

17. Stoeckel, Dieter and Weikang, Yu (1997). Superelastic Nickel-Titanium Wires. Raychem Corporation, Menlo Park, CA: p.11.

40

CHAPTER 3 SELECTING ACID-RESISTANT MATERIALS

INTRODUCTION TO THE ANODIC PASSIVATION Statistics show that worldwide annually around 30% of all metal construc-tions become useless as a result of damages caused by corrosion. Cor-roded metal equipment is taken out of operation and subjected to proc-essing in corresponding metallurgy plants. A certain part of metal equip-ment is subjected to electro chemical corrosion in electrolytic (so-called corrosive) medium. Included here are tanks designed for storage of acids, bases, salts, etc., which are subjected to electrolytic corrosion as they are surrounded by an electrolytic medium [1]. Nowadays, there are multiple possibilities to fight corrosion effectively. There are some measures to protect metal constructions that should and must be taken from the very design phase of the metal equipment by selecting the suitable material. Depending on the conditions and properties of the medium that the metal equipment is going to be subjected to various brands of steel and alloys having the appropriate corrosion resistance should be used to make it re-liable and inexpensive. This task is covered by the present assignment of selecting materials for storing diluted sulphuric acid and hydrochloric acid (10 % w/w).

It was M.V.Lomonosov (1738) and later on Bertselius and M.Faraday that described the paradoxical fact that iron easily dissolves in diluted nitric acid solutions but this process continues only until acid concentration is increased. Faraday explained this by the formation of a protective oxide layer on the iron surface, which prevents the metal from dissolving fur-ther. The process of bringing metals to a condition of increased resis-tance when subjected to various oxidizing agents is called passivation [1]. With all the practical application possibilities that this phenomenon offers, namely for fighting metal corrosion, it has been the target of multiple and comprehensive studies ever since it was initially discovered.

In addition to being subjected to oxidizing agents metals could also be passivated by means of anodic polarization, i.e. by using electric current to divert the equilibrium potential of the electrode in the positive direction. In this case we talk about anodic passivation. Modern studies on anodic passivation of metals run basically in two main directions [2]. On one side, the mechanics of the processes leading to changes in the condition of the metal surface and bringing the metal into stable (passive) state are inves-

41

tigated and the nature and morphology of oxidized layers thus formed are studied, on the other.

MECHANICS OF ANODIC PASSIVATION Dependent on the nature of the solution and the value of diversification of electrode potential in the positive direction metal passivation could take place in different ways. In active anodic dissolving of metals there may be a point of saturation of the electrolyte with respect to certain metal salts, thus forming a solid product that can be deposited on the surface of the metal [2]. If the apparent current density is maintained constant then the actual current density in the areas accessible for the electrolyte will in-crease as a result of the shield formed on the electrode surface by the deposited non-conductive solid product. The potential in these areas strongly diverts into the positive direction, which makes it possible for a thermodynamic reaction to take place and formation of a hard metal oxide or hydroxide. The oxide layer thus formed on separate areas eventually spreads on the entire metal surface preventing it from dissolving further.

Also outstanding are V.Muller’s (1933) trials that subjected iron, zinc, copper, etc. metal electrodes horizontally positioned in the electrolyte to anodic polarization in diluted sulphuric acid solutions. The polarized light studies have indicated that when there is no motion in the electrolyte hy-drated sulphates of corresponding metals deposit on the surface of the electrodes. Once these salts are formed metals transform into their pas-sive state.

Anodic passivation of metals could also be carried out without the forma-tion of the shielding solid products. If no anions are present in the electro-lyte to precipitate the metal cations that have entered the solution and if the potential of the electrode has been sufficiently diverted into the posi-tive direction to allow for a reaction to take place, then a new phase of metal oxide or hydroxide is directly deposited on the metal surface, which changes the properties of the initial metal surface and the nature of the anodic reaction. Under such conditions the metal stops to dissolve meas-urably and transfers into passive state.

Theories of passivity of metals. Wide practical applications uncovered by metal passivation have enhanced not only applications studies in this area but also investigations involving the nature of the processes taking place in this phenomenon. A large number of hypothesis were made with regards to the mechanism of passivation and the nature of the passive state of metals. Various concepts of above issues could be classified into two basic groups. One of the groups, the smaller one, comprises the con-

42

cepts according to which the passivation phenomenon is the result of a number of changes taking place in the physical properties of the metal it-self (for example, the change in the electron state of the metal and its transformation into a specific, chemically inactive allotropic state). An-other group of theoretical concepts links passivation to the formation of protective oxide layers on the metal surface. The metal does not become thermodynamically nobler during passivation but transfers into a stable state thanks to the protective layers formed on its surface, which signifi-cantly change the electrochemical properties of the metal-medium inter-face. And if the discussions on the issue of the passive state of metals still continue to be fierce they are mainly concentrated on the question: what is the nature of these protective layers? Are these two-dimensional layers of adsorbed oxygen that block the metal surface making it inactive (the adsorption theory) or these are three-dimensional oxide layers, which under the form of a separate phase cover the metal surface and purely mechanically prevent it from the actions of the corrosive medium (the phase theory) [3].

On the modern stage of development of our concepts of the mechanism of passivation and the nature of the passive state the widest popularity has the phase theory the basic ideas of which were stipulated by M.Faraday (1836). U.Evans gave direct experimental proof in support of these theoretical conceptions. He succeeded to separate from the surface of passivated iron some oxide layers and examined them directly under the microscope.

EXPERIMENTAL STUDIES – ANODIC PROTECTION OF MILD STEEL IN SULPHURIC ACID SOLUTION Experimental setup. The apparatus consists of a simple support for sus-pending two electrodes made of mild steel strip into a 600 ml beaker con-taining approx. 400 ml acid. The current supply is from a stabilized power supply source.

The current and voltage values could be measured using the power source devices and a multimeter. We increase the voltage in 0.1V inter-vals up to maximum 3.0V and we measure the current value for every voltage increase. Before we take down its value we wait for the current to stabilize. The current readings increase steadily up to the point where the passive section is reached when it suddenly drops. When 3.0V is reached we reduce the current through 0.1V intervals to see if there are some hys-teresis effects. We repeat the experiment adding to the acid a small quan-tity of concentrated brine solution. We plot the voltage variation in rela-tion to current density logarithm and we analyze the results.

43

Analysis of resulting diagrams The diagram shown in Appendix 1indicates that the curve consists of several characteristic sections, each corre-sponding to a specific metal state. We can note that when we increase the voltage current density suddenly increases from 1.5 to 3.0 log CD. This section corresponds to the active anodic dissolving of iron in sul-phuric acid. For voltage values between 0.5V and 1.3V there is a section that characterizes by the fact that the current is independent of the poten-tial of the electrode. A shielding solid product from basic iron sulphate is formed in this section, which has porous structure and dissolves in the electrolyte at a certain speed. When the speeds of formation and dissolv-ing become equal the thickness of the sediment becomes constant and independent of the electrode potential. In the 1.3V section anode current suddenly drops and the electrode goes into passive state. In the section between 1.3V and around 2V iron is passivated. We could say the current is independent of electrode potential. Despite of the fact that the metal is in its passive state some very low anode current still flows through the electrode, which is due to the dissolving of passivated iron. This current is one of the most characteristic features of metals in passive state. The po-tential corresponding to this current could be regarded as the thermody-namic limit beyond which the metal goes in a state of active dissolving and above which conditions are created for forming protection oxide lay-ers. After a voltage of 1.75V anodic deposition of oxygen starts on the passivated electrode as a result of which the current increases with every increase of electrode potential.

The opposite processes take place when current density reduces. For voltage values of 1.75V electrode potential changes in the negative direc-

Figure 1 Experimental setup diagram

44

tion and anodic metal dissolving is resumed. In this process resumption the diagram indicates a hysteresis, which could be explained by the fact that the passive layer thus formed stays stable even at lower current den-sities than those when the layer was formed. It takes some time to cover the entire electrode with the oxide layer and this time would be smaller for higher anode current densities.

When we repeat the experiment after adding a small quantity of concen-trated brine solution to the acid the acid saturates with positive hydrated ions of hydrogen and negative ions of chlorine, which are directed to the electrodes and ensure increase and eventual flow of direct current.

Applications. The results show that this experiment could be used for pro-viding anodic protection of metals against corrosion. An oxidized layer covers metals when immersed into suitable oxidizing medium (oxidation) or when used for a certain time as anodes in an electrolytic bath where their surface is oxidized by the oxide deposited on the anode (anodizing).

SELECTING MATERIALS FOR THE TANK Expensive special steels (£ 3 per kg). High-alloy steels are being pro-duced for industrial, building and domestic applications, which are both corrosion resistant and fireproof. Corrosion resistant steels are capable of resisting the destructive chemical and electrochemical action of external environment. Considering steel’s ability to resist a certain aggressive cor-rosive medium it is classified as stainless, acid resistant and scale resis-tant. Acid resistant steel exhibits high corrosion resistance against the ac-tion of various aggressive mediums [4].

Corrosion resistant steels are usually chrome- or chrome-nickel alloys containing above 12% chrome. Depending on its chemical composition steels microstructure could be ferrite, semi-ferrite and allowing structural transformations, i.e. that can be subjected to improvements (above 15% carbon, 10% to 18% chrome) having austenitic structure.

High nickel and manganese steels feature extended areas of stable aus-tenitic structure. When chrome-nickel steels are heated up to tempera-tures of 490 – 900 degrees or when being cooled down slowly in this in-terval chrome carbides are formed along the boundaries of the austenitic grains. This results in grains of rich in chrome and poor in carbon cores. As a result of this structural non-uniformity steel shows a tendency for in-ter-crystal corrosion. To avoid this drawback chrome-nickel steels have to be additionally alloyed using strong carbide-forming elements such as ti-tanium and niobium.

45

The austenite structure of chrome-nickel and chrome-nickel-manganese steels renders these materials some very essential properties, such as non-magnetic characteristics, improved strength under high temperatures and good weldability.

Chrome-nickel steels usually acquire satisfactory strength and good plas-tic characteristics after being hardened with austenite. Strength character-istics of such steels can be improved by cold-work hardening by means of cold rolling, cold drawing or stamping. Cold-work austenite steel main-tains sufficient plastic properties. Semi-finished products made of such steel could be bended, shaped or even stamped.

Selecting a high-alloy steel brand. For our application we selected X5CrNiMoCuNb18 18 steel, which exhibits highest sulphuric and other acids resistance and finds wide application in the chemical industry. We are going to give the following most essential mechanical characteristics for this steel: the strength σB, the yield strength σS, the percentage of specific elongation δ, the percentage reduction of area ψ and impact strength aK. We will also apply the chemical composition, approximate forging and temperature treatment temperatures. The data is presented in the following tables:

Steel brand σS σВ δ ψ аK ISO DIN МРа МРа % % KJ/m2

X5CrNiMoCuNb18 18 1.4505 230 750 40 35 650

Steel brand Content of elements in % ISO DIN C Si Mn Cr Ni others

X5CrNiMoCuNb18 18 1.4505 <0,07 <1,0 <2,0 16,5-18,5

16,5-18,5

Mo=2,0-2,5Cu=1,8-2,2Nb>8x%C

Steel brand Forging Hardening ISO DIN Temp. Coolant

Annealing negative Temp. Coolant

Tem-pering

X5CrNiMoCuNb18 18 1.4505 1150-750

air - 1050-1100

water -

Low-cost steels (£ 1 per kg). Good quality carbon steel is intended for the needs of all machine-building industry areas. Parts made of this steel type are usually subjected to temperature and thermo-chemical treat-ment. To meet variable and often stringent requirements in this industry these steels contain basic components that compared to regular carbon constructional steels have tighter limit deviations, smaller quantities of

46

harmful impurities, more uniform structure and higher non-metallic inclu-sions purity [4].

The basic properties and main purpose of these steels are determined by the carbon content. Low-carbon steels (C<0.25%) do not exhibit high strength but have better plastic and ductile characteristics. These are usually used for making parts involving bending, drawing, roughing, stamping and welding.

Medium-carbon steels containing above 0.25% to 0.60% carbon exhibit sufficient strength combined with good ductility. These are mainly used for making parts involving forging, hot stamping and cutting. Lower car-bon content gives steel good weldability and higher carbon content pro-vides medium to poor steel weldability.

High-carbon steel containing above 60% carbon exhibits high strength, hardness and satisfactory ductility characteristics. This is usually used for making springs and parts demanding high wear characteristics. Com-pared to alloy steel carbon steel has the advantage of being the cheapest good-quality steel but it features the following disadvantages: it features shallow hardness penetration so it is only suitable for small-diameter parts or thin-wall components; exhibits lower yield strength, fatigue strength and impact plasticity and ductility at equal tensile strength; hard-ness and strength of hardened steel quickly drops with temperature.

For our application we could select good-quality carbon steel containing up to 0.3% carbon (Ck22 steel), which features not so high strength but high ductility and very good weldability. We are going to give the most es-sential mechanical characteristics, technological properties and tempera-ture treatment conditions. The data is presented in the following tables:

Steel brand σS σВ δ ψ аK ISO DIN МРа МРа % % KJ/m2

Ck22 1.1151 245 412 25 55 - Steel brand Technological properties

ISO DIN Processing in-volving cutting Weldability

Forging tem-perature inter-val deg C

Cold process-ing ductility

Ck22 1.1151 Satisfactory. Very good 800-1300 Very good Steel brand Temperature treatment

Hardening conditions HRC hardness af-ter relaxation ISO DIN

Temp deg С Coolant

HRC hard-ness after hardening 200 deg С

Ck22 1.1151 900-920 water 34-40 32-36

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Because the selected steel is not acid-resistant for the application that we are going to use it we shall need to provide some additional protection of the metal against corrosion in the aggressive medium. We could apply passivation of the metal surface for the purpose (cover the metal with thin oxidized layer) or use protection methods. The materials we selected ex-hibit good plastic properties allowing the tank to be designed and made in the most suitable and optimum cylindrical shape. Both steel brands fea-ture good weldability, which allows for the tank to be made as a welded construction guaranteeing its surface uniformity.

The tank made of the selected brand of alloy steel X5CrNiMoCuNb18 18 could be used for storage and transportation of 10% hydrochloric acid (HCl) as for this acid this type of steel has good corrosion resistance. The tank made of Ck22 steel would not allow for storage of hydrochloric acid without additional protection. In this case we could apply cathode protec-tion (electrochemical protection) or inhibitor protection (adding admixtures to the hydrochloric acid to stop the corrosion process).

FACTORS INFLUENCING THE EFFICIENCY OF THE TANK AND ITS USAGE When designing the tank we should give consideration to all factors that influence its efficiency and affect its usage. Consideration should also be given to the efficiency of decisions made, to the possible manufacturing and assembly technology, to the operation and service conditions, to the maintenance and service life, and reliability [3].

One of the most significant factors defining the functionality, manufactur-ing and handling of the tank is its shape. The most suitable and optimum shape in this case is the cylindrical. Bending steel sheet or joining individ-ual rings can achieve this. Welding is a suitable technique to apply for joining metal parts. The materials we selected feature good ductile char-acteristics allowing us to manufacture such type of construction. Both steel types have good welding ability and the tank can be made as a welded construction to guarantee its surface uniformity. High efficiency could be achieved if universal devices and elements are used. Selected as such could be suitable handles for the tank, transportation wheels, drain valves, covers, level meters, etc.

The construction of the tank should not have excessive reserves (for strength, etc.). It should comply with the anticipated time for its service life. Nowadays, service life times for such equipment have been greatly reduced and the requirements for capabilities of operating in higher ca-pacities, reliability, efficiency, convenience of operation, ease of mainte-

48

nance, etc. have increased. All these affect the selection for the metal and tank wall thickness.

The tank should comply with the requirements for aesthetic industrial and ergonomic design. A study of available constructions should be made to help making the best decisions and introduce new solutions. The design of the tank should comply with the requirements for transportation and handling providing clamping locations and ensuring means for moving the tank and draining and filling-in acid.

The tank should meet the reliability and handling safety requirements. A warning should be provided on the outside to indicate its contents and handling safety instructions should also be indicated.

USING THE TANK FOR STORAGE OF 10% HYDROCHLORIC ACID. The tank made of the selected brand of alloy steel X5CrNiMoCuNb18 18 can be used for storage and transportation of 10% hydrochloric acid (HCl) as this type of steel provides good corrosion resistance for this type of acid. The tank made of 20 steel would not allow for storing hydrochloric acid unless some additional protection is provided. In our particular case cath-ode protection (electrochemical protection) or inhibitor protection (making additions to the hydrochloric acid to stop the corrosion process).

ADDITIONAL TANK PROTECTION TECHNIQUES Anodic protection. The nature of anodic protection lies in applying an-other metal to the tank, which would exhibit lower electrode potential and has the function of a breaking anode. Adding this side anode causes in-tensive cathode polarization of the electrodes in the micro galvanic ele-ments along the surface of the protected metal as a result of which the anodic sections of this metal become cathodes and stop corroding. In or-der for the additional metal (the protective coating) to fulfill its functions it should meet certain requirements: it should feature sufficient negative po-tential, it should be cheap, it should dissolve readily in the corresponding medium, it should feature low electrochemical equivalent and it should not form a protective layer over its own surface thus preventing itself from de-composing [4]. Zinc protective coatings are most often used for protecting steel metal constructions and are installed by means of bolts fastened to the construction. Recently, these are being replaced by aluminum- 5-10%

49

zinc alloy protective coatings. Experiments have shown that these protec-tive coatings are covered not so heavily with corrosive products and are less frequently replaced or cleaned. The effectiveness of the protective layer depends on the following factors:

1. The conductivity of the corrosive medium. Higher conductivity pro-vides wider protection range. In this case protective coatings could be placed at a larger distance one from another.

The means of its application. The protective coating should have a suit-able shape and size and should be located at an easily accessible loca-tion along the tank to allow cleaning and examination. The surface area of the protective coating is within 20 –200 cm2 depending on particular re-quirements and plate thickness is between 4 and 12 mm. Protective coat-ing protection could also be applied in the cases when the metal con-struction is covered with paint thus extending coating life.

Cathode protection. Cathode protection involves cathode polarization using electric current from an external source. The value of the polariza-tion current should be higher for more aggressive corrosive agents. An optimum polarization current density could be established for each indi-vidual case of cathode protection to provide for maximum metal protec-tion [5]. The value for the minimum current density is influenced by the nature of protected metal, the nature of the corrosive medium, etc and this value varies within a fairly wide range. The general conditions for which cathode protection could be applied are as follows:

- an electrolytic medium should be available around the metal sur-face being protected (tanks containing salts, acids, bases, etc.);

- constant contact is maintained between the electrolyte and the surface being protected and this should be a sufficiently thick layer;

- the surface to be protected should have a simple geometric shape;

- the electric current used for the protection should be safe to the construction and servicing personnel.

Cathode protection is applied to many metal constructions, for example the external and internal surfaces of tanks, cisterns, etc. The protection current voltage should not be higher than 20 to 40V and electric current – higher than 400A.

The anode is the most significant element in the cathode protection. Ei-ther steel or Cu, Al, Zn, etc. could be used for the anode. It is recom-

50

mended that the size of flat anodes (strips) is no smaller than 20mm deep and 200mm wide. For this minimum thickness value anode durability of 2 to 4 years could be guaranteed. Anode positioning should be such that the best possible current distribution is ensured. The distance between the anode and the surface being protected is not of any particular impor-tance to the corrosion protection but it is recommended that anodes are positioned at a distance from this surface not larger than 0.25m. The an-odes should be tightly secured and their contacts should be well insulated from the corrosive medium. The anodes should be directly connected to the positive pole of the current supply source. Anodes made of zinc or magnesium or aluminum alloys are used for steel protections. Zinc an-odes are manufactured with high metal purity and aluminum anodes are made using 1-1.5% zinc alloys.

Protection by adding admixtures. The protection of metal constructions that are in contact with corrosive mediums (acids or bases) could also be achieved by adding certain admixtures to them – the so-called inhibitors, which have the property of limiting the corrosion process (they prevent the metal from dissolving in a certain corrosive medium). Inhibitors find widest application for protection of metal constructions against the corro-sive action of strong acids (sulphuric acid, hydrochloric acid, nitric acid). Mixtures of organic substances are usually used for protection against the first two acids and these mixtures contain nitrogen in the form of amino acids, aldehydes (for more concentrated solutions) and sulphur-containing substances, as well as non-organic arsenic compounds (arse-nic chloride). Stibium or bismuth salts could be used as sufficiently effi-cient non-organic inhibitors. For nitric acid some alkaloids, sulphur or-ganic compounds, ethers, spirits, sugar and also additions of potassium dichromate, potassium chromate and hydrogen peroxide in specific con-centrations provide the inhibiting action. We cannot discuss of corrosion protection against solutions of alkaline bases as steel dissolves in these too slowly [5].

SELECTING THE METAL MATERIAL FOR MAKING A STORAGE TANK FOR 10% SULPHURIC ACID For a cost limit of 3 Pounds per kilogram

Alloying non-resistant to corrosion metals can render them very good cor-rosion resistance. Usually, the metal whose corrosion resistance has to be increased is added to another metal capable of passivation. This metal transfers this ability for passivation into the alloy [5]. Such alloys are high-alloy steels that exhibit good corrosion resistance. Corrosion resistant

51

steels are capable of resisting the destructive chemical and electrochemi-cal action of the environment. Acid resistant steels exhibit high corrosion resistance against various aggressive mediums.

Corrosion resistant steels are mainly chrome or chrome-nickel steels con-taining above 12% chrome. Depending on the chemical composition of the steel its microstructure is either ferrite, semi-ferrite or capable of struc-tural transformations, i.e. susceptible to improvements (above 0.15% car-bon and 10% to 18% chrome) having an austenite structure.

Steels of increased nickel and manganese content feature extended area of stable austenite structure. When heating chrome-nickel steels to tem-peratures of 450 – 900 degrees or during slow cooling down within this interval some chrome carbides are formed along the boundaries of the austenite grains [6]. Grains having rich in chrome and poor in carbon core result. As a result of this non-uniformity in structure steel shows a ten-dency for inter crystallite corrosion. To avoid this drawback chrome-nickel steels are additionally alloyed using strong carbide-forming elements, such as titanium or niobium.

Chrome-nickel steels usually gain satisfactory strength and good plastic characteristics after being alloyed with austenite. Cold-work hardening achieved through cold rolling or cold stamping or drawing could increase the strength of such steel. Cold-formed austenite steel maintains suffi-cient plastic properties. Semi-finished product made of such steel could be bended, shaped or even stamped.

The austenite structure of chrome-nickel and chrome-nickel-manganese steels renders them some good properties, such as non-magnetic charac-teristics, increased toughness under high temperatures and good welding ability.

Selecting a material for the design of the tank

We selected high-alloy steel X10CrNiMoTi1812 to suit our purpose. This type of steel is usually used for making acid resistant parts, tanks and equipment in the chemical, textile, medical and pharmaceutical industries. This type of steel exhibits high resistance against sulphuric acid and other acid types and finds very wide application.

The steel features the following most significant mechanical properties under room temperature:

Strength σB = 750 Mpa

Yield strength σS = 270 Mpa

52

Relative elongation % δ = 40%

Relative transverse shrinkage % ψ = 35% Impact strength aK = 590 KJ/m2 We are also submitting the chemical composition and the approximate temperatures of forging and temperature treatment. The data is presented underneath in the following table.

Element content in % Steel type ISO carbon silicon manganese chrome nickel others

X10CrNiMoTi18 12 <0,1 <1,0 <2,0 17,5 12,5 Mo= 2,8 Ti>5x%C

Forging Hardening Steel type

ISO Temp. Cooling

envi-ronment

Low-temp

anneal-ing

Temp. Cooling environ-

ment

Tem-pering

X10CrNiMoTi18 12 1150-750 air - 1020-1070 Air water -

For a cost limit of 1 Pound per kilogram

In this case we could use a quality carbon steel. Components made of this type of steel are usually subjected to temperature or thermo-chemical treatment. Compared to ordinary structural steels these steels contain some basic components having tighter tolerance deviations, less harmful admixtures, more uniform structure and higher non-metal inclusion purity [6].

The basic properties and the main application of these steels are deter-mined by the carbon content. Low-carbon steels (C<0.25%) do not have significant strength but are plastic and ductile. These are mainly used for making parts through bending, drawing, roughing, stamping and welding.

Medium-carbon steels containing above 0.25% up to 0.60% carbon fea-ture sufficient strength combined with good ductility. These are mainly used for making products by forging, hot stamping and cutting. Lower-carbon content steel welds well and higher carbon content steel exhibits moderate or poor welding ability.

High-carbon steel containing above 0.60% carbon features high strength, high hardness and satisfactory ductility. It is suitable for springs and parts that should have high wear characteristics.

53

Compared to alloy steel carbon steel has the advantage of being the cheapest good-quality steel but it features the following disadvantages [7]:

it features low hardness penetration so it is only suitable for small-diameter or small wall thickness parts;

it exhibits lower yield strength, fatigue limit, ductility and impact strength for equal tensile strength;

the hardness and strength of hardened steel is rapidly reduced with temperature

Selecting the steel for designing the storage tank

We selected good-quality carbon steel with up to 0.3% carbon content (C 25 steel) that features not so high strength but high ductility and very good welding ability. We will submit the most important mechanical char-acteristics, technological properties and temperature treatments condi-tions:

Strength σB = 451 Mpa

Yield strength σS = 274 Mpa

Relative elongation % δ = 23`%

Relative transverse shrinkage % ψ = 50% Impact strength aK = 880 KJ/m2

Technological properties

Steel type ISO Cutting proc-

essing Welding ability

Temperature range for forg-

ing Deg C

Ductility in cold proc-

essing

C 25 Satisfactory Very good 800 -1300 Very good Temperature Treatment

Hardening conditions

Steel type Temp deg C Cooling envi-

ronment

HRC hardness following hard-

ening

HRC hard-ness follow-ing temper-

ing 200 deg C

С 25 900 - 920 water 34 - 40 32-36 As the type of steel we selected is not acid resistant we shall need to use some additional protection of the metal against the corrosion in the ag-gressive medium for the purpose of my application. We could passivate

54

the metal surface for the purpose (deposit an oxide layer) or apply the fol-lowing protection techniques: Anodic protection Anodic protection is performed by covering the tank with another metal having lower electrode potential than the tank metal thus performing the role of a destroying anode. The additional metal should meet certain re-quirements for this purpose: it should have sufficient negative potential, it should be inexpensive, it should dissolve in electrolytic medium and it should not form a protective layer over itself.

Practical application of steel construction protection: Zinc protective cov-ers bolted down to the construction are most often used [7]. Trials indi-cate that such protective covers do not cover themselves with corrosion products and cleaning and replacing them is not performed so often. The protective coating efficiency depends on the following factors [7]:

- the electric conductivity of the corrosive medium. In higher elec-trical conductivity conditions the protective action has a wider range. In such cases the protection covers could be located at a larger distance from each other;

- the way of application. The protection cover should have suitable shape and size and should be positioned over the tank so that it allows good access for cleaning and observation. The protection cover area is within 20 – 200 cm2 depending on the particular conditions and the strip thickness is between 4 and 12mm. Pro-tection cover could also be provided in the cases when the metal construction has been painted this prolonging the life of the coat-ing.

Cathode protection For cathode protection the electrical current supplied from an external source is used to achieve cathode polarization of the metal. The more aggressive corrosive agents are the higher should be the value of the po-larization current. One optimum polarization current density could be es-tablished for every individual case to provide maximum complete metal protection.

The minimum value of current density is affected by the nature of the metal being protected, the nature of the corrosive medium and this value varies within a wide range. The conditions when cathode protection can be applied are the following:

- electrolytic medium, such as tanks containing salts, acids, bases, etc. should be available around the metal surface to be protected;

55

- the electrolyte should be in constant contact with the protected surface and sufficiently deep;

- the surface to be protected should have simple geometrical shape;

- the protection current required should be safe to the construction and the servicing personnel.

Cathode protection is applied in many metal constructions, such as the inner and outer surfaces of tanks, cisterns, etc. A safety requirement is that the protection current voltage is not higher than 20 to 40V and cur-rent power – higher than 400A.

Practical application: Steel or Cu, Al, Zn, could be used for the anode. It is recommended that the size of flat anodes (strips) is not less than 20mm in thickness and 200mm in width. For this minimum anode thickness an-ode durability of 2 to 4 years is guaranteed. Anodes should be located such that they ensure the best possible current distribution. The distance between the anode and the protected surface is not of any particular im-portance to corrosion protection but it is recommended that anodes are positioned at more than 0.25 m from this surface and that they are se-curely fastened and their contacts – well insulated from the corrosive me-dium [8]. Anodes should be connected directly to the positive pole of the current supply source. Anodes made of zinc and magnesium or aluminum alloys are typically used for protecting steel. Zinc anodes are made with high metal purity and aluminum anodes are made of aluminum alloyed with 1-1.5% zinc. Protection by using additives (inhibitors) Protection of metal constructions subjected to corrosive mediums (acids or bases) could also be provided using additives – the so-called inhibitors that have the property of canceling the corrosion process (they stop the metal from dissolving in a given medium). Most commonly applied are in-hibitors for the protection of metal constructions against the aggressive action of strong acids (sulphuric acid, hydrochloric acid and nitric acid) [8]. For the protection against the first two acids mixtures of organic com-pounds containing nitrogen in the form of amino acids, aldehydes (for more concentrated solutions) and some sulphur-containing compounds as well as non-organic arsenic compounds (arsenic chloride) are usually used. Sufficiently effective non-organic inhibitors are the salts of stibium or bismuth. For nitric acid some alkaloids, sulphur organic compounds, ethers, spirits, sugar and also additions of potassium dichromate, potas-sium chromate and hydrogen peroxide at certain concentrations provide inhibiting action. We cannot speak here about corrosion protection

56

against the action of alkali bases because steel dissolves in them too slowly.

FACTORS AFFECTING TANK EFFICIENCY AND USAGE Efficiency factors play a major role in product design and development processes. Tank efficiency involves reducing the cost of the construction as a whole, avoiding complicated and expensive solutions, using inex-pensive materials and simple manufacturing and assembling techniques.

One of the factors that influence tank efficiency is its shape. We choose a cylindrical shape that can easily be made by bending an entire sheet or welding together several face-welded cylindrical rings. This shape avoids the presence of sections subjected to internal stresses. Such sections in these constructions corrode significantly faster. This is due to the fact that as a result of the deformed (strained) grid tending to restore its initial state the metal oxide deposits being formed are constantly being destroyed and the metal this corrodes faster in this particular section. Automated or semi-automated welding providing tough and uniform welding seams hav-ing smooth surface should be applied to reduce stress. Face welding seams should be applied and to increase fatigue strength we recommend welding on both sides and avoiding accumulation of weld metal, as well as avoiding mechanical finishing of the weld seam.

Construction efficiency can also be achieved by using universal devices and elements. In our case we could choose suitable handles, wheels, valve, caps, etc. for the tank.

The certainty coefficient is not the same for the different machines and equipment. It depends on the consequences that would result from an eventual failure. But this does not imply that the tank should have exces-sive reserves. Nowadays, the servicing times for such products have been reduced and the requirements placed on the capability to operate in higher capacities, better reliability and efficiency, operational convenience and easy maintenance, have increased. That is why the construction of the tank should be in compliance with the anticipated servicing time term [9]. These requirements affect the selection of material and the tank wall thickness value (2-3mm). The bottom of the cylindrical tank could be se-lected to be flat and the recommended thickness would be around 5 mm.

All presently designed machines and equipment should meet the re-quirements of modern industrial aesthetics and ergonomics, i.e. they should have beautiful and perfect shape but still maintaining the function-ality and operational convenience as well as providing easy and conven-ient servicing and control. The tank should meet the reliability and han-

57

dling safety requirements. A warning should be provided on the outside to indicate its contents and handling safety instructions should also be indi-cated.

USING THE TANK FOR THE STORAGE OF 10% HYDROCHLO-RIC ACID For the transportation and storage of 10% hydrochloric acid (HCl) we could use the tank made of the selected alloy steel X10CrNiMoTi18 12 as this type of steel exhibits high corrosion resistance in this acid.

The tank made of 25 steel would not allow for storing hydrochloric acid unless some additional protection is provided. In this case we could apply cathode protection (electrochemical protection). We could cover the metal construction with another metal exhibiting lower electrode potential so that it can decompose (bond with chlorine ions). Such protective metal could be aluminum alloyed with 5-10% zinc. Experiments have indicated that these protective covers have lower degree of corrosion and cleaning and replacing them is not so often.

Inhibitor protection (adding additives to the hydrochloric acid to stop the corrosion process) could also be used when the tank is to be temporarily used to transport hydrochloric acid [9]. Mixtures of organic substances containing nitrogen in the form of amino acids, aldehydes (for more con-centrated solutions) and sulphur-containing compounds as well as some inorganic arsenic compounds (arsenic chloride) could be used for addi-tives.

REFERENCES

1. Bathe K., Finite element procedures in engineering analysis, Prentice Hall, Englewood Cliffs, N.J. (1982).

2. Smith I.M., Programming the finite element method, Tiptree, Essex, Anhor Brendon Ltd. (1988).

3. Ashby, M. f. AND Jones, D. R. H., Engineering Materials, Pergamon Press, Oxford (1992).

4. Hertzberg, R. W., Deformation and Fracture Mechanics of Engineer-ing Materials, 3rd edition, John Wiley, New York (1989).

5. Osgood, C. G., Fatigue Design, Pergamon Press, Oxford (1994).

58

6. Tapsell, H. J. (1992) Symposium on High Temperature Steels and Al-loys for Gas Turbines, McGraw-Hill, New York.

7. Lessels, J.M. (1996) Strength and Resistance of Metals, John Wiley, New York.

8. Andrade, E. N. (1998) The viscous flow in metallic vessels, Ch. 9 I.Mech.E., London.

9. Pomeroy, C. D. (1993) Engineering Materials & Industrial Applica-tions, John Wiley, New York.

59

CHAPTER 4 FIBRE REINFORCED COMPOSITE MATERIALS

INTRODUCTION The development of industry and material sciences in recent years more and more often involves the application of materials belonging to this vast group. Composite materials are non-metal materials made by bonding to-gether individual long known and used before or brand new materials [1]. The manufacturing processes used for the production of this constantly expanding and significant to modern industry group of materials involve both previously known and brand new production techniques.

The basic components constituting composite materials are various types of Thermoplastics and Thermosets, which were initially found and even-tually further enhanced during the 13th century. These find wide use in domestic applications and in the production of engineering goods and products for the aviation and space industry, weapon industry, radio elec-tronics industry, and scientific applications [1]. Apart from the basic con-stituent such as resin, impregnating glue, rubber, etc., composite materi-als often include in their composition a wide range of composite fibre-reinforcing additives. These include wood, wooden particles and cellulose products, such as paper, woollen or cotton textiles or fibres, glass fibres, carbon in the form of long or short fibres, and asbestos. Some super composite materials of exclusive mechanical characteristics were pro-duced based on thermosets and glass or carbon fibres [1]. Such materials feature high mechanical strength, low weight, high temperature resistance and low manufacturing cost.

Composite materials find especially wide application as sheet materials, simple or complex shape moldings or processed bulk products. Matrix materials usually involve the use of polyester, phenolic or epoxy resins. This base was used to produce “Tufnol” and other types of laminated plastics.

COMPOSITE MATERIALS AND MANUFACTURING TECH-NIQUES. Polymeric materials constitute the basic building material of composite materials. It is not usual to use pure resin to mould a plastic product. The

60

appearance and performance of most plastic and elastic polymers can be improved by the use of various additives:

Plasticisers. These are added to polymeric materials to reduce their rigidity and brittle-ness and improve their flow properties whilst being formed or molded [2]:

Primary plasticizers are used to neutralize partially the Van der Woal’s forces between adjacent molecular chains by introducing monomers whose polar groups neutralize those of the polymer groups and allow greater mobility between adjacent polymer chains.

Secondary plasticizers are monomers of a compatible but inert mate-rial without polar groups, which may be added to provide mechanical separation of the polymer chains in the same way that a lubricant separates a shaft from its bearing.

Illustration of the above is given in Figure 1.a, b.

= Strong intermolecular (Van der Waal’s) forces

Polymer chains

= Secondary plasticiser separaring the polymer chains and weakening the Van der Waal’s forces

(a)

(b)

Fig. 1

61

Secondary plasticizers may themselves be divided into two groups ac-cording to the method of application [3]:

Internal plasticization. Small amount of plasticizer are added during polymerization. For example – Polyvinyl chloride (PVC), which is a rigid rather brittle plastic material, can be made flexible by the addi-tion of 15% vinyl acetate – as a secondary plasticizer during po-lymerization.

External plasticization. This is the more common method of plasti-cization. The plasticizer in the form of a low-volatility liquid solvent is added after polymerization. It disperses throughout the plastic, filling the voids between the polymer chains and acting as a lubri-cant.

The influence of plasticizers added during the polymerization process (in the case of PVC) on the mechanical characteristics of the polymer mate-rial is illustrated in Figure 2.

10

20

30

40

50

60

100

200

300

400

500

600

0

Elong

ation

Plasticiser % High

Tens

ile s

tren

gth

[MPa

]

Tensile strength

Elon

gatio

n [%

]

Fig. 2

Fillers Fillers have a considerable influence on the properties of moldings pro-duced of any given polymeric material. They improve the impact strength

62

and reduce shrinkage during molding [3]. Fillers are essential in thermo-setting molding powders and may be present in quantities up to 80% by weight. The selection of a filler is usually determined by the properties it can impact to the plastic product and its cost also.

Some filler materials are: glass fibres, wood flour – calcium carbonate, aluminum powder (high mechanical strength), shredded: paper, cloth; Mica granules – good strength, combined with reasonable electrical insu-lation properties.

The influence of some additives on the mechanical characteristics of phe-nolic resins is illustrated in Figure 3.

10

20

30

40

50

60

0

Stre

ss

[MPa

]

0,2 0,4 0,6 0,8 1,0 1,2

Calc

ium

car

bona

te fi

ller

Wood-flour f

iller Unfille

d

Fig. 1

Fig. 3

Stabilizers, colorants, etc. additives could also be used before the polym-erization processes.

Stabilizers. They are used to prevent the degradation of polymeric materials occur-ring when they are exposed to heat, sunlight and weathering. Such deg-radation is usually accompanied by color change, deterioration in me-chanical properties, cracking and surface cracking [4]. The influence of

63

additives on the elasticity module (Young’s module) of a polyester resin is illustrated in Figure 4:

1,0

2,0

3,0

0

Flex

ural

mod

ulus

[G

Pa]

10 20 30 40 50 100% Filler................

Calcium ca

rbonate

Wood flour

Fig. 4

Antistatic agents Antistatic agents can sometimes be added to polymers to eliminate or re-duce the static charge effect. These are included to increase surface conductivity so that static charges can leak away.

Some composite manufacturing techniques A number of manufacturing techniques are available for the production of the final product: Depending on whether it is an injection-molded product, or molded or stamped product, or a laminated pre-formed (semi-finished) fiber sheet product [5]. The fiber reinforced material can also be applied to a band having the basic material applied to it (the resin of the matrix) featuring short, randomly arranged fibers. Such processes are suitable for producing sheets of composite materials to be loaded in all directions in the sheet flat plane. Such composite materials feature high characteristics depending on the characteristics of the constituent components. RIM processes are also employed.

64

GENERAL PROPERTIES OF POLYMERIC MATERIALS The properties of polymers can vary widely, but they all have the following properties in common [5]:

Electrical insulation. All polymeric materials exhibits good electrical insulation properties. However, their usefulness in this field is limited by their low heat resistance and their softness.

Strength/weight ratio. Polymeric materials vary in strength consid-erably. Some of the stronger (ex. nylon) compare with the weaker metals. All are much lighter than any of the metals used for engineer-ing purposes. Therefore properly chosen and proportioned, their strength weight ratio compares favorably with many light alloys and they are steadily taking over engineering duties, which until recently, were considered the prerogative of metals.

Corrosion resistance. All polymeric materials are inert to most inor-ganic chemicals.

INFLUENCE OF THE MATRIX (BASIC ELASTOMER) AND ADDI-TIVES ON THE QUALITY OF THE COMPOSITE MATERIAL.

Influence of the type of elastomer on the quality of the composite material.

The type of the basic material of the composite matrix greatly influences the basic characteristics of the composite material. Listed underneath are some of the elastomers being used (which are mainly required to feature high ductility and toughness rather than high strength characteristics).

Typical representatives of engineering applications are [6]:

Acrylic rubbers. These are oil, oxygen, ozone and UV radiation resistant;

Buryl rubber, Nitrile rubber. These are used for tanks for cars and aircrafts. Freon resistant;

Polychloroprene rubber. Used for tyres, cables, sealants, etc.;

Polysulphide rubber. Used for sealants, etc.;

Polyuretane rubber, Rybber hidrochloride, Silicone rubbers. Used for high and low temperature applications (from –80°С to +235°С).

65

Typical representatives of thermoplastic resins are [6]:

Polyethylene, Polypropylene, Polystyrene, Polyvinyl chloride (PVC), Polymethyl methacrylate, Polyamides, etc. Their basic properties include:

Density ≈ 900÷1300 kgm-3 ;

Tensile strength ≈ 10÷50 MPa;

Elongation ≈ 10÷800%;

Maximum service temperature ≈ -25°С ÷ +320°С, etc.

Other typical representatives of thermoplastic and thermosetting resins include:

Polyesters (Terylene), Polyacetates, Polycarbonates, Cellulose acetate, etc.

Phenol formaldehyde, Epoxides Polyacetates, Polycarbonates, Cellulose – these are all thermosetting resins having density ≈ 1500 kgm-3, Tensile strength = 35 ÷ 80 MPa, Impact value = 0,2 ÷ 1,5 and can withstand up to about 200°C.

Resol resin to which asbestos filler, etc. is added is also sometimes used for the production of composite materials. High impact composite materi-als include: polymerization type plastics with glass fiber or carbon fiber reinforcement (phenolmormalaldehyde, epoxy and polyester resins with fillers). Resin characteristics and quality significantly influence the quality of the composite material.

Influence of additive (reinforcement) material

As mentioned earlier, a large number of reinforcement materials are also used in the production of composite materials, which include [7]:

Fiberglass, wooden scrapes, carbon fiber and textiles, cotton and woolen fibers, paper and other cellulose products, nylon fibers, aluminum pow-der, etc. The overall quality of the final composite material is defined by the mechanical and other characteristics of these additives, as well as the mixing technique used in molding the final product, the arrangement of fibers in the matrix and the number of layers or method of bonding pre-pregs.

For example, in “delta wood”, which features very good mechanical char-acteristics and is usually used for making aircraft propellers and other

66

critical component parts, the arrangement between longitudinal and transverse fibers is in the ratio of 10:1.

Here are some specific features of the manufacturing techniques used for the production of the “Tufnol” type of composite material, which employs phenolic and epoxy resins fortified by the addition of some of the fillers mentioned above: Sheets of fibrous reinforcement materials are impreg-nated with the resin and they are then laid up between highly polished metal plates in hydraulic presses. The thickness of the finished sheet is determined by the number of layers of impregnated reinforcement in the laminate. Each layer of reinforcement is rotated through ninety degrees of arc so as to ensure uniformity of mechanical properties. The laminates are then heated under pressure until they become solid sheets, rods or tubes.

Manufacturing techniques are also of high significance to composite ma-terials based on epoxy or polyester resins and glass fibres for which high strength/density ratios are required for applications such as boat bodies, yachts, and aircraft component parts.

Smaller products are hot molded in semi-automatic processes, whilst lar-ger moldings are laid up individually by skilled craftsmen.

Polyester resins are the most widely used for bonding fibres together, but the epoxy resins are used where maximum strength is required and higher cost can be justified.

Several types of glass fibre reinforced materials are available featuring different characteristics, which in turn influence the quality and physical and chemical characteristics of the composite material [8]. For example:

E - glass (electrical grade) is used for the manufacture of high-grade printed circuit boards. It has excellent electrical insulation properties and dimensional stability;

C – glass is used for chemical plant moldings. It has good resis-tance to acid attack;

S – glass is a high strength;

M – glass has an exceptional high tensile modulus.

Table 1 shows some example data of glass fibre based composite mate-rials [9]:

67

Table 1

Material Relative density

Tensile strength (GPa)

Tensile modulus (GPa)

Specific strength (GPa) (1)

Specific modulus (GPa) (2)

E - glass 2,55 3,5 74 1,4 29

S – glass 2,50 4,5 88 1,8 35

Steel wire (for comparison)

7,74 4,2 200 0,54 26

where:

(1) Specific strength = Tensile strength Relative density

(2) Specific strength = Tensile modulus Relative density

As well as the direct influence of the fibre content on the tensile modulus (flexural stiffness) and the tensile strength of a GRP composite, the strength of the composite is also influenced by the orientation of the fibres [9].

Parallel yarns. All the glass strands are laid parallel to each other to pro-vide unidirectional reinforcement.

Woven cloth. Half the strands are laid at right angles to each other and half and locked by weaving. This provides bi-directional reinforcement.

Chopped strand mat. Short strands of glass fibre are arranged in a to-tally random manner to form an isotropic reinforcement, that is the rein-forcement is equal in all directions. Chopped strand mat is used where strength has to be combined with sharp curves and complex shapes.

Influence of fibre arrangement and quantity The amount of reinforcement which can be used depends upon the orien-tation of the reinforcement. With long strands laid up parallel to each other the reinforcement area fraction can be as high as 0,9. The rein-forcement area fraction is the cross-sectional area of reinforcement di-

68

vided by the total cross- sectional area as shown fig. 5 – Reinforced com-posite:

Reinforcement (area “a”)

Matrix

Reinforcement area fraction = n a

Ax

where: n = number of reinforcement a = cross sectional area of each reinforcement A = total cross sectional area of composite

Fig. 5Load

Load

Table 2 shows the influence of the type of fibre-reinforced material on the strength characteristics of the composite material.

Table 2 Properties of GRP composites [9]

Material Reinforcement (weight %)

Tensile strength (MPa)

Tensile modulus (GPa)

Chopped strand mat 10 - 45 45 - 180 15 - 15

Plain weave cloth 45 - 65 250 - 375 10 - 20

Long fibres – uniaxi-ally loaded

55 - 80 500 - 1200 25 - 50

69

RESIN-FIBRE INTERFACE (COUPLING AGENTS) In order for the composite to function properly there must be a chemical bond between the matrix and the re-inforcing fibres in order that the ap-plied load (applied to the matrix) can be transferred to the fibres (which are expected to do all the work [10]. However, the bond must not be too strong since the toughness of the composite comes from such sources as fibre pullout and fibre-matrix interfacial fracture. In 'fibre glass' the fibre is inorganic while the matrix is organic and the two do not bond readily unless the fibres are treated to modify their surface.

Silica (SiO2) is hygroscopic ie. it absorbs water onto its surface where the water breaks down into hydoxyl (-OH) groups. It is impossible to avoid the water especially as the surface modifier or ‘size' is applied in a water based solvent. It should also be stressed that water reduces the strength of SiO2 by a stress-corrosion-cracking mechanism [10]. The coupling agent takes the form of a silane (R-SiX3) where R is an organic radical that is compatible with the polymer matrix (it may even react with the ma-trix polymer; for this reason styrene groups are favored for polyesters while amine groups are preferred for epoxies) and X is a hydrolysable or-ganic group such as an alcohol. The most common silane couplant is tri-ethoxy-silane. Heat will force the elimination of water between the -OH pairs at the hydrated silica surface and the silane as well as between the adjacent silane molecules.

CONCLUSION Advanced composites are being increasingly used as alternatives for conventional materials primarily because of their high specific strength, specific stiffness and tailorable properties.

With increasing use of composites as critical-load-carrying components in a variety of structures, fully understanding of different damage mecha-nisms and developing mechanistic models that can predict damage in a generic laminate is necessary.

The mechanical properties of a composite material are determined by the properties of its constituents. Due to the strong inhomogeneity of these materials, i.e. the high stiffness ratio between fibres and matrix, and the inhomogeneous fibre distribution in a composite, the mechanical behavior is strongly influenced by phenomena occurring at a scale of microns.

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REFERENCE

1. M. Kharrat, L. Carpentier, A. Chateauminois, P. Kapsa, Evaluation of the fibre/matrix interfacial strength of a glass fibre reinforced polymer composite using a micro-indentation test." Composites Part A, 28, N°1 (1997) 39-46.

2. M. Zidi, L. Carpentier, A.Chateauminois, F.Sidoroff, Quantitative analysis of the micro-indentation behaviour of fibre reinforced com-posites: Development and validation of an analytical model.

3. Composite Science and Technology 60 (2000) 429-437

4. M. Zidi, L. Carpentier, A. Chateauminois, Ph. Kapsa, F. Sidoroff, Analysis of micro-indentation tests by means of an analytical model taking into account different interfacial responses.

5. Composites Science and Technology 61 (2001) 369-375.

6. E. Car, F. Zalamea, S. Oller, J. Miquel, E. Oñate, Numerical simulation of fiber reinforced composite materials––two procedures. International Journal of Solids and Structures, Volume 39, Issue 7, April 2002, Pages 1967-1986.

7. X. Neil Dong, Xiaohui Zhang, Y. Young Huang, X. Edward Guo, A gener-alized self-consistent estimate for the effective elastic moduli of fiber-reinforced composite materials with multiple transversely isotropic inclusions. International Journal of Mechanical Sciences, Volume 47, Is-sue 6, June 2005, Pages 922-940.

8. Z. Haktan Karadeniz, Dilek Kumlutas, A numerical study on the coeffi-cients of thermal expansion of fiber reinforced composite materials. Composite Structures, Volume 78, Issue 1, March 2007, Pages 1-10.

9. George Z. Voyiadjis, Peter I. Kattan, Mechanics of small damage in fi-ber-reinforced composite materials. Composite Structures, Volume 92, Issue 9, August 2010, Pages 2187-2193.

10. S. T. Mileiko, V. I. Glushko, Fabrication and properties of new oxide-based composite fibres (MIGL) and heat-resistant materials reinforced with them. Composites Science and Technology, Volume 58, Issue 9, September 1997, Pages 1497-1507.

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CHAPTER 5 MATERIALS & DESIGN FOR BRIDGE MODELLING

INTRODUCTION Bridge structures are widely spread in engineering practice. There is a wide variety of construction shapes and static arrangements mainly de-fined by the purpose, usage features, nature of applied loads and also, the materials used to build it. The general idea can be seen in every bridge construction, and its crane supports [1].

Materials widely used in practice to build such constructions are: stone, concrete, reinforcement concrete, metals, wood, rope and in the majority of cases, a combination of them. In many of the cases it is possible to consider them as a group of structures incorporating common calculation and design considerations and set up on common technological grounds [1]. In other cases, specific parameters and properties are used. For ex-ample, the technology and design of the construction form of stone bridges known from the past are consistent with the consideration that the arch structure should carry the load in a way that should allow for the originating torque to be reduced to a loading force applied on each of the construction elements [2].

In practice, the constructions should comply with a number of require-ments and any failure to meet them might hinder or make normal usage impossible. The following basic requirements are obligatory [2]:

- the construction shall fulfill its intended purpose and provide normal and safe use;

- the construction shall be highly reliable, i.e. it shall have sufficient strength, toughness and stability;

- the durability of the construction, both physical and moral shall be suitably consistent with the materials and funds used;

- the construction shall be efficient (optimal) to allow to build structures of minimal mass and use less expensive and scarce materials and assemblies, employ lower labor consumption to make and assemble.

INITIAL DATA In this chapter we review a model case study in which designs and con-struction of structures that extend over a short distance between two sup-

72

ports and can carry a centrally located load have been limited to the fol-lowing: construction material - pasta; adhesive agent - glue; maximum weight of structure - 50 g.

Construction geometry

Testing jig diagram For the purpose of the test the construction is rested on two supports ar-ranged at a distance of 300mm one from another. A loading structure is positioned in the centre of the bridge where components having a weight of ΔP can be attached and their number is increased until complete de-struction of the construction is achieved.

P

L- min 300mm H- max 80mm W G 50gms

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- Loading structure diagram

P P

1

2

3

4

DESIGN REQUIREMENTS It is necessary for the design to consider the requirements of the manu-facturing and assembling technique implementing most advanced and ef-ficient technological methods [3]. The designers aimed at rational repeti-tion of elements of simple shapes. Particular efficiency is achieved through the use of type-designs and design solutions, unification and standardization of resources and the optimization of production it in-volves.

The design load for the present case study is specified and so is the basic material. The main objective is to achieve the best balance between the construction own weight and its load carrying capacity, observing feasibil-ity requirements [3], minimum labor consumption [4], and considering specified limitations for maximum weight, types and ways of using raw material, and approximate geometry. We can present the load on the con-struction using the following simplified most general two-dimensional dia-gram that we can use as a pattern. Where P is the load force and q is the load distribution of its own weight [3].

P(N)

q(N/m)

1 – Body 2 – Pin 3 – Clamp 4 – Loading force

74

RAW MATERIALS & CONSTRUCTION ELEMENTS

Characteristics Pasta is available in shopping areas in a wide variety of section sizes and not so many in length and composition. It represents a sort of pastry made of a mixture of flour, water and salt and some brands may have added eggs. The shape is achieved through injection by means of noz-zles and followed by drying. It is intended for a much different purpose but the variety in size makes it a suitable means for the present study.

The material can be compared in its properties, structure (filler and bind-ing agent) and qualities to concrete. Alongside, several peculiarities in structure it should be clarified, which explains the bending test results, we have carried out with the elements. The product undergoes drying follow-ing injection. In spite of its negligible section size, drying is not taking place uniformly throughout the entire volume but starts from the surface and moves towards the middle. This is causing internal stress leading to the occurrence of micro cracks and structure unevenness [5]. From the experimental data provided underneath, the breaking strength observed for one and the same section area varied within fairly wide limits.

Material tests Consistent with the above mentioned we have classified the material as brittle with properties resembling cast iron, glass, and concrete [5]. Fol-lowing discussion over the size and shape of pasta to be used for the de-sign and construction of the artifice, we have come to the conclusion that we should limit the range even at the risk of making a mistake. We have adopted this approach with the aim to reduce the number of parameters we can use to seek optimum results in other directions, as for example various types of solutions, and construction geometry [6]. Finally, we de-cided to use four types of elements having the following dimensions:

− a pipe having D= 2,8mm OD and d=1mm ID; length L≈260mm and weight G=0.0075 gms/mm.

− A bar having height h=6mm; thickness b=1.2mm; length L≈255mm and weight G=0.01gms/mm.

− A pipe having D=5mm OD; d=3.2mm ID; length L≈32mm; weight G=0.012gms/mm.

− Vermicelli having D=1mm OD and weight G = 0.0011gms/mm.

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The last two elements in the above list are to be used to make joints. We had the idea to moisten vermicelli and thus use it for making ties and winding ropes for suspended structures but we were not successful in achieving the right technique. The material was very easy to break when moistened and it was almost impossible to achieve the desired configura-tion. Moreover, we could not manage to successfully dry the material fol-lowing moistening and the element remained with a multiply increased weight and worse mechanical characteristics [7]. After the measurement made we found that the average material density was 1,390 kgs/dm3.

Defining mechanical characteristics of brittle materials through tensile tests is hindered by the insufficient deformations and in principle they are subjected to bending tests [7]. Hence, the following diagram has been applied.

P(N)

The bending stress is determined by the equation [7]:

[ ]σ =MW

N my

max , / 2

Where M

Plmax =

4

is the maximum moment of bending of the sample (N/m);

[ ]Wd

my =⋅π 3

332

,

is the moment of axial resistance for circular sample cross section area.

[ ]σπ

=⋅

83

2Pld

N m, /

l= 100±3 mm;d= 2.8mm; P – variable P=Po+iP1 Po=48gms P1=25gms

76

The following results were measured from the tests carried out over 50 samples:

Strain to/kg 48 73 98 123 148 173 Number 2 14 9 16 6 3

We were not able to make an exact assessment of the mechanical char-acteristics from the results measured, since their variation was in a wide range and the results were influenced by above mentioned macro de-fects. Having analyzed however the results we were able to determine our work algorithm. We knew which building materials we were going to use and what their weight was so we implemented the allowed quantity of elements within the specified range of the assignment and looked for the maximum effect of the suitable construction.

CONSTRUCTIONS Possible alternatives implementing this layout can be numerous. Disre-garding the specified limitations (the sizes of building elements and the materials that should be used to make the sample product), we have classified the possible alternatives considering their construction charac-teristics in the following groups: Suspended constructions; Frameworks; and Double-beam structures [8].

A framework construction had to be prepared. We have adopted the fol-lowing designation model of the alternatives to help systemize the work in this context - B X-YY, where:

− X=1 for suspended constructions

− X=2 for frameworks

− X=3 for double-beam structures

− YY - option numbering in chronological order

Frameworks are structures made up of separate bars joined through seams or riveting (in this case we used adhesive agent). It is assumed that each assembly is a non-friction joint and the framework is considered as statically defined and geometrically unchanging hinged joint bar sys-tem [9]. This helps us achieve simplification of calculations and higher certainty. When load is applied to the assemblies, only normal strains re-sult in separate bars (tension and compression). Therefore, the material in framework construction is used much more efficiently than in other types of constructions. Practice shows that they are lighter, on the other hand, much more labor consuming compared to plate girder, for example.

77

When loading frameworks with movable loads the strain in some of the bars changes in sign which along with the high stress concentration in the assemblies significantly reduces the range of their efficient application. Therefore they are successfully used only for big apertures, low loads and statically applied external load.

Alternatives:

− Option B 2-01 The construction represents a system of three supporting plane frame-works of triangular shape mounted using horizontal and slope bars that fix the frames in space in one compact 3-dimensional system. Additional bars are implemented longitudinally in the free area between plane frameworks that together with upper framework structure comprise the supporting platform. The structure only comprises a single building ele-ment, a pipe of D=2.8mm OD and d=1mm ID and the parts only differ in length.

78

Option B 2-01

79

A 4435 mm building element is design specified for the construction and if we assume a minimum weight of the adhesive, the weight of the structure will be 33.5 g. Plane frameworks to which external loads are applied are so constructed that their geometry would copy the diagram of the moment of bending the maximum of which is in the centre of the aperture. The load diagram can be presented in the following way [9]:

T - tension bars C – compression bars

Considered in their actual application the basic provisions from the space model will still be present because in the cross-sectional area where it is most loaded, the construction is again configured as a framework at a much smaller aperture. Thus, the external load will be distributed rela-tively equally over the three longitudinal frameworks [10]. When making the model it is necessary to keep to the exact geometric element parame-ters, so that it can be made as close as possible to the design.

− Option B 2-02 As in the first option, the construction represents a system of three sup-porting plane frameworks mounted using horizontal and slope bars that fix the frames in space in one 3-dimensional system. Additional bars are implemented longitudinally in the free area between plane frameworks that together with upper framework structure comprise the supporting platform. The structure comprises the same building element. A 4462 mm building element is design specified for the construction and a theoretical weight of 34.5 g. This option is easier to make as it comprises a smaller number of parts.

80

пп

B 2-02

81

82

− Option B 2-03 The advantages and disadvantages of the third option are as for the first two ones. Also similar is the construction layout and the section of the elements. The specific feature here is that the weight of the structure is much lower compared to the previous options. A 2856 mm material and 23 g are designed for it. The arc flange of the framework assumes a more uniform distribution of strain and conditions are available for neutralization of eventual dynamic loads through a spring action of the structure. Al-though it is a framework construction it has properties of the suspended constructions as well [9]. From a technological view point the bending of the arcs can be performed by moistening of the element until softened, bending over a pattern and subsequent good drying (in a microwave, for example).

Suspended constructions

− Option B 1-01 The basic difference that distinguishes suspended constructions from other groups is that external load is reduced in the construction support-ing elements to a tension strain. Ropes are often used as a material for this structure [10]. In this context the material we are using is not the most suitable one for such type of construction.

The use of thread as a building element in Option B 1-01 does not meet the limitations defined in the assignment but I am giving it as an example to illustrate the characteristic features of this construction concept. The specific feature in this construction is that a platform is built between two high and solid supports (columns). The platform is supported in its weak-est section by means of relatively flexible elements that transmit the load to the supports bend loaded as a restrained beam [10]. If the supports are balanced by applying equivalent load in the opposite direction the torque is then eliminated from them and they can uniformly take loads and work only on stress. The efficient usage of material properties is a ground for achieving efficiency of the construction. Another advantage is the fact the dynamic loads are elastically born by the structure. A basic disadvantage of this type of construction is that it is very complicated to make in real conditions.

83

пп

B 1-01

84

− Option B 1-02 The following materials have been used for the specification of Option B 1-02:

- pipe section parts at 2.8OD/1ID

Denotation Length (mm) Quanti (number)

B 1-02-01 46 12 B 1-02-02 150 8 B 1-02-03 38 8 B 1-02-04 40 5 B 1-02-05 139 10

- rectangular section parts 6 x 1.2

Denotation Length (mm) Quanti (number)

B 1-02-06 67 23 Theoretical unit weight – 42.8 g.

The geometric characteristics of the section having 6 x 1.2 size are fea-tured by having a high inertia Jx [10]

Jx=bxh3/3,

in this case Jx=8,64.10-11 (m4).

We have considered that the working platform in this construction is to be supported by two fixtures via a bar system operating on tension. The two fixtures represent assemblies comprising two columns each statically fixed along the entire width of the unit. Due to the fact that they operate on tension the column itself represents a reinforced assembly of 5 bars adhered one against the other (section A-A). The beams that “tension” the platform are four pairs (one for each column) of bars adhered one against the other (section B-B). The most unfavorable loads in this struc-ture are in the joint areas where tension bars are tied to the columns [10].

85

AA

B

B

B 1

-02

A-A

B- B

86

− Option B 1-03

Option B 1-03 works using the same diagram. The materials used for it are:

- Pipe section parts having 2,8OD/1 ID

Denotation Length (mm) Quanti (number)

B 1-03-01 46 12 B 1-03-02 38 8 B 1-03-03 139 10 B 1-03-04 40 5

- Rectangular section parts 6 x 1,2

Denotation Length (mm) Quanti (number) B 1-03-05 173 4 B 1-03-06 67 23 B 1-03-07 70 1

The theoretical weight of the product is 41.3 g. The connection between tensioning beams here is in one point that is separate to the working plat-form. The B 1-02 and B 1-03 options are easy to make. It is possible to assemble separate parts in them in assemblies (columns, tension beams and working platform) and then assemblies can be fixed in the complete construction.

87

AA

B

B

B 1

-03

A-A

B-B

88

STRUCTURE SELECTION & MANUFACTURE The B 2-01 option has been chosen to be made due to the following rea-sons: the characteristics of the construction type were fully specified as a space framework; the construction is compact and allows for a complete usage of material properties and characteristics; it is not impossible to make when working more precise. In the same sense, this construction is requiring a precise preparation and installation of components. Otherwise, many of the advantages pointed out could become a mere good wish. The process of making the unit has been as follows:

1. We have made all sizes of product components to a maximum tolerance of +/-1mm from the nominal dimension. Precise tem-plates later used for the assembly have been drawn on paper.

2. The working platform (the upper flange of the framework) has been fixed using a template.

3. Next, we have mounted the components that form the cross sec-tional framework (in the middle of the cross section). We have performed the assembly holding the construction at 180 degrees from the surface.

4. We have fixed the long beams of the three plane frameworks and at the same time fixed them in the cross sectional area fol-lowing which they formed a triangle.

5. The diagonal and vertical bars of the framework have been care-fully fixed.

6. We left the model dry overnight and then easily bonded joint points using an adhesive. The construction was carefully cleaned and the excessive material has been removed.

ALTERNATIVE CONSTRUCTIONS Possible alternatives implementing this layout can be numerous. Disre-garding the specified limitations (the sizes of building elements and the materials that should be used to make the sample product), we have classified the possible alternatives considering their construction charac-teristics in the following groups [11]:

− Suspended constructions

− Frameworks

− Double-beam structures

In further specification of the tasks in the chapter we have undertaken to develop several options based on the construction idea of the double-

89

beam structures. This classic structure it represents a working platform build on two or more beams having a suitable section area, enough to bear external loads (forces and moments) from the platform and transmit these loads to the supports. We have specified one similar idea meaning that we have two beams but the construction is from the space framework type. Frameworks can have various configurations, for example space systems made up of three or four plane trusses.

Frameworks are structures made up of separate bars joined through seams or riveting (in our case we used adhesive agent) [11]. It is as-sumed that each assembly is a non-friction joint and the framework is considered as statically defined and geometrically unchanging hinged joint bar system. This helps us achieve simplification of calculations and higher certainty. When load is applied to the assemblies, only normal strains result in separate bars - tension and compression [12]. Therefore, the material in framework construction is used much more efficiently than in other types of constructions. Practice shows that they are lighter on the other hand, much more labor consuming compared to plate girder [12]. When loading frameworks with movable loads the strain in some of the bars changes in sign, which along with the high stress concentration in the assemblies significantly reduces the range of their efficient applica-tion. Therefore they are successfully used only for big apertures, low loads and statically applied external load.

From the type of construction we have determined the following construc-tion advantages and disadvantages:

• With regards to the manufacturing, many components can be first assembled in separate units on a different site and later be brought to the site of the mounting.

• The assembly begins with the mounting and fixing of support beams after which the elements comprising the working platform are mounted [10].

• The design of the support beam ensures efficient usage of material properties and geometric characteristics of various section areas.

• From a production point of view, the specification of such a construc-tion comprises a great number of relatively small components which is a favorable feature when serial production is involved.

• This construction type is suitable for wide aperture structures, where the framework can demonstrate its virtues [12].

• As a negative feature we could point out that when transmitting ex-ternal loads from the working platform to the supporting beams in a

90

cross-sectional direction, the beams are applied a torque that in cer-tain cases must be calculated.

F

f1 f2

Alternatives:

− Option B 3-01 The construction represents a system of a working platform mounted on two support beams fixed in space by means of supporting units at the points where it is to be positioned on the supports [11]. The system also comprises connecting bars positioned along the entire length of the struc-ture.

There are two support beams of identical design and having the shape of a prism with a triangular section and made up of pipe elements with sec-tion area of 2.8OD/1 ID. Their structure represents a space framework comprising a top and bottom flange and diagonals where the ratio be-tween height and aperture is~1/16. Counter positioned diagonals at an angle of 60 degrees are connected on the side of the left and right hand flats to the three bars that comprise the flanges of the bar structure. Hori-zontal bars were used to fix the beam base flat.

Support units were mounted in both ends of the structure and are used to strengthen the area where the structure is to be positioned over the two supports. These comprise 5 bars glued together in one plane. The work-ing platform comprises flat sections glued in specified intervals to support the beams. These also help space fixing of support beams. To increase the element length we have used connections comprising two shorter bars glued in a triangle with the base bars and positioned on a separate support beam at six points. The following components were used for the construction:

91

Denotation Section Length (mm) Quantity (number) B 3-01-01 Ф2.8/ф1 121.5 4 B 3-01-02 Ф2.8/ф1 75 2 B 3-01-03 Ф2.8/ф1 100 8 B 3-01-04 Ф2.8/ф1 125 4 B 3-01-05 Ф2.8/ф1 18 104 B 3-01-06 Ф2.8/ф1 67 10 B 3-01-07 Ф2.8/ф1 27 18 B 3-01-08 Ф2.8/ф1 25 24 B 3-01-09 6x1.2 67 13

Total weight of the structure: 49.8 g

92

AA

BB

CC

B 3

-01

A-A

B- B

C-C

93

− Option B 3-02 The structure is similar to B 3-01. Support beams have the shape of a tri-angular prism with support bars having circular section alongside and with six diagonals each having a flat section on the two side flats. The ration between height and aperture is ~ 1/8. As a disadvantage it is clearly seen that the two support beams are insufficiently reliably fixed in space [12]. The structure’s weight is 41.9 g.

− Option B 3-03 The support beams have the shape of a rectangular prism in this option. This can be described as comprising of two plane trusses having a round section bar as the bottom flange and a rectangular section bar as the top flange, each connected by means of six diagonals of the same rectangu-lar section [12]. Plane trusses are fixed between them by means of bars positioned across them. The working platform is additionally supported at the positions of connections with the diagonals by means of two rectangu-lar section bars. The rational feature of this construction is the fact that the rectangular section elements used have better geometric characteris-tics [12]. We have used the following components for this construction:

Denotation Section Length (mm) Quantity (number) B 3-03-01 Ф2,8/ф1 58 22 B 3-03-02 Ф2,8/ф1 106 12 B 3-03-03 6x1.2 255 4 B 3-03-04 6x1.2 51 8 B 3-03-05 6x1.2 50 16 B 3-03-06 6x1.2 58 4

The theoretical weight of the construction is 43.71 g.

− Option B 3-04 This construction duplicates the general schematic of B 3-03, the basic difference being that we have only used rectangular section elements as support parts and round were only the components fixing the frameworks transversely. The ratio between framework height and length is 1/8, as specified in [13]. The following parts have been specified:

Denotation Section Length (mm) Quantity (number) B 3-04-01 Ф2,8/ф1 58 22 B 3-04-02 6x1.2 106 12 B 3-04-03 6x1.2 255 4 B 3-04-04 6x1.2 51 24 B 3-04-05 6x1.2 50 16

The structure weight is 52.73 g.

94

B 3

- 02

95

B3 -

03

96

B 3

- 04

97

In view of reducing structure weight down to the maximum permissible value, unnecessary elements can be eliminated from the construction [14]. The height of the structure can also be optimized so that the weight is achieved within the specified limits keeping its strength characteristics [14].

STRUCTURE SELECTION & MANUFACTURE In choosing the structure model we considered the B 3-01 and B 3-04 op-tions as the most suitable ones. At first sight, the B 3-04 option seems tougher due to the suitable element section pattern it was designed for. On the other hand, we were apprehensive for the stability of the structure in its cross section and any mistake during assembling or lack of preci-sion during testing might have caused it to become unstable and twisted in the specified plane. Then it would be easy to destroy a connection and spoil the job. That is the reason why we considered adopting the B 3-01 option as being elaborately designed.

The work in making the structure model has continued in the following or-der: All sizes of components were made for the structure to maximum precision. We divided the structure into separate assemblies and began assembly mounting. The extension bars without the reinforcement were made. We assembled the two support beams, by initially making one plane truss and then forming the frame and finally gluing the diagonals and horizontal beams of the framework. The reinforcement bars were glued to the extensions. We have separately prepared the supports each consisting of five bars glued one to another in one plane. We mounted the two support beams on their supports precisely. We glued the working platform, and following final adjustment - cleaning the structure was ready.

REFERENCES

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2. Tommy H.T. Chan, K.C. Chung, Development of Simplified Method for Composite Bridge Design in Hong Kong. Advances in Steel Structures (ICASS '96), 1996, Pages 513-518.

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3. A. M. Memari, H. H. West, Computation of bridge design forces from influence surfaces. Computers & Structures, Volume 38, Issues 5-6, 1991, Pages 547-556.

4. C. Menn, An approach to bridge design. Engineering Structures, Vol-ume 13, Issue 2, April 1991, Pages 106-112.

5. Manabu Ito, Cable-supported steel bridges: Design problems and so-lutions. Journal of Constructional Steel Research, Volume 39, Issue 1, August 1996, Pages 69-84.

6. Bench-top CMM has moving-bridge design. Precision Engineering, Volume 11, Issue 4, October 1989, Page 246.

7. Yingwei Liu, Fred Moses, Bridge design with reserve and residual reli-ability constraints. Structural Safety, Volume 11, Issue 1, November 1991, Pages 29-42.

8. An integrated bridge design system: Will, K M, Virk, S P, Schelling, D R and Emkin, L ZProceedings of the first international conference on computing in civil engineering ASCE, New York, NY, USA (1981). Computer-Aided Design, Volume 14, Issue 2, March 1982, Page 123.

9. BDES: a bridge design expert system: Biswas, M and Welch, J GEng. with Comput. Vol 2 No 3 (1987). Computer-Aided Design, Volume 20, Issue 1, January-February 1988, Page 45.

10. Y. L. Mo, R. H. Han, Cyclic load tests on prestressed concrete model frames. Engineering Structures, Volume 18, Issue 4, April 1996, Pages 311-320.

11. Jacques Brozzetti, Design development of steel-concrete composite bridges in France. Journal of Constructional Steel Research, Volume 55, Issues 1-3, July 2000, Pages 229-243.

12. Meng-Hao Tsai, Si-Yi Wu, Kuo-Chun Chang, George C. Lee, Shak-ing table tests of a scaled bridge model with rolling-type seismic isola-tion bearings. Engineering Structures, Volume 29, Issue 5, May 2007, Pages 694-702.

13. B. Semper, Finite element methods for suspension bridge models. Computers & Mathematics with Applications, Volume 26, Issue 5, Sep-tember 1993, Pages 77-91.

14. Ming-Hui Huang, David P. Thambiratnam, Nimal J. Perera, Vibration characteristics of shallow suspension bridge with pre-tensioned ca-bles. Engineering Structures, Volume 27, Issue 8, July 2005, Pages 1220-1233.