Disclosure to Promote the Right To Information

Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public.

इंटरनेट मानक

“!ान $ एक न' भारत का +नम-ण”Satyanarayan Gangaram Pitroda

“Invent a New India Using Knowledge”

“प0रा1 को छोड न' 5 तरफ”Jawaharlal Nehru

“Step Out From the Old to the New”

“जान1 का अ+धकार, जी1 का अ+धकार”Mazdoor Kisan Shakti Sangathan

“The Right to Information, The Right to Live”

“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता है”Bhartṛhari—Nītiśatakam

“Knowledge is such a treasure which cannot be stolen”

“Invent a New India Using Knowledge”

है”ह”ह

IS 15643 (2006): Non-destructive examination of polymerbased composite materials - Code of practice [MTD 21:Non-Destructive Testing]

IS 15643:2006

Wi’d?u%’m

Indian Standard

NON-DESTRUCTIVE EXAMINATION OF POLYMERBASED COMPOSITE MATERIALS —

CODE OF PRACTICE

Ics 19.100

,,, ,

(3 BIS 2006

March 2006

BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

Price Group 6

Non-destructive Testing Sectional Committee, MTD21

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the dratl finalized by the Non-destructiveTesting Sectional Committee had been approved by the Metallurgical Engineering Division Council.

Fibre reinforced polymeric materials have come into wide spread use for applications where high strength-to-weight ratios and considerable stiffhess are needed. In addition, the ability to use a variety of fibre and matrixmaterials, and to alter the fibre lay up, enables the material properties to be tailored to the specific applications.

The detection of flaws in composites is particularly difficult because of the in-homogeneous, anisotropic andlayered nature of the material. Moreover, the defects are of multiple geometries, hence, composite materials mustbe regarded as a different media from metals when considering which NDT methods are appropriate.

The following abbreviations shall apply in the standard:

Acoustic emissiontesting AET

Carbon fibre reinforced plasitc CFRP

Fibre reinforced plastic FRP

Glass fibre reinforced plastic GFRP

Non-destructive testing NDT

Radiography testing RT

Ultrasonic testing UTVisual Test VT

Infrared thermography IRT

In preparation of this standard assistance has been drawn from the following documents:,,

MIL-HDBK-732 A :1991

MIL-HDBK-733 :1986

MIL-HDBK-731 :1984

MIL-HDBK-793 :1989

ADAMS (RD) AND CAWLY (P)

BAR-COHEN (Y)

JONES (TS), POLANSKY (D)

AND BERGER (H)

RUSSEL-FLOYD ANDPHILIPS (MG)

SCOTT (IG) AND SCALA (CM)

Radiography, Department of Defence, USA.

Non-destructive testing methods of composite materials — Acousticemission. Department of Defence, USA.

Non-destructive testing methods of composite materials — Radiography.Department of Defence, USA.

Non-de&ructive testing methods of composite materials— Thermography.Department of Defence, USA.

Non-destructive testing techniques for structural composites. Departmentof Defence, USA.

The code sub group of the European Working Group on Acoustic Emission.EWGAE codes for acoustic emission examination: code IV and code V.NDT International. 18,4; 1985; 185-194.

A review of defect types and non-destructive testing techniques forcomposite and bonded joints. NDT International, 21,4:1988:208-221.

NDE of fibre reinforced composite materials—A review. MaterialsEvaluation. 44, March; 1986; 446-454.

Radiation inspection methods for composites. NDT International.21, 4;

1988; 277-281.

A critical assessment of acoustic-ultrasonics as a method of non-destructiveexamination for carbon fibre reinforced thermoplastic laminates. NDTInternational. 21, 4; 1988; 247-257.

A review of non-destructive testing of composite materials. NDTInternational. 15, 2; 1982; 75-85.

IS 15643:2006

Indian Standard

NON-DESTRUCTIVE EXAMINATION OF POLYMERBASED COMPOSITE MATERIALS —

CODE OF PRACTICE

1 SCOPE

1.1 This Code describesthe non-destructive examinationmethods forpolymer based fibre reinforced composites.Some of the NDT techniques available for the testingof fibre reinforced composites are described in amanner that outlines the techniques, their advantagesand limitations. Lastly the applications of thesetechniques on products is explained.

Several techniques have been developed over the pastcouple of years for NDT of composites, which includeradiography, ultrasonic, infrared thermography,acoustic emission and acousto-ultrasonics. Only thesetechniques have been covered in this standard.

1.2 The contents of this document have been dividedin the following three sections:

Section 1 Composite material

Section 2 Non-destructive testing techniques

Section 3 Application of non-destructive testingtechniques

2 TERMINOLOGY

-For the purpose of this standard, the followingdefinitions shall apply.

2.1 Delamination — Separation of the layers ofmaterial at the edge of a laminate.

2.2 Debond — Separation of two distinct layersindividually-processed, observed at the joining planes.

2.3 Wrinkles — In a laminate an impregnation thathas the appearance of a wave moulded into one or moreplies, fabric or other reinforcement material.

2.4 Crack — An actual separation of the laminate,visible on opposite surface and extending through thethickness.

2.5 Porosity — Presence of numerous visible pits (pinholes).

2.6 Inclusion — Metallic or non-metallic particlesincluded in a laminate which are foreign to itscomposition.

2.7 Void — Air entrapment within and between theplies of reinforcement, usually spherical in shape.

2.8 Chip — A small piece broken off on edge orsurl%ce.Low density regions between two plies filledcompletely with resin.

2.9 Resin Rich Lines (Resin Leanness) — Lo\vdensity region between two plies having intermittentresin filling.

2.10 Lack of Material—Non-availability of materialdue to insufficient machining allowance.

SECTtON 1 COMPOSITE MATERIAL

3 COMPOSITE MATERIALS

3.1 Overview

Composites are defined as material systems consistingof two or more constituents, each of which isdistinguishable at microscopic level. The constituentsretain their individual identities in the composite ,andare separated by a detectable interface. Generally, oneof the constituents acts as a reinforcing agent, and theother serves as the matrix or the binder. The propertiesof the composite materials are derived from thecombination of properties contributed by theconstituents but modified by their synergistic effects.The loads are taken up by the reinforcements while thematrix holds the fibres together, transfers load fromone fibre to the other and protects them againstenvironmental attack and damage due to handling.

The composite materiak described here are polymerbased having ‘advanced high modulus or high-p.erformance reinforcements’. The reinforcementsinclude glass, carbon and aramid fibres.

The type of composite structures considered includesolid laminates, wound structures and sandwichconstructions.

3.2 Reinforcements

Reinforcements are available in various forms but thebasic form is fibre from which all other forms are made.

3.2.1 Glass Fibre

Fibre glass generally utilizes either E-glass (a lime-alumina-borokilicate glass) or S-glass (a silica-alumina-magnesia glass) fibre and is produced as a continuous

IS 15643:2006

monofilament bundle by drawing molten glass through polymeric matrix material used in composites has, asa rmdtihole bushing. The size of the holes will determine its main role, the responsibility of absorbing andthe diameter of the fibre. The monofilament bundles transmitting loads to the reinforcement fibre.can then be prepared in different forms such as rovings, However, secondarily, the matrix material also

> multifilament strands, and woven and non-woven controls many other properties, such .as the visco-fabrics. elastic behaviour. creem stress relaxation, and in-plane

1.

Glass is the most widely used reinforcement. It is lightand interlaminar shear. The chemical, thermal, electrical

in weight, has high tensile strength and high heatand environmental ageing characteristics of the

resistance. It is virtually tire proof, inexpensive andcomposites are also properties that are controlled by

available in various forms.the matrix material.

3.2.2 Carbon Fibre 3.4 Manufacturing Metltods

High performance carbon fibre is produced “by the 3.4.1 Hand Lay-Up Process

pyrolisis of .polyacrylo nitrile (PAN) fibres under In this process, components are normally produced ontension at temperatures of 2033-3033 Kin controlled a metal mould coated with a release agent. Afteratmosphere. The properties of the fibre are a function applying a gel coat, reinforcements are added andof both the tension and the temperature used during wetted with suitable resin. The component after cure/pyrolisis. Like glass fibres, carbon fibres can also be post-cure is removed from the mould and trimmed toobtained in various forms. shape.

Carbon fibres have strength vastly superior to that ofglass fibres and have excellent heat stability too. Amongthe fibres presently available, carbon tibre has thehighest modulus. As they have high dimensionalstability over widely ranging temperature differencesor thermal cycles, they find applications in electronicinstrumentation and spacecraft. Some of the other widerange of applications include aircraft structuralcomponents, missiles and armament components, X-rayequipments, various automotive applications, sportsand recreational equipment.

3.2.3 Aramid Fibre

Aramid is a generic term denoting a class of polyamidematerials. The aramid fibres are produced byconventional textile spinning methods. Kewlar,Twaronand Nomex are some of these materials. Kevlar is theregistered trade-mark for a new family of aromaticpolyimide fibre introduced by M/s Dupont, USA. It isa product derived from paraphenylene diamine andterephthaloyal chloride. Kevlar-49 is the principalart!mid tibre currently used in advanced compositeapplications. The fibre is pale yellow in colour and itis available as yams, rovings and woven fabrics.

Aramid fibres are used in composite applicationsbecause of their outstanding combination of lowdensity, high strength and good stiffness. The fibre isinherently flame resistant and does not melt, but willdecompose at about 733 K. Application areas includehigh performance structures for aerospace, automobiles,rocket, missile cases and sporting goods.

3.3 Matrix Materials

The principal matrix materials used in composites areepoxy, polyester, phenolics and polyamides. The

3.4.2 Vacuum Bagging

This technique is used for fabricating components andstructures having complex shapes, including doublecurvature, and relatively large parts. The flexible plasticmembrane, that is, placed over the surface of the lay-up forms a vacuum bag. The bag is evacuated, andatmospheric pressure is used to consolidate the lay-upagainst the mould. .,

3.4.3 Filament Winding

Precise lay-up of continuous reinforcements topre-described patterns is the basis of the filamentwinding method. It is a process in which resinimpregnated continuous rovings/tows (gathered strandsof fibre) are wound over arotating male mandrel. Thereinforcement may be wrapped either in adjacent bandsor in repeating bands that are stepped by multiple widthof the band and which eventually cover the mandrelsurface. In this technique, it is possible to vary tkwinding tension, wind angle or resin content in eachlayer of reinforcement in order to obtain the desiredthickness and resin content of the composite withrequired directional strength.

3.4.4 Tape Winding

A tape winding machine essentially consists of ahead-stock, tail-stock and a modified tiltable bedwith hydraulically operated compound slide. Pre-impregnated carbon/phenolic or silica/phenolic tapesare fed through rollers on to a cylindrical or conicalmandrel. The winding-can be carried out either parallelto the axis or at an angle. A hydraulic device providesnecessary pressure to the lay up. A heating system alsois generally employed at the time of winding to softenthe matrix.

2

3.4.5 Sandwich Process

A sandwich structure is made up of two outer skins ofcomposite laminate. These skins are bonded to aninternal structural material such as honeycomb core,foam, etc (see Fig. 1).

4 DEFECTS IN COMPOSITES

4.1 Types of Defects

The types of defects occurring in fibre reinforcedcomposites are different from those found inconventional materials, as the nature of the defectsis very much dependent on the manufacturingmethods.

-IS 15643:2006

SECTION 2 NON-DESTRUCTIVETESTING TECHNIQUES

5 VISUAL TEST

5.1 Overview

Visual inspection was probably the first method ofnon-destructive testing employed and even today itremains foremost among all the other NDT techniquesdeveloped so far, to detect and evaluate defects.Luminous energy NDT tests are used primarily (a)for inspecting accessible surfaces of opaque materials,and (b) for inspecting the interior of transparent testobjects.

Y FACE SKIN\

HONEYCOMB CORE

\

‘.’.!, ,

ADH

Y I

- FACE SKIN

FIG. 1 TYPICALSANDWICHCONSTRUCTION(HONEYCOMB)

3

IS 15643:2006

5.2 Visual Aids

Lamps of proper luminance, magnifying glasses, etc,are employed for the visual inspection of all materials.Endoscopes and fibroscopes are widely used now-a-days for the visual inspection of surfaces which are notaccessible to the human eye.

5.3 Remote Visual ”Inspection (RVI)

Various NDT methods exists, including radiography,ultrasonics, infrared thermography, etc, but perhaps thesimplest to use, easiest to interpret and most economicalto adopt is the remote visual inspection. Endoscopes andfibroscopes act purely as an extension to the eye and donot involve extensive training, complex electronics ordetailed analysis of results. since there are only a fewmoving parts, breakdown is rare and the inspection canusually be completed in minutes via existing accesspoints. Theendoscope or fibroscope is a precision opticalinstrument with self contained illumination and is usedfor the visual inspection of a wide variety of internalsurfaces such as bores, deep holes, recesses and otherinaccessible areas, through openings.

5.4 The Limitation

Visual NDT is one of the most important and simplestforms of inspection. It is used to observe the surfacedefects of the material or product under test. But thistechnique is highly operator dependent and hence theoperators should be trained to know what they arelooking for and what any variation might mean to thereliability of the final product.

6 TAP TEST

6.1 The tap test is one of the oldest methods of non-destructive testing and it is regularly used for the testingof laminated structures, honeycomb construction andbonded joints. In the tap test, themrface of the structureis tapped by hand using a hard blunt object such as atapping hammer or a coin. The operator taps each pointof the structure and listens to the resulting soundradiated by the structure. Defective areas sound‘hollow’ in comparison with a good area. This methodis always used as a fkst inspection method, which isfollowed by one or more other suitable techniques toevaluate completely the test object.

6.2 Limitation

The main limitation of this technique is that it calls formuch skill and only a qualified person havingexperience can carry out this test successfully.

7 RADIOGRAPHIC NDT

7.1 Overview

Radiographic non-destructive testing is the techniqueof producing a picture on a sensitive surface by a form

of radiation other than visible light. The most widelyknown form of radiography is X-ray. Other forms ofradiography NDT are fluoroscopic radiography,neutron radiography and gamma radiography.

7.2 X-Ray Radiography

7.2.1 X-ray radiography is one of the most commonlyused non-destructive method of detecting the presenceand location of internal flaws like cavities, inclusionsor other disccmtinuities in materials. When X-radiationis directed normal to the surface of a specimen, part ofit is absorbed and the rest is transmitted. If the specimenis of uniform thickness, the transmitted beam will beof equal intensity. However, if the specimen containsany internal flaws, there will be -a variation at thosepoints. This results in variation of the emergent X-raybeam, which makes possible, to study the internalstructure of matter.

7.2.2 Limitations

Due to the geometry of the defects ocwrring incomposites, X-ray radiography has potential mainly inthe tangential mode. Normal exposures do not detectdelaminations/debonds as the gap due to delamination/debonds is not sufficient to cause a variation in theemergent X-ray beam.

In the radiography of composites, which comprise oflow and medium density materials, scattered radiatidnforms a high percentage of the total radiation reachingthe film. This is.dueto the nature of the soft, low voltageradiation used to radiograph these materials, which isinherently less penetrating and more subjective toscatter. However, if higher, more penetrating voltagesare used excessive overall film density could occur,resulting in reduced contrast and the radiographbecomes unreadable in terms of flaw detection. Scatter,is therefore, a major problem during radiography ofcomposites. This can be solved to a certain extent byplacing lead foils on both sides and in close contactwith the X-ray film. The foil nearer to the radiationsource typically has a thickness of 0.02-1.15 mm. Whilethe foil on the back of the screen is to absorb scatteredradiation, the front screen is to generate electrons andsecondary radiation in order to increase the intensitytransmitted to the radiographed object.

7.3 Real Time Radiography

7.3.1 Real time radiography is quick and enableshundred percent inspection within a short time. Thereal time display systems are cost effective whereverthey are used because they eliminate the use of filmsand their development costs and provide instantinformation. Real time X-ray systems, when combinedwith computers and image enhancement techniques,allow obtaining defects information not obtainable

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IS 15643:2006

otherwise. Over the past ~ew years, a number ofcomputer based image enhancement and analysissystems have been developed, some of which providevarious forms of image enhancement and others whichprovide both enhancement and image analysis.Advances in the design and construction of X-rayimage intensifier tubes have resulted in largeimprovements and hence of late, real time radiographyis gettingincreased attention. However, this techniquealso has a serious limitation of low sensitivity level.

7.3.2 Limitations

In a real time radiography system, the radiation emittedby an X-ray -source, after penetrating the sample,impinges on the input phosphor screen of an imageintensifier tube. This screen converts the radiation intoa light image. The phosphor is in direct contact with aphoto cathode. Photo cathodes emit electrons whenexcited by the input phosphor light. These electronsare accelerated and focused into a phosphor outputscreen which converts the electrons back to light ofhigh intensity. The output from the camera is amplified,digitised, stored and enhanced by a digital imageprocessor, which is then presented on a televisionmonitor.

7.3.3 Gamma Radiography

Gamma radiography is generally used in the samemanner as X-ray radiography. This is used for in-situ

applications and for cost reduction.

7.3.4 Neutron Radiography

Neutron radiography, like X-ray radiography, dependson the differential absorption of neutrons by thespecimen material. Neutrons, particularly those withlow velocities (thermal neutrons), exhibit a randomvariation of absorption from element to element. Therange of absorption coefficients for neutrons is 0.03 to90, considerably exceeding that for X-rays (O.13 to4.0), which allows for greater flexibility and excellentcontrast in neutron radiographs. Furthermore, elementswith adjacent atomic numbers may have widelydifferent neutron absorption coefficients, and some lowatomic number elements (such as hydrogen) canattenuate neutrons much more strongly than some highatomic number elements (such as lead). Thus, thickersections of high atomic number materials can beradiographed with neutrons in a much shorter time thanwith X-rays. Also neutron radiographs can exhibit asharp contrast between elements which have similarabsorption coefficients for X-rays but quite differentones for neutrons. Hence neutron radiography is apromising technique for composites in which the fibreand matrix are having similar coefficients ofabsorption. However, this technique is yet to becomepopular for thelQDT of composites.

8 ULTRASONIC NDT

8.1 Overview

Advanced composites tire of low density and do notlend themselves to non-destructive evaluation withconventional high frequent y (2.25 MHz) ultrasonicinspection-methods. However, there are a few ultrasonicequipments which utilize the lower frequencies of theultrasonic spectrum (20 to 40 KHz) and are able topenetrate lower density materials like composites.

Perhaps ultrasonic technique is the most widely usedNDT technique for composites. The test maybe carriedout either with a single transducer in pulse echo mode

or with hvo transducers in through-transmission mode.In either case, it is necessary that the transducers arecoupled to the test material via a liquid or a suitablesolid medium because of the severe impedancemismatch between air and solid materials. In manycases, particularly in aerospace structures, non-application of couplant is most essential as they aredifficult to be cleaned completely after the test. Thismay hamper any further processing on the product. Inspacecraft applications the minute particles of thecouplant may outgo in space and settle on nearbyelectronic component this affecting the normalfunctioning of the components and hence the satellite,In such situations, dry-scan ultrasonic systems (whichdo not use a liquid couplant) are employed. Theultrasonic testing is broadly based on (a) pulse-eeho,(b) through-transmission, and (c) resonance methods.All the three methods are employed for the NDT ofcomposites.

8.2 Pulse-Echo Method

In pulse-echo method, evenly times pulsed of ultrasonicwaves are transmitted into the material being tested.These pulses reflect tlom discontinuities in their path,or from any boundary of the material on which theystrike. The received reflections or echoes are thenusually displayed on a CRT. A single transducer is usedas both the transmitter and the receiver, althoughsometimes two transducers are used, one acting as thetransmitter and the other as the receiver.

Ultrasonic equipments are available for point contactinspection in the pulse-echo method. The point contactinspection dispenses with the application of a,couplantfor the testing to be carried out. The transmitter andreceiver transducers are point tipped and are housed ina single casing. This facilitates the probe to be seatedproperly on curved surfaces also. The transmittergenerates a wavefront in the material under test. Afier itpropagates through the internal structure, it strikes thereceiver. The energy upon arrival at the receivingtransducer, is sensed and electronically evaluated.Changes in the-material properties such as thickness

5

IS 15643:2006

and debonds, etc, cause changes in the received energyphase and/or amplitude. These changes are sensed bythe alarm circuitry and produce audible and visiblealarms.

8.3 Through-Transmission Method

The through-transmission method requires the use oftwo transducers. One transducer is used as a transmitterwhile the other is used as a receiver. As in pulse-echo,short pulses of ultrasonic waves are transmitted intothematerial, although continuous waves are sometimesused. Echoes returning to the transmitting transducerare not used. The receiving transducer is aligned withthe transmitting transducer to pickup the sound waveswhich pass directly through the material. The soundnessor quality of the material being tested is in terms ofenergy lost by a sound beam as it travels through thematerial.

Dry-scan ultrasonic systems are available which arevery effective in through-transmission mode. In thismethod, the ultrasonic energy is measured after it passesthrough the material under test. Reduction of energydue to defect results in reduced amplitude of the peak.This is used/preferred for thicker parts perpendicularto the direction of ultrasonic transmission. In suchsystems, the transducers have soft rubber tips whichact as the coupling agents for the probe to the materialunder tests. This is a completely dry techniqueproviding a remarkable break-through in the NDT ofcomposites. Tests can be conducted with speed andconvenience as there is no requirement for liquidacoustic coupling.

8A Resonance Method

ResonanGe can be defined as the characteristic of avibrating body to—Under certain conditions—Resonate or vibrate in sympathy with vibration source.

INCIDENT~RAOIATlON T1

A resonant condition will exist any time a continuouslongitudinal wave is introduced into a specimen andreflected’ in-phase’ with the incoming wave. Standingwaves are set up and the specimen vibrates withconsiderable increase in amplitude. Resonance willoccur any time a continuous longitudinal wave istransmitted into a specimen and the frequency is variedtill standing waves are set up.

The equipment based on resonance method is aninspection tool which provides information aboutcohesion stiffness of bonded structures. It comprisesof an instrumerit which imposes a definite band offrequencies on a piezoelectric crystal to vibrate in oneof its own natural resonance frequencies. When thisfreely vibrating crystal is coupled by means of a liquidto the structure to be tested, the natural resonancefrequency will be influenced by the mass coupled to it.The resonance frequency and the voltage over thecrystal are made visible by means of leder displays.

8.5 Limitation

The limitations of the above-mentioned equipments arethat the part being tested must be manually scannedand manually marked to identify the defect areas asrecorded information from these equipments are noteasily obtained.

9 INFRARED THERMOGRAPHY

9.1 Overview ,,, ,

Infrared thermography is one of the most promisingnon-destructive testing techniques for compositeapplications. Heat is applied to one surface and allowedto flow inwards. If any inhomogeneity ispresent withinthe material, the heat flow characteristics changeaccordingly and cause a change in the temperature onthe surface (see Fig. 2). Either through-transmission.orsingle surface temperature transients can be used. The

SURFACETEMPERATURE

T2 DISTRIBUTION 7.

~TRANSMITTED ABSORBED L

RADIATION RADIATION

FIG. 2 HEATTRANSFERTHROUGHA MATERIALWITHDEFECT

6

IS 15643:2006

changes in temperatures are detected and displayed by the detector material no longer emits any current, thatmeans of an infrared thermography system. is it is not sensitive to wave lengths above the cut-off

It is a fast and non-contact method. Hence large objectswhich are inaccessible can be scanned thoroughly.Various types of defects such as delamination, debonds,internal voids and inclusions can be easily detected.

9.2 Theory

All objects emit heat (infrared radiation) which isconstantly being absorbed and re-emitted by ourselvesand everything around us. Infrared thermography is thetechnique that uses to make an object visible by meansof this heat radiation. Infrared radiation occurs at thered end of the visible region of the electromagneticspectrum and its wavelength varies from 0.75to -1000 microns. This infi-ared radiation follows thesame laws and has similar characteristics as visible light.Thermography ensures that the picture focused by thescanner can be interpreted by human eye. Thermogram‘infrared’ or ‘heat’ picture gives the heat distributionof an object.

When a thermal input to an object, the heat flow patternis impaired by inhomogeneities which results in acorresponding surface temperature distribution, andhence radiant emittance.

The Stefan-Boltzmann Law states that the total radiantemittance from a black body per unit area varies directlyas the fourth power of the absolute temperature. Toexpress it mathematically, the total radiant emittanceper unit area of a grey body is:

W= EoT4

where

E = emissivity of the grey body, and

c = stefan-Boltzmann constant.

It is the relative temperature distribution that isrepresented on the display in the form of athermogram.

9.3 Thermography System

The thermography system consists of two main parts,scanner and display unit. The scanner is an infraredcamera which converts infrared radiation from an objectinto electronic signals. It has a detector-iridiumantirnonide/mercury cadmium telluride-which actuallysenses the infrared radiation. The detector in an infraredscanner comprises a material which produces an electriccurrent when exposed to infrared radiation.

The electric current can be used in many different waysto produce an infrared picture. The current is dependenton the intensity and wavelength of the radiation. Acharacteristic of-the detector is its cut-off wavelength.If the incident radiation arriving at the detector materialis of a wavelength that exceeds the cut off wavelength,

wavelength.

There is a fundamental feature for semi conductingmaterial. It always emits a certain disturbance currentcalled noise, which is dependent on the temperature ofthe material. Such noise is often called thermal agitationnoise. To minimize the effect of this noise, the detectoris mounted into the side of a flask containing liquidnitrogen. This has a temperature of 77 K. The effect ofcooling ensures that the noise is considerably reduced.A further advantage of cooling is that the cut-off rangeis also increased. Electronically cooled detectors arealso available.

The detector constantly monitors different points andthe electrical current from the detector varies in strengthwith the radiation of the object at every point. Thiscurrent and its variations are used to produce a picturein a grey scale where the most intensity radiating pointsare the brightest (hottest areas). The scanner isconnected to a display unit wtilch presents an intlaredpicture of the object on a television type screen. Undernormal mode, the hot areas of the object appear white,the cold areas appear black and vice-versa underinverted mode.

9.4 Advantages

Infrared thermography is an attractive and relativelysimpler technique for the non-destructive testing ofcomposites. The main advantages of this techniqueare the speed with which large areas can be scannedin lesser time and the non-essentiality of direct contactwith the object to be inspected. This technique hasgot very wide range of applications which includedefense, industry, medicine, scientific research anddevelopment, etc. The tests can be recorded on a videotape for further analysis and preservation. The testscan also be photographed using a polaroid camera.Computer aided analysis helps to simplify the muchcontained information in the therrnographic images.All these make infrared thermography a powerful andpromising NDT technique for the evaluation ofcomposites,

9.5 Limitation

The main limitation of this technique is that only planerdefects can be easily detected and that defect thicknessdepth of the defect cannot be ascertained. This is onlyan initial scanning method and firther tests are requiredto get complete details regarding the size and shape ofdefects. In many cases radiography, has to be resortedto for getting the depth and thickness of defects.

Uniform heating of the specimen is most essential. Heatperturbations occurring during heating are attributed

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to the defects present. Non-uniform heating gives riseto improper surface temperature distribution on thespecimen and this makes it difficult to do properinterpretation of the display.

10 ACOUSTIC EM-ISSION NDT

10.1 Overview

Acoustic emission (AE) testing is one particular areaof composite testing that is progressing rapidly.Technological advances in sensor technology, datacollection and manipulation are.increasing the amountof information obtainable by this method. Combinedwith a growing sophistication in interpretation of AEdata and correlation with information from other testmethods, these advances are helping AET to be veryversatile in locating and identifying a growing rangeof defect phenomena. Acoustic emission testing is usedfor the proof testing of composite strictures for globalidentification of structural defects. AE tests not onlyshow the areas of growing defects but also provideinsight into improved structural design.

10.2 Principle

Composite materials release discrete bursts of energywhen stressed, some of which are audible. Much of theacoustic emission (AE) activity in composites isinaudible to the unaided ear because it is too high infrequency or too low in amplitude. With piezo-electrictransducers affixed to the surface of a compositespecimen, these emissions may be electronicallymonitored and recorded for subsequent analysis withthe assistance of computer data reduction andmanipulation techniques.

In fibre reinforced composites, the acoustic emissionis generated by cracking of the matrix, debonding ofthe matrix from the fibres, delamination, separation,etc. Signals from these growing flaws travel within thestructure and are detected by the surface mountedpiezoelectric sensors. Proper instrumentation canidentify flaw location by collecting the data obtainedfrom the multiple sensors on the test specimen and usingsource location methodologies.

10.3 Stress Application

The stress-applied to the composite structures to causethe material to emit acoustic emissions is generallyaccomplished by applying a mechanical force such asbending, tension or torsion on the item. For proof testingof composite pressure vessels, they are internallypressurized to emit the acoustic signals.

11 ACOUSTO-ULTRASONIC NDT

11.1 Overview

The acousto-ultrasonic (AU) technique is a newly

developed non-destructive evaluation method thatcombines the features of both acoustic emission andultrasonics.

In the case of acoustic emission technique, one has towait for spontaneous signats and defects which aregrowing only can be monitored. Hence material has tobe deformed in order to get acoustic emission events.In the case of ultrasonics, when the defect concentrationis more, the distance between the peaks obtained is verysmall and in some cases they overlap one over the other.So exact location of the defect becomes difficult.

In the case of acousto-ultrasonics, the ultrasonic pulseswhich resemble acoustic emission waves in the materialare injected inside the materials. These pulses areallowed to interact with the material medium whichmakes it possible to characterize the material qualitywithout loading it. -Byusing AUT, the degradation inmechanical performance of a structure can also beassessed. This is a distinct advantage over the otherNDT techniques.

Flaw detection is done to find out whether thecomponent will perform satisfactorily under workingcondition. This is because flaws affect the strength andhence the performance of the component. But thestrength itself can be predicted using AUT.

In some cases a composite structure may be free ofreadily detectable flaws but exhibit low strength. andprobably it is unsuitable for use. In such situations, AIJTplaces an important part of determining the strength ofthe material.

11.2 Principle

In acousto-ultrasonic technique, the ultrasonic pulsesthat are injected inside the material resemble thespontaneously generated acoustic emissions signalswhich arise due to deformation, crack growth, residualstresses, etc. Therefore, it is quite natural that if theseartificially generated stress waves are allowed tointeract with the internal features of the material (thatis, microstructural environmental within the material),the resultant modulated signal characterises the materialquality. The measurement of these simulated acousticemission waves correlates strongly with materialmicrostructure and mechanical strength in the case ofcomposite material.

Using this acousto-ultrasonic technique, strength canbe measured indirectly by means of a materialcharacterization parameter known as stress wave factor(SWF). Higher values of SWF denote higher materialstrength.

11.3 StressWave Factor

Stress wave factor is the product of repetition rate, time

8

interval and ringdown counts. It is mathematicallyexpressed as,

SWF = R. T. N

where

R = repetition ~ate of ultrasonic pulses that areinjected inside the material;

T = time interval between which counting has beenmade; and

N = number of threshold crossings.

The acousto-ultrasonic measurements are made bymeans of the stress wave factor. The value of SWF isused to quantify mechanical properties like strength andfracture resistance. The SWF will indicate regionswhere strain energy is likely to concentrate and resultin crack nucleation and fracture. Stress wave factormainly depends on the dampening effect of the materialmedium of ultrasonic pulses, such as microstructure,morphology, porosity, bond quality, cure state, microcracks and so on.

SECTION 3 APPLICATION OFNON-DESTRUCTIVE TESTING TECHNIQUES

12 OVERVIEW

Non-destructive testing of all finished products is anecessity to find out their structural integrity. Table 1lists some of the defects that would affect the stmctura]integrity of composite structures and the NDTtechniques that may be used to detect these defects.

13 RADIOGRAPHY

Radiography is widely used for the detection of internaldefects in products which -is represented as cross-sectional density variations. Delamination and

IS 15643:2006

debonds are easily identified provided they are orientedparallel to the X-ray beam. Foreign materials andcrushed honeycomb core in sandwich structures canalso be detected. Radiographs are expensive in termsof time and materials, but they are permanent recordthat the engineer can use these to evaluate problemsthat may occur at a later date. Film”based radiographyis possible only at discrete locations in a product,but with the fluoroscope systems it is possible tohave 100 percent scanning.

14 ULTRASONIC

The dryscan ultrasonic system uses a completely drytechnique providing a remarkable break-through in theNDT of advanced materials. Tests can be conductedwith speed and convenience as there is no Requirementfor liquid acoustic coupling. The system has soft rubbertipped transducers which act as the coupling agent forthe probe .to the material under test.

The transmitterand the receiver are positioned exactlyopposite to each other over a good region as shown inFig. 3(A). The beat pa~ern observed in the CRT isadjusted to Full Screen -Height (FSH) with a suitablegain. The probes are then placed over a delaminatedregion as shown in Fig. 3(B). As the medium throughwhich the ultrasonic pulses travel is not continuous dueto the presence of a delamination, very littleultrasonicenergy is transmitted to the receiver. Consequently,no beat patterns are obtained. The beat patternscorresponding to the good and delaminated regions areas given in Fig. 4(A) mrd Fig. 4(B).

15 INFRARED THE-RMOGRAPHY

15.1 Overview

To apply the thermography technique for non-destructive testing, one must introduce a thermal

Table 1 Defects Detected by Various NDT Techniques

(Clause 12)

Tap Tests Radiography Ultrasonic Infrared Accoustic Accousto-Defect Radiography Thermography Emission Ultrasonic

(1) (2) (3) (4) (5) (6) (7)

Delamination x x x x x x

Debond x x x x x xFibre Misalignment x

Damaged Fibres x

Variation in Thickness x x x xVariation in Density x xVoids and Porosity’ x x x xCrushed Core x x x

9

IS 15643-:2006

? /

GOOD REGION

3(A) Transducer Position at a Good Region

? \ {

UELAMINAIION

3(B) Transducer Position at a Delamination

FIG. 3 TRANSDUCERPOSITION

k!mmc4(A) Beat Pattern at a Good Region 4(B) Beat Pattern at a Delamination

FIG. 4 BEAT PATTERN

pattern in the test object to be studied, observe the the test object is treated as a path for heat from externalrelative temperature distribution on the surface of heat source. In this, flaws will cause local differencesthe examined test object and properly interpret the in thermal conductivities which will evidenceresults. themselves in the thermal mapping. It is this method

15.2 Active and Passive Methodswhich is widely used in the thermographic NDT ofcomposites.

Thermographic NDT-techniques may be categorized15.3 Basic Principles of Operation

as active or passive when referring to the means bywhich heat is produced in the object to be tests. A A specimen (with various inbuilt defects) is uniformlystructural material may become hot under mechanical heated to about 10to 15°Cabove the room temperature.loading in regions where damage is occurring. Such a Defects in the specimen are revealed by correspondingsituation in which heat is produced actively in the differences in heat emission pattern by the specimen.specimen by damage process is called active technique. The heat emission pattern is detected by the scannerSimilarly, passive method refers -to those for which and transmitted to the display unit.

10

IS 15643:2006

15.4 Heat Emission Patterns

A matter of importance in passive thermography is thechoosing of steady state or transient conditions in orderto develop the heat-emission patterns. For a steady statemethod, the heat source, the specimen and all of theheat loss mechanisms are given sufficient time to reachthermal equilibrium before the observation ofthermograms is made. At steady state conditions, thetemperature distribution remains constant as a fimctionof .tirne. Such a procedure has the advantage of beingmore easily reproducible and more amenable toquantitative interpretations and comparisons. Once theentire system has reached thermal equilibrium, thenumber of operational or system dependent variablesthat enter into the interpretation of the data recorded isminimized. Hence the interpretation of the data isgreatly facilitated. Though this is the ideal method ofproducing heat pattern for repeatability and betterinterpretation of thermograms, it is not always possibleto achieve steady-state conditions with appreciabletemperature differentials corresponding to the flaws.

On the other hand, in transient heating method, thelargest temperature differential in a specimen occursat the’‘first instant when a heat source is applied to thespecimen. Following that first point of time, energy istransferred according to Fourier’s law and distributedin such a way-so as to balance the energy input againstconduction, convection and radiation losses. Thetemperature gradients are reduced by this process untilthermal equilibrium is reached. Hence it is not veryeasy to interpret the thermograms at steady-stateconditions and transient method is resorted to in mostof the applications, Infrared lamps or hot air blowerscan be used to heat the specimens uniformly. Heatingand scanning are done simultaneously from the sameside of the specimen. Lack of heat transfer due to thepresence of defects gives rise to hot spots. In transientheat method, as the heat source passes the area of thespecimen surface over a flaw, that portion cools slowerthan the surrounding material. These localized hot spots

appear as white areas on the display screen duringscanning. The excellent sensitivity of the thermographysystem, which will detect temperature differences ofas small as O.1°C ensures that defects in the specimenare clearly identifiable.

16 ACOUSTIC EMISSION

The primary failure modes in composite structuresinclude matrix cracking, delamination and fibrebreakage. These failures occur during the pressurizationof a fibre reinforced pressure vessel. AE can be usedfor inspecting all the three modes of failure. AE testinginvolves the positioning of AE transducers at properlocations on the structure and then applying a load ora stress. As the load increases, micro failures occurand the transducers pick up the signal. The source ofthe signal and hence the defect can be located fromthe time required for the signal to reach the varioustransducers. By use of standard specimens representinga structure, AE counts versus load traces can beprepared and then compared to the counts obtainedduring pressure testing to help to predict the strengthor weakness of a structure. Acoustic emission basedproof testing of composite structures is well knownand a preferred method for global identification ofstructural defects.

17 ACOUSTO-ULTRASONIC ,,, ,

A material free of any defects allow the stress waves topass through it more easily than when it is a defectiveone. In other words a good quality material dampensthe stress waves to a lesser extent as compared to amaterial which has grossly distributed defects. Hencehigher values of SWF are obtained for a good qualitymaterial. During the passage of ultrasonic pulses if itfinds any defects such as voids and delamination, thenthe pulses are scattered and hence the peak amplitudeof the signal is decreased; as a result the number ofthreshold crossing (N) is also decreased. Hence stresswave factor decreases.

,,.,.‘(..

11

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