Light Weight Hollow Sphere Composite Materials
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Transcript of Light Weight Hollow Sphere Composite Materials
Lightweight hollow sphere composite(HSC) materials 2010
ABSTRACT
LIGHTWEIGHT, HOLLOW-SPHERE-COMPOSITE (HSC) MATERIALS FOR ENGINEERING APPLICATION
Lightweight structure is a new trend in machine tool design to ensure higher speed and higher
acceleration of elements. The drive and control systems in mechanical engineering requires
lightweight design provided by the recently developed light materials thus resulting in
economical advantages. The hollow-sphere-composites (HSCs) consist of hollow spheres up to
80 of the volume and a reactive resin system as binder. The recently developed HSC materials,
the hollow sphere bodies, are made from ceramics, silicates, plastics or metals and provide a
range of structural materials of different chemical composition, grain size distribution, density,
bulk density, softening temperature and compression. Therefore, a vast palette of HSC-variants
can be obtained with different properties for a variety of applications. The mechanical properties
of HSC materials depend on the properties of the spherical hollow bodies. The mechanical and
thermal behavior of HSC materials can be characterised by using dynamic mechanical analysis
(DMA), differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The
thermal and mechanical properties of selected HSC structures, e.g. machine tool components,
robot arms, demonstrate the flexibility and application feasibility of this new material.
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Lightweight hollow sphere composite(HSC) materials 2010
TABLE OF CONTENTS
1. Introduction…………………………………………………………(3)
2. Hollow sphere composites…………………………………………..(4)
3. Properties of hollow sphere composites……………………….…….(5)
4. Thermal properties…………………………………………….…….(6)
5. Mechanical properties………………………………………….…...(10)
6. Application of HSC in mechanical engineering……………….……(15)
7. Conclusion…………………………………………………….……(17)
8. References…………………………………………………………. (18)
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Lightweight hollow sphere composite(HSC) materials 2010
CHAPTER IINTRODUCTION
In mechanical engineering, including automotive and aircraft manufacture, the same lightweight
building principles are used to meet various and often complex demands in shape-, structure-,
material coupled with the need for optimized production process selection for technology needs
and financial considerations. The optimised design of machine tools using finite element
methods may lead to substantial improvements in the acceleration or damping behaviours. The
application of new, alternative materials in machine tool design provides dramatic improvements
in mass reduction through the full utilisation of material, high strength and stiffness as well as
maximum functional integrity and economy. The requirements for the lightweight machine
structures are characterised by the optimal use of material quantity. These demands can rarely be
satisfied with monolithic structures. As a result, the application of cellular materials, e.g.
honeycomb, metal foams or syntactic foams will soon gain significance. A combination of
metals and fibrous materials can be used adaptively to different conditions, similar to natural
structures, like the hand bones as shown in Fig. 1. This is a foam structure connected with the
supporting system, where muscles and sinews are utilised for movements.
Fig. 1. Cellular structure of human hand.
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CHAPTER II
HOLLOW-SPHERE-COMPOSITES
An alternative method in reducing the mass of materials is to use a mixture of high percentage
volume of hollow spheres containing air or gas, and a reactive resin system. In this research
hollow-sphere-composites consisting of corundum based (0.5–1 mm) macro-hollow-spheres and
aluminium-silicate Fillite (5–300_m) micro-hollow-spheres are used as shown in Fig. 2.
In the recent research programme 12 different types of hollow spheres were used in combination
with cold and warm hardener epoxy resin (EP) and with and without fibre reinforcement,
resulting in excess of 20 HSC-variants with different properties. The hollow spheres vary in
diameter between 10 and 2000 _m and the wall thickness is only 10% of the diameter size. The
round shape of the spheres provides a high package density and a minimal viscous drag.
Fig. 2. (a) Bulk material of corundum 0.5–1 mm; (b) interior of Fillite (SEM); (c) hollow-sphere-
composite (corundum and Fillite); (d) interior of hollow-sphere-composite (SEM).
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CHAPTER III
PROPERTIES OF HOLLOW-SPHERE-COMPOSITES
In order to establish the application areas of HSC in mechanical engineering, it is extremely
important to characterize the thermal and mechanical behaviour of the material and to determine
the characteristic values, which are necessary for the FE-calculations of the machine elements.
Due tolack of standards on HSC materials, the thermal and mechanical tests must be governed by
the appropriate standards for polymer concrete and plastic materials. The German Standards DIN
51290 prescribe that the minimum dimensions of the sample shouldn’t be smaller than three
times the maximum grain size of the used filler. The result is that the preferred sample geometry
based on plastic standards must be modified to apply to HSC.
Thermo elastic properties of hollow sphere composites are studied based on the uniform matrix-
field concept proposed here. Some connections between local thermal and mechanical fields
produced by certain homogeneous boundary conditions are derived, and furthermore, exact
relations are also obtained between the effective thermo elastic properties of the composites. For
a macroscopically isotropic composite with a certain ratio of the outer radius to the inner radius,
it is found that the effective bulk modulus and the linear coefficient of thermal expansion can be
exactly determined, if the thermal expansion coefficient of the matrix and that of the sphere are
the same
Hollow sphere structures (HSS) are novel lightweight materials within the group of cellular
metals (such as metal foams) which are characterised by high specific stiffness, the ability to
absorb high amounts of energy at a relatively low stress levels, potential for noise control,
vibration damping and thermal insulation. Combination of these different properties opens a
wide field of potential multifunctional applications e.g. in automotive or aerospace industry.
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CHAPTER IV
THERMAL PROPERTIES
Investigations were carried out to obtain the thermal behavior and hardening of epoxy resins and
HSC using differential scanning calorimeters (DSC 200). The obtained typical temperatures are:
glass transition temperature (Tg), cured temperature (Tcure), temperature at the beginning of
thermal degradation (Tox). The obtained temperatures and their effects on residual reaction heat
of the remaining reactants (_Hr) are shown in Fig. 3. It can be stated that the thermal behaviour
of HSC is mainly governed by the epoxy resin used.
Fig. 3. DSC-scan of 11.1 mg epoxy resin Ebalta (1) and 12.3 mg HSC consist of corundum and
Fillite (2) with heating rate of 20 K/min in air.
The linear thermal expansion coefficient for Tg(α1) and linear thermal expansion coefficient
over Tg(α2) can be measured using thermomechanical analysis (TMA). Fig. 4 demonstrates, that
with increased percentage volume of fillers from 65% (Sample 2) to 78% (Sample 3) the α1- and
α2-values will be smaller, which is attributable to the smaller thermal expansion values of the
78% HSC material used in the research.
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Fig. 4. TMA-curves of epoxy resin and HSC-variants with different resin volume fractions.
In order to minimise the thermal distortion of machine tool elements it is important to know the
α1-values. Table 1 includes the α1-, Tg and α2-values for some HSC variants. These values
depend on the base materials used and can be determined from the following equations
α =Σviαi
where vi is the volumetric percentage, αi the thermal expansion coefficient.
SAMPLE COMPOSITION α1(×10^−6
K^−1)
Tg (◦C) α2(×10^−6
K^−1)
CURING
TIME (DAYS)
1 Only epoxy resin Ebalta 70.9 60 105.3 30
2 65 vol.% Fillite + corundum 33.1 51.5 64.1 28
3 78 vol.% Fillite + corundum 22.3 52.4 51.9 30
4 78 vol.% Fillite 34.5 62.6 49.1 21
5 78 vol.% corundum 0–2mm 23.4 51.3 30.8 19
Table 1
α1-Values of epoxy resin and HSC
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The vi and αi values of the components are normally available, but in this case the thermal
expansion coefficient of the fillers and the influences of the encapsulated gas in HSC on α1 are
unknown. However, the α-value of the corundum (α-Al2O3) is 9.5 × 10^−6 K^−1.
The calculated α-value of epoxy resin is 70 × 10^−6 K^−1. The calculated α1-value of Sample 5
is 22.8 × 10^−6 K^−1, which agrees well with the experimentally obtained value of 23.4×10^−6
K^−1. The dynamic mechanical analysis (DMA) investigations of three-point-bending-samples
of epoxy resin (a) and of HSC-Sample 3 (b) are shown in Fig. 5. At higher frequencies the Tg
moves to higher temperature values and due to the sensitivity of the DMA-methods two Tg
points are found for the semi-cured samples. At the start of the Tg area the microbrown
movements takes place followed by an entropy elastic state, where the dependence of the elastic
modulus on the temperature is less significant. It is notable that the fillers improve the stiffness
(E_) of Sample 3 (HSC) in comparison to Sample 1 (epoxy resin).
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Fig. 5. Elastic bending modulus (E’), loss modulus (E”) and log decrement (D) of epoxy resin
(Sample 1) (a) and HSC (Sample 3) of (b).
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CHAPTER V
MECHANICAL PROPERTIES
The elasticity modulus (E) of epoxy resin Ebalta 120/TL (EP) and HSC were obtained from
mechanical tests and are shown in Table 2. The mechanical properties of epoxy resin and HSC-
samples are shown in Table 2, along with steel (St), glass fibre (GF) and carbon fibre (CF)
materials for purpose of comparison. The density (ρ) of materials indicates that HSC are
lightweight materials. The ratio of stiffness (E) to density is an important parameter for material
selection. To compare the compression strength of two bars of equal dimension but different
materials the equation is simplified to 3√ E/g . It is clear from the table that HSC-Samples 2–4
have higher compression modulus than either steel or glass fibre . If GF or CF is manufactured as
laminate, then its mechanical properties becomes much smaller. A clear disadvantage of CF is its
anisotropy, whereas HSC is isotropic in all directions.
VALUE EP,
SAMPLE
1
HSC Steel GF CF
SAMPL
E
2
SAMPLE
3
SAMPLE
4
SAMPLE
5
ρ (g/cm3) 1.15 0.95 0.9 0.65 1.16 7.8 2.6 1.78
E (GPa) 3.5 7.8 6.8 4.1 8.7 210 73 235
3√ E/g(
3√(Mpa)
cm3/g)
13.2 21.4 21 24.6 18.7 7.6 16 34.5
Table 2Density and Young’s modulus (E) of epoxy resin and HSC in comparison to steel (St), glass fibre (GF) or
carbon fibre (CF)
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Fig. 6 shows the tensile strength (σt) and specific strength of epoxy resin and HSC-Samples 2–5.
The tensile test specimen was 250mmin length, 10mmin thickness and 25mmin width. The
tensile strength tests were carried out with a speed of 5 mm/min according to DIN EN ISO 527-
3. The specific strength of Sample 3 (Fillite and corundum 0.5–1 mm) and Sample 4 (Fillite) are
higher than that of epoxy resin. The result is, than using the same mass of material, a higher
volume of component can be made when using Samples 2–4, and it withstands the same tensile
strength as a component made from Sample 1.
Fig. 6. Tensile strength and specific strength of EP (Sample 1) and HSC (Samples 2–5).
Compression tests were conducted with test pieces having a length of 100 mm, a thickness of
30mm and a width of 30 mm. The speed of compression tests was 1 mm/min. The compressive
stress–strain curves of selected HSC-variants are presented in Fig. 7. The symbols of circle,
square, etc. mark the mean values of the compressive strength (σc) and the corresponding mean
values of compression-strain of Samples 5–10 of each variant. The σc-values in Fig. 7 are
greater than that of σt in Fig. 6 because in compression tests the pores will be closed and they
stop the propagation of the cracks. Samples 4 and 5 in Fig. 7 show that two typical stages occur
during deformation in the course of compression test of cellular solids such as polymer foams or
metal foams. Following an almost linear-elastic behaviour at low strains the curve shows a long
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plateau with almost constant load, but in comparison to the another cellular solids the HSC
material is superior in withstanding compression. Sample 4 filled with the smaller filler type
Fillite behaves better under compression than the filled with corundum, because Sample 5 has a
higher porosity. Samples 2 and 3 have high packing density thus providing higher compressive
strength values. The increase in the volumetric percentage of resin in Sample 2 improves the σc-
values. The smaller the size of the spheres the more marked the plateau areas are, as in this case
the crack propagation can be rapidly stopped by impediments (spheres or pores). This explains
why the samples filled with smaller particles cracks appear to be diagonal, while samples filled
with greater fillers develop transversal cracking develops. It has to be noted that adhesion bonds
between fillers and binders are of paramount importance. If the stiffness of the spheres is higher
than the stiffness of the resin then cracking starts in the resin and vice-versa.
Fig. 7. Typical compressive stress–strain curves of HSC variants and test samples after
compression test.
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The damage propagation can be explained using the scanning electron micrograph (SEM) images
of the fracture surfaces of Samples 3, 4 and 5 in Fig. 8. The Fillite spheres of Sample 4 in Fig. 8a
are broken. Due to the different wall thickness of the ceramic hollow spheres of Sample 5 in Fig.
8b, the spheres are broken at different levels. The space between the greater corundum spheres of
Sample 5 are greater than the space between the smaller Fillite spheres of Sample 5. A better
packing density of the fillers is shown in Fig. 8c, where Sample 3 is filled with different grain
size of spheres of known volumetric percentage fraction, thus causing to improve mechanical
properties of Sample 3 in comparison to Samples 4 or 5.
Fig. 8. SEM images of fracture surfaces among bending HSC-samples.
Fig. 9. Bending strength values for epoxy resin, HSC with and without carbon fibre or glass fibre.
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The bending stress in Fig. 9 was determined using three-point-bending samples with following
dimensions: 240mm length, 20mm width and 12mm height, according to the DIN EN ISO 178,
with a proof-speed of 4.8 mm/min. The bending strength values of HSC are smaller than that of
epoxy resin. Some HSC variants at the opposite side of the applied force were reinforced with
carbon or glass fibre to improve tensile properties.
Sample 3 is a mixture of ceramic and aluminium silicate hollow spheres and presents better
mechanical properties than Samples 4 or 5, which were filled with a single filler type. The
thermal expansion coefficient of Sample 3 is smaller in comparison to Sample 2 or 4. Sample 3
was selected as construction material for machine tool components and other engineering parts.
CHAPTER VI
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APPLICATION OF HSC IN MECHANICAL ENGINEERING
On the research programme a number of machine elements, such as jigs of milling tables and
robot arms for SCARA Adept robots were developed. These components were successfully
tested and the application of HSC materials in mechanical engineering was demonstrated. The
finite element program COSAR provided indications for the need of design changes regarding
the direction of carbon fibre reinforcements and the aluminium connection elements. The models
in Fig. 10 were loaded with 1000MPa bending force and the developed stresses remained below
acceptable limit. Based on the results obtained, two robot arms were made from HSC reinforced
with carbon fibre or aluminium alloys bars. These robot arms were 10 and 25% lighter in weight
than as the original aluminium alloy arms.
Fig. 10. Finite element models and robot arms made from aluminium alloy (a) and HSC (b).
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A milling machine table was successfully developed from HSC to replace a steel table. The
developed HSC table was designed with reinforcing steel elements and carbon fibre laminates to
withstand the typical tensile strengths. The achieved mass reduction is between 30 and 80%, thus
enhancing dynamic characteristics. The damping properties of the HSC table are superior to that
of cast iron table, which is partly attributed to the ply structure as shown in Fig. 11
Fig. 11. Table of a milling machine made from HSC, steel plate and carbon laminates.
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CHAPTER VIICONCLUSION
“Lightweight” is a major trend in machine tool design to ensure higher speed and higher
acceleration of elements, which results from state-of-the-art technology, such as the new linear
drive and the control system. Research is being carried out in institutes worldwide into
lightweight construction by either design and/or choice of material. One type of advanced
lightweight engineering material to reduce the mass of the moving parts of machine tools is
hollow-sphere composites. Investigations of their thermal and mechanical properties show the
superior quality of HSCs compared with alternative materials
It can be stated that HSC materials combined with metal or fibre reinforcements promise a
successful alternative to light metals or metal foams. In this research a number of machine
building parts with good dimensional accuracy have been produced and tested with good results.
The spherical form of the hollow materials provided a considerably smoother surface than that of
fibrous or irregular fillers and the resin consumption was significantly reduced. The application
of HSC materials is advantageous for the user because of the low material and production costs.
The excellent vibration and damping properties coupled with very low heat conductivity and
resultant heat distortion predestines the HSC materials to be used successfully in a variety of
engineering areas. The chemical resistance and the ease of recycling are further advantages of
this material by changing the composition of the matrix material and the volume.
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CHAPTER VIII
REFERENCES
[1] S. Klaeger, E. Baumeister, Untersuchung von Hohlkugelkomposit als Leichtbauwerkstoff.
Internationale Fachtagung “Polymerwerktsoffe”, Halle, Saale, 2001.
[2] P. Menz, Maschinenbaugruppen aus Kompositmaterial, Dt-Patent No. 1 952 367 (1996).
[3] N.N., Information from companies Omya and Treibacher, 2000.
[4] DIN 52190: Prüfung von Reaktionsharzbeton, Teil 3. Prüfung gesondert hergestellter
Probekörper, 1991.
[5] E.A. Turi, Thermal Characterisation of Polymeric Materials, vol. 2, Brooklyn, 1997.
[6] T.A. Osswald, G. Menges, Materials Science of Polymers for Engineers, Hanser Publishers,
Munich, 1995.
[7] H. Salmang, H. Scholze, Keramik. Teil 1. Allgemeine Grundlagen und wichtige
Eigenschaften. Sechste, verbesserte und erweiterte Auflage, Springer-Verlag, Berlin, 1982.
[8] S. Knappe, DMA-measurement from Netsch company, unpublished, 2000.
[9] B. Knauer, A. Wende, Konstruktionstechnik und Leichtbau. Methodik–Werkstoff–
Gestaltung–Bemessung, Berlin, 1988.
[10] N.N., R&G Faserverbundwerkstoffe GmbH, Waldenbuch, 2002.
[11] L.J. Gibson, M.F. Ashby, Cellular Solids, Structure and Properties, 2nd ed., Cambridge,
1997.
[12] H.-P. Degischer, B. Kriszt, Handbook of Cellular Metals, Production, Processing,
Applications, Wiley–VCH, Weinheim, 2002.
[13] E. Baumeister, Hollow-spheres-composites—as new lightweight materials for mechanical
engineering, in: Werkstoffwoche- Partnerschaft GbR (Ed.), in: Proceedings of the MATERIALS
WEEK 2002. Werkstoff-Informationsgesselschaft mbH, Frankfurt, 2003.
[14] Z. Bako, Polymer Concrete and Hollow Sphere Composites for Manufacturing of Machine
Tools, Otto-von-Guericke-University, Magdeburg and University of Miskolc, 2000.
[15] L. Bährend, Leichtbau im Maschinenbau am Besipiel der Konstruktion und experimentellen
Untersuchung eines Fräsmaschinetisches aus Hohlkugelkomposit, Otto-von-Guericke-
University, Magdeburg, 1998.
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