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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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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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



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.



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.



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



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).



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.



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.



CHAPTER VIII

REFERENCES

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Munich, 1995.

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[10] N.N., R&G Faserverbundwerkstoffe GmbH, Waldenbuch, 2002.

[11] L.J. Gibson, M.F. Ashby, Cellular Solids, Structure and Properties, 2nd ed., Cambridge,

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[12] H.-P. Degischer, B. Kriszt, Handbook of Cellular Metals, Production, Processing,

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[13] E. Baumeister, Hollow-spheres-composites—as new lightweight materials for mechanical

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[14] Z. Bako, Polymer Concrete and Hollow Sphere Composites for Manufacturing of Machine

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Untersuchung eines Fräsmaschinetisches aus Hohlkugelkomposit, Otto-von-Guericke-

University, Magdeburg, 1998.


light weight hollow sphere composite materials

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