Damping Analysis on Hybrid of Natural Rubber Particle and Epoxy Filled Glass Fiber Composites

 

Ravikumar L^*, Chandramohan D

Department of Mechanical Engineering, Sri Sairam Engineering. College, Chennai 600044, Tamilnadu, India

 

Abstract:

Vibration damping has gained importance for achieving improved vibration and noise control, dynamic stability, fatigue and impact resistance in advanced engineering systems. In the present work, the effect of natural rubber particle inclusions in epoxy glass fibre composites, on the mechanical and damping properties of epoxy glass fibre composites is investigated. Test specimens are fabricated with inclusion of natural rubber particles of different sizes and Dynamic Mechanical Analysis (DMA) is carried out for storage modulus and loss factors (tan δ) to study the influence of size of the particle rubber inclusions on these properties at various frequencies keeping temperature constant and at various temperatures keeping frequency constant. The analysis is carried out for two different conditions in 3-point bending and in shear. Among the selected particle sizes 0.254mm exhibits highest damping with a little reduction in stroge modulus.

 

 

 

INTRODUCTION:

Fibre-reinforced composites are receiving greater importance in the present day industry because of their low density, high stiffness, high strength and damping characteristics. The chief advantage of fibre reinforced composites is ability to tailor the properties. Recent researches are taking place to improve the damping capacity of fibre reinforced composites to reduce the vibrational levels and noise in fibre reinforced composites structures. Though the stiffness and strengths of polymers are improved by the addition of high modulus fibres, their damping capacity is reduced to sustain the vibrations. Many works reported for enhancement of damping capacity of the fibre reinforced composites.

 

Hawng and Gibson ^[1] improved the damping of FRC (Fibre reinforced composites) by orienting high proportion of fibers at ± 45 ^o to the primary loading direction. Though damping is improved, considerable reduction in stiffness is observed in their work. There is always part off between the improvement in damping and stiffness and the drive ever been to reduce this gap. Viscoelastic damping tapes were attached to the surface of the base structures after fabrication by Mantena et al ^[2] and these tapes were sandwiched between the base structure and constrained layer to enhance the damping in the structure. The ratio of tape length to beam length and the optimum location of the viscoelastic tape was investigated by the authors. In addition to considerable improvement in damping in case of surface treatments techniques additional weight is added to the structure. The embedded co-cured viscoelastic layers can avoid the drawback of weight addition.

 

Rotz and Barret ^[3] designed composite structures with the required damping by using damping hat (selection of damping layers location and stacking sequences). The drawback with co-cured layers is the delamination at the interphases of viscoelastic layers. The behavior of unidirectional and symmetric angle-ply carbon fibre-epoxy laminates with interleaved layers was studied by Liao et al. ^[4] and reported that the introduction of the PEAA layer significantly improved the damping. The effectiveness of interleaving increases with the flexural modulus of the outer layer. Experimental results revealed that proper selection and placement of interleaving material with appropriate laminate configuration and constituent properties significantly improve overall damping of the interleaved composite laminate. The hybridization of laminates improves both damping and rigidity of composites ^{[5, 6].}

 

These hybrid laminates in the reported work consists of both polyethylene SPECTRA fibre reinforced laminae and graphite fibre reinforced laminae. Experimentally it was proved that the position of the damping layers is an important factor in influencing the loss factor of the composite structure. The high shear strain energy stored at interphase plays a major role in damping characteristics of fibre reinforced composites due to a mismatch of the properties. Hence the fibres are coated with viscoelastic materials to improve the damping in composites. Experimental and analytical models to study the effect of the fibre coatings on damping were reported by Finegan and Gibson ^[7]. Jinhai et al. ^[8] investigated the damping behavior of magnesium matrix composites reinforced with copper coated and uncoated Sic particulates and concluded that at high temperatures the damping of composite with uncoated Sic exhibited better than the pure Mg matrix. Even though the vibration damping capacity and fracture toughness involve the process of energy dissipation both are different mechanical properties. Damping capacity is the measure of cyclic dissipation of vibration energy whereas fracture toughness is the energy dissipated at the crack tip as crack grows.

 

Kim et al. ^[9] improved the fracture toughness of epoxy composite with addition of carbon black and micro clay particles. It was found that reinforcement with micro particles improved the fracture toughness at the room temperature, but decreased the fracture toughness at the cryogenic temperature in spite of their toughening effect. A variety of methods have been used to improve the toughness in polymer matrix composites. Rubber particles were included in polymers to improve the impact strength [^10]. The dissipation mechanisms in rubber particle included polymeric composites are crazing, rubber bridging, and cavitation of rubber particles which causes massive shear bonding of the matrix around the particles.

 

The rubber particles in polymeric composites act as craze initiators responsible for energy dissipation and improvement in damping. With recycled rubber particles Zhou Hong et al ^[11] fabricated a composite sound absorber with good attenuation property to dampen the noise. Even though both vibration damping and fracture toughness involve the process of energy dissipation, studies on vibration damping are few. The importance of dynamic analysis (DMA) as a tool in the study of behavior of composite structures is paramount. It has been proved to be an effective method to study the relaxations in polymers there by the behavior of materials under various stresses, temperature and phase composition of composites and its role in mechanical properties. DMA is simply described as applying an oscillating force to a sample and analyzing the material response to that force. From this one calculates the properties like the tendency to flow (called viscosity) from the phase lag and the stiffness (modulus) from the sample recovery. These properties often described as ability to lose energy as heat (damping) and ability to recover from deformation (elasticity). These properties enhanced especially in polymeric composites due to relaxation of polymer chains.

 

The DMA measurement consists of the observation of time dependent deformation behavior x (t) of sample under periodic, mostly sinusoidal deformation force with very small amplitudes F (t).  Thus it is possible to calculate Young’s modulus E’ (storage modulus) and E” (loss modulus) as well as mechanical loss factor Tan δ (damping) dependence on temperature and deformation frequency. Dipa ray et al. ^[12] carried out dynamic mechanical and thermal analysis of vnylester resin matrix composites reinforced with untreated and alkali treated jute fibres. They studied dynamic properties of composites as function of temperature using DMA. Grant et al. ^[13] produced an intrinsically damped composite with the viscoelastic properties of an epoxy resin system. Through the addition of chain extension modifiers, peak loss factors up to 0.4 have been achieved compared to 0.005 for traditional glass reinforced epoxy materials. Maries et al. ^[14] determined the dynamic and static mechanical properties of randomly oriented intimately mixed shirt banana/sial hybrid fibre reinforced polymer composites. Dynamic properties such as the storage modulus, damping behavior and static properties such as tensile, flexural and impact properties were investigated as a function of total fibre volume fraction and the relative volume fraction of the two fibres. The storage modulus was found to increase with fibre volume fraction above glass transition temperature of the matrix and maximum value was obtained at a volume fraction of 0.4. Tensile modulus and flexural strength were found to be highest at 0.4 volume fraction.

 

Bleach et al. ^[15] developed a bioabsorbable self reinforced polylactide/biphasic calcium phosphate composite for fracture fixation plates. As the filler content increased, the failure strain decreased due to a reduction in the amount of ductile polymer present and the ultimate tensile strength decreased because of agglomeration and void formation at higher filler content. The matrix glass transition temperature increased due to polymer chain absorption and immobilization onto the biphasic calcium phosphate particles.

 

Sui et al. ^[16] introduced liquid nano reinforcements and hybrids of nanofillers with different size to simultaneously improve the reinforcement and toughing of epoxy resin. The flexural strength and breaking strain of the epoxy composites with 0.3wt% reactive carbon nanofibres and 0.2wt% carbon nanotubes increased by 45% and 64% respectively. DMA results showed an increase in storage modulus and glass transition temperature for the hybrid nanocomposites compared to that of the neat epoxy resin. Damping characteristics of nylon and reinforced nylon with glass and carbon fibre spur gears were investigated through DMA by senthilvelan et al ^[17]. They concluded that addition of high modulus materials such as carbon and glass fibers increases the vibration and noise levels. The main object of their work was to reduce the gap between the reduction in damping and improvement in stiffness due to addition of high modulus fibers.

 

Upinder et al ^[18] prepared some novel composites with polypropylene and silicon rubber with wollastonite as fibrous filler having aspect ratio 5:1 in different proportions. The results show that there is a marginal change in mechanical properties of the untreated wallastonite filled polypropylene composite on addition of silicon rubber and malicanhydride, whereas the enhancement in the heat deflection temperature was significant. Though the enhancement of damping is reported through constrained layer treatments, co-cured layers and fibre coating techniques addition of excess mass, delaminations at interleaved layers and reduction in stiffness are the major drawbacks in each of the cases respectively. Improvement in toughness of plastics using rubber inclusions is reported by various works but the studies on damping are few. Vibration tests that are carried out for evaluation of structural damping, significantly depends on type of structure and boundary conditions imposed on the structure. In the present work epoxy glass fibre fabric composites are fabricated with the inclusions natural rubber particles of different sizes to enhance the material damping and to reduce the gap between the reduction in stiffness and improvement in damping. Dynamic mechanical analysis is carried out using DMA Q 800 on both neat composites and rubber particle included composites to investigate the effect of particle size on loss factors and storage moduli in   two different modes a) three point bending b) shear. These properties are investigated for their dependence on frequency, and temperature.

 

MATERIALS AND METHODS:

Sample Preparation:

Glass fabric of 5 mil plain mat and epoxy resin (lapox 12) with hardener K6 is taken for the fabrication of the samples. The nominal content of glass fibre in the composite is set at 40% by weight and the remaining 60% resin. Natural rubber particles (cross-linked and approximately spherical in shape) are sieved and four different particles 0.9, 0.45, 0.25, 0.0975 mm (nominal diameter of the pore sizes of the sieves) in size are segregated. These particles 10% by weight are added to the 90% glass fibre composite (glass fibre to resin ratio is 40:60) as inclusions. Rubber particles are mechanically mixed with epoxy resin at room temperature and followed by mixing with sonicator for 60 min. The thickness of the each composite sample is 4 mm.

 

The composite sample plates are fabricated in rectangles with the glass fibre fabric mats are oriented at 0⁰ and 90⁰ towards length and width directions. Hand layup technique followed by compression molding was used for fabrication of composite. A stainless steel mould measuring 300 mm x 250 mm x 4 mm length, width & depth respectively was used for preparing composites. Releasing agent Poly Vinyl Alcohol (PVA) was sprayed evenly on to the surface to facilitate easy removal. Samples are fabricated in the form of plates of 300 mm x 180 mm x 4 mm with different sizes of rubber particles as inclusions in between the layers, and 16 layers are lied up and cured at room temperature for 24 hr. For DMA analysis, rectangular samples having size 15 mm x 10 mm x 4 mm for three point bending and 10 mm x 10 mm x 4 mm for shear tests were cut from the fabricated plates.

The morphology of the cross section of natural rubber filled epoxy glass fibre composites were examined using Scanning electron microscopy (SEM), ZEISS EVO® MA15, in order to examine distribution of rubber particles in the composite. Samples are coated with gold using plasma sputter apparatus.

 

The viscoelastic properties such as storage modulus and loss factor of neat composite and rubber particle included composites with different sizes of inclusions were measured using DMA Q800.The analysis was done in three point bending  and shear mode. In both the cases variation of loss factor and storage modulus with frequency at room temperature and variation with temperature at constant frequency of 10 Hz were obtained. For temperature dependent properties the samples are heated up to 180⁰ C at a heating rate of 1⁰C/min.

 

RESULTS AND DISCUSSION:

Morphology studies reveal the aspects of fibre bonding, adhesion between fibre and matrix and distribution of particle rubber in the glass epoxy composite. Fig.1 shows SEM micrograph of the cut surface of glass fibre-reinforced epoxy resin composite specimen with rubber particles. From the figure 1 it is observed that rubber particles are well distributed in the matrix.

 

 

Fig1.SEM Micrograph of rubber filled epoxy glsss fiber composite.

 

Bending loss factor (measure of damping) in glass epoxy composite at various frequencies in the range of 0-200 Hz is reported in fig 2.

 

 

Fig 2. Variation of flexural loss factor with frequency in 3 point bending

 

Loss factor increased with frequency up to 110 Hz and then decreased for all the specimens. Bending loss factors increased with rubber particle inclusions at all frequencies considered when compared with neat composite. Shear loss factor various frequencies in the range of 0-200 Hz is reported in fig 3.

 

 

Fig 3. Variation of flexural loss factor with frequency in shear

 

This is also increased with frequency up to 100 Hz and then decreased for all the specimens. Loss factor is increased with rubber particle inclusions at all frequencies considered when compared to neat composite. In both cases, loss factor is decreased with increase in size of particle inclusions except with the sizes 0.254 mm and 0.0975 mm. A specific observation is that the maximum loss factor for 0.254 mm among the selected particle size. Since loss factor is representation of damping, 0.254 mm rubber particle inclusions may give maximum damping ratio among the selected one.  For a fibre composite system the rule of mixtures formula for calculating damping is

 

Q = Q_f  V_f + Q_m V_m+ Q_i  V_i                                                                                                                                                                                             (1)

 

Where Q is damping capacity of composite, Q_f is damping capacity of the fibre, V_f is volume fraction of fibre, Q_m is damping capacity of the matrix, V_f is volume fraction of matrix, Q_i is damping capacity of the interphase, V_i is volume fraction of interphase [17]. It manifest that the damping is determined by the interphase in addition to matrix and fibre. The viscoelastic rubber particles present in interphase will produce micro-slip, which may generate damping. The reason for exhibiting better damping with 0.254 mm particle size compared to 0.0975 mm is that fine rubber particles may not exist in the fibre matrix interphase layer which is an order of 50 nm [19].  Similarly at higher rubber particle size the energy dissipation due to micro-shear yield mode of deformation is suppressed [20] and decreases the damping. Hence, 0.254 mm rubber particle inclusions improved damping of the composite among the selected particle sizes. Figure 4 shows the variation of the storage modulus in flexural mode with frequency for all composite specimens.

 

 

Fig 4. Variation of flexural storage modulus with frequency in 3 point bending

 

It is observed that the flexural storage modulus for all specimens is almost constant with the frequency up to a range of 100 Hz to 110 Hz and then increased slightly. The rubber particle inclusions in glass epoxy composite reduced the flexural storage modulus. The micro-slip between the rubber particles and interphase layer is attributed for this reduction.  Reduction in flexural storage modulus increased with size of the rubber particle inclusions. At smaller particle inclusions i.e. 0.0975 mm and 0.254 mm the reduction is at about 10% to 15% where as at particle sizes 0.45 mm and 0.9 mm it is about 30% to 40%. This may be due to stress raising tendency of larger particles because of abrupt changes in material properties. The experimental values presented in fig 5 explain the effect of particle size on storage modulus in shear with frequency.

 

 

Fig 5. Variation of shear storage modulus with frequency in shear

 

The storage modulus in shear mode is almost constant with the frequency considered. The rubber particle inclusions reduce the shear storage modulus at all frequencies as in the case of three point bending. The effect of size of rubber particle inclusions is similar as in flexural storage modulus. The shear storage modulus is reduced 10% to 20% to that of neat composite with all sizes of rubber particle inclusions. Among the selected particle sizes, the shear storage modulus is reduced with increase in size of inclusions. This is also may be due to stress raising tendency of larger particle sizes.

 

The effect of size of rubber particle inclusions in glass epoxy composite on loss factors in three point bending and in shear with temperature at10 Hz is reported in fig 6 and 7 respectively. It is observed that the glass transition temperatures T_g is shifted to lower side due to addition of rubber inclusions. T_g for neat composite is about 140⁰C and for rubber filled composites is around 90⁰C in three point bending. The size of particle inclusions on T_g is insignificant in case of shear loading. The T_g for neat composite is about 130⁰C and for rubber filled composites ranges from 80⁰C to 110⁰C at different particle size.

 

 

Fig 6. Variation of flexural loss factor with temperature in 3 point bending

 

The shift in T_g is due to increase in mobility of rubber particles. This shift is less in case of shear compared to three point bending because of better load transfer. Magnitude of loss factor is more with particle size 0.254 mm as in the case of different frequencies. The magnitude of the peak is decreased with increase in particle size. At temperatures more than 90⁰C, micro-slip at interphase of fiber, matrix and particle inclusions increased significantly there by increases damping considerably.

 

The DMA results revealed (fig 8 and 9) that storage modulus in three point bending and in shear decreased slightly with temperature for all composite specimens.

 

 

Fig 7. Variation of shear loss factor with temperature in shear

 

 

Fig 8. Variation of storage modulus with temperature in 3 point bending

 

 

Fig 9. Variation of storage modulus with temperature in shear

 

The glass transition temperature T_g is 120⁰C for a neat composite and 80⁰C for rubber filled composites in three point bending. At T_g the molecular relaxations of viscoelastic rubber particles takes place and these molecular motions at interface attributes for damping. In shear, T_g is 110⁰C for neat composite and 90⁰C for composite with rubber particle inclusions. Once crossing these temperatures the reduction in storage modulus is drastic. On passing through the T_g great fall of both the storage modulus is observed for all composite specimens. 

 

CONCLUSIONS:

DMA tests are carried out on neat composite and rubber particle filled composites, from the experimental results, Bending loss factor and shear loss factor are increased up to 100 Hz and then decreased with frequency. Both loss factors are improved with rubber particle inclusions and particle size of 0.254 mm exhibited better results among the selected specimens. Storage modulus in three point bending is constant up to 100 Hz and then increased slightly. This property decreased with inclusion of rubber particles in the composite. The reduction is storage modulus for 0.0975 mm and 0.254 mm particle filled composite is 10% to 15%, whereas for 0.45 mm and 0.9 mm particle inclusions reduction in storage modulus is 30% to 40%. Glass transition temperature of neat composite is around 140⁰C in three point bending and for rubber particle included composite is around 85⁰C to 90⁰C and in shear mode is about 100⁰C to 125⁰ C at different particle sizes. The peak magnitude of loss factor is highest at 0.254 mm size rubber particle filled composite among the selected particles. Storage modulus is decreased with increase in temperature up to certain level and then dropped drastically in three point bending and shear on passing through glass transition temperature T_g^.

 

 

 

 

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Received on 21.04.2018       Modified on 29.05.2018 Accepted on 06.07.2018      ©A&V Publications All right reserved Research J. Science and Tech. 2018; 10(4):253-260. DOI: 10.5958/2349-2988.2018.00036.0