Fracture behaviour of fly-ash filled FRP composites

Composite Structures 10 (1988) 271-279

Fracture Behaviour of Fly-Ash Filled FRP Composites

V. K. Srivastava, ° P. S. S h e m b e k a r ~ & R . P r a k a s h ~ ~Department of Mechanical Engineering, hSchool of Biomedical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India

A BSTRA CT The major objective of this study was to determine the fracture toughness and fracture surface energy of epoxy, epoxy~fly-ash, epoxy~carbon,fibre, epoxy/ carbon fibre/fly-ash, epoxy~glass fibre and epoxy/glass fibre/fly-ash composites. The quality of composite specimens was evaluated by the ultrasonic' method. The results show that a fly-ash particle can arrest the crack path and thus improve the fracture properties of fibre reinforced plastic (FRP) composites. The results of this study have further significance in view of the fact that fly-ash powder is far cheaper than carbon fibre, glass ,fibre and epoxy resin.

1 INTRODUCTION T h e strength and toughness of a composite depends upon the shape and size of filler, the a m o u n t which is c o m p o u n d e d with the plastic, the bonding b e t w e e n the filler and the plastic, the toughness of the plastic and sometimes the toughness of the filler. Composite materials containing long filaments, or short fibres surrounded by a matrix, are attractive for structural applications where a high stiffness-to-weight ratio is required. An understanding of the fracture behaviour of composites is important for several reasons. Certain composites can be designed and fabricated to have unusually high resistance to crack propagation, and a composite of elements 'A' and 'B' can actually be tougher than either A or B individually (for example, fibre glass). This behaviour offers exciting possibilities for applications where high structural reliability is required. 1 Parhizgar et al. 2 have reported experimental 271 Composite Structures 0263-8223/88/$03.50© 1988Elsevier Science Publishers Ltd, England. Printed in Great Britain

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V. K. Sri~'astava, P. S. Shembekar, R. Prakash

results on fracture toughness of unidirectional glass/epoxy composites. Konish e t a l . ~ have presented similar results for graphite/epoxy composites. Thorat and Lakkad 4 have also measured fracture toughness of unidirectional glass fibre/carbon fibre epoxy composite. They found that fracture is triggered by a critical stress intensity factor, which is a characteristic of the crack orientation and is dependent on initial crack length and specimen geometry. Peters 5 has performed studies on increasing fracture toughness with increasing notch length of cross-ply graphite/epoxy laminates. Donaldson 6 has studied the combined mode I (crack opening) and mode I1 (forward shearing) fracture behaviour of unidirectional T300/1034C graphite/epoxy and graphite reinforced APC-1 polyetheretherketone. The modified three rail shear test is used to obtain the in-plane mode II fracture toughness and results for the epoxy and thermoplastic system are compared.7 The results show that the area of prima~ interest of the mixed mode interaction is the high shear area. Recently, sheet moulding compounds (SMC) have been widely used in structural applications and their mechanical reliability and performance have become increasingly important. Watanabe and Yasuda ~~ have reported that a point of incleation (knee point) exists on the SMC stress/strain curve, which identified the elastic and plastic region in tension. It was observed by them that the cracking in composites made from SMC may be reduced by using a matrix with high failure strain and the strength and modulus of these SMC composites may be increased by increasing the glass fibre content. Their modulus could be expressed as a function of resin modulus and volume fractions of filler and glass fibre. Ramsteiner and Theysohn ~" have also studied the tensile behaviour of filled composites. They reported that the influence of particle shape on Young's modulus, tensile strength and elongational viscosity can be understood on the basis of very simple models with a shape factor, determined for the applied filler type. Use of various non-destructive evaluation (NDE) methods is very. important in the case of fibre reinforced plastic (FRP) composites because, unlike metals, properties of FRP composites vary from point to point and from part to part. In case of metallic alloys it is very easy to add alloying elements to the molten metal and thus get homogeneous properties in the bulk alloy material. In the case of composites, however, it is very, difficult to control the number of defects present or to control the fibre distribution, e t c . ~1 Stone and Clarke 12 have evaluated the quality of composites by the ultrasonic technique in carbon fibre reinforced plastic (CFRP) composites. Their results show that the ultrasonic attenuation is sensitive enough to pick-up void percentage inside the materials. The aim of the present research programme is to improve the fracture

Fracture behaviour O[ifly-ashfiHed FRP composiws

273

properties of fibre reinforced plastic composite by adding fly-ash powder. Also, the quality of each specimen is evaluated by the ultrasonic method.

2 EXPERIMENTAL

2.1 Materials 2.1.1 Fibre & matrix Commercially available, E-glass fibre woven mats were obtained and unidirectional fibre strands were pulled out from these woven mats for making unidirectional GRP composites. Carbon fibres (Type-I high modulus) were used for making unidirectional CFRP composites. Epoxy resin (CY-205 resin and HY-951 hardener) was used as matrix material. 2.1.2 f')'ller--fly-ash powder The fly-ash was obtained from Obra Thermal Power Station, Mirzapur, Uttar Pradesh, India. The fly-ash powder is a mixture of different chemical constituents such as silica (56.04 wt%), aluminium (24-90 wt%), calcium oxide (2-22 wt%), ferric oxide (1.26 wt%), magnesium oxide (0.94 wt%). The physical properties of fly-ash include density (3.385 g cm -~), porosity (0-38) and particle diameter (63/xm to 105/xm). 2.2 Testing specimen Five different composites were used for the present study. These were epoxy/fly-ash, epoxy/carbon fibre/fly-ash, epoxy/carbon fibre (CFRP), epoxy/glass fibre/fly-ash and epoxy/glass fibre (GRP) composites. Unfilled epoxy resin specimens were used for the present study. Using hand lay-up technique unidirectional epoxy/glass fibre (GRP) and epoxy/carbon fibre (CFRP) composites were moulded. Hybrid composites were prepared with E-glass fibre, carbon fibre, fly-ash powder and epoxy resin by the hand lay-up technique. Fly-ash powder was used as filler. The ratio of fly-ash and epoxy resin was 0.069 by volume. First, the fly-ash powder and epoxy resin were mixed properly. Using this mixture, E-glass fibre and carbon fibre, unidirectional epoxy/carbon fibre/fly-ash and epoxy/glass fibre/fly-ash composite specimens were moulded at room temperature. The fibre volume fraction of each composite was 0-48. Finally, particulate composite specimens of epoxy/fly-ash (6.5 vol%) were also prepared for the present study. Single edge notched specimens with nominal dimensions: length = 100ram; thickness = 5ram; initial crack length = 5 mm and width = 10 to 20 mm were used for measuring the fracture properties of

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V. K. Srivastava, P. S. Shembekar, R. Prakash

P~

Fig. 1. Geometry of fracture specimen. P, tensile load: L, length; a, crack length; t, thickness; w. width.

each specimen as shown in Fig. 1. The value of the fibre volume fraction (46-5%) was maintained for all the composite materials.

2.3 Ultrasonic testing Ultrasonic tests were carried out using a through transmission technique. Two immersion type ultrasonic probes, 10 mm in diameter and having a frequency of 10 MHz, were used for all the tests, which were conducted in a water bath. A commercially available ultrasonic flaw detector (model UFD67B) was used for ultrasonic attenuation measurements. Five values of ultrasonic attenuation were obtained for each specimen, the coefficient of variation between the readings being 0.865. Average ultrasonic attenuation values were taken in order to determine the quality of each specimen and to correlate with the fracture stress of each type of specimen.

2.4 Fracture tests The fracture load of each specimen was tested on a commercially available Hounsfield Tensometer. Tensile load was applied very gradually by rotating the handle of the tensometer. Curves were directly obtained from the tensometer of load versus elongation for each specimen. The fracture 10ad was thereby identified. The fracture toughness for each single edge notched specimen was calculated using the following expression: ~3 K1 = Y P ~ w't

(1)

where P = applied load, t = thickness, w -- width, a = crack length, and

The correction factor Y corresponding to the case of anisotropic material has been used in all conditions; the effect of anisotropy has been neglected.

Fracture behaviour of fly-ash filled FRP composites

275

The fracture surface energy per unit plate area for each single edge notched specimen was calculated using the following expression :14 Y~ =

~

(2)

where cry = nominal fracture stress, and E = modulus of elasticity.

3 RESULTS AND DISCUSSION The load/elongation curves are shown in Fig. 2 of the epoxy, epoxy/fly-ash, epoxy/carbon fibre, epoxy/carbon fibre/fly-ash, epoxy/glass fibre and epoxy/glass fibre/fly-ash composite specimens. The results show that the total elongation of the epoxy resin and epoxy/fly-ash was one quarter of that of the unidirectional GRP, CFRP, epoxy/carbon fibre/fly-ash hybrid and epoxy/glass fibre/fly-ash hybrid composites. The total elongation of the GRP composite is increased by adding the fly-ash as a filler. However, the total elongation of the CFRP composites is less than that of the epoxy/carbon fibre/fly-ash hybrid composites. It is clear that the elastic region of the hybrid composite is dependent on elongation of epoxy resin and fibres. Thus, the fly-ash particle has improved the quality of glass fibre reinforced plastic (GRP) composite. Probable reasons for the lower elasticity of CFRP include stress concentrations at carbon fibre ends, residual stresses from moulding and the presence of interfaces between carbon fibres and filler (fly-ash) particles, etc. Figure 3 shows that the ultrasonic attenuation increases in epoxy, epoxy/ fly-ash, epoxy/carbon fibre, epoxy/carbon fibre/fly-ash, epoxy/glass fibre and epoxy/glass fibre/fly-ash composites, which exhibit decreasing nominal fracture stress, confirm that ultrasonic attenuation is a sensitive indicator of defects in composites. 12.15Ultrasonic attenuation is not very sensitive to fibre volume fraction. This in turn means that in practice ultrasonic attenuation measurements can be made to evaluate quality of the composite specimens. These observations are in line with those of Stone and Clarke. J2 Figure 4 shows the relationship between the fracture toughness of epoxy, epoxy/fly-ash, epoxy/carbon fibre, epoxy/carbon fibre/fly-ash, epoxy/glass fibre and epoxy/glass fibre/fly-ash composites and the ratio (a/w). It shows that the fracture toughness increases with increasing the ratio of initial crack length and width of the specimen. When a fibre composite is loaded, fracture generally initiates either by fibre fracture, debonding, matrix shear fracture or matrix tensile fracture. 1The higher fracture toughness means the area of crack propagation is greater. The results also show that fracture

276

V. K. Srivastava, P. & Shembekar, R. Prakash

1800

1600

140(; C

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1000

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ELONGATION (ram) Fig. 2, Load versus elongation for epoxy, epoxy/fly-ash, epoxy/carbon fibre, epoxy/carbon fibre/fly-ash epoxy/glass fibre and epoxy/glass fibre/fly-ash composites (a/w = (1.417L

toughness is affected by reinforcing fibres, fibre diameter, matrix and size of filler, etc. The fracture toughness of composites is increased by increasing the toughness of the epoxy resin. The fracture toughness of GRP Composite is increased by adding fly-ash filler (6.5 vol%) but the fracture toughness of CFRP is decreased by adding fly-ash filler. The toughness properties of CFRP composite are higher than those of the GRP composite. However, the experimental results conclude that the epoxy/glass fibre/fly-ash hybrid

Fracture behaviour c~ffly-ash,filled FRP composites

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15

Unidirectional &- Epoxy / • - Epoxy/fly- ash q// O- Epoxy/carbon fibre / / O - Epoxy/carbon f i b r e / f / y - a s h / J D - EPOXy/gLoss fibre /

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278

V. K. Srivastava, P. S. Shembekar, R. Prakash

Unidirectionot A - EPOXy • - Epoxy/fty-osl~ 5-0 - 0 - Epoxy/carbon fibre • - E p o x y / c o r b o n fi~e/fty-

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composite can be used in the place of CFRP composite as regards toughness. The hypothesis behind increasing the fracture toughness of the epoxy/glass fibre/fly-ash hybrid composite is that fly-ash arrests the crack path and also reduces the percentage of voids of GRP composites. Therefore, the fracture surface energy of epoxy/glass fibre/fly-ash is greater than that of GRP and other composite specimens, as shown in Fig. 5. The surface fracture energy per unit area (YF) also increases with increasing area of crack propagation.

4 CONCLUSIONS It is concluded from the experimental results that the fracture toughness and fracture surface energy of unidirectional glass fibre reinforced plastic (GRP) composites are increased by adding fly-ash filler (6-5 vol%) as compared to other materials. Thus, the fly-ash has served a dual purpose, viz. that of a filler material as well as that of a material which improves the fracture properties. In other words, fracture properties are enhanced by lowering the cost of the composite.

Fracture behaviouroffly-ashfilled FRP composites

279

ACKNOWLEDGEMENTS The authors are grateful to Obra Thermal Power Station, Mirzapur, Uttar Pradesh, for providing the fly-ash powder, and also to the Department of Mechanical Engineering and School of Biomedical Engineering for providing the research facilities.

REFERENCES 1. Tetelman, A. S., Fracture processes in fibre composite materials, Composite Materials." Testing and Design, ASTM STP 460, ASTM, 1969, p. 473. 2. Parhizgar, S., Zachary, L. W. & Sun, C. T., Application of the principles of linear fracture mechanics to the composite materials, Int. J. Fracture, 20 (1982) 3. 3. Konish, H. J., Jr, Swedlow, J. L. & Cruse, T. A., Experimental investigation of fracture in an advanced fibre composite, J, Composite Materials, 6 (1972) 114. 4. Thorat, H. & Lakkad, S. C., Fracture toughness of unidirectional glass/carbon hybrid composites. J. Composite Materials, 17 (1983) 2. 5. Peters, P. W. M., On the increasing fracture toughness at increasing notch length of 0/90 and 0/___45/I) graphite/epoxy laminates, Composites, 14 (1983) 365. 6. Donaldson, S. L., Fracture toughness testing of graphite/epoxy and graphite/ peck composites, Composites, 16 (1985) 103. 7. Lakshminarayana, H. V., A symmetric rail shear test for mode I1 fracture toughness (GIIC) of composite materials--finite element analysis, J. Composite Materials, 18 (1984) 227. 8. Watanabe, T. & Yasuda, M., Fracture behaviour of sheet moulding compounds, Part l. Under tensile load, Composites, 13 (1982) 55. 9. Watanabe, T. & Yasuda, M., Fracture behaviour of sheet moulding compounds, Part 2. Influence of constituents on mechanical properties, Composites, 13 (1982) 59. 10. Ramsteiner, F. & Theysohn, R., On the tensile behaviour of filled composites. Composites, 15 (1984) 121. 11. Cohen, Y. B., NDE of fibre-reinforced composite materials----A review, Materials Evaluation, 44 (1986) 446. 12. Stone, D. E. W. & Clarke, B., Ultrasonic attenuation as a measure of void content in carbon fibre reinforced plastics, Non-Destructive Testing, 8 (1975) 137. 13. Agarwal, B. D. & Broutman, W., Analysis and Performance of Fibre Composites, John Wiley and Sons, New York, 1980, p. 300. 14. Richardson, M. O. W., Polymer Engineering Composites, Applied Science Publishers Ltd, London, 1977, p. 49. 15. Prakash, R., Non destructive testing of composites, Composites, 10 (1980) 217.