Role of matrix resin in delamination onset and growth in composite laminates

measurements. The maximum fibre volume fraction, Vf, at which orientation control is effective is found to increase markedly as the fibre aspect ratio decreases since interaction between larger fibres at high Vj, hinders their relative movement. Simultaneous application of an ultrasonic field is found to increase the maximum Vf by vibrating to untangle fibres. The authors suggest that the technique could be applied to dispersion of whiskers in a matrix with continuous fibre reinforcement. Fretting fatigue studies on carbon fibre/epoxy resin laminates: III - microscopy of fretting fatigue failure mechanisms Schulte, K., Friedrich, K. and Kutter, K. Composites Science and Technology Vol 3 No 3 (1988) pp 155-176 Fretting fatigue studies were carried out on CFRP laminates differing in stacking sequence: [+45, 0, ±453, 90, -F4512s or [02, 902, 02, 902]s. Microscopic examination of failure mechanisms is reported and discussed in detail. It was found that where fretting had occurred against 0° plies fatigue life had been reduced but where 0° plies had been protected by 45° plies the reduction in fatigue life had been less severe. Friction and wear of rubber/epoxy composites Lhymn, C. and Lhymn, Y. Journal o f Materials Science Vol24 (1989) pp 1191-1194 In friction and wear situations where the asperity to asperity temperature rises, material loss by the mechanism of evaporation can become dominant over the numerous other microstructural mechanisms cited in the literature. The present paper examines this process for a rubber particle reinforced epoxy matrix composite system. Results suggest that a fatigue - shear model recently proposed accounts for observations in wear rate and sliding velocity, indicating a thermally activated process. Crack propagation and interfacial reactions have also been studied by SEM, as well as the effect lubrication has on these processes. Low-velocity hard-object impact of filamentwound Kevlar/epoxy composite Meyer, P. I. Composites Science and Technology Vol 33 No 4 (1988) pp 279-293 Specimens of filament-wound Kevlar/epoxy material as used in a missile motor case were subjected to two types of low-velocity impact test: quasistatic indentation and dynamic impact (drop-weight single impact). Fractional energy loss and absorbed energies were calculated and the damage was inspected both visually and by optical microscope. It is reported that the specimens exhibited a complex response to impact but that in all samples the fibres remained intact. Step increases in fractional energy loss were observed and the difference in fractional energy loss between the two types of impact were attributed to the ability of the Kevlar/ epoxy to 'cold flow' at the low strain rates of the indentation test, relieving stress in plastic deformation as opposed to fracture. Low voltage electrical properties of polypropylene filled with stainless steel fibres as a model of sample conductivity based on fibre geometry Bridge, B., Folkes, M. J. and Jahankhani,

COMPOSITES. SEPTEMBER 1989

H. Journal o f Materials Science Vo124 (1989) pp 1479-1485 The low voltage characteristics of three composites with different volume fractions of stainless steel fibres in a polypropylene matrix have been investigated to examine both their shielding properties to radio and microwave frequencies and to determine if such composites obey Ohms Law. Percolation conditions are produced at low fibre volume fractions due to their adopting a helical shape which increases the probability of forming continuous chains. A model based on perfectly formed helices of the same dimensions predicts the percolation threshold which is in good agreement with experimental observations. The fibre length and helix diameter are also found to vary threshold resistivity and again agrees with the proposed model assuming a low number of fibre contacts as a result of processing. Mechanical and electrical behaviour of Magnex DC conductive polymer composite Abdelazeez, M. K., Ahmed, M. S. and Zihliz, A. M. Journal o f Materials Science Vol 24 (1989) pp 1309-1315 Conductive polymer composites are attracting much interest at present with a view to utilising both mechanical and electrical properties in engineering applications. Such a composite is Magnex DC (trade mark of Diamond Shamrock Corp), which is examined in the present paper with particular reference to its mechanical and AC impedance behaviour at low frequency. The variation of elastic modulus and yield stress are determined as a function of temperature. In addition the yield stress at high strain rate values suggests a single activation process occuring for which the activation energy and activation volume are determined. The AC impedance shows a frequency dependence that is explained by considering space charges around steel fibres, and that AC conductivity is independent of frequency below 10 kHz but then increases parabolically with increasing frequency. Micromechanisms of crack extension in unmodified and modified epoxy resins Low, I-T., and Mai, Y-W. Composites Science and Technology Vol 33 No 3 (1988) pp 191-212 The failure behaviour of pure epoxy and modified epoxy resins was studied using fracture mechanics and SEM. The epoxy matrix was modified with a dispersion of rubbery and/or rigid fillers. The rubbery phase was a carboxyl-terminated butadieneacrylonitrile rubber; the rigid fillers were either ZrO2 particles or short alumina fibres. The fillers are shown to have the ability to control crack resistance and to interact with crack fronts. Relationships between fracture micromechanisms and microstructures are also identified and correlated. A mixed mode specimen for interlaminar fracture testing Benzeggagh, M. L., Davies, P., Gong, X. J., Roelandt, J. M., Mourin, M. and Prel, Y. J. Composites Science and Technology Vol 34 No 2 (1989) pp 129-143 The mixed load loading of a 20mm thick unidirectional glass fibre (60 vol % E-glass) reinforced epoxy resin was studied. The material, whose interlaminar fracture resistnace had previously been characterized

under pure mode I and pure mode II loading, was tested using a specimen which enabled the shear component Gn to be increased from 44 to 56% of the total fracture energy by varying the free length of the specimen. A finite element analysis was used to quantify mode separation. It is shown that as the proportion of mode II is increased, the delamination resistance increases Property and processing performance of graphite/PEEK prepreg tapes and fabrics Silverman, E. M. and Jones, R. J. S A M P E Journal Vol 24 No 4 (July~August 1988) pp 33-40 Different forms of graphite/PEEK composite materials were compared. Laminates were prepared from unidirectional prepreg tapes, woven fibre bundles of thermoplastic filaments and graphite filaments (co-woven) and woven hybrid yarns of intimate blends of PEEK filaments and graphite filaments (comingled) and tested for mechanical properties, thermal properties and quality as shown by examination by SEM. It was found that laminates fabricated from the prepreg tapes exhibited high flexural and transverse tensile properties (attributed to good fibre/ resin adhesion). Lower mechanical properties were exhibited by the laminates fabricated from co-woven and comingled fabrics, with the latter giving slightly higher results than the former. Role of matrix resin in delamination onset and growth in composite laminates Ye, L. Composites Science and Technology Vol 33 No 4 (1988) pp 257-277 The effect of the matrix resin on the onset and growth of delamination in composite laminates was investigated. Two epoxy resins, both reinforced with carbon fibres, with very different properties were used in two different lay-ups and the onset of delamination and its growth were studied in detail. It is shown that laminate resistance to delamination can be enhanced and delamination growth rate can be significantly reduced, without a marked loss in stiffness, by using a tough resin system. Scratch hardness as an evaluation of cure temperature for glass fibre reinforced polyester Briscoe, B.J. and Evans, P. D. Composites Science and Technology Vol 34 No 1 (1989) pp 73-90 Scratch hardness tests were carried out on samples of glass fibre-reinforced polyester and the results analysed in detail. It was found that ductile flow, brittle damage, and total frictional work were at a minimum and viscoelastic recovery at a maximum at a state of composite cure described as optimum cure. It is therefore suggested that the scratch hardness technique may be used as a tool in determining the material properties of a thermosetting resin surface. It is also suggested that at the state of optimum cure the material will also have achieved a state close to its maximum stiffness, consistent with its maximum toughness. A stochastic approach to fatigue damage of carbon fiber composites Ihara, C., Misawa, T. and Shigeyama, Y. Engineering Fracture Mechanics Vo133 No 3 (1989) pp 467-476

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