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The flexural behaviour of aramid fibre hybrid composite materials
The results of compression tests on a range of truncated conical shells made from a chopped strand mat glass/polyester material are reported. Cones with a wall thickness greater than 2 mm fail by progressive crushing failure starting at the small end of the cone and no trigger is necessary. The specific energy absorbed during crushing is seen to vary with cone angle, wall thickness and section diameter in a complex way. The crush zone morphology is seen to vary with cone angle and trigger geometry but it has not yet been found possible to explain the variation in energy absorption with cone geometry in terms of failure mode.
Effect of matrix toughness on crack propagation during stress corrosion of glass reinforced composites Price, J.N. and Hull, D. Composites Science and Technology Vo128 No 3 (1987) pp 193-210 The propagation of stress corrosion cracks in specimens of unidirectional E-glass fibres in a range of polyester matrices, made by filament winding around a flat plate mandrel, in 0.6N HCI environment were monitored using a travelling microscope and the cracked specimens examined fractographically. The effects of the toughness of the matrices are discussed in detail. In general, it was found that increasing matrix toughness leads to a reduction in crack rate.
The flexural behaviour of aramid fibre hybrid composite materials Fischer, S. and Marom. G. Composites Science and Technology Vo128 No 4 (1987) pp 291-314 The elastic-plastic model of flexural behaviour of beams has been applied to hybrid composites containing aramid fibres. As with aramid fibre-reinforced composites, the neutral axis was shifted towards the tensile face, the shift being dependent upon the quantity and placement of the aramid fibre. Both the analyses and experimental work are directed towards two different sandwich hybrids: one with aramid fibres in the skins and carbon or glass fibres in the core and the other with aramid fibres in the core and glass or carbon fibres in the skins. The effect of the placement of the aramid layer and the relative thickness of the skins on the stresses and the elastic-plastic behaviour is discussed in detail.
Fracture ehuracterisation of random short fibre reinforced thermoset resin composites Mower, T.M. and Li, V.C. Engineering Fracture Mechanics Vo126 No 4 (1987) pp 593-603 The applicability of classical linear elastic fracture mechanics (LEFM) approaches to toughness characterisation of random short fibre-reinforced polymer composites is examined on the basis of the experimental investigations of several workers. It is concluded that for long fibres (24 or 50 mm) LEFM is not applicable. For short fibres (0.5 mm) the process zone is sufficiently small that LEFM toughness tests are valid with standard specimen sizes. The tension softening curve which is a constitutive relation between the tensile stress and the separation distance in the process zone is suggested as an alternative approach to toughness characterisation.
Frequency effects on the fatigue life and damage of graphite/epoxy composites Tsai, G.C., Doyle, J.F. and Sun, C.T. Journal of
344
Composite Materials Vol 21 (January 1987) pp 1-13 A damage accumulation model, which includes the effects of fatigue frequency and stress level. is developed for (_+45)2s graphite/epoxy laminates. The changes in dynamic modulus and creep strain as measures of damage are used to develop two equations which relate damage to each of these variables. It is shown that frequency effects are important only in stress controlled situations because of the frequency dependence of Young's modulus. Although both changes in dynamic strain and creep strain may provide a good indication of damage development, the two relationships combine in a non-linear manner. Mechanical properties of oriented porous polypropylene filled with modified CaCO 3 Mitsuishi, I~, Kodama, S. and Kawasaki, H. Journal of Materials Science Letters Vol 6 (1987) pp 434-436 The effect of filler content ( 1.6-14.5 volume %) and filler modification (PCE or PCP, 1.6-68 volume %) on the relative modulus of CaCO 3 filled oriented polypropylene was studied. It was found that to obtain the high loading effect, the CaCO 3 must be surface modified. Surface modification decreases the volume fraction of voids upon elongation due to improved adhesion between the polypropylene and the CaCO 3. The relative modulus (oriented PP composite/oriented PP) decreased with increasing volume fraction of voids and the values were smaller than that of the undrawn polymer.
Microstructural details and the effect of testing conditions on the fracture toughness of injection moulded poly(phenylene-sulphide) composites Karger-Kocsis, J. and Friedrich, K, Journal of Materials Science Vol 22 No 3 (March 1987) pp 947-961 The matrix layer structure of PPS and composites containing 30wt% short glass and carbon fibres (as determined by optical microscopy) is found to depend on processing conditions, such as thermal treatment. Annealing at 230°C for 8 h increases crystallinity and simplifies the layer structure, and also improves the fracture toughness and modulus of the composites. The modulus of both filled and unfilled PPS does not change with strain rate but decreases at Tg (90°C). Results are presented in terms of failure maps and fracture toughness-modulus maps.
Nondestructive evaluation of strength and separation modes in adhesively bonded automotive glass fiber composite single lap joints Williams Jr, J.H., Lee, S.S. and Wang, T.K. Journal of Composite Materials Vol 21 (January 1987) pp 14-35 The effects of flawing (undercure of urethane adhesive and excessive mould release on adherends) in adhesive joints was studied using acoustic emission and ultrasonic techniques. Flawed bonds were found to have lower strength than unflawed bonds, and three modes of failure were associated with the type of flaw - - delamination in unflawed specimens, interfacial in specimens with excessive mould release and cohesive failure in undercured joints. The fracture strengths were correlated with results from ultrasonic tests (throughthickness attenuation, pulse-echo and stress wave factor) and 'acoustic emission load-delay" results; the latter being shown to be capable of
distinguishing between the modes of failure occurring in the joints. Thus, these nondestructive evaluation techniques may be used to characterize adhesively-bonded FRP materials.
Prediction of material damping of laminated polymer matrix composites Sun, C.T., Wu, J.K. and Gibson, ILF. Journal of Materials Science Vol 22 No 3 (March 1987) pp 1006-1012 This analytical determination of damping in laminated composites uses a force-balance approach based on a classical lamination theory previously applied by the authors to the case of short fibre composites. Damping is expressed in terms of the in-plane, coupled and flexural damping from the IAI, IBI and ID1 stiffness matrices and the elastic/viscoelastic correspondence principle. Several graphite/ epoxy laminates with different stacking sequences are considered. Prediction of moisture diffusion parameters in composite materials under stress Neumann. S. and Marom G. Journal t~/' Composite Materials Vol 21 (January 1987) pp 68-80 A model is proposed which predicts the changes in the diffusion parameters rate, maximum absorption capacity, and the Fickean diffusion coefficient as a function of changes in stress level. Two factors are considered in the model: that stress-dependent free volume changes in the matrix affect the moisture absorption capacity of the matrix; and that the diffusion coefficients in both stressed and unstressed states are expressed by a Doolittle-type equation connecting the diffusion coefficient and the free volume. Experimental work is reported on moisture absorption in stressed unidirectional carbon fibre-reinforced epoxy laminates in which the fibre content and fibre orientation were varied. It was found that increasing the fibre angle increased the effect of external stress on the diffusion coefficient. This stress effect is increased when the fibre volume fraction is increased.
Properties of thin polypropylene films loaded with finely dispersed gold Hogarth, C.A. and Al-lsmail, S.A.Y. Journal of Materials Science Letters Vol 6 (1987) pp 409-412 Electrical measurements have been made on standard thin film sandwich samples and on planar structure samples of polypropylene containing finely dispersed gold particles (3 pm). The parameters estimated werc the frec electron concentration, density of trapping states, trapping coefficient, carrier mobility, activation energy (0.48 eV) for conduction and permittivity. The density of states in the conduction band was also estimated. The voltage at which the space charge limited current was observed was 150V. It is generally concluded that the addition of less than 1wt% of finely dispersed metal increases the permittivity by more than twice compared with evaporated films of polypropylene measured at room temperature. Planar samples showed a significant sensitivity to light.
Pull-out of steel fasteners from a laminated composite subjected to external pressure Parry. T.V., Wilson, J.M. and Leek, D.C.
COMPOSITES
. SEPTEMBER
1987
Effect of matrix toughness on crack propagation during stress corrosion of glass reinforced composites Price, J.N. and Hull, D. Composites Science and Technology Vo128 No 3 (1987) pp 193-210 The propagation of stress corrosion cracks in specimens of unidirectional E-glass fibres in a range of polyester matrices, made by filament winding around a flat plate mandrel, in 0.6N HCI environment were monitored using a travelling microscope and the cracked specimens examined fractographically. The effects of the toughness of the matrices are discussed in detail. In general, it was found that increasing matrix toughness leads to a reduction in crack rate.
The flexural behaviour of aramid fibre hybrid composite materials Fischer, S. and Marom. G. Composites Science and Technology Vo128 No 4 (1987) pp 291-314 The elastic-plastic model of flexural behaviour of beams has been applied to hybrid composites containing aramid fibres. As with aramid fibre-reinforced composites, the neutral axis was shifted towards the tensile face, the shift being dependent upon the quantity and placement of the aramid fibre. Both the analyses and experimental work are directed towards two different sandwich hybrids: one with aramid fibres in the skins and carbon or glass fibres in the core and the other with aramid fibres in the core and glass or carbon fibres in the skins. The effect of the placement of the aramid layer and the relative thickness of the skins on the stresses and the elastic-plastic behaviour is discussed in detail.
Fracture ehuracterisation of random short fibre reinforced thermoset resin composites Mower, T.M. and Li, V.C. Engineering Fracture Mechanics Vo126 No 4 (1987) pp 593-603 The applicability of classical linear elastic fracture mechanics (LEFM) approaches to toughness characterisation of random short fibre-reinforced polymer composites is examined on the basis of the experimental investigations of several workers. It is concluded that for long fibres (24 or 50 mm) LEFM is not applicable. For short fibres (0.5 mm) the process zone is sufficiently small that LEFM toughness tests are valid with standard specimen sizes. The tension softening curve which is a constitutive relation between the tensile stress and the separation distance in the process zone is suggested as an alternative approach to toughness characterisation.
Frequency effects on the fatigue life and damage of graphite/epoxy composites Tsai, G.C., Doyle, J.F. and Sun, C.T. Journal of
344
Composite Materials Vol 21 (January 1987) pp 1-13 A damage accumulation model, which includes the effects of fatigue frequency and stress level. is developed for (_+45)2s graphite/epoxy laminates. The changes in dynamic modulus and creep strain as measures of damage are used to develop two equations which relate damage to each of these variables. It is shown that frequency effects are important only in stress controlled situations because of the frequency dependence of Young's modulus. Although both changes in dynamic strain and creep strain may provide a good indication of damage development, the two relationships combine in a non-linear manner. Mechanical properties of oriented porous polypropylene filled with modified CaCO 3 Mitsuishi, I~, Kodama, S. and Kawasaki, H. Journal of Materials Science Letters Vol 6 (1987) pp 434-436 The effect of filler content ( 1.6-14.5 volume %) and filler modification (PCE or PCP, 1.6-68 volume %) on the relative modulus of CaCO 3 filled oriented polypropylene was studied. It was found that to obtain the high loading effect, the CaCO 3 must be surface modified. Surface modification decreases the volume fraction of voids upon elongation due to improved adhesion between the polypropylene and the CaCO 3. The relative modulus (oriented PP composite/oriented PP) decreased with increasing volume fraction of voids and the values were smaller than that of the undrawn polymer.
Microstructural details and the effect of testing conditions on the fracture toughness of injection moulded poly(phenylene-sulphide) composites Karger-Kocsis, J. and Friedrich, K, Journal of Materials Science Vol 22 No 3 (March 1987) pp 947-961 The matrix layer structure of PPS and composites containing 30wt% short glass and carbon fibres (as determined by optical microscopy) is found to depend on processing conditions, such as thermal treatment. Annealing at 230°C for 8 h increases crystallinity and simplifies the layer structure, and also improves the fracture toughness and modulus of the composites. The modulus of both filled and unfilled PPS does not change with strain rate but decreases at Tg (90°C). Results are presented in terms of failure maps and fracture toughness-modulus maps.
Nondestructive evaluation of strength and separation modes in adhesively bonded automotive glass fiber composite single lap joints Williams Jr, J.H., Lee, S.S. and Wang, T.K. Journal of Composite Materials Vol 21 (January 1987) pp 14-35 The effects of flawing (undercure of urethane adhesive and excessive mould release on adherends) in adhesive joints was studied using acoustic emission and ultrasonic techniques. Flawed bonds were found to have lower strength than unflawed bonds, and three modes of failure were associated with the type of flaw - - delamination in unflawed specimens, interfacial in specimens with excessive mould release and cohesive failure in undercured joints. The fracture strengths were correlated with results from ultrasonic tests (throughthickness attenuation, pulse-echo and stress wave factor) and 'acoustic emission load-delay" results; the latter being shown to be capable of
distinguishing between the modes of failure occurring in the joints. Thus, these nondestructive evaluation techniques may be used to characterize adhesively-bonded FRP materials.
Prediction of material damping of laminated polymer matrix composites Sun, C.T., Wu, J.K. and Gibson, ILF. Journal of Materials Science Vol 22 No 3 (March 1987) pp 1006-1012 This analytical determination of damping in laminated composites uses a force-balance approach based on a classical lamination theory previously applied by the authors to the case of short fibre composites. Damping is expressed in terms of the in-plane, coupled and flexural damping from the IAI, IBI and ID1 stiffness matrices and the elastic/viscoelastic correspondence principle. Several graphite/ epoxy laminates with different stacking sequences are considered. Prediction of moisture diffusion parameters in composite materials under stress Neumann. S. and Marom G. Journal t~/' Composite Materials Vol 21 (January 1987) pp 68-80 A model is proposed which predicts the changes in the diffusion parameters rate, maximum absorption capacity, and the Fickean diffusion coefficient as a function of changes in stress level. Two factors are considered in the model: that stress-dependent free volume changes in the matrix affect the moisture absorption capacity of the matrix; and that the diffusion coefficients in both stressed and unstressed states are expressed by a Doolittle-type equation connecting the diffusion coefficient and the free volume. Experimental work is reported on moisture absorption in stressed unidirectional carbon fibre-reinforced epoxy laminates in which the fibre content and fibre orientation were varied. It was found that increasing the fibre angle increased the effect of external stress on the diffusion coefficient. This stress effect is increased when the fibre volume fraction is increased.
Properties of thin polypropylene films loaded with finely dispersed gold Hogarth, C.A. and Al-lsmail, S.A.Y. Journal of Materials Science Letters Vol 6 (1987) pp 409-412 Electrical measurements have been made on standard thin film sandwich samples and on planar structure samples of polypropylene containing finely dispersed gold particles (3 pm). The parameters estimated werc the frec electron concentration, density of trapping states, trapping coefficient, carrier mobility, activation energy (0.48 eV) for conduction and permittivity. The density of states in the conduction band was also estimated. The voltage at which the space charge limited current was observed was 150V. It is generally concluded that the addition of less than 1wt% of finely dispersed metal increases the permittivity by more than twice compared with evaporated films of polypropylene measured at room temperature. Planar samples showed a significant sensitivity to light.
Pull-out of steel fasteners from a laminated composite subjected to external pressure Parry. T.V., Wilson, J.M. and Leek, D.C.
COMPOSITES
. SEPTEMBER
1987