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Building tough composites
Composites containing 20 volume % SiC whiskers in a l u m i n i u m 6061 were extruded in cylindrical form; the fibres were generally aligned in the extrusion direction. Specimens were cut so as to have different fibre orientations and were subjected to various thermal treatments before testing in tension and in Charpy V-notch (CVN) tests. As-received and heat-treated composites showed superior modulus, yield stress and UTS over wrought Al 6061 alloy. However, the wrought alloy h a d CVN energy values of an order of magnitude greater than the composites. Kic of all asreceived and treated composites was half that of the alloy. The highest toughness value was obtained for the fibre orientation with fewest SiC whiskers in the crack plane, ie, with greatest mean free path between reinforcements.
The fabrication and properties of metal-matrix composites based on aluminium alloy infiltrated alumina fiber preforms Bader, M.G., Clyne, T.W., Cappleman, G.R. and Hubert, P.A. Composites Science and Technology Vo123 No 4 (1985) pp 287-301 It is shown that metal-matarix composites may be conveniently fabricated from &alumina preforms and several aluminium alloys by a squeeze-casting liquid metal infiltration process. The fibre reinforcement is found to enhance the stiffness of the aluminium alloy and this enhancement is retained at temperatures up to 300°C. High temperature tensile load-bearing capacity of unidirectional continuous-fibre composites: significance of a specimen-end effect Masaki, N., Wakashima, K. and Umekawa, S. Journal of Materials Science Vol 20 No 11 (November 1985) pp 4123--4129 A significant specimen-end effect is noted in continuous metal fibre-reinforced metal-matrix composites when determining high temperature tensile load-bearing properties; this is due to viscous sliding at the fibre/matrix interface resulting from differential strain between fibre and matrix at elevated temperature. A tungstenfibre/Cu composite is used as a model system in which is is found that the UTS measured in a tensile test and four-point bend test up to 1000°C is significantly smaller than the UTS calculated by the rule of mixtures. A new casting process for carbon (or SiC-based) fibre-nluminium matrix low cost composite materials Rocher, J.P., Quenisset, J.M. and Naslain, R. Journal of Materials Science Letters Vol 4 (1985) pp 1527-1529 The authors outline a new process for making complex a l u m i n i u m matrix composites reinforced with carbon-based fibres derived from low pressure, light alloy foundry techniques. The method is based on pretreatment of fibres with an aqueous solution of fluorides. Experiments on a 2 D carbon preform and preforms made from Nicalon SiC-based fibres with aluminium alloy show complete densification and good wetting of fibres. It is believed that fibre degradation remains low enough to preserve 70% of the fibre strength.
ORGANIC FIBRES The effects of filament diameter variability on the failure of Kevlnr 49-- epoxy strands Schwartz, P., Rosensaft, lVt and Wagner, I-I.D. Journal of Materials Science Letters Vol 4 (1985) pp 1409-1411
COMPOSITES. APRIL 1 9 8 6
The letter reports on the variability of failure stress of epoxy-impregnated Kevlar 49 strands taken from two different spools. The Weibull shape and scale parameters and WeibuU plots are also presented. The variability between strands was found to be the same for both spools and no differences in yarn strength were observed. Limited strain rate tests indicated a slight reduction in the coefficient of variation at slower strain rates and an increase in the Weibull shape parameter. The authors conclude that any effect of filament diameter variability is not apparent Gel drawn fibres of poly(vinyl alcohol) Cebe, E and Grubb, D. Journal of Materials Science Vol 20 No 12 (December 1985) pp 44654475 Gels of PVA were drawn at temperatures of 140-160°C up to draw ratios of 14. Gels were made from solutions in which the concentration of polymer varied from 2-15%, thus varying the entanglement density. Drawing increased crystallinity from 20-55% in the isotropic material to 55-80%. Superior mechanical properties were obtained from gels with low entanglement density;, Young's modulus increased with draw ratio and was independent of draw temperature or degree of crystallinity. High modulus/high strength poly(-p-phenylene benzobisthiazole) f i b r e s Part 2 structure property investigations Allen, S.R, Farris, R.J. and Thomas, E.L. Journal of Materials Science Vol 20 No 12 (December 1985) pp 4583--4593 This paper details the morphological changes occurring during heat treatment of PBT fibres (discussed in part 1) and investigates the relationship between fibre structural characteristics and mechanical properties. Wide- angle X-ray diffraction is used to examine enhancement of lateral molecular order in the fibres on heat treatment; tensile strength increased as order increased. However, this increase in strength and also modulus depended not only on heat treatment conditions but also on the application of tension during treatment which improved lateral orientation. Improved mechanical behaviour in ultra high modulus polyethylenes by controlled crosslinking Woods, D.W., Busfield, W.K. and Ward, I.M. Plastics and Rubber Processing and Applications Vol 5 (1985) pp 157-164 Electron irradiation has been used to produce marked improvements in the mechanical behaviour of ultra- high modulus polyethylene fibres. Results are presented for irradiation in vacuo and in acetylene and show substantial improvement in creep, fatigue and thermal performance of the fibres.
RESIN MATRICES Acoustic emission monitoring of damage progression in graphite-epoxy laminates Shippen, N.C. and Adams, D.F. Journal of Reinforced Plastics and Composites Vol 4 No 3 (July 1985) pp 242-261 Acoustic emission from five different laminates including cross plies and angle plies was monitored using a system which included count rate and amplitude distribution capabilities interfaced with a computer-based data acquisition system. X-ray photographs of damage progression are also presented. The results are
discussed and methods of eliminating extraneous noises are suggested.
Building tough composites Klein, A.J. Advanced Materials and Processes Vol l No 2 (1985) p 10 This paper outlines the range of matrix materials available for use in tough composite systems In semicrystalline thermoplastic resins, mechanical properties vary with morphology;, degree of crystallinity and spheroid size are sensitive to cooling rate and temperature gradients. Epoxy resins have improved G[c due to the addition of elastomers which decrease cross-link density. Other methods of improving toughness include bending two types of epoxy which cure independently (interpenetrating polymer network, IPN) leading to a m i n i m u m increase in viscosity, blending thermoplastic and thermoset resins (semi-IPN) and interleafing tough resin prepreg with discrete layers of very tough resin. Characterization of damage initiation and propagation in graphite/epoxy laminates by acoustic emission Garg, A. and Ishai, O. Engineering Fracture Mechanics Vol22 No 4 (1985) pp 595-608 Transverse cracking and Mode I delamination in multidirectional and unidirectional graphite/epoxy composites was investigated by acoustic emission (AE). The effects of moisture on AE were also investigated. It was found that in delamination AE response was continuous with damage growth but discontinuous for transverse cracking. The trend in transverse cracking was found to be significantly affected by moisture; in delamination moisture reduced the AE response per unit area of damage growth. Composites review:, characterization of the cure of carbon fibre/epoxy composite prepregs by differential scanning calorimetry Morgan, ILJ., Walkup, C.M. and Hiheisel, T.H. Journal of Composites Technology and Research Vol 7 No 1 (Spring 1985) The paper reports a systematic DSC study of the constituents of boron tetrafiuoride monoethylamine (BF3:NH2C2Hs)-catalysed T G D D M - D D S epoxies and their mixtures, the nature of the catalysed cure reactions and the environmental sensitivity of the catalyst. Results indicated that in the BF3:NH2C2H 5catalysed TGDDM-DDS/carbon fibre commercial prepregs the cure reactions occur by BF3:NH2C2Hs-catalysed, BF~ZNH3C2H5catalysed, and non-catalysed reactions Catalytic activity was found to vary considerably between prepreg lots and within a prepreg lot Moisture was found not to directly inhibit the BF3:NH2C2Hs-catalyst activity or modify the cure reactions Contribution to the micromechanical interpretation of fracture work of short-fibre-reinforced thermoplastics Lauke, B., Schultrich, B. and Barthel, R. Composites Science and Technology Vo123 No 1 (1985) pp 21-35 A model for the work of fracture, which takes into account debonding, sliding at the interface and local plastic deformation of matrix bridges, is proposed. Starting from a micromechanical analysis of the debonding and sliding length, fracture energies are calculated. It is shown that, depending on the relative contributions to the total energy, which itself depends upon the loading rate, the composite fracture energy
165
The fabrication and properties of metal-matrix composites based on aluminium alloy infiltrated alumina fiber preforms Bader, M.G., Clyne, T.W., Cappleman, G.R. and Hubert, P.A. Composites Science and Technology Vo123 No 4 (1985) pp 287-301 It is shown that metal-matarix composites may be conveniently fabricated from &alumina preforms and several aluminium alloys by a squeeze-casting liquid metal infiltration process. The fibre reinforcement is found to enhance the stiffness of the aluminium alloy and this enhancement is retained at temperatures up to 300°C. High temperature tensile load-bearing capacity of unidirectional continuous-fibre composites: significance of a specimen-end effect Masaki, N., Wakashima, K. and Umekawa, S. Journal of Materials Science Vol 20 No 11 (November 1985) pp 4123--4129 A significant specimen-end effect is noted in continuous metal fibre-reinforced metal-matrix composites when determining high temperature tensile load-bearing properties; this is due to viscous sliding at the fibre/matrix interface resulting from differential strain between fibre and matrix at elevated temperature. A tungstenfibre/Cu composite is used as a model system in which is is found that the UTS measured in a tensile test and four-point bend test up to 1000°C is significantly smaller than the UTS calculated by the rule of mixtures. A new casting process for carbon (or SiC-based) fibre-nluminium matrix low cost composite materials Rocher, J.P., Quenisset, J.M. and Naslain, R. Journal of Materials Science Letters Vol 4 (1985) pp 1527-1529 The authors outline a new process for making complex a l u m i n i u m matrix composites reinforced with carbon-based fibres derived from low pressure, light alloy foundry techniques. The method is based on pretreatment of fibres with an aqueous solution of fluorides. Experiments on a 2 D carbon preform and preforms made from Nicalon SiC-based fibres with aluminium alloy show complete densification and good wetting of fibres. It is believed that fibre degradation remains low enough to preserve 70% of the fibre strength.
ORGANIC FIBRES The effects of filament diameter variability on the failure of Kevlnr 49-- epoxy strands Schwartz, P., Rosensaft, lVt and Wagner, I-I.D. Journal of Materials Science Letters Vol 4 (1985) pp 1409-1411
COMPOSITES. APRIL 1 9 8 6
The letter reports on the variability of failure stress of epoxy-impregnated Kevlar 49 strands taken from two different spools. The Weibull shape and scale parameters and WeibuU plots are also presented. The variability between strands was found to be the same for both spools and no differences in yarn strength were observed. Limited strain rate tests indicated a slight reduction in the coefficient of variation at slower strain rates and an increase in the Weibull shape parameter. The authors conclude that any effect of filament diameter variability is not apparent Gel drawn fibres of poly(vinyl alcohol) Cebe, E and Grubb, D. Journal of Materials Science Vol 20 No 12 (December 1985) pp 44654475 Gels of PVA were drawn at temperatures of 140-160°C up to draw ratios of 14. Gels were made from solutions in which the concentration of polymer varied from 2-15%, thus varying the entanglement density. Drawing increased crystallinity from 20-55% in the isotropic material to 55-80%. Superior mechanical properties were obtained from gels with low entanglement density;, Young's modulus increased with draw ratio and was independent of draw temperature or degree of crystallinity. High modulus/high strength poly(-p-phenylene benzobisthiazole) f i b r e s Part 2 structure property investigations Allen, S.R, Farris, R.J. and Thomas, E.L. Journal of Materials Science Vol 20 No 12 (December 1985) pp 4583--4593 This paper details the morphological changes occurring during heat treatment of PBT fibres (discussed in part 1) and investigates the relationship between fibre structural characteristics and mechanical properties. Wide- angle X-ray diffraction is used to examine enhancement of lateral molecular order in the fibres on heat treatment; tensile strength increased as order increased. However, this increase in strength and also modulus depended not only on heat treatment conditions but also on the application of tension during treatment which improved lateral orientation. Improved mechanical behaviour in ultra high modulus polyethylenes by controlled crosslinking Woods, D.W., Busfield, W.K. and Ward, I.M. Plastics and Rubber Processing and Applications Vol 5 (1985) pp 157-164 Electron irradiation has been used to produce marked improvements in the mechanical behaviour of ultra- high modulus polyethylene fibres. Results are presented for irradiation in vacuo and in acetylene and show substantial improvement in creep, fatigue and thermal performance of the fibres.
RESIN MATRICES Acoustic emission monitoring of damage progression in graphite-epoxy laminates Shippen, N.C. and Adams, D.F. Journal of Reinforced Plastics and Composites Vol 4 No 3 (July 1985) pp 242-261 Acoustic emission from five different laminates including cross plies and angle plies was monitored using a system which included count rate and amplitude distribution capabilities interfaced with a computer-based data acquisition system. X-ray photographs of damage progression are also presented. The results are
discussed and methods of eliminating extraneous noises are suggested.
Building tough composites Klein, A.J. Advanced Materials and Processes Vol l No 2 (1985) p 10 This paper outlines the range of matrix materials available for use in tough composite systems In semicrystalline thermoplastic resins, mechanical properties vary with morphology;, degree of crystallinity and spheroid size are sensitive to cooling rate and temperature gradients. Epoxy resins have improved G[c due to the addition of elastomers which decrease cross-link density. Other methods of improving toughness include bending two types of epoxy which cure independently (interpenetrating polymer network, IPN) leading to a m i n i m u m increase in viscosity, blending thermoplastic and thermoset resins (semi-IPN) and interleafing tough resin prepreg with discrete layers of very tough resin. Characterization of damage initiation and propagation in graphite/epoxy laminates by acoustic emission Garg, A. and Ishai, O. Engineering Fracture Mechanics Vol22 No 4 (1985) pp 595-608 Transverse cracking and Mode I delamination in multidirectional and unidirectional graphite/epoxy composites was investigated by acoustic emission (AE). The effects of moisture on AE were also investigated. It was found that in delamination AE response was continuous with damage growth but discontinuous for transverse cracking. The trend in transverse cracking was found to be significantly affected by moisture; in delamination moisture reduced the AE response per unit area of damage growth. Composites review:, characterization of the cure of carbon fibre/epoxy composite prepregs by differential scanning calorimetry Morgan, ILJ., Walkup, C.M. and Hiheisel, T.H. Journal of Composites Technology and Research Vol 7 No 1 (Spring 1985) The paper reports a systematic DSC study of the constituents of boron tetrafiuoride monoethylamine (BF3:NH2C2Hs)-catalysed T G D D M - D D S epoxies and their mixtures, the nature of the catalysed cure reactions and the environmental sensitivity of the catalyst. Results indicated that in the BF3:NH2C2H 5catalysed TGDDM-DDS/carbon fibre commercial prepregs the cure reactions occur by BF3:NH2C2Hs-catalysed, BF~ZNH3C2H5catalysed, and non-catalysed reactions Catalytic activity was found to vary considerably between prepreg lots and within a prepreg lot Moisture was found not to directly inhibit the BF3:NH2C2Hs-catalyst activity or modify the cure reactions Contribution to the micromechanical interpretation of fracture work of short-fibre-reinforced thermoplastics Lauke, B., Schultrich, B. and Barthel, R. Composites Science and Technology Vo123 No 1 (1985) pp 21-35 A model for the work of fracture, which takes into account debonding, sliding at the interface and local plastic deformation of matrix bridges, is proposed. Starting from a micromechanical analysis of the debonding and sliding length, fracture energies are calculated. It is shown that, depending on the relative contributions to the total energy, which itself depends upon the loading rate, the composite fracture energy
165