- Email: [email protected]
Behaviour of tungsten-silver composite under cyclic loading
Materials testing: helping set design guide lines Bader, M. G. Design Engineering, pp 59 - 61 {February 1971) An introductory article to a series, this paper discusses theoretical and practical aspects of materials testing and interpretation of data. A definition of the material elastic constants is given and typical values for c o m m o n materials are tabulated. This is followed by brief discussions of metal plasticity data and polymer testing. (7111/39) The strength of carbon fibres Cooper, G. A. and Mayer, R. M. Journal of Materials Science, Vol 6, No 1, pp 60 - 67 {January 1971) The authors consider PAN-based carbon fibres and discuss the relationship between fibre microstructure and strength and modulus properties. It appears that the relationship is qualitatively understood in the modulus case but influences on the strength are less well understood. An alternative failure mechanism, involving the shearing of graphite planes in the fibre crystallites, is examined and could account for failures in the more graphitic carbon fibres. It is suggested that increases in strength of fibres failing by this mode could be achieved by a reduction in flaws and b y application of the principles of solid-solution or dispersion hardening. Some results from irradiated and boron-doped fibres are presented as examples. (71/1/40) Strength properties of discontinuous fiber composites Chen, P. E. Polymer Engineering and Science, Vol 11, No 1, p 51 (January 1971) The finite element method is used to calculate the longitudinal strengths of unidirectional discontinuous fibre composites and the results are compared with experimental data from composites of tungsten-copper, boronaluminium, glass-epoxy and boronepoxy. A modification of the JacksonCratchley equations is proposed for the calculation of the strengths of randomly oriented discontinuous fibre composites. (71 / 1/41 ) Tensile properties of graphite fibers from several acrylic yarns Ezekiel, H. M. Fibre Science and Technology, Vol 3, No 3, p 243 (1971) Graphite fibres of high tensile strength and modulus have been made from acrylic yarns of varying composition and orientation which is contrary to the earlier belief that yarns of high tenacity and orientation were required as precursor material. The moduli of fibres from one yarn are reported to vary widely with graphitization temperature and yarn tension. (71/1/42)
The tensile properties of nickel-coated carbon fibres Perry, A. J., de Lamotte, E. and Phillips, K. Journal of Materials Science, Vol 5, No 11, pp 945-950 [November 1970) The tensile properties of nickel-coated Grafil-HT carbon fibres were studied as a function of coating thickness, taking into account the diameterdependence of the uncoated fibre properties. The stress-strain curve exhibited three stages: (1) fibre and coating elastic; (2) coating yields, fibre elastic, coating plastic; (3) fracture of fibre plus coating. The stress/strain behaviour is explicable by the law-ofmixtures and coating grain size. (71/1/43) Thermal expansion behaviour of twophase solids Fahmy, A. A. and Ragai, A. N. Journal o f Applied Physics, Vol 41, No 13, pp 5108-5111 {December 1970) The thermal expansion behaviour of lead/fused silica and aluminium/ silicon, has been investigated experimentally above and below room temperature. The coefficients always fall below those predicted by a simple rule of mixtures. A model is proposed which accounts for this behaviour. Composites which are rich in inclusions show certain tendencies that could be explained in terms of microplastic yielding. (71 / 1/44) Thermal expansion of composite materials Rummala, R. R. and Friedburg, A. L. Journal o f Applied Physics, Vol 41, No 13, pp 5 1 0 4 - 5 1 0 7 (December 1970) An equation is presented for predicting the thermal expansion coefficient of dilute dispersions of elastic spheres in a matrix. The application o f this and other existing equations to ceramic-glass systems etc is discussed. (71/1/45) Time dependent static strength and refiability for composites Halpin, J. C., K o p f and Goldberg, W. Journal of Composite Materials, 1Iol 4, pp 4 6 2 - 4 7 4 {October 1970) A study of the fracture properties of laminated composites under uniaxial tension revealed that the distribution of breaking strength under constant or variable stress rate can be defined by the Weibull distribution. The shape parameter of the distribution is not affected significantly by loading conditions, temperatures and strength degradations. Having established a relationship between the shape parameter and the breaking-strength/lifetime curve, it was possible to develop a reliability plan. A p r o o f test experiment was performed which enabled
weak members to be removed from the population. It is concluded that composite materials can be designed to yield structures with a higher reliability per unit weight than the equivalent metal structure. (71 / 1/46)
Work-hardening theory of fibre-reinforced composites Neumann, P. and Haasen, P. Philosophical Magazine, Vol 23, No 182, p 285 (I 9 71) The flow stress of the matrix of a composite reinforced by elastic fibres is calculated by means of a model in which dislocations pile upon slip planes at 45 ° to the fibre direction. The work hardening rates as functions of fibre diameter and volume fraction of fibre agree well with the experimental results of Kelly and Lilholt with the interplaner distance of the slip planes as the adjustable variable in the model. (71/1/47) CERAMIC MA TRICES Silicon nitride ceramic composites with high toughness Lindley, M. W. and Godfrey, D. J. Nature, Vol 229, No 5281, pp 192 - 1 9 3 {January 1971) Silicon carbide/silicon nitride composites have been fabricated with a range of densities by flame spraying silicon powder and filament winding the silicon carbide fibres, followed by nitriding. Low density composites can also be prepared by slip casting and vibratory compaction. The toughness (absence of catastrophic brittle fracture) can be increased to a level comparable with cast iron by lowering the bond strength between fibre and matrix. This leads to a lower failure strength but is still considerably better than unreinforced silicon nitride. (71/2/1) METAL MA TRICES Behaviour of tungsten-silver composite under cyclic loading Bhattacharyya, S. and Parikh, N. M. Fibre Science and Technology, Vol 3, No 3, p 209 {January 1971) Composites of tungsten filaments in a silver matrix were made by either plating, cold pressing or hot pressing followed by sintering, and their behaviour under cyclic tensile loading conditions examined. Under these conditions the secondary proportional limit is increased whereas the primary is hardly affected and both of the elastic moduli decrease slowly during testing. The latter effect is attributed to the formation of voids during stress cycling. (71/3/13)
COMPOSITESJune 1971 ] 25
The tensile properties of nickel-coated carbon fibres Perry, A. J., de Lamotte, E. and Phillips, K. Journal of Materials Science, Vol 5, No 11, pp 945-950 [November 1970) The tensile properties of nickel-coated Grafil-HT carbon fibres were studied as a function of coating thickness, taking into account the diameterdependence of the uncoated fibre properties. The stress-strain curve exhibited three stages: (1) fibre and coating elastic; (2) coating yields, fibre elastic, coating plastic; (3) fracture of fibre plus coating. The stress/strain behaviour is explicable by the law-ofmixtures and coating grain size. (71/1/43) Thermal expansion behaviour of twophase solids Fahmy, A. A. and Ragai, A. N. Journal o f Applied Physics, Vol 41, No 13, pp 5108-5111 {December 1970) The thermal expansion behaviour of lead/fused silica and aluminium/ silicon, has been investigated experimentally above and below room temperature. The coefficients always fall below those predicted by a simple rule of mixtures. A model is proposed which accounts for this behaviour. Composites which are rich in inclusions show certain tendencies that could be explained in terms of microplastic yielding. (71 / 1/44) Thermal expansion of composite materials Rummala, R. R. and Friedburg, A. L. Journal o f Applied Physics, Vol 41, No 13, pp 5 1 0 4 - 5 1 0 7 (December 1970) An equation is presented for predicting the thermal expansion coefficient of dilute dispersions of elastic spheres in a matrix. The application o f this and other existing equations to ceramic-glass systems etc is discussed. (71/1/45) Time dependent static strength and refiability for composites Halpin, J. C., K o p f and Goldberg, W. Journal of Composite Materials, 1Iol 4, pp 4 6 2 - 4 7 4 {October 1970) A study of the fracture properties of laminated composites under uniaxial tension revealed that the distribution of breaking strength under constant or variable stress rate can be defined by the Weibull distribution. The shape parameter of the distribution is not affected significantly by loading conditions, temperatures and strength degradations. Having established a relationship between the shape parameter and the breaking-strength/lifetime curve, it was possible to develop a reliability plan. A p r o o f test experiment was performed which enabled
weak members to be removed from the population. It is concluded that composite materials can be designed to yield structures with a higher reliability per unit weight than the equivalent metal structure. (71 / 1/46)
Work-hardening theory of fibre-reinforced composites Neumann, P. and Haasen, P. Philosophical Magazine, Vol 23, No 182, p 285 (I 9 71) The flow stress of the matrix of a composite reinforced by elastic fibres is calculated by means of a model in which dislocations pile upon slip planes at 45 ° to the fibre direction. The work hardening rates as functions of fibre diameter and volume fraction of fibre agree well with the experimental results of Kelly and Lilholt with the interplaner distance of the slip planes as the adjustable variable in the model. (71/1/47) CERAMIC MA TRICES Silicon nitride ceramic composites with high toughness Lindley, M. W. and Godfrey, D. J. Nature, Vol 229, No 5281, pp 192 - 1 9 3 {January 1971) Silicon carbide/silicon nitride composites have been fabricated with a range of densities by flame spraying silicon powder and filament winding the silicon carbide fibres, followed by nitriding. Low density composites can also be prepared by slip casting and vibratory compaction. The toughness (absence of catastrophic brittle fracture) can be increased to a level comparable with cast iron by lowering the bond strength between fibre and matrix. This leads to a lower failure strength but is still considerably better than unreinforced silicon nitride. (71/2/1) METAL MA TRICES Behaviour of tungsten-silver composite under cyclic loading Bhattacharyya, S. and Parikh, N. M. Fibre Science and Technology, Vol 3, No 3, p 209 {January 1971) Composites of tungsten filaments in a silver matrix were made by either plating, cold pressing or hot pressing followed by sintering, and their behaviour under cyclic tensile loading conditions examined. Under these conditions the secondary proportional limit is increased whereas the primary is hardly affected and both of the elastic moduli decrease slowly during testing. The latter effect is attributed to the formation of voids during stress cycling. (71/3/13)
COMPOSITESJune 1971 ] 25