- Email: [email protected]
Composite PTCR thermistors utilizing conducting borides, silicides, and carbide powders
degrees of freedom than the finite element method to obtain the same accuracy and compares well with the experimental results. The effect of various parameters (material, number of layers, fibre orientation and curvature) on the frequencies is reported. Variational analysis of interfacial stress intensity factors for finite bimaterial plates under biaxial load Wang, W.-C. and Chen, J.-T. Composite Structures Vol 17 No 2 (1991) pp 157-179 The use of the eigenfunction expansion variational method to calculate interfacial stress intensity factors for finite bimaterial plates with a crack situated along the interface under biaxial load is reported. The effects of biaxial load factors, material properties and higher order terms of the complex series on interracial stress intensity factors are discussed. The isochromatic fringe patterns were also analytically constructed to visualize the whole-field stress distributions.
METAL MA TRICES Dislocation mechanisms for the relaxation of thermal stress at metal---ceramic interfaces Shieu, F.-S. and Sass, S.L. Acta Metallurgica et. Materialia Vo139 No 4 (1991) pp 539-547 The dislocation structure in NiO/Pt interfaces that survived large temperature changes was studied in this paper. This was in order to understand how a metal-ceramic couple accommodates thermal stresses exceeding the yield stress of the metal and the fracture stress of the ceramic. A mechanism of dislocation movement based on slip and glide was suggested to produce the change in spacing observed and relieve the thermal strain. This was consistent with experimental observations. Failure by ductile cavity growth at a metalceramic interface Tvergaard, V. Acta Metallurgica et. Materialia Vo139 No 3 (1991) pp 419-426 A metal layer bonding two ceramic surfaces was analysed numerically under tensile loading normal to the layer. The elasticplastic deformation in the metal layer was found to correlate with a uniaxial strain state with the associated development of a high level of triaxial tension. This was apart from small regions near free edges of the specimen. The uniaxial strain state was used in a cell model analysis to study ductile failure in the metal layer by the growth of cavities at initially unbonded points at the interface. Small isolated cavities were discussed with reference to studies of cavitation instability in elastic-plastic solids subject to highly triaxial stress states. The predictions from the analyses were also compared with appropriate experimental results. Fracture characteristics of AI/Zircon particulate composites Pillai, U.T.S. and Pandey, R.K. Composites Science and Technology Vol 40 No 4 (1991) pp 333-354 The effects of volume fraction and size of Zircon particulates on the fracture toughness and micromechanisms of fracture in AI/Zircon particulate composites has been studied. The composites are prepared by a liquid metallurgy technique using volume
330
COMPOSITES. JULY 1991
fractions of Zircon in the range 0.06-0.18 and particle sizes between 75 and 250 p.m. The study was conducted on composites in the cast and forged conditions. These AI/Zircon composites are compared with Al/graphite composites in terms of strength and fracture toughness as a function of volume fraction of filler, and regions of optimum performance are identified. On plastic relaxation of thermal stresses in reinforced metals Dunand, D.C. and Mortensen, A. Acta Metallurgica et. Materialia Vol 39 No 2 (1991) pp 127-139 A model system of silver chloride containing alumina fibres or glass microspheres was used to study matrix plasticity induced by thermal mismatch in metal-matrix composites. Matrix dislocations were rendered opaque by exposure to stroboscopic light, enabling their observation using transmitted light microscopy. Plastic deformation around the inclusions was found to take the form of rows of prismatic dislocation loops punched into the matrix around the particles, and/or a plastic zone containing tangled dislocations. From the number of loops punched by spheres, the temperature interval over which slip of prismatic loops was operative was calculated to be 100+30 K wide. From calculation of the stress in the plastic zone around the fibres it was determined that the matrix was undergoing local strain hardening. A model was proposed based on these findings to predict the radius of the plastic zone around embedded cylinders and spheres. This was compared with the experimental data. Overview No. 91 Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices - - 1. Optimisation of barrier structure Kieschke, R.R., Somekh, R.E. and Clyne, T.W. Acta Metallurgica et. Materialia Vo139 No 4 (1991) pp 427-435 The use of thin surface coatings on fibrous reinforcement to act as a diffusion barrier, specifically the effect of deposition conditions on barrier structure, was discussed in this paper. Other aspects of this technique were considered in two further papers contained in the same edition: System stress state, pp 437443; and Microstructural stability in composites based on magnesium and titanium, pp 445-452. Role of interfaces in creep of fibre-reinforced metal-matrix composites - - I. Continuous fibres Goto, S. and McLean, M. Acta Metallurgica et. Materialia I/o139 No 2 (1991) pp 153-164 A model was proposed that considered the effect on composite creep deformation of the interface between the fibre and matrix, with its properties being distinct from those of the matrix and fibre phases. Continuous fibres were considered in this paper with short fibres discussed in part II of the paper, pp 165-177, within the same edition. The fibres were assumed to deform only elastically with the matrix subject to power law creep. Important microstructurai and property characteristics of the constituent phases that can significantly influence creep were identified within the model. Predictions of the model were found to be consistent with the limited experimental data available.
RESIN/VIA TRICES Characterisation of the vibration damping loss factor of glass and graphite fibre composites Crane, R. and Gillespie Jr., J.W. Composites Science and Technology Vol 40 No 4 (1991) pp 355-375 In this paper the authors study the material damping of laminated composites using the half power bandwidth method for a cantilever beam specimen excited with an impulse excitation. To investigate the effect of fibre and matrix properties on the damping of composites, unidirectional, continuous fibre, 0° and 90° laminates were fabricated from glass/ epoxy, graphite/epoxy and graphite/PEEK. The loss factor is measured as a function of frequency. Composite PTCR thermistors utilizing conducting borides, silieides, and carbide powders Shrout, T.R., Moffatt, D. and Huebner, W. Journal o f Materials Science Vol 26 No 1 (1991) pp 145-154 Composite samples were fabricated from MoSi2, NbB 2, NbSi2, TiB 2, TiC, WbSi2 and ZrB 2, all conducting ceramics, and matrices of semicrystalline and amorphous polymers. All the composites exhibited percolation behaviour and the positive temperature coefficient effect, except NbB 2 and NbSi2 in the amorphous polymer. The surface area, size and degree of agglomeration of the powders were characterized and were found to have a marked effect on the resulting properties. Microstructures were characterized by scanning electron microscopy and the effect of ceramic-polymer surface chemistry was noted. The wettability and mixing capability particularly affected the homogeneity and so the final properties of the composite. A failure criterion for composite materials Feng, W.W. Journal o f Composite Materials Vo125 No I (1991) pp 88-100 The strain invariants of finite elasticity were used as the basis for a failure criterion for composite materials. It was assumed that the material was transversely isotropic and that failure occurred when the strain energy density reaches its maximum value. The fibre and matrix failure modes were assumed to be decoupled, simplifying the criterion. A special case was also introduced, the failure criterion for infinitesimal-strain elasticity theory. The failure surfaces in the strain invariants' space were determined for unidirectional IM7/8551-7 graphite/epoxy and Kevlar 49/epoxy composites. Formation and growth of 90 ° ply fatigue cracks in carbon/epoxy laminates Lafarie-Frenot, M.C. and Henaff-Gardin, C. Composites Science and Technology Vol 40 No 3 (1991) pp 307-324 This study concentrates on the fatigue mechanisms of matrix cracking in carbon/epoxy cross-ply laminates. Tension-tension fatigue tests were conducted on two equivalent layups, differing only in the stacking sequence and in the resulting 90° ply thickness. Transverse ply crack onset, accumulation and growth were measured. The 90° ply thickness is shown to have a considerable influence on the crack distribution in the transverse ply
METAL MA TRICES Dislocation mechanisms for the relaxation of thermal stress at metal---ceramic interfaces Shieu, F.-S. and Sass, S.L. Acta Metallurgica et. Materialia Vo139 No 4 (1991) pp 539-547 The dislocation structure in NiO/Pt interfaces that survived large temperature changes was studied in this paper. This was in order to understand how a metal-ceramic couple accommodates thermal stresses exceeding the yield stress of the metal and the fracture stress of the ceramic. A mechanism of dislocation movement based on slip and glide was suggested to produce the change in spacing observed and relieve the thermal strain. This was consistent with experimental observations. Failure by ductile cavity growth at a metalceramic interface Tvergaard, V. Acta Metallurgica et. Materialia Vo139 No 3 (1991) pp 419-426 A metal layer bonding two ceramic surfaces was analysed numerically under tensile loading normal to the layer. The elasticplastic deformation in the metal layer was found to correlate with a uniaxial strain state with the associated development of a high level of triaxial tension. This was apart from small regions near free edges of the specimen. The uniaxial strain state was used in a cell model analysis to study ductile failure in the metal layer by the growth of cavities at initially unbonded points at the interface. Small isolated cavities were discussed with reference to studies of cavitation instability in elastic-plastic solids subject to highly triaxial stress states. The predictions from the analyses were also compared with appropriate experimental results. Fracture characteristics of AI/Zircon particulate composites Pillai, U.T.S. and Pandey, R.K. Composites Science and Technology Vol 40 No 4 (1991) pp 333-354 The effects of volume fraction and size of Zircon particulates on the fracture toughness and micromechanisms of fracture in AI/Zircon particulate composites has been studied. The composites are prepared by a liquid metallurgy technique using volume
330
COMPOSITES. JULY 1991
fractions of Zircon in the range 0.06-0.18 and particle sizes between 75 and 250 p.m. The study was conducted on composites in the cast and forged conditions. These AI/Zircon composites are compared with Al/graphite composites in terms of strength and fracture toughness as a function of volume fraction of filler, and regions of optimum performance are identified. On plastic relaxation of thermal stresses in reinforced metals Dunand, D.C. and Mortensen, A. Acta Metallurgica et. Materialia Vol 39 No 2 (1991) pp 127-139 A model system of silver chloride containing alumina fibres or glass microspheres was used to study matrix plasticity induced by thermal mismatch in metal-matrix composites. Matrix dislocations were rendered opaque by exposure to stroboscopic light, enabling their observation using transmitted light microscopy. Plastic deformation around the inclusions was found to take the form of rows of prismatic dislocation loops punched into the matrix around the particles, and/or a plastic zone containing tangled dislocations. From the number of loops punched by spheres, the temperature interval over which slip of prismatic loops was operative was calculated to be 100+30 K wide. From calculation of the stress in the plastic zone around the fibres it was determined that the matrix was undergoing local strain hardening. A model was proposed based on these findings to predict the radius of the plastic zone around embedded cylinders and spheres. This was compared with the experimental data. Overview No. 91 Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices - - 1. Optimisation of barrier structure Kieschke, R.R., Somekh, R.E. and Clyne, T.W. Acta Metallurgica et. Materialia Vo139 No 4 (1991) pp 427-435 The use of thin surface coatings on fibrous reinforcement to act as a diffusion barrier, specifically the effect of deposition conditions on barrier structure, was discussed in this paper. Other aspects of this technique were considered in two further papers contained in the same edition: System stress state, pp 437443; and Microstructural stability in composites based on magnesium and titanium, pp 445-452. Role of interfaces in creep of fibre-reinforced metal-matrix composites - - I. Continuous fibres Goto, S. and McLean, M. Acta Metallurgica et. Materialia I/o139 No 2 (1991) pp 153-164 A model was proposed that considered the effect on composite creep deformation of the interface between the fibre and matrix, with its properties being distinct from those of the matrix and fibre phases. Continuous fibres were considered in this paper with short fibres discussed in part II of the paper, pp 165-177, within the same edition. The fibres were assumed to deform only elastically with the matrix subject to power law creep. Important microstructurai and property characteristics of the constituent phases that can significantly influence creep were identified within the model. Predictions of the model were found to be consistent with the limited experimental data available.
RESIN/VIA TRICES Characterisation of the vibration damping loss factor of glass and graphite fibre composites Crane, R. and Gillespie Jr., J.W. Composites Science and Technology Vol 40 No 4 (1991) pp 355-375 In this paper the authors study the material damping of laminated composites using the half power bandwidth method for a cantilever beam specimen excited with an impulse excitation. To investigate the effect of fibre and matrix properties on the damping of composites, unidirectional, continuous fibre, 0° and 90° laminates were fabricated from glass/ epoxy, graphite/epoxy and graphite/PEEK. The loss factor is measured as a function of frequency. Composite PTCR thermistors utilizing conducting borides, silieides, and carbide powders Shrout, T.R., Moffatt, D. and Huebner, W. Journal o f Materials Science Vol 26 No 1 (1991) pp 145-154 Composite samples were fabricated from MoSi2, NbB 2, NbSi2, TiB 2, TiC, WbSi2 and ZrB 2, all conducting ceramics, and matrices of semicrystalline and amorphous polymers. All the composites exhibited percolation behaviour and the positive temperature coefficient effect, except NbB 2 and NbSi2 in the amorphous polymer. The surface area, size and degree of agglomeration of the powders were characterized and were found to have a marked effect on the resulting properties. Microstructures were characterized by scanning electron microscopy and the effect of ceramic-polymer surface chemistry was noted. The wettability and mixing capability particularly affected the homogeneity and so the final properties of the composite. A failure criterion for composite materials Feng, W.W. Journal o f Composite Materials Vo125 No I (1991) pp 88-100 The strain invariants of finite elasticity were used as the basis for a failure criterion for composite materials. It was assumed that the material was transversely isotropic and that failure occurred when the strain energy density reaches its maximum value. The fibre and matrix failure modes were assumed to be decoupled, simplifying the criterion. A special case was also introduced, the failure criterion for infinitesimal-strain elasticity theory. The failure surfaces in the strain invariants' space were determined for unidirectional IM7/8551-7 graphite/epoxy and Kevlar 49/epoxy composites. Formation and growth of 90 ° ply fatigue cracks in carbon/epoxy laminates Lafarie-Frenot, M.C. and Henaff-Gardin, C. Composites Science and Technology Vol 40 No 3 (1991) pp 307-324 This study concentrates on the fatigue mechanisms of matrix cracking in carbon/epoxy cross-ply laminates. Tension-tension fatigue tests were conducted on two equivalent layups, differing only in the stacking sequence and in the resulting 90° ply thickness. Transverse ply crack onset, accumulation and growth were measured. The 90° ply thickness is shown to have a considerable influence on the crack distribution in the transverse ply