L. Monette

Effect of modulus and cohesive energy on critical fibre length in fibre-reinforced composites

The effect of fibre modulus and cohesive energy on critical fibre length and radius in ceramic-fibre-reinforced brittle composites has been investigated employing both analytical theory and computer simulation. The theory consists of a shear-lag analysis in which an energy failure criterion is incorporated. The simulation consists of a two-dimensional computer model based upon a discrete network of grid points. Failure is also defined in terms of an energy criterion, where the energy is calculated on the basis of a two- and three-body interaction between the grid points. Both theory and simulation show that a minimum critical aspect ratio is found as a function of the elastic moduli ratio, Ef/Em, with a divergence occurring at both low- and high-modulus values. As the modulus ratio is increased, there is a transition in failure mechanism from tensile-dominated failure in the matrix to shear-dominated failure at the fibre-matrix interface. In addition, families of critical aspect ratio curves are obtained as a function of the cohesive energy ratio, Uf/Um. Larger cohesive energy ratios shift the critical aspect ratio curve to larger values. These features potentially explain trends in the experimental results reported by Asloun et al., where the critical fibre aspect ratio was measured for fibre/matrix systems having different modulus and toughness ratios.

THE YOUNG'S MODULUS OF SILICA BEADS/EPOXY COMPOSITES : EXPERIMENTS AND SIMULATIONS

The effect of particulate volume fraction vp and diameter dp on the composite Young’s modulus Ec is studied both experimentally, using a silica bead/epoxy system, as well as with the help of computer simulations. The experimental and simulation results show that for a given particulate size, the overall Ec vs vp curve displays a concave upward shape and not a linear shape. This superlinear trend of the data implies that the average strain normalized to the applied strain λ=ep/ec transferred to the particulates increases with volume fraction. The above finding is explained in terms of a mean‐field picture, where a single particle interacts with an effective medium consisting of the remaining particles embedded in the matrix. As the modulus of the effective medium surrounding a reference particle increases with vp, the modulus mismatch between the reference particulate and the medium is consequently reduced. This leads to an overall increase in the normalized average strain λ transferred to each particulat...

Effect of interphase modulus and cohesive energy on the critical aspect ratio in short-fibre composites

The effect of interphase modulus and cohesive energy on critical fibre length in short-fibre reinforced brittle composites has been investigated employing computer simulation. The simulation consists of a two-dimensional computer model based upon a discrete network of grid points. Failure is defined in terms of an energy criterion, where the energy is calculated on the basis of a two- and three-body interaction between the grid points. Simulation results show that for a whisker-type fibre, a thick interphase (i.e. Ai>Af where A represents the cross sectional area) with an elastic modulus less than that of the matrix in combination with an increased interphase toughness greatly reduce the critical aspect ratio, for both metal-matrix and ceramic-reinforced brittle polymer composites. The results also show a variation in the failure mode from tensile failure in the matrix to tensile and shear failure in the interphase as a function of the fibre-interphase modulus ratio. In particular, a significant increase in the load transfer efficiency in metal-matrix composites is found, for an interphase modulus Ei less than the matrix modulus Em. Better load transfer properties in metal-matrix composites cause the yield point to occur at higher values of applied strain, and hence may significantly increase the toughness (area under the stress-strain curve) for certain metal-matrix composites. The computer results are compared with the predictions of Coxs shear-lag theory as well as with a new theoretical development presented in this work. The new theory is found to provide a better description of the fibre and matrix stress distribution.

Effect of volume fraction and morphology of reinforcing phases in composites

Computer modeling has been employed to study the effect of volume fraction and morphology of second‐phase constituents on composite stiffness and strength. It is found that the efficiency of load transfer to the second‐phase constituent increases with volume fraction vf for particulate composites. For aligned short‐fiber composites, the efficiency of load transfer reaches a limiting value with increasing volume fraction for homogeneous fiber dispersions, while for fiber distributions which allow for fiber‐rich and matrix‐rich regions, the efficiency of load transfer decreases. The saturation or decrease in load transfer efficiency is due to fiber confinement, by which the interfiber matrix material is constrained by the presence of neighboring fibers. Hence, the amount of shear tractions and load transferred to a given fiber is altered by the local fiber distribution, as compared to the case of an isolated fiber (dilute limit). The strength of brittle particulate composites is reduced for most particulate...