Papers by Author: Jacques Lamon

Abstract: Previous works have shown that ceramic matrix composites are sensitive to delayed failure during fatigue in oxidizing environments. The phenomenon of slow crack growth has been deeply investigated on single fibers and multifilament tows in previous papers. The present paper proposes a multiscale model of failure driven by slow crack growth in fibers, for 2D woven composites under a constant load. The model is based on the delayed failure of longitudinal tows. Additional phenomena involved in the failure of tows have been identified using fractographic examination of 2D woven SiC/SiC composite testspecimens after fatigue tests at high temperatures. Stochastic features including random load sharing, fiber overloading, fiber characteristics and fiber arrangement within the tows have been introduced using appropriate density functions. Rupture time predictions are compared to experimental data.

Abstract: The purpose of the study was to increase the thermal conductivity of multilayered and self-sealing ceramic matrix composites via the silicon melt infiltration process. The first step of the process consisted in filling porosity using various organic xerogels by the sol-gel route. Carbon xerogels obtained by subsequent pyrolysis may reduce and homogenize the porous network within the composite. Cracking of the xerogels due to volumic shrinkage occurring during air drying may be decreased by controlling the initial parameters as concerns the gel solution and/or by operating a second impregnation/pyrolysis step. Filling of such composites by liquid silicon revealed that a specific route and particular conditions are necessary to eliminate porosity by controlling gas production species from pore surface at high temperature. This may be achieved through a directional flow and using highly viscous silicon (thanks to a localized wick), and by keeping the sides of the materials permeable to gas. This led to composite materials with a thermal conductivity which was four times as high as that of those materials densified via CVI. An increase in mechanical properties was also observed.

Abstract: Interfaces are a key element in ceramic matrix composites, but also in several material assemblies, such as, for instance, multilayers. This is not only because they can cause crack deflection and make materials damage tolerant, but also because they can be tailored with regard to material performances. An approach to crack deflection at interfaces or within interphases is proposed on the basis of the following Cook and Gordon’s mechanism: a crack is nucleated along an interface, ahead of a propagating crack; deflection of this crack then results from coalescence with the interface crack. The stress state induced by a crack was computed in a cell of bimaterial using the finite element method. The cell represents a matrix and a fiber, or an interphase and a fiber or two layers in a multilayer. A master curve was established. It represents the debonding condition based on strengths and elastic moduli of constituents. Then a deviation potential was defined. Deviation potentials were calculated for various fibre/matrix or layer combinations.