EFFECT OF MICROCRACKING ON THE MACROSCOPIC BEHAVIOR OF CERAMIC MATRIX COMPOSITES - ULTRASONIC EVALUATION OF ANISOTROPIC DAMAGE

This paper studies the effect of matrix microcracking within a SiC-SiC composite on its macroscopic behaviour. Experimentally, ultrasonic evaluation under load is used. Ultrasonic velocities are measured for varying propagation directions as a function of applied stress. The anisotropic stiffness degradation due to the applied stress is determined from ultrasonic velocity measurements. This anisotropic stiffness degradation is a function of microcrack evolution and accumulation in the damaged composite. The theoretical approach is developed in the framework of continuum damage mechanics. Two internal variables are introduced to distinguish between microcrack initiation, distribution and growth on one hand, and microcrack evolution with applied stress, closing or opening without growth on the other hand. This model taking into account the initial state of the material is well adapted to the behaviour of this SiC-SiC composite for which matrix microcracking is the fundamental mechanism of degradation. Coupled with ultrasonic evaluation, this model allows to assess completely the accumulated damage in the investigated principal plane and in particular to find the loading history of the material.