Anisotropic damage evolution in a 0°/90° laminated ceramic-matrix composite

Anisotropic damage evolution in a 0 degrees /90 degrees laminated Nicalon(TM) SiC fiber-reinforced calcium aluminosilicate (CAS) glass-ceramic composite during uniaxial tensile deformation has been investigated using a variety of non-invasive characterization techniques. The elastic constant reduction in the three principal directions was measured from in situ laser-generated ultrasonic velocity measurements in various sound propagation directions. They indicate that, in addition to a large drop in elastic stiffness in the loading direction, the constants characterizing the nominal elastic stiffness transverse to the loading direction were also degraded. Surface replicas taken intermittently during loading revealed that transverse softening of the elastic stiffness was associated with fiber/matrix interface damage mainly in the 0 degrees plies, while the large softening of the elastic stiffness in the loading direction was the result of multiple matrix cracking in both the 0 degrees and 90 degrees plies. While the ultrasonic data allowed a detailed characterization of the anisotropic damage evolution in this laminate, acoustic emission measurements and surface replica data identified the crack initiation stress in the 90 degrees plies and correlated it to macroscopically observable deviations of the stress-strain curve from linear elastic behavior. These matrix cracks were found to have initiated preferentially in the weak 90 degrees plies near the 90 degrees /0 degrees ply boundaries. (C) 2000 Acm Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.