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

Anisotropic damage evolution in a 0 degrees /90 degrees laminated Nicalon(T M) SiC fiber-reinforced calcium aluminosilicate (CAS) glass-ceramic composi te during uniaxial tensile deformation has been investigated using a variet y of non-invasive characterization techniques. The elastic constant reducti on in the three principal directions was measured from in situ laser-genera ted ultrasonic velocity measurements in various sound propagation direction s. They indicate that, in addition to a large drop in elastic stiffness in the loading direction, the constants characterizing the nominal elastic sti ffness transverse to the loading direction were also degraded. Surface repl icas 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 st iffness in the loading direction was the result of multiple matrix cracking in both the 0 degrees and 90 degrees plies. While the ultrasonic data allo wed a detailed characterization of the anisotropic damage evolution in this laminate, acoustic emission measurements and surface replica data identifi ed the crack initiation stress in the 90 degrees plies and correlated it to macroscopically observable deviations of the stress-strain curve from line ar elastic behavior. These matrix cracks were found to have initiated prefe rentially in the weak 90 degrees plies near the 90 degrees /0 degrees ply b oundaries. (C) 2000 Acm Metallurgica Inc. Published by Elsevier Science Ltd . All rights reserved.