Anisotropic damage evolution in unidirectional fiber reinforced ceramics

Damage transverse to the loading direction of a unidirectionally reinforced ceramic matrix composite has been observed and characterized by ultrasonic methods and acetate replicas. A laser-ultrasonic technique approach was developed and used to measure the ultrasonic velocity anisotropy, thus the complete elastic stiffness tensor, during uniaxial tensile loading/unloading of calcium aluminosilicate (GAS) composites reinforced by NicalonTM SIC fibers. Acoustic emission and surface replica observations link degradation of the axial stiffness to the accumulation of matrix cracks normal to the loading (and fiber) direction. The substantial loss of transverse stiffness is proposed to originate from the fiber/matrix interface debonding near matrix cracks. These observations may significantly impact the constitutive models used to design multiaxially loaded ceramic matrix composites. (C) 1997 Acta Metallurgica Inc.