Multi-scale Investigation of Damage Mechanisms in SiC/SiC Ceramic Matrix Composites.

Abstract

Fiber reinforced ceramic matrix composites (CMCs) are desirable materials for high-temperature, structural engineering applications because of their low density, high melting temperature, and relatively high toughness. In order to employ them commercially, as in aircraft engines, deformation and damage mechanisms must be well characterized. This requires comprehensive assessments of damage initiation (which is detectable only at microscopic length scales), damage accumulation (the detection of which spans microscopic and macroscopic scales), and failure (which is best characterized at macroscopic scales). A multi-scale approach was adopted to characterize each of these stages of damage in continuous fiber silicon carbide (SiC/SiC) CMCs. Thermomechanical testing, conducted in-situ in a scanning electron microscope and augmented with digital image correlation (DIC), revealed when and where, with respect to stress state and microstructure respectively, cracks initiated. Microscopic analyses further revealed toughening mechanisms in the constituent fibers and a statistical correlation between crack propagation and microstructure was established. Using full-field deformation data from DIC to evaluate the J-integral, toughness in cross-ply and unidirectional CMCs was characterized and fiber-bridging laws were developed. Macroscopic damage analyses elucidated crack accumulation behavior in both the linear and non-linear loading regimes and through final failure. Experimental testing at the macroscale provided a resource for validating numerical damage models, characterizing crack speed in the presence of toughening constituents, and, when combined with fractography, developing a three dimensional perspective of crack propagation. Comparing deformation data collected from the same specimen but at different length scales, it was shown that the reduced spatial resolution of the macroscale masks underlying damage that is present at the microscale. As each length scale has strengths and weaknesses, it is argued that a comprehensive characterization of damage in CMCs must account for behavior in both microscopic and macroscopic regimes.