Crack opening behavior in ceramic matrix composites

Abstract

The evolution of matrix cracks in a melt-infiltrated SiC/SiC ceramic matrix composite (CMC) under uniaxial tension was examined using scanning electron microscopy (SEM) combined with digital image correlation (DIC) and manual crack opening displacement (COD) measurements. CMC modeling and life prediction strongly depend a thorough understanding of when matrix cracks occur, the extent of cracking for given conditions (time-temperature-environment-stress), and the interactions of matrix cracks with fibers and interfaces. In this work, strain relaxation due to matrix cracking, the relationship between CODs and applied stress, and damage evolution at stresses below the proportional limit were assessed. Direct experimental observation of strain relaxation adjacent to regions of matrix cracking is presented and discussed. Additionally, crack openings were found to increase linearly with increasing applied stress, and no crack was found to pass fully through the gage cross-section. This calls into question the modeling assumption of through-cracks for all loading conditions and fiber architectures, which can obscure oxidation mechanisms that are active in realistic cracking conditions. Finally, the combination of SEM with DIC is demonstrated throughout to be a powerful means for damage identification and quantification in CMCs at stresses well below the proportional limit. K E Y W O R D Sceramic matrix composites, cracks/cracking, damage, silicon carbide

Static fatigue of Hi‐Nicalon™‐S fiber at elevated temperature in air, steam, and silicic acid‐saturated steam

“…19,21,22 Oxidation of SiC in air, low pO 2 , CO, CO 2 , and steam has been studied. 18,20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] Much work has been done concerning effects of oxidation on SiC matrix and fiber properties in CMCs, 18,[20][21][22][40][41][42][43][44][45][46][47][48] but questions about fundamental mechanisms still remain. There is abundant evidence that subcritical crack growth is an important SiC fiber degradation mechanism, 45,[49][50][51] particularly at intermediate temperatures in oxidizing environments.…”

Damage evolution in SiC/SiC unidirectional composites by X‐ray tomography

“…[5][6][7][8][9][10][11][12] Typically, damage accumulation in CMCs is directly observed using surface microscopy techniques either in situ or postfracture. [13][14][15][16] For example, damage mechanisms such as crack opening displacement and matrix crack density have been studied using DIC and manual crack opening displacement measurements by Sevener et al 17 They used these measurements to study the in-situ progression of matrix cracking in woven SiC/SiC CMCs under tensile load using an SEM and DIC techniques. Other nondestructive methods, such as acoustic emission (AE) and electrical resistance have been utilized to study the types of damage that occur in CMCs under various types of loading and environmental conditions.…”

TIME-DEPENDENT MATRIX MULTI-FRACTURE OF SiC/SiC CERAMIC-MATRIX COMPOSITES CONSIDERING INTERFACE OXIDATION

“…Continuous fibre-reinforced ceramic-matrix composites (CMCs), by incorporating fibres in the ceramic matrices, not only exploit their attractive high-temperature strength, but also reduce the propensity for catastrophic failure [1,2,3]. The environment inside the hot section CMC components is harsh and the composite is typically subjected to complex thermomechanical loading, which can lead to matrix multi-cracking [4,5]. These matrix cracks form paths for the ingress of the environment oxidising the fibres and leading to a premature failure [6,7,8,9,10].…”