Toughening Behavior of a Two-Dimensional SiC/SiC Woven Composite at Ambient Temperature: I, Damage Initiation and R-Curve Behavior

Abstract

The damage initiation and R-curve behavior for a two-dimensional (2-D) SiC/SiC woven composite are characterized at ambient temperature and related to in situ microscopic observations of damage accumulation and crack advance. Matrix cracking and crack deflection/branching are observed and dominate fracture behavior in the early loading stage such that primary crack extension occurs at apparent stress intensity values as high as 12 MPam1/2. Linear elastic fracture mechanics (LEFM), though questionable, was assumed to be valid in the early stages of damage initiation prior to primary crack advance, but was clearly invalid once primary crack extension had occurred. Such a high primary crack extension toughness value is confirmed by a renotch technique whereby the crack wake is removed and the fracture resistance drops close to the initial value. Based on microstructural observations, multiple matrix cracks are found to be arrested at fiber bundles. The key to toughening appears to be associated with the mechanics of crack arrest at fiber bundles in the woven architecture. Toughening mechanisms include multiple matrix cracking (similar to microcracking), crack branching, and crack deflection in the crack frontal zone. Application of models to evaluate toughening based on these mechanisms results in values comparable to experimental data. In the regime of primary crack extension, a J-integral technique was applied to investigate the R-curve behavior and results showed a rising JR-curve which started at 1500 J/m2 and reached 6150 J/m2 after about 13 mm of primary crack extension. There was evidence of substantial crack bridging by fiber tows and fibrous pull-out in this regime of crack advance.