TOUGHENING BEHAVIOR OF A 2-DIMENSIONAL SIC SIC WOVEN COMPOSITE AT AMBIENT-TEMPERATURE - I, DAMAGE INITIATION AND R-CURVE BEHAVIOR/

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 obs erved and dominate fracture behavior in the early loading stage such t hat primary crack extension occurs at apparent stress intensity values as high as 12 MPa.m(1/2). Linear elastic fracture mechanics (LEFM), t hough questionable, was assumed to be valid in the early stages of dam age 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 t he initial value. Based on microstructural observations, multiple matr ix cracks are found to be arrested at fiber bundles. The key to toughe ning appears to be associated with the mechanics of crack arrest at fi ber bundles in the woven architecture. Toughening mechanisms include m ultiple matrix cracking (similar to microcracking), crack branching, a nd crack deflection in the crack frontal zone. Application of models t o evaluate toughening based on these mechanisms results in values comp arable to experimental data. In the regime of primary crack extension, a J-integral technique was applied to investigate the R-curve behavio r and results showed a rising J(R)-curve which started at 1500 J/m(2) and reached 6150 J/m(2) after about 13 mm of primary crack extension, There was evidence of substantial crack bridging by fiber tows and fib rous pullout in this regime of crack advance.