Effect of Fiber Bridging and Fiber Fracture on Fatigue Cracking in a Titanium-Matrix Composite

摘要

Constant amplitude fatigue tests were performed on (0)_4 and (0/90)_sSCS-6/TIMETAL 21S titanium-matrix composite specimens to study fiber bridging of matrix fatigue cracks. Crack length was monitored throughout the tests, and dispalcements near the crack surface were measured periodically by placing an Elber gage (1.5-mm gage length point extensiometer) across the crack at a number of positions. Specimens were removed prior to failure and mechanically polished to the frist layer of fibers and the extent of fibers and the extent of fiber bridging observed. While some cracks were fully bridged, other cracks contained broken fibers among the intact, bridging fibers. A model has been developed to study the mechanics of a cracked unidirectional composite with any combination of intact and broken fibers in the wake of the matrix carck. Displacements near the crack surface predicted by the model agree with the Elber gage measurements for cracks that were fully bridged. Predictiosn were also performed for bridged cracks with discrete fiber breaks in the crack wake based on the geometry observed after polshing with good correlation between the predictions and the experiemntal measurements.