FATIGUE MECHANISMS IN GRAPHITE SIC COMPOSITES AT ROOM AND HIGH-TEMPERATURE/

Some deductions have been made from fractographic evidence about mecha nisms of low-cycle mechanical fatigue in plain woven graphite/SiC comp osites at room and high temperature in vacuum. At both room temperatur e and 830-degrees-C, fatigue appears to be confined to the crack wake, where attrition reduces the efficacy of bridging fibers. It is inferr ed that the crack tip advances at some critical value of the crack tip stress intensity factor, as in monotonic growth, rather than by any i ntrinsic fatigue mechanism in the matrix. However, the manifestations of attrition are very different at room and high temperatures. At high temperature, wear is greatly accelerated by the action of SiC debris within the crack. This distinction is rationalized in terms of the tem perature dependence expected in the opening displacement of a bridged crack. This argument leads in turn to plausible explanations of trends in load-life curves and the morphology of cracks as the temperature r ises.