A COMPUTERIZED TEST SYSTEM FOR THERMAL-MECHANICAL FATIGUE CRACK-GROWTH

A computerized testing system to measure fatigue crack growth under thermal-mechanical fatigue conditions is described. Built around a servohydraulic machine, the system is capable of a push-pull test under stress- or strain-controlled conditions in the temperature range of 25 to 1050 degree C. Temperature and mechanical strain are independently controlled by the closed-loop system to simulate complex in-service strain-temperature relationship. A dc electrical potential method is used to measure crack growth rates. The correction procedure of the potential signal to take into account powerline and rf-induced noises and thermal changes is described. It is shown that the potential drop technique can be used for physical mechanism studies and for modeling crack tip processes.