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A test method is described for performing uniaxial low-cycle thermal mechanical fatigue tests at elevated temperatures on nickel-base superalloys. Both smooth specimen crack initiation and notched specimen crack propagation testing is discussed. Standard closed loop servohydraulic testing machines with closed loop electric induction heaters are utilized. The temperature and mechanical strain are independently controlled, allowing for a wide range of relationships between temperature and mechanical strain.
Tubular specimens are directly inductively heated in air and cooled with compressed air. The temperature is measured with a radiation pyrometer which eliminates thermocouples and premature failures caused by welded thermocouples. Axial strain in the specimen is measured from machined internal ridges on the specimen utilizing a linear variable differential transformer and a quartz internal extensometer.
An electronic system compensates for the thermal expansion component of strain for both the control loop and the recorders. This system permits direct recording of the load-induced mechanical strain. Load-strain hysteresis loops and strain-temperature plots can simply be recorded during the test.
thermal fatigue, mechanical tests, nickel alloys, crack initiation, crack propagation
mechanical engineer, Failure Analysis Associates, Palo Alto, Calif