Development of a thermal-mechanical fatigue testing facility

Components in the hot section of gas turbines undergo complex temperature-strain-time histories while in service. This leads to fatigue crack growth under conditions of varying temperature and cyclic strain, referred to as thermal-mechanical fatigue (TMF). A computerized TMF testing facility and testing procedures are described. The TMF testing facility is capable of conducting fully reversed strain-controlled, in-phase (IP) and out-of-phase (OP) TMF, and isothermal low-cycle fatigue (IT-LCF) in the 600°C to 1000°C temperature range. TMF and IT-LCF tests were performed on bare IN738LC nickel-base superalloy at strain ranges from 0.4 to 1 percent to commission the TMF testing facility. The tests were conducted in air, through a temperature range of 750°C to 950°C at a constant strain rate of 2 x 10-5 sec-1. The results of these tests demonstrate the successful application of the testing procedures and prove the testing system is capable of cycling specimens under various programmed thermal-mechanical loading histories. The fatigue lives were found to differ with strain-temperature phasing and strain range. The results of IP-TMF tests correspond well to the cyclic life observed during IT-LCF loading. Predominantly intergranular crack initiation and propagation were observed under all loading conditions.