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A technique for conducting strain-controlled, thermochemical, axial-torsional fatigue tests on thin-walled tubular specimens was developed. Three waveforms of loading, namely, the axial strain waveform, the engineering shear strain waveform, and the temperature waveform were required in these tests. The phasing relationships between the mechanical strain waveforms and the temperature and axial strain waveforms were used to define a set of four axial-torsional, thermomechanical fatigue (AT-TMF) tests. Real-time test control (three channels) and data acquisition (a minimum of seven channels) were performed with a software program written in C language and executed on a personal computer. The AT-TMF testing technique was used to investigate the axial-torsional thermomechanical fatigue behavior of a cobalt-base superalloy, Haynes 188. The maximum and minimum temperatures selected for the AT-TMF tests were 760 and 316°C, respectively. Details of the testing system, calibration of the dynamic temperature profile of the thin-walled tubular specimen, thermal strain compensation technique, and test control and data acquisition schemes, are reported. The isothermal, axial, torsional, and in- and out-of-phase axial-torsional fatigue behaviors of Haynes 188 at 316 and 760°C were characterized in previous investigations. The cyclic deformation and fatigue behaviors of Haynes 188 in AT-TMF tests are compared to the previously reported isothermal axial-torsional behavior of this superalloy at the maximum and minimum temperatures.
thermomechanical fatigue, axial-torsional fatigue, computer-controlled testing, data acquisition, thin-walled tubular specimen, testing techniques, hysteresis loop, cyclic deformation, cobalt-base superalloy, fatigue (materials), fracture (materials), testing, deformation (materials), multiaxial fatigue
Senior research engineer, NYMA, Inc., NASA Lewis Research Center, Cleveland, OH
Materials research engineer, U.S. Army Research Laboratory, Vehicle Propulsion Directorate, NASA Lewis Research Center, Cleveland, OH