Abstract: With regard to the structures subjected to severe mechanical-thermal coupling fatigue conditions, the consistency between service conditions and loading conditions was the prerequisite for the fatigue properties test and fatigue life prediction. A miniature piezoelectric-driven instrument integrating with a tripodal piezoelectric actuator and envelope-type heating function was developed to investigate the mechanical-thermal coupling fatigue properties and obtain the fatigue life of materials under approximate service conditions. A physical model was described to quantitatively calculate the alternating deformation under the mechanical-thermal coupling tensile-bending combined condition. Accordingly, a fatigue life prediction method based on the Coffin-Manson equation under tensile-bending combined loading was proposed to estimate the mechanical-thermal coupling fatigue life in terms of the total coupling deformation and plastic strain amplitude. The feasibility of the proposed instrument was verified through the analysis of displacement responses of high entropy alloy specimens under the thermal-mechanical coupling tensile-bending combined condition at temperatures ranging from RT to 600 °C. The comparison between fatigue lives directly measured through thermal-mechanical coupling experiments and theoretically predicted fatigue lives indicated the availability of the modified Coffin-Manson equation.

Key words: Fatigue testing instrument, Mechanical-thermal coupling, Combined loading, Fatigue life prediction, Modified Coffin-Manson equation