Stress relaxation mechanism and life evaluation of titanium alloy springs under long-term thermo-mechanical coupling conditions

Abstract

The primary failure mode for helical compression springs under prolonged static thermo-mechanical coupling conditions is stress relaxation rather than fatigue failure. Unfortunately, for the investigation of spring failure modes, previous research has mainly focused on fatigue failure, while lacking exploration of the evolution of its microscopic mechanisms underlying stress relaxation and the resultant changes in mechanical properties. To elucidate the microstructure evolution mechanism of dual-phase titanium alloy springs exposed to long-term thermo-mechanical coupling conditions and to evaluate their life and reliability, a series of stress relaxation tests on Ti-6Al-4V titanium alloy springs were carried out. The degradation of mechanical properties in Ti-6Al–4V springs under various temperatures and compressive deformations was analyzed. Additionally, microscopic mechanisms were further examined using transmission electron microscopy (TEM) and electron backscattering diffraction techniques (EBSD). Based on these investigations, a stress-life model for Ti-6Al-4V springs was established through maximum likelihood estimation (MLE) and the Power-Exponential equation, followed by an evaluation of its reliability and life. The findings of this paper can provide engineering application guidance into the safe and reliable operation of dual-phase titanium alloy springs under thermo-mechanical coupling conditions.