Analysis and Modeling of Thermal Signatures for Fatigue Damage Characterization in Ti–6Al–4V Titanium Alloy

Abstract

Low cycle fatigue (LCF) damage in titanium alloys has been traditionally assessed based on driving parameters such as stress, strain or hysteresis energy. These mechanical parameters sometimes fail to quantify the true contribution of stored damage energy which is the main cause of LCF damage in these alloys. Hence in the present investigation, thermal evolution captured on-line using lock-in infrared-thermography has been used to characterize low cycle fatigue damage of Ti–6Al–4V under a range of applied total strain amplitudes (0.8–1.8 %). The analysis of thermal signatures indicated that the contribution of thermo-elasticity decreased and inelasticity increased with an increase in strain amplitude due to significant change in damage micromechanism from quasi-cleavage to ductile mode. Increase in applied strain resulted into increased fatigue damage and consequently higher temperature increase. An inverse relationship similar to standard strain amplitude-fatigue life equation was established between temperature increase and fatigue life. The damage energy was evaluated using first law of thermodynamics i.e. law of energy conservation. Damage energy increased with an increase in applied strain amplitude. A relationship was also developed between damage energy and strain amplitude.