Lock-in Infrared Thermography for Fatigue Limit Estimation in Ti–6Al–4V Alloy

Abstract

Lock-in infrared thermography was used to estimate the fatigue limit for Ti–6Al–4V alloy at room temperature. The method detected infrared emitted from specimen during cyclic loading, i.e., temperature change related to frequency (f) of the loading. The temperature change contained reversible component and irreversible component, which related to thermos-elastic effect and plastic deformation, respectively. The latter component was divided from the former one by lock-in analysis to estimate fatigue limit. Estimated fatigue limits corresponded to those obtained from conventional fatigue tests at several stress ratios. A new assessment line was identified as σ_a = σ_w(1 − σ_m/σ_B)^1/n, where σ_a represents the stress amplitude, σ_w signifies the fatigue limit, σ_m denotes mean stress, σ_B expresses tensile strength, and the n exponent for the alloy is about 2.3. The non-failure region of the new diagram was smaller than that of the modified Goodman diagram because of a reduced fatigue limit at near-zero stress ratios in the titanium alloy.