Rapid Determination of Fatigue Limit Using Temperature Second Harmonic

Background The rapid determination of the fatigue limit using temperature second harmonic, which is the temperature variation with twice the frequency of the cyclic load, is attractive in terms of time and cost effectiveness. However, the temperature second harmonic contains non-fatigue-related factors (e.g., internal friction and load second harmonic), in addition to fatigue-related factors (e.g., plastic deformation); therefore, this presents challenges in uniquely determining the fatigue limit. Objective An experimental procedure for uniquely determining the fatigue limit based on temperature second harmonic measurements is developed. Methods In the developed procedure, the temperature second harmonic amplitude is accurately measured using the present phase double frequency (2f) lock-in thermography, and the fatigue limit is then uniquely determined based on the generation mechanism of the temperature second harmonic. The present phase 2f lock-in thermography requires only simple post-processing, considering the phase information of both the non-fatigue-related and fatigue-related factors. The fatigue limit estimation accuracies of the developed procedure and four existing methods are quantitatively evaluated in comparison with the actual fatigue limit for heat-treated carbon steel plate specimens subjected to annealing, normalizing, and quenching-tempering. Results By performing simple post-processing, the present phase 2f lock-in thermography easily eliminates the influence of internal friction and load second harmonic. The developed procedure exhibits the second-best estimation accuracies among all the methods for all the heat-treated specimens. Conclusions The developed procedure is validated through its reasonable estimation accuracy.

Automated summary

An experimental procedure for uniquely determining the fatigue limit based on temperature second harmonic measurements is developed. The present phase double frequency (2f) lock-in thermography accurately measures the temperature second harmonic amplitude, and the fatigue limit is then uniquely determined based on the generation mechanism of the temperature second harmonic. This procedure effectively eliminates the influence of non-fatigue-related factors and provides reasonable estimation accuracy.