Is the temperature plateau of a self-heating test a robust parameter to investigate the fatigue limit of steels with thermography?

A critical aspect of standard test methods for fatigue characterization is that they do not provide any information on heat dissipation in the material and involve very expensive experimental campaigns in time and costs. In recent years, thermographic methods capable of reducing testing time have been developed, also providing more information on damage occurring in the material. A commonly used approach is based on the assessment of the temperature plateau during a stepwise loading procedure. At times, however, this approach can fail if temperature stabilization is not achieved. In this regard, in this paper, a new approach based on the assessment of 3 different thermal indexes was proposed to estimate the fatigue limit of 3 stainless steels: AISI 316, 17-4PH, and ASTMA890 grade 4a, respectively, exhibiting fully austenitic, fully martensitic, and duplex biphasic microstructure. The fatigue tests were carried out by using a stepwise loading procedure, under loading ratio of 0.5. The analysis demonstrated the possibility to further reduce testing time and, consequently, the fatigue experimental campaign. Moreover, some ideas are discussed about how to justify the different thermal behaviour of a biphasic stainless steel, in total temperature variation. Moreover, a discussion of results regarding the various thermal behaviours of the investigated steels and a possible correlation with the microstructure has been proposed.