Rapid Determination of the Fatigue Behavior at Different Stress Ratios of Steels by Measuring the Energy Release

Structural integrity of mechanical devices is of fundamental importance for reliability under the action of service loads. To properly design a mechanical device against fatigue failure, a long test campaign involving many specimens and time must be performed according to traditional fatigue tests protocol. However, fatigue is a very dissipative phenomenon in which a large amount of energy is dissipated in the surrounding environment. Moving from this assumption, the adoption of infrared thermography can dramatically decrease the amount of time to obtain reliable information regarding the fatigue life of materials and components. Risitano Thermographic Method (RTM) links the superficial temperature during a fatigue test with the dissipated energy for a given stress level. The whole fatigue life of a specimen is represented by an Energy Parameter, strictly dependent on the test frequency and stress ratio, and this allow to obtain, even with one specimen, the entire fatigue curve. The Static Thermographic Method (STM) allows to assess the first damage in a specimen subjected to static tensile test by monitoring the superficial temperature evolution. The obtained limit stress could be directly related with the onset of fatigue damage within the material if cyclically stressed. The aim of the present work is to investigate the relation between the energy release and the damage at different stress ratios within a stainless steel AISI 316L, both under static tensile and fatigue tests using RTM and STM. Moreover, microstructure analysis is carried out to identify possible failure sites.