A new methodology proposed to study the evolution of fatigue damage in metallic alloys

Abstract

The aim of this work was to develop and demonstrate the efficiency of an accessible and simple methodology, adapted from small punch fatigue (SPF) concepts, to study the evolution of the fatigue damage in metallic alloys. This demonstration was carried out using a 10 mm thick hot rolled sheet of a lean duplex stainless steel (UNS S32304). The methodology proposed consisted of the design and construction of a SPF device that was adapted to a traditional mechanical testing machine for carrying out fatigue tests in small discs of 10 mm of diameter and 1 mm thickness. Microstructural analyses were performed by Scanning Electron Microscopy (SEM) before and after the fatigue tests loading with predefined interruptions to monitor the fatigue degradation evolution until it was considered the complete failure of the specimen, when the dominant cracks exceeded 1 mm length. In addition, samples of this steel, before and after being subjected to cyclic fatigue loading, were analyzed by Transmission Electron Microscopy (TEM). The methodology proved to be efficient, allowing to identify and monitor, from the initial stages, the events of cyclic plastic deformation that occurred on the surface of the test specimens. Furthermore, it was also possible to identify the initial stages of nucleation and crack growth as well as their relationships with the microstructural aspects of the steel and its thermal history.