Use of instrumented indentation testing to study local mechanical properties of 304L SS welded joints subjected to low-cycle fatigue loadings

In this paper, a series of experiments including room-temperature low-cycle fatigue tests, instrumented indentation measurements, optical microscopy examinations, etc. were carried out to study the local mechanical properties and microstructures in particular zones of 304L SS welded joints in both the as-welded and cyclic straining conditions. An analytical algorithm proposed by Dao et al. [12] was utilized to estimate the basic mechanical properties (E, σy and n) from the indentation load-depth data. It is shown that in the as-welded condition, the weld metal has a higher strength and strain hardening capacity but a lower stiffness than does the base metal, while the material within HAZ exhibits a significant gradient of the mechanical properties. A strain amplitude-dependent variation in the local mechanical properties in particular joint regions was also obtained. Optical microscopy examinations showed that the microstructure in particular zones of 304L SS welded joints was not only decided by the metallurgical condition resulting from the welding process, but also determined by the strain amplitude applied during cyclic straining. In this study, an empirical criterion for judging the fracture locations of a weldment based on the yield stress mismatch ratio was also introduced to assess the integrity of the present 304L SS welded joints subjected to low-cycle fatigue loadings. The research work performed in this paper would further expand the applications of instrumented indentation technique in fatigue strength analysis of welded components.