Micromechanical analysis of plastic damage under stress-triaxiality considering void growth and void-inclusion interaction

Abstract

Single crystal nickel-based super alloys produced by directionally solidification (DS), cast or selective electron beam melting (SEBM) methods, contain substantial amount of pores. With respect to production processes, the morphology of pores as well as their distribution varies. These pores are found to be detrimental to creep resistance, fatigue life, and after all, the failure of components. Moreover, it was found that some hard precipitates evolve during high temperature deformation. During loading, these precipitates interact with the evolving pores causing significant strength reduction of the material. To assess the influence of pores and precipitates on the structural damage and failure, a micromechanical based understanding is needed, which explains the evolution of localized damage in terms of local change of porosities, and interactions of pores and precipitates. In this thesis detailed numerical analyses were performed to elucidate the effects of pores and precipitates on the evolution of local stress-strain and material softening leading to failure. To this goal, a cell model was developed that incorporates different shaped voids and precipitates, varied by different volume percent and their location. The influence of void growth on the local stress-strain evolution was studied for different stress-triaxialities taking systemic arrangements of pores and precipitates into account. From the analysis a direct correlation was made among the local stress-triaxiality, void growth, and structural necking. The results give a clear micromechanics-based understanding that how the internal pores and precipitates play significant roles on the macroscopic damage and failure. Further, local failure strain for different shapes of pores and participates were established for critical void volume fraction. Using these data, several damage parameters can be estimated for a well-known porosity-based Gurson-Tvergaard-Needleman model (GTN) damage model.