Mechanistic studies of high-temperature crack initiation in single crystal materials

In this work, the results of mechanistic studies of surface crack initiation due to the coalescence of microcracks from casting defects are presented. Two approaches are considered; namely, a continuum damage (CD) one and another where a microvoid is explicitly introduced in the vicinity of a notch. In both cases, a rate-dependent crystallographic theory is relied upon to describe the visco-plastic behavior of the single crystal. The numerical studies are conducted on a notched compact tension (CT) specimen with and without a single casting defect, idealized as a cylindrical void close to the notch surface. CD predictions of the formation of surface cracks under constant far-field loading, linked to the nucleation and coalescence of microcracks from internal porosities, are obtained from a recently proposed mechanistic anisotropic void growth model. In the explicit-void modeling approach, the time to crack initiation under constant load has been predicted using a strain-based failure criterion. Finite element analysis of the CT specimen revealed that, due to the strong localization of inelastic strain at the void, a microcrack will initiate in the vicinity of the void rather than at the notch surface. The numerical results have also shown that the time to crack initiation depends strongly on the applied load level and the void location. These results and those obtained from the CD predictions are compared and discussed. The applicability of a failure assessment approach, based on the linear elastic stress intensity factor to predict the crack initiation time under creep loading, is also considered.