Experimental evaluation of effective surface energy for cleavage microcrack propagation across grain boundary in steels

Abstract

Fracture tests and finite element analysis were conducted to evaluate the temperature dependence of the effective surface energy γmm corresponding to microcrack propagation across the grain boundary of steels. Steels containing coarse cementite particles were prepared to enable γmm evaluation. The critical fracture stress of microcrack propagation across the grain boundary is considered larger than the critical fracture stress of microcrack propagation across the interface between a cementite particle and a ferrite grain in these steels because of the large cementite particles; therefore, local fracture stress estimated by finite element analysis and fracture initiation site obtained by fractography reflect the critical fracture stress of microcrack propagation across the grain boundary. Thus, the accuracy of γmm calculated from local fracture stress is observed to be enhanced by employing the steels prepared for the present study. In addition, the effect of nickel on γmm was studied using steels containing different concentrations of nickel. Values of γmm were evaluated considering both the direction of a neighboring facet relative to the crack initiation facet and the mixed mode stress intensity factors. The authors proposed a new temperature dependence ofγmm. Additionally, the negligible effect of nickel addition on γmm was observed. The results of the present study can be used effectively to develop a probabilistic cleavage fracture model based on microstructural information.