Micromechanical simulation of fracture behavior of bimodal nanostructured metals

Nanostructured (NS) metals with bimodal grain size distribution that consist of coarse grained (CG) and nano-grained (NG) regions have proved to have both high strength and good ductility. In this paper a numerical investigation, using the combination of a mechanism-based strain gradient plasticity theory, a micromechanics composite model, and the Johnson-Cook failure model, is conducted to investigate the effects of the distribution of the CG inclusions and their shape on fracture behavior of a bimodal NS copper. Load-response relations are employed to evaluate the loading history stability, while apparent crack length and strain energy history are used to analyze the fracture resistance. This study shows that both crack bridging in the CG inclusions and crack deflection in the NG matrix can significantly toughen the bimodal NS Cu. Our simulations also show that there exists a critical volume fraction of CG inclusions for some microstructures at which the fracture resistance of the bimodal NS Cu is at its minimal state and thus it should be avoided in material design.