Theoretical analyses of nanocrack nucleation near the main crack tip in nano and micro crystalline materials

Abstract

Nano or micro crack nucleation in the process of metallic fracture is a common phenomenon and it affects fracture behaviors of the materials. The mechanisms of nanocrack nucleation are different for metals with nanoscale and microscale grain sizes. In this paper, according to the deformation mechanisms of materials with different grain sizes, theoretical models are established to investigate nanocrack nucleation near a main crack tip under mode I load. The theoretical solution is presented based on the distributed dislocation technique. For nanocrystalline metals, the process of nanocrack nucleation due to grain boundary sliding is analyzed and the effect of grain boundary structure on nanocrack nucleation is studied. The results show that nanocracks may be preferentially nucleated at the triple junctions which are located on the upper or lower sides of the main crack plane, instead of the front of the crack tip. A larger nanocrack may be nucleated at triple junctions with a larger characteristic abutting angle of the triple junctions. For microcrystalline metals, nanocrack nucleation due to lattice dislocation pileup is considered. The problem of inclusion cracking due to lattice dislocation pileup is analyzed. The results show that lattice dislocation pileup causes the entire soft inclusion to crack easily, but a relatively small nanocrack can be only nucleated in the hard inclusion.