The Minimum Energy Density Criterion for the Competition between Shear and Flat Fracture

Abstract

It is generally recognized that the fracture toughness KIC of materials represents the crack resistance during fracture, i.e., the energy consumption. In this study, AISI 4340 steels with different strength are applied to systematically investigate the intrinsic mechanism of fracture toughness. It is found that the fracture surface morphologies of AISI 4340 steels change from quasi‐cleavage fracture to dimple fracture with increasing the tempering temperature. While, the specimen size has almost no effect on the fracture mechanism of shear/flat fracture region, demonstrating that the fracture toughness depends on the proportion of shear and flat fracture region. Additionally, the shear lip width of the KIC specimen will increase to a constant value with enhancing the specimen thickness. Based on the energy balance principle, the maximum shear lip width can be obtained at the minimum fracture energy density of materials; consequently, there is an essential relationship between the shear fracture and fracture toughness, i.e., the minimum energy density criterion. The new finding, which provides a scaling law to well evaluate the plane strain fracture toughness of new materials with high toughness (e.g., nanostructured materials and metallic glasses), may broad the understandings on the fracture toughness.