Controlling factors for roughness increases on cleavage fracture surfaces and crack branching in polycrystalline steel

Abstract

Brittle fracture in carbon steel has serious impact on the fracture safety of steel structures. Thus, technology that arrests crack propagation is the final defence for these structures. Conditions that can reliably stop crack propagation should be thoroughly clarified. Considering the background of the social importance of the issue, many experimental and theoretical studies have been conducted from both the mechanical and microstructural viewpoints.Fracture surface roughness is an important factor in studying the physics of dynamic high-speed crack propagation. For example, in resin materials, fracture surface roughness increases with crack propagation speed, called mirror-mist-hackle, and when the crack propagation speed further accelerates, cracks macroscopically branch off. In this study, the authors conclude that crack branching occurs under conditions of high stress triaxiality, not under the high-speed conditions seen in resin materials, and the critical conditions for branching in steel are revealed. To elucidate the controlling factors that increase fracture surface roughness and branching, detailed discussion using numerical analysis is carried out.