Table of Contents
Materials and Structure
The photograph shows the result of a brittle fracture process in a low-carbon steel tested at low temperature. The top photograph is a microscopic view of the fracture surface, and the lower pair of photographs show the sample after fracture. The fracture surface is normal to the applied tensile stress in the sample and each "grain" in the polycrystalline metal sample may fail along its boundary with neighboring grains (intergranular fracture) or fail through its bulk on particular crystal planes (transgranular fracture). Fracture in this mode is associated with rapid crack propagation through the material and a "critical crack length" is associated with failure at a given stress through the Griffith criterion that was developed for glass-like materials. This states that:                    sC = (2Eg/pc)0.5 where sC is the critical stress, E is Young's modulus, g the surface energy of the crack, and c the length of the critical crack in the material. The surface energy may be extended to include any additional work that is done in near crack surface plastic deformation in less brittle materials such as metals.
From: Schaffer et al, "The Science and Design of Engineering Materials," McGraw Hill (1999)