Chapter 7: Fracture of Metals

Fracture of Metals begins by establishing the theoretical cohesive strength of metals, derived from atomic attractive and repulsive forces, and explains why actual fracture strengths are significantly lower due to the presence of flaws and cracks, a concept solidified by the Griffith theory of brittle fracture. The text details the energy balance required for crack propagation, where the release of elastic strain energy must overcome the surface energy required to create new crack surfaces, and discusses Orowan's modification to include plastic work for metallic materials. Significant attention is given to the dislocation theories of brittle fracture, describing how dislocation pile-ups at grain boundaries acts as stress concentrators (Zener and Stroh models) to nucleate microcracks, and how this relates to the Petch relationship regarding grain size dependence. The chapter explores the metallographic aspects of fracture through fractography, identifying characteristic features such as river markings in cleavage fracture and dimpled rupture in ductile failure, which occurs via the nucleation, growth, and coalescence of microvoids. Furthermore, it analyzes the ductile-to-brittle transition in body-centered cubic metals, influenced by temperature, strain rate, and stress state, specifically utilizing Cottrell's theory to explain the transition temperature. The discussion extends to notch effects, which introduce triaxial tensile stresses and plastic constraint that can elevate the yield stress (notch strengthening) or promote brittle failure. Finally, the chapter covers fracture under combined stresses, comparing criteria like maximum-normal-stress and maximum-shear-stress, and reviews the effects of high hydrostatic pressure, which generally suppresses crack initiation and enhances ductility.