ENB331 Lecture Notes - Lecture 4: Stress (Mechanics), Plane Stress, Stress Concentration
Document Summary
Most failures in engineering structures are due to: Yielding dominant failure: formation of micro-voids and cavities at grain boundaries and/or interfaces between material and impurities. Coalescence of voids and cavities to form elliptical crack, in doing this, extensive plastic deformation around crack tip until critical crack length. Rapid propagation of crack by shear deformation at ~45 to tensile axis. Fibrous central region consisting of multiple dimples (left over from micro-voids). Shear lip (tearing) of the material at the outer region. Defects in yielding dominant failure are microscopic: dislocations, interstitials, grain boundaries, precipitates. Fracture dominant failure: crack propagates rapidly and perpendicular to tensile axis. Little or no plastic deformation around crack tip. Flaws are macroscopic i. e. weld defects, porosity (holes), inclusions, steel corrosion, cracks etc. In polycrystalline materials this can give a faceted appearance as different crystallographic planes are exposed. Cracks propagate along the grain boundaries: intergranular fracture. e. g. solidification of brittle films along grain boundaries can lead to intergranular fracture.
