Linear Elastic Fracture Mechanics (LEFM) applies when the nonlinear deformation of the material is confined to a small region near the crack tip. For brittle materials, it accurately establishes the criteria for catastrophic failure. However, severe limitations arise when large regions of the material are subject to plastic deformation before a crack propagates. Elastic Plastic Fracture Mechanics (EPFM) is proposed to analyze the relatively large plastic zones.
Elastic Plastic Fracture Mechanics (EPFM) assumes isotropic and elastic-plastic materials. Based on the assumption, the strain energy fields or opening displacement near the crack tips are calculated. When the energy or opening exceeds the critical value, the crack will grow.
Please note that although the term elastic-plastic is used in this approach, the material is merely nonlinear-elastic. In others words, the unloading curve of the so called elastic-plastic material in EPFM follows the original loading curve, instead of a parallel line to the linear loading part which is normally the case for true elastic-plastic materials.