AN ELASTO-PLASTIC FRACTURE MECHANICS APPROACH TO FATIGUE CRACK PROPAGATION

Abstract

The elasto-plastic fracture mechanics approach to fatigue crack propagation gives the following relationship ; Δa/Δn = c'·w*m L or Δa/Δn = c·wmL, where, Δa/Δn ; propagation rate of fatigue crack, w*L, wL ; cyclic plastic zone length and monotonic plastic zone length at crack tip, respectively (w*L∝wL is assumed), C, C', m ; material constants, which are related to the ductility, elastic modulus and yield strength. In addition to above facts, authors insist that the plastic constraint around the crack tip is important in fatigue crack propagation, for the following reason ; the larger the plastic constraint, the faster the propagation rate of fatigue crack, because the crack propagates in plane strain state without ductility ; and the less the plastic constraint in plane stress state, the slower the rate. The transition phenomena exists between both states, depending on plastic zone width and the thickness of the plate. Above mentioned facts can be proved experimentally.