Limits of applicability of LEFM: numerical investigation on the crack-tip-yielding in a hollow-cylindrical specimen

Even if most Fatigue Crack Growth analyses are performed within the frame of Linear-Elastic Fracture Mechanics (LEFM), in some practical cases the boundaries of applicability of LEFM can be exceeded. This study analyses a fatigue crack growth under complex stress state, in a hollow cylindrical specimen with initial semielliptical surface crack, for which the LEFM limits are exceeded. The stresses in the surrounding of the crack tip, obtained by means of an elastic-plastic FE-analysis, are used to calculate the size of the plastic region and its shape. The analysis shows how the plastic zone size turns out to be comparable in size with residual ligament and therefore the LEFM boundaries are exceeded. This outcome explains the misalignment found between the crack growth rates simulated by LEFM and the corresponding experimental results.