A theoretical and experimental investigation of the influence of crack tip plasticity on fatigue crack closure

A correlation is assumed to exist between the true crack tip stress intensity factor range existing at the crack tip, and the instantaneous crack growth increment. In order to find the hue crack tip stress intensity range crack closure induced by local plasticity is modelled using a simplified model in which the yielding is confined to a line, and boundary elements, together with a quadratic programming procedure, are used to determine both the extent of plasticity and crack closure effects. A means of determining crack growth rate is then developed, which may use data obtained experimentally at a different R-ratio from that in the prototype. Confirmation of the general correctness of the hypothesis is obtained by conducting 4-point bend fatigue tests with varying degrees of load reversal, and moire interferometric measurements of the crack tip displacement field. This enables correlations to be made between the measured values of the crack tip closure conditions and the values predicted by the model. Good agreement is found. The approach may be directly extended to cases where there are occasional under or overloads, or a complex loading regime is applied.