Computation of the Crack-Tip Energy Release Rate for Cyclic Crack Growth

Abstract

The elastic-plastic finite-element method, employing the mechanical sublayer theory of plasticity, was used to compute a local crack-tip parameter which shows significant promise for characterization of the cyclic crack growth process. The proposed cyclic crack growth rate characterizing parameter is the crack tip value of the energy release rate. This crack-tip energy release rate (G0), has been shown to be equivalent to the global energy release rate (G), for the case of monotonic loading of a stationary crack. This work extends the proposed definition of the crack tip energy release rate to cyclic loading, including the important effects of crack growth. Elastic-plastic incremental finite-element analyses of a model which represents the elastically-bounded crack-tip region for the case of small-scale-yielding (SSY) have been performed, for both stationary and growing cracks. The crack-tip energy release rate G0, computed by the simple technique described here, was compared with the magnitude of the remote elastic K-field loading which was applied at the outer boundary of the SSY model, under plane-strain conditions. The effects of plasticity-induced crack closure upon G0 were investigated by allowing the crack tip to advance into the prior residual plastic zone in several steps, until a steady-state condition was achieved. The occurrence of crack face closure during the unloading portion of each loading cycle and of the subsequent reopening of the crack during the next loading cycle was accurately determined. From these computational results, a modified definition of G0, embodying the effects of plasticity-induced crack closure, was developed. The results of these investigations show, generally, that the crack-tip energy release rate, when appropriately defined and computed in the context of cyclic loading, crack advance and crack face closure, provides a consistent and rational description of the local driving force acting upon the tip of a growing crack. As such, G0 should prove to be a viable cyclic crack growth characterizing parameter.