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The small scale yielding (SSY) relationships for stationary cracks cannot describe the local elastic-plastic behavior at a crack tip when irregular loading is considered. As a consequence, linear models for crack growth prediction fail. Nonlinear crack growth behavior (retardation, acceleration) is due mainly to load interaction effects which are governed by memory rules. As to stationary cracks, memory which is based on the local behavior (closed hysteresis loops) can be described by reaching certain load levels. Extended to crack growth problems, the “structural memory” criterion is described by striking elastic-plastic boundaries. This points up the central role that the development of plastic zone sizes as a function of crack length and load history plays. The concern of the paper is with those structural analysis aspects under the assumption of perfectly plastic material properties. SSY plastic zone formulae are derived for the peak overload/underload case which are shown to be of central importance for describing the nonlinear effects due to any irregular loading history. It is demonstrated, in principle, how crack closure can be taken into account.
fracture mechanics, cyclic loads, plasticity, strain concentration, plastic zone, residual stress, closure, material memory, fatigue (materials), crack propagation, overload, underload
Scientific research engineer, Laboratorium fur Betriebsfestigkeit, Darmstadt,
Professor, Institut für Statik und Stahlbau, Technical University of Darmstadt,