Observations on assessing ductile instability margins in the presence of fatigue

The current situation in respect of analysing a structure against ductile t earing instability in the presence of fatigue is reviewed. Although in its main sections, R6 advises against such analysis under normal operating load s, in its status notes, it does allow the analysis under certain provisos. A brief review of the real behaviour of structures is performed. It is appa rent that ductile instability is determined by the material's resistance cu rve, while the crack growth to that point is determined by the influence of tearing on the fatigue laws. It is shown that restricting an analysis to d uctile initiation is arbitrary and gives no indication of the real margins. Indeed, if this is done, margins could be very small without knowing it. Some of the issues associated with assessing for ductile instability in the presence of fatigue are addressed, particularly those associated with vari able amplitude loading. It is shown that although it is not possible to ass ess the instantaneous effect of any load applied to a structure where crack ing has entered the tearing-fatigue regime, because of the unknowns associa ted with variable amplitude load excursions, the limiting case can be asses sed, and this can be used to determine margins and associated factors. Uncertainties may arise because of the effects of temperature and hold time s on plasticity, and where the crack growth and instability laws have not b een properly verified. So long as steps are taken to safeguard the situatio n against these uncertainties, possibly by means of verification tests wher e needed, it is more appropriate to use tearing under normal operating cond itions than to limit the analysis to initiation. (C) 2000 Elsevier Science Ltd. All rights reserved.