On the Time and Loading Rate Dependence of Crack-Tip Fields at Room Temperature—A Viscoplastic Analysis of Tensile Small-Scale Yielding

A finite-element analysis of stationary cracks in Mode I small-scale yielding is presented. It is asserted that the deformation of structural metals at room temperature and quasi-static loading rates can be significantly time dependent and so the theory of viscoplasticity based on total strain and over-stress is employed. Loading levels are controlled by the elastic stress-intensity factor K while loading rates are governed by the time rate of change of K. For loading rates varying over six orders of magnitude in the quasi-static range followed by subsequent periods of sustained loading, significant time dependence of the crack-tip fields is predicted. Interpreting the results using two different fracture criteria, it is predicted that the value of K at which crack growth is initiated is loading-rate dependent. Moreover, it also follows that strain-induced crack growth could be initiated during periods of sustained load. The results are strongly suggestive that time-dependent deformation should be considered in design against fracture even at room temperature. Appropriate design approaches, based on the special features of the viscoplasticity model employed, are proposed.