APPROXIMATE INELASTIC ANALYSIS OF DEFECTIVE COMPONENTS

In the inelastic analysis of defective components at high temperature, the crack tip characterising parameters are of interest for a range of loading conditions. In this Paper, four situations are examined: (1) steady state creep under constant load; (2) the transitional behaviour under constant load prior to attainment of steady state conditions; (3) behaviour under displacement control; and (4) behaviour under displacement-controlled cycling with hold periods. For steady state creep, it is shown by comparison with finite-element solutions and experimental data that a reference stress approximation may be used to predict the parameter C* with reasonable accuracy. The transitional period prior to steady-state conditions is examined by finite-element calculations, and the time-dependent parameter C(t) [C(t) --> (C* as t --> infinity] is evaluated for a number of geometries under constant load. The results are predicted by extending the steady state reference stress approximation. Comparisons are made between C(t) and the C(t) parameter of Saxena. Finite-element results are also reported for geometries creeping under a constant applied displacement. Under this loading, the parameter C(t) falls rapidly with time. The reference stress method is used to predict the rate at which the applied loading falls, and then to estimate the value of C(t). It is shown that the amount of crack growth associated with displacement-controlled loading is much less than under load control at the initial load. A simplified estimate is made of the total crack growth during a period of constant displacement, and compared with the experimental data for steels of both high and low creep ductility. Under displacement-controlled cycling, finite-element calculations involving hold periods would be extremely time consuming. It is shown that the simplified results developed for constant-displacement loading may also be applied to predict the experimental behaviour under cyclic loading provided the material creep data used in the estimates are obtained under relevant cyclically hardened or softened conditions.