PREDICTIONS OF NONSTEADY ASYMPTOTIC CRACK-TIP FIELDS IN POWER-LAW CREEPING MATERIALS

The applicability of accepted engineering procedures to predict the am plitude of singular crack tip fields during the transitional regime pr ior to steady state has been investigated. Estimates of both transient and steady state values of the C(t)-integral for simplified geometrie s are compared with results obtained from detailed finite element anal yses. Different creep exponents and both primary (mechanical) and seco ndary (displacement-controlled) loadings are considered. Estimates of the steady state value C were obtained from Nuclear Electric's high t ermperature assessment procedure, R5, which is based on reference stre ss concepts, and from the EPRI's fully plastic solutions for J-integra ls. The transient behaviour of C(t) was estimated using approximations given by Ehlers and Riedel (Advances in Fracture Research, Proc, Fift h Int. Conf, on Fracture, Vol. 2, Pergamon, New York, 1981, pp. 691-69 8), Saxena (Fracture Mechanics: Seventeenth Volume, ASTM STP 905, ASTM , Philadelphia, 1986, pp. 185-201), and the R5 procedure (Ainsworth et al., Fatigue Fract. Eng. Struct. 10 (1987) 115-127). In most cases, t he transient estimates given by the latter two were found to conservat ively predict the finite element results, although some underpredictio ns were encountered in the planar geometries shortly after loading. Th e recommended use of plane stress reference stresses with the R5 proce dure, however, provides overall conservatism in the values of C(t). Eh lers and Riedel estimates, though, are generally non-conservative, exc ept for the lowest crack-tip constraint configuration analysed (i.e, c entre cracked plate).