Prediction of scatter in creep crack growth data from creep failure strain properties

Procedures are available for determining the creep crack growth (CCG) properties of materials experimentally in terms of the creep fracture mechanics parameter C*. When these properties have not been measured, predictions can be made from models of the cracking process. These models show that creep crack growth rate at a given value of C is most sensitive to the creep ductility E; appropriate to the state of stress at a crack tip. For plane stress conditions, a is found that epsilon(f)* can be taken to be equal to the uniaxial creep ductility et of a material and for plane strain to be given by epsilon(f)/130. Hence, in this paper, uniaxial creep properties on type 316 L(N) austenitic stainless steel and two 9% Cr steels, designated P91 and P92, in the temperature range 500-750 degrees C are reported and scatter in the data identified from statistical analysis. For each steel, the dependence of creep ductility on stress and temperature is determined, It has been found that none of the rupture time (t(R)), creep strain rate (epsilon*), and net section stress (sigma(net)), parameters has a better relationship with the creep failure strain. Hence, none of these parameters characterizes better than the other the creep ductility's behavior. Additionally, it has been observed that although the creep failure strain of 316 L(N) is independent of these parameters, it increases with temperature. In contrast, the creep failure strain of P91 is slightly sensitive to sigma(net) but is overcompensated by the scatter and so can be assumed stress independent. The creep failure strain of P91 is seen to be also insensitive to temperature. For P92, epsilon(f) goes from a lower to an upper shelf through a transition region. Thus the best solution to characterize the failure strain behavior is to use a polynomial solution. Creep crack growth properties obtained on cracked compact tension and C-shaped specimens are also presented. Probabilistic analysis, based on Monte Carlo simulation methods, is employed to predict scatter in the creep crack growth data from scatter in the uniaxial creep properties. In most instances, close agreement is found between the experimentally measured scatter in the cracking results and that predicted from the scatter obtained from creep ductility data.