Probabilistic analysis of creep crack initiation and growth in pipe components

Abstract

In this paper creep data have been collected on uniaxial specimens of a carbon manganese (C–Mn) steel and a 9% Cr steel, designated P91, at 360 and 625 °C, respectively. Additional tests have been performed at these temperatures on precracked compact tension (CT) specimens and pressurized pipes and tubes containing axial cracks to measure the creep crack initiation (CCI) and creep crack growth (CCG) rate properties of these steels. All the data have been analysed statistically assuming log–normal distributions in properties. The results of this analysis have been incorporated into a Monte-Carlo simulation to predict the cracking behaviour of the pressurized pipes from the uniaxial and ‘benchmark’ CT specimen data. Good agreement has been obtained for the P91 steel but not for the C–Mn steel. The discrepancy observed for the C–Mn steel has been attributed to constraint effects, possible extensive plasticity which occurred on loading and to uncertainties in the determination of the creep fracture mechanics parameter C ∗ used for characterising the cracking behaviour. Further comparisons have been made with deterministic calculations involving different combinations of material properties in a sensitivity study to establish probabilities of failure in the pipes based on the ‘benchmark’ CT specimen.