Creep crack growth of seam-welded P22 and P91 pipes with artificial defects. Part II. Data analysis

Abstract

This paper examines several methods for assessing experimental creep and fatigue crack growth data obtained on P22 (2.25Cr1Mo) and P91 (9Cr1MoVNb) axially notched, seam-welded pipes tested at 565 and 625 °C, respectively [Creep crack growth of seam-welded P22 and P91 pipes with artificial defects—part I: experimental study and post-test metallography. Second International HIDA Conference, Advances in Defects Assessment in High Temperature Plant, MPA, Stuttgart, Germany, 4–6 October, 2000]. The overall objective of this work is to identify the nature of any correlation between component and conventional testpiece creep crack growth rates and thereby provide a supplementary tool for structural integrity analysis. Creep crack growth rate of the notch located in the heat-affect-zone of the weld was assessed in terms of both stress intensity factors, K I, and the C ∗-integral. To estimate the C ∗-integral, reference stresses were developed by deriving limit load solutions which reconcile the different collapse loads of the axially notched pipes. Both minimum and average creep rate laws were utilised in the analysis to accommodate the strain rate in the C ∗ relation. Each test was examined independently, but the general conclusion from each analysis was the same, in that C ∗-integral, rather than the stress intensity factor, gave better correlation with respect to conventional data generated using compact tension (CT) specimens. The assessment of creep crack growth demonstrates one particular aspect of interest. In terms of the C ∗ based correlation of creep crack growth rates, the analysis was found to be relatively independent of the stress state and correlates well with CT specimen data when appropriate reference stresses are used. In addition, cracking in the tested pipes was observed to occur between plane stress and plane strain conditions, inferring that both creep ductility and ligament straining contribute towards the failure mechanism.