TDFAD Analysis of Creep Crack Initiation in Pre-Strained and As-Received Type 316H Stainless Steel

Abstract

The time dependent failure assessment diagram (TDFAD), an extended form of the low temperature FAD used in the R6 procedure, has been developed to allow creep crack initiation to be predicted using a FAD-based approach. The TDFAD has a number of advantages compared to other methods for predicting creep crack initiation time: detailed calculations of crack tip parameters such as C* are not needed; it is not necessary to establish the fracture regime in advance and the TDFAD can indicate whether initiation occurs under small-scale or widespread creep conditions. The TDFAD approach relies on the availability of appropriate creep toughness, Kcmat, values. In this work, creep toughness data have been derived from experimental creep crack growth (CCG) tests on compact tension fracture mechanics specimens made of Type 316H stainless steel in as-received (AR) material and material that has been pre-compressed (PC) by up to 8% plastic strain. The influence of material pre-conditioning on creep toughness is examined. TDFADs are then constructed for each material condition and creep crack initiation (CCI) time predictions are obtained for the CCG tests performed on the Type 316H steel at 550 °C. The influence of material condition on the predicted CCI mode is examined and discussed.