Developments in Quantification and Use of Environmental Cracking Mechanisms for Design and Lifetime Prediction

Abstract

ABSTRACTFollowing a long period of qualitative assessment of environmental cracking phenomenology, there has been recent emphasis on developing quantitative, mechanistically-based predictive models. This focus is driven by scientific and engineering concerns. For example, neither pressure vessel design nor evaluation codes incorporate the contribution of environmental cracking understaticloading, nor do they differentiate between the large possible variations in both environment and material chemistry. Similarly, while numerous scientific hypotheses have been proposed to explain the crack advance process and related, contributing phenomena, ultimately their validity must withstand the scrutiny of quantification and comparison to measured cracking kinetics and observed component lifetimes.The benefits and approaches involved in predicting environmental cracking based on mechanistic modeling of crack advance and its underlying phenomena are discussed. The benefits and approaches involved in predicting environmental cracking based on mechanistic modeling of crack advance and the underlying phenomena are discussed. The formulation of a quantitative model from fundamental mechanisms has progressed most rapidly for the film rupture / slip dissolution mechanism of environmental crack advance. Specifically, for iron and nickel base alloys in high temperature water, identification, quantification and integration of the important underlying parameters has led to algorithms which predict crack growth rates over a wide continuum in static and dynamic stressing, metallurgical microstructure and solution chemistry applicable to light water reactors. For widely studied systems, e.g., stainless and low alloy steels in 288°C water, the predictions are within a factor of 2–3 for 90% of the data which span “initiation”, the possible roles of contributing, “secondary” crack advance processes in oxidation-based mechanisms, such as environmentally enhanced cleavage, spontaneous oxidation, etc.Similar efforts to identify and quantify the critical parameters and underlying processes in crack advance mechanisms involving hydrogen embrittlement, environmentally assisted cleavage, etc. are summarized. Conceptual approaches to and progress toward integrating these models with in-situ chemical and crack monitoring devices, with the goal of providing on-line evaluation, control and lifetime prediction of environmental cracking, are also discussed.