Study of Fatigue Initiation of Austenitic Stainless Steel in a High Temperature Water Environment and in Air Using Blunt Notch Compact Tension Specimens

Abstract

Fatigue life assessment procedures for components exposed to high temperature pressurised water environments are typically based on NUREG/CR-6909 or broadly similar codes (e.g. proposed ASME code case N-792). The effects of a high temperature water environment on the fatigue life are accounted for by simply adjusting the fatigue life in ambient temperature air by an environmental factor (FEN). This adjustment assumes that the environment affects both initiation (nucleation) and propagation equally, which is potentially over-conservative. Blunt notch compact tension (CT) specimens (along with direct current potential drop (DCPD) crack detection) have been proposed as a means of determining the “true” fatigue initiation life, enabling the relative impact of the environment on initiation and growth to be characterised and the level of conservatism in the FEN approach assessed. The current work uses a combination of finite element analysis and fatigue testing in both air and water to assess the feasibility of blunt notch CT testing to detect initiation and to quantify the environmental impact. This work indicates significant difficulties with the blunt notch CT test methodology both in terms of quantifying the applicable strain and in terms of detection of the very early stages of initiation which preclude the quantitative application of the technique to study true initiation. Qualitatively, the results suggest that there is still a significant impact of the high temperature water environment on the earliest detectable stages of crack growth in austenitic stainless steels; however the earliest defects detectible by DCPD techniques still involve a significant contribution from short crack growth. Nevertheless, the technique provides a valuable insight into initiation and residual life of components subjected to through wall strain gradients.

Final Report on CFD and Thermal-Mechanical Stress Analysis of PWR Surge Line under Transient Condition Thermal Stratification and an Evolutionary Cyclic Plasticity Based Transformative Fatigue Evaluation Approach without Using S~N Curve: Rev. 1

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Finite Element Based Full-Life Cyclic Stress Analysis of 316 Grade Nuclear Reactor Stainless Steel Under Constant, Variable, and Random Fatigue Loading

“…In recent years, several improvements to fatigue design (S $ N) curves have been recommended by various researchers. Some of the improvements are considering the effects of environments [6,7], surface finish [8], mean stress [9], and hold time [10]. To assess fatigue damage under variable/random amplitude loading using the fatigue design (S $ N) curves, currently, the linear damage accumulation rule [11] along with rainflow cycle counting method [12] is used.…”

Study the Cyclic Plasticity Behavior of 508 LAS under Constant, Variable and Grid-Load-Following Loading Cycles for Fatigue Evaluation of PWR Components

“…One method of accomplishing this has been factoring out the effect of the reactor coolant on the fatigue life estimations based on S~N curves [2][3][4]. Recent work by various researchers [5][6][7][8][9][10][11][12][13][14] has further demonstrated the value of the experimental and S~N based approach to fatigue life estimation.…”