Abstract
The fatigue life design curves in nuclear codes are generally derived from uniaxial straincontrolled fatigue test results. Evidently, the test conditions are very different from the actual components loading context, which involves much more complex thermo-mechanical loading including mean stress, static load holding time and variation in water chemistry, etc. In this work, the mean stress and environmental effects on fatigue life of 316L austenitic stainless steel in air and light water reactor (LWR) environment were studied using hollow fatigue specimens and testing under load-controlled condition. Both positive (+50 MPa) and negative (-20 MPa) mean stresses showed beneficial effect on fatigue life in LWR environment and in air. This is tentatively attributed to mean stress enhanced cyclic hardening, which leads to smaller strain response at the same loading force. -20 MPa mean stress was found to increase fatigue limit, whereas the effect of +50 MPa mean stress on fatigue limit is still unclear. The preliminary results illustrate that the environmental reduction of fatigue life is amplified in load-controlled fatigue tests with tensile mean stress.
Highlights
Fatigue is a common mode of failure of structural materials
It relies on the use of fatigue curves and endurance limits, derived mainly from straincontrolled low-cycle fatigue (LCF) tests with small, smooth specimens in air at room temperature, which do not explicitly consider the possible effects of light water reactors (LWR) environments [1]
The design margins of 2 and 20 on stress/strain and cycles, respectively, in the ASME Code design curve were intended to cover the effects of some variables that can influence the fatigue life of components, but were not investigated in the tests, which provided the original data for the curves
Summary
Fatigue is a common mode of failure of structural materials. Austenitic stainless steels (SSs) are extensively used as structural materials in aggressive environments, such as in pressure-boundary components in the primary reactor coolant circuit of light water reactors (LWR). The design margins of 2 and 20 on stress/strain and cycles, respectively, in the ASME Code design curve were intended to cover the effects of some variables (e.g., surface finish/roughness, material variability, load sequence, size effects and atmosphere) that can influence the fatigue life of components, but were not investigated in the tests, which provided the original data for the curves They did not intend to cover the effects of LWR environments. Based on laboratory investigations, different proposals [7, 8], such as Environmental Factors Approach, US NRC Regulatory Guide 1.207 and NUREG/CR-6909 or other national equivalents (JSME in Japan), were established for incorporating environmental effects into the fatigue design procedure of the ASME Code These procedures are related to some relevant uncertainties and potential undue conservatism and their practical application is rather complex. Load- and strain-controlled fatigue tests were performed with hollow specimens in air and hydrogenated high-temperature water and the first preliminary results of these experiments are discussed in this paper
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