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Title: Fatigue design criteria for austenitic stainless steels based on simplified elastic-plastic analysis
Author: Clarkson, David Macarthur
ISNI:       0000 0004 9357 1661
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2020
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Pressure-retaining components of civil light-water reactor (LWR) plants are susceptible to low-cycle fatigue damage throughout their operational life. In the UK civil nuclear industry, the assurance of such components against fatigue failure has traditionally been achieved by satisfying the elastic design-by-analysis (DBA) criteria outlined in Section III of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC). The demonstration of a fatigue usage factor of unity against an S-N fatigue design curve forms the basis for establishing acceptable designs. The ASME III procedure is deterministic, and it is presumed that uncertainties are accounted for by conservatism arising from the largely unquantified margins imposed by the original Code authors, and accumulated from the use of pessimistic input variables and methodological assumptions in the assessment. Whilst this conservatism was tolerable in the past, the emergent industry understanding of the deleterious effect of the LWR coolant environment on fatigue life and its strong dependence on strain rate and temperature for austenitic stainless steels has spawned additional regulatory requirements to incorporate LWR environmental effects into traditional Code fatigue assessments. Consequently, the application of extant assessment methods, now exacerbated by environmental fatigue penalty factors, can pose difficulties in satisfying Code requirements for some critical components, potentially introducing unnecessary design constraints and an additional in-service inspection burden. It is well understood that the current ASME III procedure for fatigue evaluation neglects several key variables, is often very conservative and provides an unquantified design margin, and thus does not provide a consistent measure of component risk. The desire for longer plant design life and the potential for civil plants to adopt flexible modes of operation has increased the urgency to develop a more accurate fatigue evaluation procedure, recognising that the traditional design margins, and indeed the acceptance criterion itself, may not be fit for purpose when considering modern plant performance requirements and economic constraints Accordingly, several actions have been initiated as part of the 'ASME 2025 Nuclear Code' initiative to modernise the existing fatigue design rules. This includes future code development to adopt a risk-informed design methodology based on probabilistic methods with target reliability as an acceptance criterion for fatigue. To this end, ASME has commenced development of a new plant system design standard for establishing plant system and component reliability targets. In the UK, probabilistic methods for fatigue assessment are also gaining traction within industry, and are currently under consideration by the Department for Business, Energy and Industrial Strategy (BEIS) and the Technical Advisory Group on the Structural Integrity of High Integrity Plant (TAGSI) in anticipation of their future application in nuclear plant safety cases. Adopting a risk-informed design methodology will require improved accuracy of predicting fatigue crack initiation, which in pressure vessels is strongly influenced by the plastic strain range experienced on the component surface. ASME III prescribes simplified elastic-plastic analysis procedures wherein the plastic strain range may be estimated from elastic analysis using a plasticity correction factor (Ke). However, the existing approach is recognised to be very conservative, especially for ductile materials such as austenitic stainless steels. The aim of this work is to investigate these conservatisms and develop alternative approaches for simplified elastic-plastic fatigue analysis of austenitic stainless steel components with improved accuracy and practicality, suitable for future application to probabilistic fatigue initiation analysis. Extant Ke methods prescribed within various nuclear and non-nuclear codes and standards are reviewed to understand their relative advantages and limitations. A framework is proposed for calculating the actual plasticity correction factor (KeFEA) implied by detailed elastic-plastic analysis. A large number of elastic-plastic finite element analyses are performed for a range of case studies considering plant representative components and loading conditions. The performance of the various code Ke factors are evaluated and compared. Two alternative approaches -€“ the Global Plasticity Correction Factor (Fg) and Stress-Modified Neuber (SMN) methods -are proposed and validated against the compiled KeFEA results. Both approaches are shown to be fully compatible with existing methods for assessing environmentally assisted fatigue. Through a benchmark problem, the proposed methods are demonstrated to give a more appropriate evaluation of fatigue usage, enabling significant improvements in component design life.
Supervisor: Mackenzie, Donald Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral