Environmental Influences on the Fatigue Assessment of Austenitic and Ferritic Steel Components Including Welds

Abstract

The fatigue assessment of safety relevant components is of importance for ageing management with regard to safety and reliability. For cyclic stress evaluation, different country specific design codes and standards provide fatigue analysis procedures to be performed considering the various mechanical and thermal loading histories and geometric complexities of the components. For the fatigue design curves used as limiting criteria, the influence of different factors like e.g. environment, surface, temperature and data scatter must be taken into consideration in an appropriate way. In this context there is a need of consolidating and increasing the current knowledge. In the framework of an ongoing three years German cooperative project performed by Materials Testing Institute MPA Stuttgart and AREVA GmbH (Erlangen) it is the aim to both improve the state of the art based on an experimental program on the factors mentioned above including hold-times at transient free static load and on the derivation of a practicable engineering fatigue assessment concept. Emanating from a review of the current state of the art the cooperative project is split up into three major parts: 1) Experimental investigations concerning the influence of loading parameters and environmentally assisted fatigue (EAF) effects (light water reactor environment) on the fatigue strength of ferritic steels including weldments. 2) Experimental investigations concerning the influence of long hold times and the EAF effects on the fatigue strength of austenitic and ferritic steels. 3) The results of the outlined experimental program and published results will constitute the input for the proposal of an engineering fatigue assessment concept. This concept includes the differentiation between numerous factors of influence as an essential feature. In this context the margins between mean data curves and design curves are to be discussed in detail considering the factors of influence in general and EAF in particular. Based on a comprehensive consolidation of the state of the art and previous investigations in air and in light water reactor environment an experimental program is set up with the following key aspects: - Strain controlled fatigue tests on welded (microstructure of the weldment excluding microscopic and macroscopic weld notch effects) and unwelded smooth laboratory specimens subjected to constant and variable strain amplitude loading in air and light water reactor environment. - Strain controlled fatigue tests on notched specimens for the consideration of multi-axiality effects in air and light water reactor environment. - Strain controlled fatigue tests on smooth round laboratory specimens in air and in light water reactor environment focusing on long (power plant relevant) hold time effects.