Numerical Approach for the Consideration of Hold Time Effects

In recent years the Environmentally Assisted Fatigue (EAF) became an item, which has to be considered additionally in terms of ensuring a conservative determination of the actual component’s health status resp. the CUF. For practical application, the consideration of the so called Fen-factor leads to the reduction of the admissible cycles in fatigue calculations. Beyond that the influence of elevated temperatures has been identified as one parameter having a negative influence on the admissible cycles as well. For example the German KTA 3201.2 defines for austenitic steels separate fatigue curves for temperatures above 80°C and for temperatures below 80°C. In summary on the one hand parameters influencing component’s lifetime negatively have to be considered in terms of conservative calculations. On the other hand, there are other parameters which influence the component’s fatigue lifetime in a positive manner. As such positive effects are neglected so far, CUF allowing for EAF tend to become over conservative leading to oversized components. Therefore, positive effects should be considered as well in the framework of a comprehensive and detailed analysis making sure not to overdesign components.When taking a closer look on the operational behavior of primary circuit components, fatigue loading is mainly defined by long steady-state periods with no significant changes in the loadings and by normally short outage periods with no thermal loading. For example fatigue of a PWR surge-line is mostly caused by short in-surge and out-surge events during start-up or shut-down of the plant. Normal operation transients mostly not cause fatigue relevant events in the surge-line. Fatigue of PWR spray-lines is primarily generated by very few spray-events during a one-year period of operation. Spray events are mainly caused by significant load ramps. Subsequently the fatigue status of primary circuit components is controlled by long periods with no fatigue relevant loading at operating temperature and few additional loading patterns in between. Experimental investigations have shown that hold time effects have a positive influence on fatigue lifetime of austenitic stainless steel materials.Anyhow, no quantification of these effects has been published in recent years. Within this publication an engineering based approach will be developed to quantify the hold time effect based on literature and published data. On the basis of a practical example the influence of hold time effects will be quantified and a direct comparison to lifetime reducing effect of EAF and temperature will be drawn.© 2014 ASME