Notch Support for LCF-Loading: A Fracture Mechanics Approach

Abstract The demand for an increase in flexible operation of power plants provides a key challenge. Solutions for new components as well as for units in operation have to be developed to safely allow for more load cycles and faster start-up times. One way to realize this is to improve the lifetime assessment methods by reducing implied conservatisms. The presented work focuses on the description of notch support effects at elevated temperatures for the LCF-regime. At first, a tailored experimental set-up using notched round-bars is introduced. Those tests are performed under global strain control and are equipped with an Alternating Current Potential Drop (ACPD) measurement system. The experimental results show that the crack growth phase up to a so called "technical crack size" is of major relevance to describe notch support effects. That is why within the second part of this paper, a concept is introduced to consider this early crack growth phase within a lifetime assessment approach. Within this concept, fracture mechanical parameters are determined by applying a novel energy-based Finite Element approach. Suggestions are presented to define proper values of the cyclic J -Integral as well as of the amount of Plasticity Induced Crack Closure (PICC). Finally, the concept is applied and validated by comparing the simulation approach with the experimental results.