Correlations between crack initiation and crack propagation lives of notched specimens under constant and variable amplitude loading

摘要

This paper starts with an overview of the application of the three guidelines (GL) of the German Research Association of Mechanical Engineers (FKM). Each of these provides algorithms for calculating fatigue lives of components under constant or variable amplitude loading, however, with underlying different failure criteria, that is, technical crack initiation life (GL-nonlinear), fatigue crack growth life (GL-fracture mechanics), and total fracture life (GL-linear). Since the GL-linear only considers linear-elastic material behavior in the fracture fatigue life calculation, the guideline is not capable for the LCF and is not able to take sequence effects into account. For a better approximation of fracture fatigue lives, the elastic-plastic material behavior has to be taken into account. This paper introduces the U-Concept which has been evaluated from a large structural durability database from the literature with more than 1,500 experiments for constant and more than 700 experiments for variable amplitude loading. The U-Concept is a small add-on to the local strain approach (LSA) which is the backbone of the GL-nonlinear. It enables (1) to directly calculate the fatigue life to total fracture based on elastic-plastic material behavior according to the LSA or (2) to estimate the remaining fatigue life from crack initiation to fracture without a crack growth simulation. Furthermore, the U-Concept needs no experimental determined crack growth parameters, the only necessary material parameter is the ultimate tensile strength R_m. The U-Concept is therefore easy applicable for industrial users and increases the accuracy of the approximated fatigue lives by considering the elastic-plastic material behavior.