Most fatigue life prediction concepts are based on the transfer of some characteristic material data to the component under consideration. The implicitly assumed equivalence can be limited by various factors including different surface conditions, sizes, residual stress fields, and cumulative damage-related items such as transient endurance limit and load-dependent failure mechanisms. This paper concentrates on aspects related to cumulative damage and presents a model for cumulative damage assessment. It allows prediction of crack initiation life where crack initiation means the occurrence of cracks of an engineering size (≃ 1 mm). Though formulated in terms of a cumulative damage calculation, it is based on the consideration of short crack growth behavior. The main features are: • Consideration of load sequence dependent crack opening and closing levels controlled by the elasto-plastic strain history. • Damage sum (i.e., crack length) dependent decrease of the fatigue limit. • Derivation of a crack-driving parameter based on elasto-plastic fracture mechanics.
An experimental test program including two steels and an aluminum alloy and several types of loading spectra revealed the improved accuracy of the model in comparison with previous approaches. The improvement can be attributed to the consideration of the most important aspects determining the physical process of the formation and growth of short cracks under variable amplitude loading.