Multiaxial Plastic Hardening Models used in Shape Optimization with respect to Fatigue Life

Abstract

The total fatigue life Nf of notched machine or structural elements is given as a sum of two phases (stages): the initiation stage Ni (fatigue crack initiation), and the second phase, fatigue crack propagation Np. For non sharply notched parts, 50% (some sources report even 85-90%) of the whole lifetime is connected with the initiation phase. Hence, the problem of predicting the critical number of loading cycles corresponding to crack initiation in a structural element is of fundamental importance for rational design with specified service life. When the elastic local stress and strain exceed the elastic limit, an elastic-plastic stress evolution occurs. The crack initiation is then dependent on the plastic dissipated energy and the stress at the notch root. A closely related problem is that of rational design of notch shape in order to maximize the critical number of cycles corresponding to crack initiation.