Numerical modeling of fatigue damage for automotive steels

Today, machine elements are predominantly exposed to multiaxial cyclic loading. For this reason, it is especially important from the industrial perspective to determine the damage caused by fatigue. In recent years, with the development of computer technology, damage estimations are carried out with the finite element method. However, nowadays, finite element-based software that makes damage prediction requires a separate analysis for each loading condition and also uses linear elastic approaches for stress-strain results at the root of the notch. This situation causes time and accuracy loss in damage estimations. In the scope of this thesis, a finite element modelling method based on cyclic plasticity to determine fatigue damage and crack initiation is proposed. The proposed method is applied through utilization of user defined subroutines. The main innovation brought within the scope of the doctoral dissertation is that the proposed method can realize the complete solution of equilibrium and constitutive equations in finite element analysis based on the combined use of cyclic plasticity and damage theories. At the same time, the integrated damage models can be used to estimate the fatigue life of metal alloys in both body-centered cubic and face-centered cubic structures, and the desired damage model can be integrated into the method. Within the scope of the thesis study, firstly, the cyclic plasticity and damage models used in the study were implemented into a user-defined subroutine with the help of a fortran code. The proposed method was first applied in the damage estimation of three different test samples known in the literature. As a result of the studies carried out, it has been shown that the proposed method gives successful results in different notch geometries and the method can be applied easily compared to traditional damage estimation methods and it prevents time losses due to the fact that it can be solved with more than one damage model at the same time. As a last step the method is applied on an industrial part and it has been determined that the method is also applicable for complex problems.