Abstract
Micromechanical fatigue lifetime predictions, in particular for the high cycle fatigue regime, require an appropriate modelling of mean stress effects in order to account for lifetime reducing positive mean stresses. Focus of this micromechanical study is the comparison of three selected fatigue indicator parameters (FIPs), with respect to their applicability to different total strain ratios. In this work, investigations are performed on the modelling and prediction of the fatigue crack initiation life of the martensitic high-strength steel SAE 4150 for two different total strain ratios. First, multiple martensitic statistical volume elements (SVEs) are generated by multiscale Voronoi tessellations. Micromechanical fatigue simulations are then performed on these SVEs by means of a crystal plasticity model to obtain microstructure dependent fatigue responses. In order to account for the material specific fatigue damage zone, a non-local homogenisation scheme for the FIPs is introduced for lath martensitic microstructures. The numerical results of the different non-local FIPs are compared with experimental fatigue crack initiation results for two different total strain ratios. It is concluded that the multiaxial fatigue criteria proposed by Fatemi-Socie is superior for predicting fatigue crack initiation life to the energy dissipation criteria and the accumulated plastic slip criteria for the investigated total strain ratios.
Highlights
Martensitic high-strength steels are distinguished by a high strength to weight ratio, an excellent weight and cost-effectiveness as well as a more environmental friendly energy balance than aluminium alloys
The micromechanical model described in the previous section is used to investigate the early stage of the fatigue crack initiation behaviour of the considered material, in a fundamental way
Comparisons of the three different non-local fatigue indicator parameters (FIPs) are performed for two considered strain ratios, Rε = −1 and Rε = 0
Summary
Martensitic high-strength steels are distinguished by a high strength to weight ratio, an excellent weight and cost-effectiveness as well as a more environmental friendly energy balance than aluminium alloys These attributes are of particular importance in today’s challenges such as the design of sustainable transportation systems [1]. Among these martensitic high-strength steels, the quenched and tempered SAE 4150 plays a pivotal role due to its excellent strength and toughness. These specific material properties are directly related to the unique hierarchical martensitic microstructure being an assembly of of laths, blocks and packets [2,3]. A fundamental prerequisite for the prediction of components fatigue life is the adequate modelling of the influence of varying load ratios on the cyclic fatigue behaviour due to the lifetime
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