Micromechanical Modeling of Fatigue Crack Nucleation around Non-Metallic Inclusions in Martensitic High-Strength Steels

Abstract

Martensitic high-strength steels are prone to exhibit premature fatigue failure due to fatigue crack nucleation at non-metallic inclusions and other microstructural defects. This study investigates the fatigue crack nucleation behavior of the martensitic steel SAE 4150 at different microstructural defects by means of micromechanical simulations. Inclusion statistics based on experimental data serve as a reference for the identification of failure-relevant inclusions and defects for the material of interest. A comprehensive numerical design of experiment was performed to systematically assess the influencing parameters of the microstructural defects with respect to their fatigue crack nucleation potential. In particular, the effects of defect type, inclusion–matrix interface configuration, defect size, defect shape and defect alignment to loading axis on fatigue damage behavior were studied and discussed in detail. To account for the evolution of residual stresses around inclusions due to previous heat treatments of the material, an elasto-plastic extension of the micromechanical model is proposed. The non-local Fatemi–Socie parameter was used in this study to quantify the fatigue crack nucleation potential. The numerical results of the study exhibit a loading level-dependent damage potential of the different inclusion–matrix configurations and a fundamental influence of the alignment of specific defect types to the loading axis. These results illustrate that the micromechanical model can quantitatively evaluate the different defects, which can make a valuable contribution to the comparison of different material grades in the future.

Investigation of Size Effects Due to Different Cooling Rates of As-Quenched Martensite Microstructures in a Low-Alloy Steel

“…For the experimental investigations in this work, referring to previous studies, the material 50CrMo4 (SAE 4150) is used [1,22,40,41]. For the sake of completeness, mean values of the chemical composition are documented in Table 1.…”

Creep-Fatigue Crack Initiation Simulation of a Modified 12% Cr Steel Based on Grain Boundary Cavitation and Plastic Slip Accumulation

“…Fatigue indicator parameters (FIPs) can be used to reflect the mesoscale deformation mechanism of fatigue crack initiation as well as the fatigue damage evolutions, as a means of correlating the local microstructure with the most likely locations of fatigue crack initiation, and have been a dominant approach to modelling fatigue crack initiation at the grain scale recently [ 46 , 47 ]. The basic assumption is that irreversible slip generated in the slip system will lead to fatigue damage.…”

Fatigue Design and Defects in Metals and Alloys

“…Twelve research articles have been published in this Special Issue of Metals. The subjects are multidisciplinary, including (i) the effect of microstructural defects on the high and very high cycle fatigue strength of metallic materials [1][2][3][4][5], (ii) probabilistic fatigue prognosis incorporating the size of defects and control volume [2,4,6], (iii) the effects of plasticity, high temperature, and aggressive environment on the fatigue response [7][8][9], (iv) thermo-chemo-mechanical treatments to mitigate the detrimental effect of defects on the fatigue strength [10,11], and (v) non-destructive techniques to detect fatigue critical defects [12].…”