Martensite is applied as the main structure of high-strength steel to satisfy the demand for lightweight machines. As the fatigue crack extension life takes almost the whole fatigue life, the complex fatigue extension behavior needs to be further studied. This paper is planned to clarify the effect of the hierarchical microstructures and their interfaces on the fatigue extension and the fatigue crack extension mode for the safe and long-lasting use of this material. To achieve the proposal, the rotating bending fatigue tests of 18% Ni martenstic steel was carried out. Fatigue crack behavior and micostrcture near crack path was observed on the specimen surface. The crack extension was found to be discontinuous and was processed by the sub-crack initiation and coalescence with main crack. By the observation of the microstructure around crack path, the observed sub-crack was found to be the inter-granular crack. The proposed reason for the extension process was thought to be the strain localization by the slip along {110} plane and high angle microstructure intrefcae resistance to dislocation motion. Besides, the crack path included inter-granular and trans-granular crack. And the crack extension mode in this material was thought to be damage accumulation mode.
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To achieve high-strength steels, their microstructures are complicated. However, with effort, these high-strength steels do not exhibit the fatigue limits expected from their hardness or tensile strength. The low fatigue limit due to inclusions in the steels can be predicted as a fatigue limit problem for metallic materials with small defects. However, the threshold stress intensity factor range of high-strength steel of a long crack is still not as high as expected from the hardness. Currently, there is no clear explanation for this reason. Therefore, the material cannot be used with confidence. The authors propose that this is due to a different crack extension mechanism. In other words, the authors point out the existence of a different mechanism of fatigue crack extension from the generally accepted mechanism of fatigue crack extension due to plastic deformation by alternating slip. Based on the mechanism, the mode of fatigue crack extension is called damage accumulation mode fatigue crack propagation. This name differs from the conventional name focusing on the loading mode, i.e., Modes I, II, and III, and is focused on the extension mechanism. This study discusses a method to predict the fatigue crack propagation behavior.
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This work introduces a computational fracture model for Ti64 alloy based on coupled Crystal Plasticity Phase Field model for fracture but also considers the atomistic mechanisms of plasticity at the crack tip. Atomistic simulations are conducted to identify the crack-tip mechanisms of plasticity and the continuum scale phase field model is augmented to account for this. Using the data generated using atomic scale Molecular Dynamic simulations, a functional form describing the evolution of dislocation density nucleating from the crack tip is obtained using Bayesian Inference and Genetic Programming based Symbolic Regression. The effect of nucleated dislocations in crack path and rate of crack propagation is evaluated. The additional plastic strain at the crack tip is also validated with results from Molecular Dynamics.
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Numerous different crack growth modeling approaches have been developed to consider the short crack and long crack behaviors by accounting for the stress intensity range-based crack driving forces or the crack closure concept. However, those methods lacked a proper systematic approach to accurately account for the behavior of short cracks. Based on the recent systematic study performed in the authors’ group, a new generalized two-parameter driving force model is proposed to account for crack growth driving forces and corresponding crack growth thresholds to predict both short crack and long crack propagation behaviors. The model predicted crack growth rates are compared with crack growth data set of Ti-6Al-4V titanium and 2024-T3 aluminum alloys. Predicted results show good agreement with experimental crack growth data for these materials.
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