Deformation-induced martensitic transformation (DIMT) has been used for designing high-performance alloys to prevent structural failure under static loads. Its effectiveness against fatigue, however, is unclear. This limits the application of DIMT for parts that are exposed to variable loads, although such scenarios are the rule and not the exception for structural failure. Here we reveal the dual role of DIMT in fatigue crack growth through in situ observations. Two antagonistic fatigue mechanisms mediated by DIMT are identified, namely, transformation-mediated crack arresting, which prevents crack growth, and transformation-mediated crack coalescence, which promotes crack growth. Both mechanisms are due to the hardness and brittleness of martensite as a transformation product, rather than to the actual transformation process itself. In fatigue crack growth, the prevalence of one mechanism over the other critically depends on the crack size and the mechanical stability of the parent austenite phase. Elucidating the two mechanisms and their interplay allows for the microstructure design and safe use of metastable alloys that experience fatigue loads. The findings also generally reveal how metastable alloy microstructures must be designed for materials to be fatigue-resistant.
Quantitative thermographic methodology (QTM), which takes energy dissipation as a fatigue indicator, has been successfully applied to predict the fatigue life of materials and welded joints under constant amplitude loading. This study advances the QTM approach for predicting the fatigue life under variable amplitude loading in both low and high cycle fatigue regimes. Experimental data, obtained by fatigue tests under variable amplitude loading, were used in order to apply the developed QTM approach and to demonstrate that it is able to take into account the loading sequence effect. Good predictions of the fatigue life were achieved.
This paper focuses on a comparative study of the critical plane approach (CPA) and the dissipated energy approach (DEA) for multiaxial fatigue evaluation on an AISI 316L steel. It is a follow-up to our previous study on the development of a DEA model for multiaxial fatigue life prediction. The results demonstrate that the developed DEA model can provide overall better performance compared with some classic CPA models both in terms of prediction accuracy and versatility to multiaxial loading paths. An in-depth crack mechanism investigation reveals that the multiaxial fatigue life is mostly spent in the formation of micro-cracks, which exhibits distinct characteristics under the different loading paths examined. This effect can be better considered by using DEA model compared with the CPA models adopted.
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