Abstract
The unzipping analysis, based on the alternate shear deformation process of two intersecting shear planes at a crack tip, is extended to study fatigue crack growth in a two-phase martensitic-ferritic steel. The unzipping crack increment Δa uz is directly related to ΔK and ΔJ in the case of small scale yielding. It is preferrable to use Δa uz is directly related to ΔK and ΔJ in the case of small scale yielding. It is preferable to use Δa uz as a physical parameter to correlate with the growth rates of micro-cracks and fatigue cracks in a multi-phase material. In the case of micro-cracks, ΔK is often not applicable because of extensive plastic deformation; and in the case of multi-phase material, neither ΔK nor ΔJ is applicable because of material inhomogeneity. The effective ΔK, ΔK eff, is defined in terms of Δa uz. The relations between the endurance limit of a two-phase steel and crack nucleus size, ferrite layer thickness, the constraint by the strong martensite on crack tip deformation in the ferrite domain, and ΔK th's of the martensite and ferrite are analyzed.
Résumé
Une analyse de rupture progressive et continue des liaisons, basée sur un processus de déformation de cisaillement alterné de deux plants de cisaillement s'intersectant à l'extrémité d'une fissure, a été étendue à l'analyse de la propagation des fissures de fatigue dans un acier martensito-ferritique à deux phases. L'accroissement de la fissure Δa est directement en relation avec ΔK et ΔJ dans le cas de déformation plastique de faible étendue. II est préférable d'utiliser Δa comme paramètre physique en corrélation avec les vitesses de croissance de microfissures et des fissures de fatigue dans un matériau à phases multiples. Dans le cas de microfissures, ΔK n'est souvent pas applicable en raison de la déformation plastique importante. Dans le cas de matériau multiphase ni ΔK ni ΔJ ne sont applicables en raison de l'inhomogénéité du matériau. La valeur effective ΔK eff est définie en terme de Δa. Les relations entre la limite d'endurance d'un acier à deux phases et la taille du nodule de fissuration, l'épaisseur de la couche de ferrite, la contrainte qu'exerce une zone martensitique dure sur le domaine ferritique, sur la déformation à l'extrémité de la fissure en domaine ferritique, et les valeurs de ΔK de la martensite et de la ferrite sont analysées.
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Yang, C.Y., Liu, H.W. The unzipping model of fatigue crack growth and its application to a two-phase steel. Int J Fract 17, 157–168 (1981). https://doi.org/10.1007/BF00053518
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DOI: https://doi.org/10.1007/BF00053518