The unzipping model of fatigue crack growth and its application to a two-phase steel
Tóm tắt
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.
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