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Crack closure is a phenomenon in fatigue loading, during which the crack remains in a closed position even though some external tensile force is acting on the material. During this process the crack opens only at stress above a particular stress. This is due to factors such as plastic deformation or phase transformation during crack propagation, corrosion of crack surfaces, presence of fluids in the crack, or roughness at cracked surfaces. This provides a longer life for fatigued material than expected, by slowing the crack growth rate. The crack closure effect helps explain a wide range of fatigue data. It has become the default interpretation of load ratio〔 effects. It is used in almost all fatigue life prediction models. However, it is virtually impossible to predict the effects of crack closure experimentally. ΔKth = ΔKmax - ΔKmin ΔKeffective = ΔKmax – Kopening ΔKeffective ≤ ΔKth ==Plasticity induced crack closure== Plasticity-induced closure results from compressible residual stresses developing in the plastic wake. This concept which was advanced and generally accepted in the 1970s assumes that a plastically transformed area is formed at the crack tip which leaves a wake of plastically deformed zone along the crack length. This zone has residual compressive stress induced by the elastic and plastic deformation of the material during unloading. During the next cycle, while loading, the crack tip does not open unless the applied load is enough to overcome the residual compressive stress present in the plastic wake zone. Thus the effective stress at the crack tip is lowered.〔•Engineering Fracture Mechanics by Prof. K. Ramesh, Department of Applied Mechanics, IIT Madras〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Crack closure」の詳細全文を読む スポンサード リンク
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