Crack closure

Crack closure is a phenomenon in fatigue loading, where the opposing faces of a crack remain in contact even with an external load acting on the material. As the load is increased, a critical value will be reached at which time the crack becomes open. Crack closure occurs from the presence of material propping open the crack faces and can arise from many sources including plastic deformation or phase transformation during crack propagation, corrosion of crack surfaces, presence of fluids in the crack, or roughness at cracked surfaces.. Crack closure is a phenomenon in fatigue loading, where the opposing faces of a crack remain in contact even with an external load acting on the material. As the load is increased, a critical value will be reached at which time the crack becomes open. Crack closure occurs from the presence of material propping open the crack faces and can arise from many sources including plastic deformation or phase transformation during crack propagation, corrosion of crack surfaces, presence of fluids in the crack, or roughness at cracked surfaces.. During cyclic loading, a crack will open and close causing the crack tip opening displacement (CTOD) to vary cyclically in phase with the applied force. If the loading cycle includes a period of negative force or stress ratio R {displaystyle R} (i.e. R < 0 {displaystyle R<0} ), the CTOD will remain equal to zero as the crack faces are pressed together. However, it was discovered that the CTOD can also be zero at other times even when the applied force is positive preventing the stress intensity factor reaching its minimum. Thus, the amplitude of the stress intensity factor range, also known as the crack tip driving force, is reduced relative to the case in which no closure occurs, thereby reducing the crack growth rate. The closure level increases with stress ratio and above approximately R = 0.7 {displaystyle R=0.7} , the crack faces do not contact and closure does not typically occur. The applied load will generate a stress intensity factor at the crack tip, K {displaystyle K} producing a crack tip opening displacement, CTOD. Crack growth is generally a function of the stress intensity factor range, Δ K {displaystyle Delta K} for an applied loading cycle and is Δ K = K max − K min {displaystyle Delta K=K_{ ext{max}}-K_{ ext{min}}} However, crack closure occurs when the fracture surfaces are in contact below the opening level stress intensity factor K < K op {displaystyle K<K_{ ext{op}}} even though under positive load, allowing us to define an effective stress intensity range Δ K eff {displaystyle Delta K_{ ext{eff}}} as Δ K eff = K max − K op {displaystyle Delta K_{ ext{eff}}=K_{ ext{max}}-K_{ ext{op}}} which is less than the nominal applied Δ K {displaystyle Delta K} . The phenomenon of crack closure was first discovered by Elber in 1970. He observed and confirmed that a contact between the fracture surfaces could take place even during cyclic tensile loading. The crack closure effect helps explain a wide range of fatigue data, and is especially important in the understanding of the effect of stress ratio (less closure at higher stress ratio) and short cracks (less closure than long cracks for the same cyclic stress intensity). The phenomenon of plasticity-induced crack closure is associated with the development of residual plastically deformed material on the flanks of an advancing fatigue crack..