Crack closure

Crack closure is a phenomenon in fatigue loading, during which the crack will tend to remain in a closed position even though some external tensile force is acting on the material. During this process the crack will open only at stress above a particular crack opening stress. This is due to several factors such as plastic deformation or phase transformation during crack propagation, corrosion of crack surfaces, presence of fluids in crack, or roughness at cracked surfaces etc. this will provide a longer fatigue life for the material than expected, by slowing the crack growth rate.

The crack closure effect has been successful in empirical determination of 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 in an experimental method.

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. In this concept it is assumed as a plastically transformed area is formed at the crack tip which leaves a wake of plastically deformed zone along the crack length. This zone will be having 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 until 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.

Phase transformation induced crack closure

this kind of crack closure is common in pressure vessels and other fluid related areas. In this concept while opening of the crack occurs under loads, the crack is filled with fluid from the surrounding which wedges open the crack during unloading. Hence, in cyclic loading the effective stress required for opening the crack is increased.

Oxide induced crack closure

this occurs in aggressive environment, where rapid corrosion of crack occurs during crack propagation. Here also the effect is to wedge a crack open during the fatigue loading, due to corroded particles present in crack. And hence the effective stress required is lowered as in the case of fluid induced crack closure.

Roughness induced crack closure

this concept is developed to explain the crack closure phenomena in mode 2 type of loading. Due to heterogeneity in micro structure there will be microscopic roughness of fatigue fracture surfaces. As a result, mismatch can occur between the upper and lower crack faces during displacement undergone in mode 2 type of loading. These mismatch wedges open the crack resulting in crack closure.