Crack closure

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.

ΔK = ΔKmax - ΔKmin

ΔKeffective = ΔKmax – Kopening

ΔKeffective ≤ ΔK

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.

Phase transformation induced crack closure

This kind of crack closure is common in pressure vessels and other fluid related areas. In this concept while the crack opens under loads, the crack is filled with fluid from its surroundings that 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 where rapid corrosion occurs during crack propagation. Here the effect is to wedge a crack open during fatigue loading, due to the presence of corroded particles in the crack. And hence the effective stress required is lowered as in the case of fluid induced crack closure.

Roughness induced crack closure

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

Roughness induced crack closure is hence a result of slipping of crack faces leading to faceted crack morphology and is justifiable or valid when the roughness of the surface is of same order as the crack opening displacement. It is influenced by grain size, loading history,material mechanical properties and load ratio. Also, the slip of crack faces occur at low ΔK and R ratios. Ageing of the specimen also influences the crack closure. Comparing under aged and hyper overaged conditions crack closure due to roughness was greater in under aged specimens in which planar slip dominated.

It was also found that crack closure due to surface roughness was influenced by grain size to a greater extend. The extent of crack closure was found to increase with increase in grain size, especially at low load ratios. Specimen type is also a factor influencing crack closure. Roughness induced crack closure is generally used to describe contact of faceted fracture features which are dimensionally small (of the order of grain size). However there are situations where crack closure can occur due to crack branching or deflection.