Crack closure

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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.[1]

The crack closure effect helps explain a wide range of fatigue data. It has become the default interpretation of load ratio[1] 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[edit]

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.[2]

Phase transformation induced crack closure[edit]

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.[3]

Oxide induced crack closure[edit]

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.[4][5]

Roughness induced crack closure[edit]

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.

References[edit]

  1. ^ a b "Unified Approach to Fatigue Damage Evaluation - U.S. Naval Research Laboratory". Nrl.navy.mil. 2010-02-01. Retrieved 2013-05-14. 
  2. ^ •Engineering Fracture Mechanics by Prof. K. Ramesh, Department of Applied Mechanics, IIT Madras
  3. ^ Lecture 35 on YouTube
  4. ^ • Engineering Fracture Mechanics by Prof. K. Ramesh, Department of Applied Mechanics, IIT Madras
  5. ^ Effects of fracture loading on YouTube