Crack closure

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.

for an applied loading cycle and is However, crack closure occurs when the fracture surfaces are in contact below the opening level stress intensity factor

He observed that a contact between the fracture surfaces could take place even during cyclic tensile loading.

These steels transform from the austenitic to the martensitic lattice structure under sufficiently high deformation, which leads to an increase of the material volume ahead of the crack tip.

[9] This transformation-induced closure is strongly influenced by the size and geometry of the test specimen and of the fatigue crack.

It is caused when the base material at the fracture surface is exposed to gaseous and aqueous atmospheres and becomes oxidized.

[10] Although the oxidized layer is normally very thin, under continuous and repetitive deformation, the contaminated layer and the base material experience repetitive breaking, exposing even more of the base material, and thus produce even more oxides.

As such, the volume of the oxides can be interpreted as a wedge inserted into the crack, reducing the effect stress intensity range.

[10] Due to the anisotropy and heterogeneity in the micro structure, out-of-plane deformation occurs locally when Mode II loading is applied, and thus microscopic roughness of fatigue fracture surfaces is present.

The misfit in the fracture surfaces also takes place in the far field of the crack, which can be explained by the asymmetric displacement and rotation of material.