Mechanism of the two stage plastic deformation following an overload in fatigue crack growth

The plastic deformation behaviour at a fatigue crack tip following a tensile overload has been dynamically analysed in pure titanium using an electrochemical method. Using this method, the occurrence of plastic deformation can be recognised by an increase in polarisation current. Two stage plastic deformation, which was associated with the load increase, is observed during the retardation period. The two stage plastic deformation is due to the difference in the load levels at which the plastic deformations start in the interior and at the surfaces of the specimen. The plastic deformation associated with the lower of these two load levels is caused at the plane stress portions and the plastic deformation associated with the higher load level is caused at the plane strain portion of the crack front. The changes in the plastic deformation behaviour is explained by the changes in the shape of the crack front.