MECHANISM OF THE 2-STAGE PLASTIC-DEFORMATION FOLLOWING AN OVERLOAD INFATIGUE-CRACK GROWTH

The plastic deformation behaviour at a fatigue crack tip following a t ensile overload has been dynamically analysed in pure titanium using a n 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 inc rease, is observed during the retardation period. The two stage plasti c 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 pl ane strain portion of the crack front. The changes in the plastic defo rmation behaviour is explained by the changes in the shape of the crac k front. (C) 1998 Published by Elsevier Science Ltd.