POSITRON ANNIHILATION STUDY ON THE DISTRIBUTION OF DEFECTS IN THE PLASTIC ZoNE A-T CRACK TIP

The plastic zone of crack tip of wedge opening loading specimens were studied by positron annihilation technique. The line-shape parameters of Doppler broadening of annihilating radiation were measured. The value of S parameters was calibrated by specimens with known amount of strain. Then the distribution of the plastic strain in the plastic zone as a measure of the density of defects was determined. Experimental results indicate:1. The dimension of plastic zone is exactly the same as that determined by fracture mechanics.2. The distribution of strain in the plastic zone is not homogeneous.3. There is a high strain area at the very tip of the crack.4. The strain in the plastic zone is increased after electrochemical hydrogenation. Iso-strain curves are expanded preferentially in the direction of greatest shear stress. It is concluded that hydrogen facilitates the propagation of dislocations or the sliding of movable dislocations and vacancies can generate at the same time.