The
fatigue crack propagation, the
plastic deformation at the crack
tip and the acoustic
emission behaviour for each cycle of crack growth were studied for a low strength steel A537 in both air and 3.5%NaC1 aqueous solution. The reduction in the speCimen thickness on the fracture surface was measured as a function of the crack length in order to determine the influence of corrosive medium on the plastic deformation of crack tip material. The laser speCkle interferometry was used to characterize the plasticdeformation behaviour on the surfaCe of the
plastic zone at crack tip in air and 3.5% NaC1 solution under the hydrogen charging conditions, and the mono-direction tension behaviour of smooth specimens in two environments was examined.The results showed that
acoustic emission count rate increased with the fatigue crack
propagation rate in almost a linear way in both air and solution environments. The distinct effect of corrosive solution was to increase the fatigue crack propagation rate and at the same time to decrease the acoustic emission count rate. In both air and corrosive solution the fracture surface took on a morphology of ductile striations, which shows that CF crack propagation takes place due to alternative plastic blunting and resharpening of the crack tip by dislocation slip. The fact that the thickness on the fracture surface reduced with increasing crack length indicated the plastic zone size at the crack tip increased with increasing crack length. In the same crack length the reduction on the specimen thickness in 3.5% NaC1 solution was obviously smaller than that in air, which suggested a decrease of the plastic zone size at the crack tip during CF process. The comparison of the plastic zone sizes at the crack tip by laser speckle interferometry in air and in corrosive solution under hydrogen charging condition showed that the existence of hydrogen at crack tip caused a significant decrease of the plastic zone size. The experimental results of mono-direction tension tests demonstrated that both yield stress and hardening exponent increased obviously after 6h hydrogen charging. The reduction of the' acousitic emission events during corrosion fatigue crack propagation is associated with the change of the plastic zone size at the crack tip.
More abstracts about the REDUCTION OF PLASTIC ZONE SIZE AT CRACK TIP AND ACOUSTIC EMISSION FOR A537 STEEL IN 3.5% NaC1 SOLUTI