Modeling of fatigue behavior in pre-corroded AZ31 magnesium alloy

In this paper, the mechanical and fatigue behavior of pre-corroded wrought AZ31 magnesium alloy was studied. For this purpose, the standard 3.5 wt.% NaCl corrosive solution was used. The samples were immersed for 3-24 hours to characterize the effect of immersion time on the mechanical properties of AZ31 alloy. Standard specimens were also immersed for 1-3 hours for the fatigue testing. Results of tensile tests showed that thorough the immersion of 0-24 hours, the deviation of ultimate tensile stress and yield stress were less than 4% and 6%, respectively. Moreover, the deviation of elastic modulus was less than 20%. Although, the elongation was deviated by 81% through the immersion of 0-24 hours. A drastic decrease was observed in the fatigue lifetime of pre-corroded alloy compared to the bare alloy. As the immersion time increased, the fatigue lifetime decreased. Maximum reduction in fatigue strength occurred when the immersion time was 3 hours and the stress amplitude was 82.5 MPa. Fatigue results also showed that the Levenberg-Marquardt was a good method to find the materials' constants, as the maximum and average relative errors were 10.28% and 2.78%, respectively. The fatigue fracture surfaces of pre-corroded specimens indicated the brittle fracture. The Basquin model was used for fatigue lifetime prediction. A new model was proposed with a new parameter, initial virtual crack size, to relate the immersion time to the fatigue lifetime using the Paris equation. The fatigue lifetime of 1-3-hour pre-corroded AZ31 magnesium alloy was estimated by the new model with acceptable relative errors.