FATIGUE OF AZ91E-T6 CAST MAGNESIUM ALLOY

The purpose of this research was to obtain room temperature fatigue behavior of AZ91E-T6 cast magnesium alloy and to determine if commonly used models that depict fatigue behavior are applicable to this cast alloy. Axial strain-controlled fatigue behavior using cylindrical specimens were employed to determine low cycle fatigue behavior with strain ratios R = epsilon(min)/epsilon(min) = 0, -1, and -2. The conventional log-log total strain low cycle fatigue model properly represented the R = -1 axial fatigue data. Significant mean stress relaxation occurred for all R = 0 and -2 axial fatigue tests. However, for the smaller strain amplitude tests with R = 0, sufficient mean stresses were retained such that fatigue life was reduced. The mean strains/stresses had little influence on the cyclic stress-strain curve which exhibited cyclic strain hardening. Mean stress effects were analyzed using the Morrow, SWT and Lorenzo-Laird models and similar, but oftentimes nonconservative, calculations resulted. Region I and II fatigue crack growth behavior was determined using C(T) specimens with load ratios R = P(min)/P(max) = 0.05 and 0.5. Values of DELTAK(th) and (DELTAK(th)eff were less than 1.5 MPa square-root m and the Paris equation slopes were between 3.3 and 3.9. Quasi-cleavage was predominant for both fatigue crack growth and final fracture regions. The commonly used low cycle fatigue and fatigue crack growth models appear to reasonably represent most of the results with this AZ91E-T6 cast magnesium alloy.