Damping behaviour of AZ91 magnesium alloy with cracks

The amplitude-dependent damping of a commercial magnesium cast alloy AZ91 was determined at room temperature by measurement of the logarithmic decrement of free decaying vibrations of bending beams clamped at one side. In order to generate cracks in the specimens they were subjected to (1) isochronal heat treatments for 1 h at temperatures above 400 °C with succeeding quenching in cold water and (2) controlled fatigue bending loading in the same equipment also used for the damping measurements. After both treatments, the amplitude-dependent damping curves show a maximum for strains 10⁻⁵ to 10⁻³, which can be correlated with the presence of cracks and can be explained by a simple rheological model based on crack damping. This maximum is enhanced when the number of loading cycles or the quenching temperature is increased which can be explained by crack nucleation. Crack growth with increasing number of loading cycles shifts the maximum to lower strains.