Damage in fiber reinforced and unreinforced AZ91 studied by internal friction

The influence of load cycling on the amplitude dependent damping and possible causes of fatigue in unreinforced and Saffil fibre reinforced magnesium alloy AZ91 was investigated. The unreinforced AZ91 alloy was investigated in the as cast and in the homogenized state. For an increasing number of cycles N, the amplitude dependent component of the logarithmic decrement decreases until a critical N (=NC). For higher cycle numbers (N > NC), a maximum in the strain amplitude dependent damping curves increases and shifts to lower strain amplitudes with increasing N, which is attributed to the growth of cracks. These findings are consistent with the decrease of the resonant frequency during crack growth.