Magnesium alloys are being increasingly used for engineering applications. Fatigue crack-growth data have therefore been obtained for a high strength magnesium-Zr ahoy and a medium strength, weldable magnesium-Mn alloy. The results of tests on sheet material are presented in terms of the range of stress intensity factor AK. Critical values of hK necessary for fatigue crack growth AZ& were also obtained. The behaviour of the two alloys was similar; both rates of crack growth and AK, were sensitive to mean stress. Fatigue crack growth was entirely on a 90" plane with no sign of the transition to crack growth or 45" planes usually observed in sheet materials. This was ascribed to the effects of preferred orientation of the crystal structure.
NOMENCLATURE
constants critical value of o,sa necessary for fatigue crack growth stress intensity factor (opening mode) range of 4 in fatigue cycle critical value of AK for fatigue crack growth crack length final crack length initial crack length number of cycles gross stress alternating stress mean stress testpiece width plastic zone radius yield stress (taken as 0.2 percent proof stress) net section stress stress ratio (ot. -ad/( Us + u,J.