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The work focuses on experimental examination of the fatigue behavior of magnesium alloy AZ31 produced by three different procedures: squeeze casting (SC), hot rolling (HR), and equalchannel angular pressing (ECAP). The microstructures produced were studied by light and transmission electron microscopy (TEM). Squeeze-cast AZ31 had low porosity and coarse grains, while hot-rolled material showed microstructure with grain size of 3 to 20 lm. The finest grain structure with the average grain size of about 1 to 2 lm was found in the material pressed 4 times at 200°C using the ECAP technique, route B c . It was shown that low-and high-cycle fatigue behavior under symmetric loading at room temperature and with loading frequency of 20 Hz is strongly dependent on the technique employed in producing the alloy. The ECAP was shown to improve the fatigue life of the material in the low-cycle region over that of the squeezecast material. However, the fatigue life of AZ31 after ECAP was slightly lower than that of the hot-rolled material. In the high-cycle region, the hot-rolled material and the material that underwent ECAP exhibit the same fatigue strength, which is superior to that of the squeeze-cast alloy. Fatigue crack initiation and the character of fracture were examined by means of scanning electron microscopy.

The notch size effect (i.e. the decrease of the notched fatigue limit with increasing notch size for the same stress concentration factor) was quantitatively derived by describing the threshold conditions for the propagation of a short semi-elliptical crack nucleated at the notch root. A close relation between the Kitagawa-Takahashi diagram for the short crack threshold stress and the dependence of the notched fatigue limit on the notch size was shown. The derived relation for the notch size effect was experimentally verified for several specimen/notch geometries in the cases of pressure vessel steel and copper. NOMENCLATURE I = crack size I, , = maximum non-propagating crack size for unnotched specimens Con = maximum non-propagating crack size for notched specimens Lo = crack size corresponding to the knee point in the Kitagawa-Takahashi diagram p = notch radius po = maximum non-damaging notch radius K, = elastic stress concentration factor u = stress uc = plain fatigue limit uen = notched fatigue limit K = stress intensity factor Q = shape factor for semi-elliptical cracks Kath = amplitude of the threshold stress intensity factor

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