In order to propose a prediction expression of a threshold stress intensity factor range (AKlh) for magnesium alloys with small crack, rotating bending fatigue tests in three magnesium alloys which have a different hardness and low carbon steel (JIS S10C) were carried out. In these tests, the AK of a crack propagation lower limit from an annealed pre-crack as the AKlh was measured. One of the three magnesium alloys have the same Vickers hardness (HV) as the low carbon steel, however the AKth was half of the low carbon steel. In order to make this difference clear, the authors focused on an effective threshold stress intensity factor range (AKeffth), because there is large difference in the AKeffth which is not negligible to evaluate the AKlh between magnesium alloys and steel. Considering the difference in the AKefflh, a prediction expression was proposed based on Murakami's prediction expression which can predict the AKlh in various metals by applying only two parameters i.e. the HV and an initial crack size. However, the prediction expression cannot evaluate the AK th of the magnesium alloys. In this expression, Young's modulus which is proportional to the AKeffth was used to modify Murakami's prediction expression. In addition the new prediction expression was speculated to be accurate.
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