Characteristic of threshold stress intensity factor range for small crack on magnesium alloys

In order to propose a prediction expression of a threshold stress intensity factor range (AK_lh) 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 AK_lh was measured. One of the three magnesium alloys have the same Vickers hardness (HV) as the low carbon steel, however the AK_th 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 (AK_effth), because there is large difference in the AK_effth which is not negligible to evaluate the AK_lh between magnesium alloys and steel. Considering the difference in the AK_efflh, a prediction expression was proposed based on Murakami's prediction expression which can predict the AK_lh 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 AK_effth was used to modify Murakami's prediction expression. In addition the new prediction expression was speculated to be accurate.