To clarify the effect of the mean stress on the fatigue strength of various magnesium alloy castings at room temperature and high temperature, tension-compression fatigue tests were carried out under various mean stresses by using specimens cut from 6-20-mm-thick castings. Consequently, a method of drawing endurance limit diagrams has been developed. The main results obtained are as follows.

The endurance limit, σ_wo, under completely reversed stress showed a good correlation with 0.2% proof stress, σ_0.2, which can be expressed by the following equation. σwo = 0.63σ_0.2.

In the case of room temperature, at which no creeps occur, the endurance limit diagram can be estimated by a straight line which passes the true fracture strength, σ_T, on the horizontal axis and the endurance limit, σ_wo, of the completely reversed stress on the vertical axis.

In the case of high temperature ( max. 423 K ), at which creeps occur, the endurance limit decreases abruptly on the region of high mean stress. The endurance limit diagram can be estimated by the same way as mentioned above, under the condition of σ_m ( mean stress ) ≤ σ_ε2.5 ( 2.5% creep strain strength ). However, under the condition of σ_ε2.5 , it can be presented as a straight line which passes σ_ε2.5 ( horizontal coordinate ) on the line estimated by the above mentioned method and the creep rupture strength, σ_c, on the horizontal axis.