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In order to clarify the crack initiation and propagation behaviour of asqueeze cast aluminum-silicon-magnesium alloy, fatigue tests of smooth specimens cut fromAl alloy car wheels formed by the squeeze casting were carried out under the constantstress amplitudes. The behaviour of microcracks was monitored successively by the plasticreplication method. In addition to the constant stress amplitude tests, fatigue tests under thetwo-step loading (low-to-high block and high-to-low block loading) were performed. Thephysical basis of the effect of stress change on fatigue damage was investigated based onthe behaviour of small cracks.