The semi-solid rheomolding AZ91D auto tire active safety device parts were prepared.The initiation and propagation behaviors of fatigue crack in the semi-solid rheomolding AZ91D magnesium alloy auto parts were investigated through"25 000 km road test of cars equipping with the sample parts"and"50 km road test of cars after sudden deflation of their tires".The results indicate that

the fatigue cracks are mainly initiated at the inclusions and the inclusions/matrix interface

the second phase particle/matrix interface and stomas.The stress control nucleation mechanism is an important factor for the crack initiating from larger size inclusion

and the plastic strain control mechanism is dominant for the crack initiating from the second phase particle/matrix interface or small inclusion/matrix interface.The mechanically small crack propagation is extended by the convergence of adjacent short fatigue cracks in the initial stage.The crack closure effect exists in the mechanically small crack propagation process

which is attributed to plastic-induced crack closure mechanism.