The influence of tensile pre-strain on the fatigue strength of a squeeze cast aluminum alloy cut out from a car wheel was investigated under rotating bending through the successive observations of the specimen surface by the plastic replication method. A crack was initiated from either surface slip bands or at an interface between eutectic Si particles and the matrix but was suppressed by eutectic Si particles in the materials without pre-strain. With tensile pre-strain, cracking or weakening occurred at an interface between eutectic Si particles and the matrix, and consequently the initiation of a fatigue crack was accelerated. However, the propagation process of a longer crack was suppressed by pre-strain.
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