Crack initiation and propagation of cast A356 aluminum alloy under multi-axial cyclic loadings

Mechanical fatigue tests were conducted on specimens of A356-T6 casting alloys under uniaxial and multi-axial cyclic loadings. SEM and quantitative metallography technique were used to examine fracture surfaces and statistically to analyze particle cracking after fatigue tests, respectively. The existence of casting defects has considerable influence on fatigue behavior, and the dominant fatigue crack preferentially nucleates from porosity and oxide films near the outside surface of the specimen. In the absence of these defects, the crack nucleation occurs from the large and cracked eutectic silicon particles. The number of cracked particles increases with the number of fatigue cycles, but the damage rate depends on the particular loading paths. Large and elongated particles with their major axes parallel to the tensile axis show the greatest tendency to cracking. The cracks in particles can be regarded as micro-cracks in this material which can coalesce together and provide a weak path for fatigue crack propagation. Final fracture occurs when the percentage of cracked particles increases to a threshold level during the fatigue process, over 50 pct in this study.