A study of crack intiation sites in high cycle fatigue of 319 aluminum alloy castings

High cycle fatigue performance of aluminum castings is of great importance for the casting designer. Several recent fatigue studies in Al-Si-Mg alloys have found that in addition to porosity, oxide films are viable fatigue crack initiators. The current study analyzed the high cycle fatigue performance of 319 (Al-Si-Cu-Mg) alloy test bars produced from gravity poured sand moulds. The fatigue life was found to be related to the size of defect acting as the crack initiation site. Scanning electron microscopy and energy dispersive spectroscopy were used to identify the nature of crack initiation sites. A linear elastic fracture mechanics crack growth model was found to be particularly successful at predicting the life of fatigue samples that initiated at oxide films. Having crack-like geometry, and a minute crack tip radius, oxide films effectively acted as preformed cracks. Consequently there was an absence of crack nucleation time, explaining the correlation of predicted propagation life to fatigue life.