A COMPARISON OF CAST ALUMINUM BULKHEAD FATIGUE RESISTANCE: THE EFFECT OF SPECIMEN GEOMETRY

Automotive cylinder block bulkheads were obtained from two aluminum casting suppliers for a study on the high-cycle fatigue resistance of these materials. The fatigue strengths were measured via a staircase protocol using cylindrical round bars. These values were similar to 20-30% lower than previously measured data where an hour-glass fatigue sample geometry was used. A model to predict the fatigue life and strength (at a given number of cycles) was developed and revealed that the hour-glass geometry had a predisposition to higher fatigue strengths due to the lower probability of sampling a casting pore local to the region with minimum radius and highest stress. It was concluded that previous fatigue data on the bulkhead materials were inflated due to the choice in fatigue specimen geometry. More conservative values, which better represent the expected material behavior and intrinsic fatigue resistance, were obtained using the cylindrical bar geometry.