Super-long life fatigue properties of aluminum alloy die-casting related with internal or surface defects

By the ultrasonic fatigue test for Al-Si-Cu die-casting, the influence of internal or surface defects on super-long life properties was investigated. The results were as follows : (1) When surface defects were the crack starting points, the S-N curve leveled off around 107 cycles. On the other hand when the defects were internal defects, the S-N curve showed continued degradation as Nf increase. (2) When the crack starting point was an internal defect. ODA (Optically Dark Area) was observed around the crack initiation point after 108 cycles. However, ODA was not observed when the compulsory fatigue was applied for no-break sample that was tested over 1010 cycles of Nf. This suggests that the incidence of fatigue crack from the defect point or its progress were not recognized. (3) By using the following formula, which compares the relationship between ΔKCD ∝ (√area)1-3 and ΔK th(√ area)13 based on the √area parameter model, it is possible to explain the breaking or non-___breaking phenomenon caused by the casting defect in the super-long life area (over 107 cycles).