Estimation of Fatigue Limit of a A356-T6 Automotive Wheel in Presence of Defects

The automotive wheel is a critical safety component in the vehicle and, for such a reason, it has also to meet strict requirements about technological properties. Aluminum wheels are produced by low pressure die casting technique and the casting defects related to the process have to be properly considered having a high effect in decreasing both static and cyclic resistance of the component. Effectively, casting defects as porosities influence the fatigue crack initiation and strongly affect the fatigue life too. One of the most common problem in the real component is the mismatch between the experimental and literature data about fatigue life. In fact, many scientific researches are usually carried out on small samples produced in a controlled condition and therefore it is difficult to direct transfer the laboratory results to a real cast component with a well-defined shape and different thicknesses. In the present study, an aluminum alloy A356-T6 wheel was analyzed in order to assess its fatigue performance, taking into account the casting defects. The fatigue limit of the component was calculated by rotating bending fatigue tests executed on the whole wheels. Microfractographic analyses on the broken wheels were carried out on the fracture surfaces using a Scanning Electron Microscope in order to identify the crack initiation zone: it was recognized that the crack always started from shrinkage porosities. The statistical population of these defects was therefore investigated on samples taken from the wheel in crack nucleation positions of the spoke and the maximum expected defect size on the component was estimated by the statistics of extreme values. The experimental fatigue limit was finally compared with the theoretical value predicted with the Murakami’s method.