Notch fatigue behaviour in a Sb-modified permanent-mold cast A356-T6 aluminium alloy

The purpose of the present study is to evaluate the effect of notch on fatigue strength and to understand the crack initiation and small crack growth behaviour from the notch root in a permanent-mold cast Al-Mg-Si alloy, A356. Axial fatigue tests have been performed using three types of notched specimens with different stress concentration factors, Kt, of 2.13, 3.38 and 4.9. As commonly observed, all the notched specimens showed lower fatigue strength than the smooth specimens, but nearly the same fatigue strength regardless of stress concentration factor. Cracks initiated and grew at the mid-thickness of the notch root and then reached the surface as a through-thickness crack. Such behaviour depended strongly on stress level and stress concentration factor. At higher stresses, small cracks grew faster than the corresponding large cracks characterized in terms of the effective stress intensity factor range, ΔKeff, but at a low stress, the growth behaviour for small cracks coincided with that for large cracks characterized in terms of ΔKeff. While the smooth specimens showed significantly faster small crack growth than large cracks due to different fracture mechanism, all the notched specimens did not reveal such behaviour. Furthermore, it was indicated that the present alloy displayed a very low notch sensitivity.