Crack growth and closure behavior of surface cracks in 7075-T6 aluminium alloy is investigated under axial loading, taking into account the difference in fatigue growth behaviour at the maximum crack depth point and at the surface intersection point and also the difference between the behaviors of the surface crack and the through-thickness crack. The plane strain closure response at the point of maximum depth of a surface crack is monitored using an extensometer spanning the surface crack at the midpoint of its length. The plane stress closure at the surface intersection point is observed by multiple strain gauges placed at appropriate intervals ahead of the crack tip and is continuously monitored without interrupting the fatigue test. The crack opening ratio is found to be about 10% greater at the maximum depth point than at the surface intersection point. Under axial loading, the difference in plane strain crack closure behaviour between the surface crack and the through-thickness crack is relatively small. Growth rates of surface cracks can be well described by the effective stress intensity factor range based on the closure measurements made in this study. The growth rates in terms of the effective stress intensity factor range seem to be slightly slower in surface cracks than in through-thickness cracks. A simple model for surface crack growth prediction is suggested. The prediction model provides conservative estimation for fatigue life within factor of two and the predicted crack geometry variations agree well with the observed results. As a result, the prediction model based on simple assumption is considered to be useful for engineering application.
