Notched multiaxial fatigue of Al7050-T7451: on the need for an equivalent process zone size

The aim of this work is to investigate stress gradient effects on the fatigue life estimation of notched Al 7050-T7451 specimens under combined torsion and push-pull loading conditions. Initially, simple push-pull and torsion fatigue tests in plain and notched specimens were independently conducted not only to obtain the material properties necessary to calibrate a standard multiaxial critical plane based model, but also to raise the critical distance versus life curves in tension (L? – Nf) and in torsion (L? – Nf). This latter step also required a Finite Element Elastic Stress Analysis of the notched specimens tested in the medium high-cycle fatigue regime. Then, proportional multiaxial fatigue tests were carried out using this same notched geometry. The combination of a multiaxial model with the theory of critical distance (TCD) was applied to estimate fatigue lives. For this aluminium alloy, neither the use of the L? – Nf nor L? – Nf combined with the predictive multiaxial model was able to estimate lives in an accurate way.