CUMULATIVE DAMAGE BEHAVIOR OF ANISOTROPIC AL-6061-T6 AS A FUNCTION OFAXIAL-TORSIONAL LOADING MODE SEQUENCE

Two types of low cycle fatigue tests were conducted along two principa l orthotropic directions of an orthotropic Al-6061-T6 plate in strain control at room temperature: (1) reference fatigue tests under three l oading conditions: push-pull, torsion, and combined push-pull/torsion in-phase; (2) sequential fatigue tests in which different sequences of push-pull and torsion were performed. Fatigue cracking behavior was o bserved during all of the fatigue tests. Shear cracking dominated the damage of the material. Anisotropic constitutive relations of the mate rial were used in the evaluation of several multiaxial fatigue damage models. The predictive capabilities of these models were assessed base d on the results of reference fatigue tests. The damage accumulation b ehavior of the material was found to depend on the sequence of the loa ding mode. For the sequence of torsion then push-pull, the damage summ ation was greater than unity. However, for the sequence of push-pull t hen torsion, the damage accumulation was near unity as predicted by th e linear damage rule. A nonlinear damage accumulation rule could repre sent the results of the sequential fatigue tests.