Strain-Based Fatigue for High-Strength Aluminum Alloys

A study is reported of the cyclic deformation, fatigue, and mean stress relaxation behavior of extruded material of aluminum alloys 7075-T6511 and 7249-T76511. Experimental data and curve fitting constants are given for the cyclic stress-strain and strain-life curves. The stress-life (elastic strain) component of the strain-life curve shows a transition to a shallow slope at short life, necessitating a two-segment fit. Mean stress effects are included in the stress-life fit by applying the Walker mean stress method, and this is incorporated into the strain-life curve. Other mean stress relationships are also compared to the test data, specifically those of Goodman, Morrow, and Smith-Watson-Topper. Mean stress relaxation was observed even at quite low strain amplitudes where there is no measurable plastic deformation, with a transition in behavior to a strong relaxation effect at relatively large strains. Hence, the data are partitioned around εa = 0.006 and different sets of relaxation constants using the Landgraf method are fitted above and below this level.