Programmed Load Fatigue Tests on Notched and Welded Specimens of Al-Zn-Mg Alloys

The report describes constant amplitude and programmed load fatigue tests on notched and welded specimens in two high strength aluminium-zinc- magnesium alloys. It is shown that, with very few exceptions, the hypothesis that sigma(n/N) = 1 consistently underestimates life. For the notched specimens using the basic quadratic programme sigma(n/N) varied from 0.8 to 14.8. For welded specimens using the same programme the value of sigma(n/N) varied much less and was approximately 2.6 to 7.0 for pulsating tension loading. Changes in the order of application of programme blocks had a negligible effect on the values of sigma(n/N) obtained. However, both for half tensile and for alternating loading, lower values of sigma(n/N) were obtained. When a spectrum parallel to the constant amplitude S-N curve was used high values of sigma(n/N) were obtained, except when the lowest stresses in the programme were omitted. This fact led to the belief that a possible reason for the high values of SIGMA(n/N) is coaxing at the lowest stress levels in the programme.