PROGRAMMED MANEUVER-SPECTRUM FATIGUE TESTS OF AIRCRAFT BEAM SPECIMENS

The bending fatigue properties of a group of 7075-T6 AL ALLOY BEAM SPECIMENS WERE FOUND TO BE SIMILAR TO THOSE OF TYPICAL AIRRAFT STRUCTURES. Test results warrant the following conclusions regarding the constant-load-amplitude fatigue properties of the beam specimens. Static prestressing at 100 pct lim load improved the fatigue life of the specimens at fatigue load levels of 60 pct lim load or less, provided that the preload was applied in the same direction as the subsequent fatigue loads. Periodic single overstressing at 100 pct lim load affected the fatigue properties in the same way as prestressing did, only more so. Peiodic repeated understressing at 25 pct lim load produced no significant changes in the fatigue life under fatigue lods applied in the same direction as the understressing. There were indications that positive load levels below a certain cutoff point in the spectrum did not introdce significant fatigue damage. The spectrum cutoff concept applies only to positive load levels. (Author)