Experimental study on the damage tolerance of stepped and ply-drop tapered skin doubler repairs under fatigue loading

This paper presents an experimental study on the damage tolerance of stepped and ply-drop tapered skin doubler repairs when subjected to fatigue loading. Graphite/epoxy ply was used for the doubler and parent adherend with a stacking sequence of (45/0/0/-45/90)_3S. Each specimen was 360 mm long and 20 mm wide. The bondline damage at the tip of the doubler repair was simulated by a Teflon tape. Various flaw sizes were selected by changing the flaw length and flaw width. A total of thirty specimens were tested consisting of fifteen specimens with stepped doubler repair and the other fifteen specimens with ply-drop doubler repair. A constant amplitude fatigue loading was applied to all specimens. The crack propagation was recorded and the fatigue life was reported. The effect of the initial flaw on the damage tolerance of the two types of skin doubler repairs was discussed.