Fatigue damage tolerance of two tapered composite patch configurations

Abstract To avoid high peel stresses around the perimeter of a bonded composite patch repair, the patch thickness often tapers to a fine edge. This paper investigates and compares the damage tolerance of two different configurations of taper design under fatigue loading with different size of initial bondline flaws. The bondline damage at the tip of the doubler repair was simulated by an initial flaw which was created through a Teflon tape. A constant amplitude fatigue loading was applied to all specimens. The crack propagation against the number of cycles was recorded and the fatigue life when the crack reached 100 mm from the doubler tip was reported. Microscope investigations were conducted and provided micro scale evidence on the effects of initial flaw size on the crack initiation pattern of the two doubler design configurations. Finally, the damage tolerance of the two configurations were compared and conclusions were drawn with implications for the design of composite patch repairs. It was found that ply-drop doubler joints showed better damage tolerance than stepped doubler joints under fatigue loading. The experimental results suggested a threshold size of 5 mm for the doubler tip flaw, if any inspection technology should be adopted for damage detection.