Failure Analysis of A Trainer Aircraft Wing Tip

A departure of a wing tip led to a failure investigation of the remnant wing tip. These wings (the torque box) are single units from tip to tip, constructed of aluminum alloy spars, ribs, and skins with control surfaces reinforced with internal honeycomb cores. The wing tip structure consists of a rib, internal doublers on both the upper and lower surfaces, the wing tip skins, and on many wing tips external repair doublers on both the upper and lower surfaces. The analysis of the failed wing tip revealed two major mechanisms contributing to the failure event: failure of the composite adhesive and fatigue fracture. Separation of the wing tip from the wing at the upper structure was a result of cohesive failures of fillet and node bonds. These failures are theorized to result from internal pressure built up from repairs that ultimately exceeded the flatwise tensile strength of the adhesive. Flatwise tension stresses can cause face to core debonding, and flatwise compression stresses can cause core crushing. The honeycomb core separated from the upper structure at an outboard rivet line; an indication of the possibility of moisture entry from a repair associated with the external doubler. Wing tip separation from the lower structure was a result of fatigue and overload. Fracture at the remnant lower wing tip structure was localized to the wing tip skin through the row of rivets. Fatigue cracking initiated from multiple rivet holes and exhibited an overall crack length of 5.250 inches. Cracking initiated from the outer surface of the skin. The adhesive bond strength at the lower structure was stronger than the fatigue strength of the wing tip skin, leading to the bond between the honeycomb core and lower structure remaining intact.