Fractographic examination of coupons representing aircraft structural joints with and without hole cold expansion

In an effort to establish the potential benefits of hole cold expansion and interference fit fasteners for long term fatigue performance, coupons representing three types of aircraft structural joints were designed, manufactured and assembled. These joints, manufactured using 7075-T6 aluminium alloys for main members and 7075-T73 for doublers or spacers, were representative of: No Load Transfer joints, simulating “non-structural” joints, such as brackets, supports for conduits, fuel lines, and wire bundles. Medium Load Transfer joints, simulating symmetrical doubler, stiffener and rib joints; and High Load Transfer joints, simulating symmetrical splices in major load carrying members. The joints were assembled using Hi-Lok™ interference fit fasteners, with and without hole cold expansion, and were fatigue tested to complete failure using a representative mission mix service load spectrum for a coastal patrol aircraft wing fatigue sensitive location. The failure surfaces were examined with the aid of an optical microscope at various magnifications. The fractographic descriptions for the three types of joints examined document the crack nucleation sites, mechanisms of crack formation, and extent of damage at catastrophic failure, including the effects of manufacturing, such as direction of hole cold expansion and direction of interference fit fastener insertion. Examination of fracture surfaces for fractographic evidence provides researchers and engineers with valuable insights into the failure process, which will enable them to perform more accurate and representative life predictions of joint performance.

► Fractographic assessment of nucleation sites for representative aircraft joints with interference fits and hole cold expansion. ► Interference fit fasteners can damage holes on the insertion side, exacerbating crack nucleation by fretting fatigue mechanisms. ► Hole conditions will affect where cracks originate and how they propagate, affecting not only fatigue life, but also requisite safety of flight inspections. ► Investigate material characteristics (grain orientation, ductility, etc.) and geometric design details (edge margins) for their hole cold expansion potential before indiscriminate use of this technology.