Evaluation of Skin-Spar Joint Resistance to Hydrodynamic Ram

Abstract : This paper presents the application of a new dynamic joint loading methodology that uses a RamGun device. Projectile-induced hydrodynamic ram can generate fuel-tank pressures in excess of 10,000 psi. This event is potentially catastrophic for aircraft fuel tanks designed to survive sustained pressures of little more than 50 psi. While the magnitude of ram pressure is dictated by a combination of fuel-level and projectile threat, skin-spar joint design is the primary means by which damage can otherwise be controlled. Damage resistant joints restrict the spread of damage and assist aircraft survival. Joint resistance to ram is conventionally evaluated using a combination of two methods: a) T-joint pull-off tests and b) ballistically-tested wingboxes. While T-joint tests are a low-cost method of ranking skin-spar joints according to their load at failure, realism is traded away in favor of an economical and easily understood test. T-joint pull-off tests are symmetrically performed quasi-statically at a strain rate of less than 0.01 in/in-sec, whereas projectile-generated ram events involve asymmetric high-rate loading conditions on the order of 100 in/in-sec. Contrary to T-joint pull-off tests, wingbox ram tests with actual threat projectiles are fully realistic, but these tests come with a price tag in excess of $250,000 (inclusive of tooling, structural materials, test costs, and labor). This is too expensive for wholesale evaluation of competing joint concepts. When attempting to model high-strain rate events associated with ram, conventional static failure criteria has proven inadequate. The required dynamic failure criterion is typically 2-4 times that of the static case. This test series demonstrated that the RamGun can be used with confidence to perform comparative testing, design downselection, and failure metrics for competing joint concepts.