Research on the damage to aircraft composite material fuel tanks due to penetrating projectiles (hydraulic ram) at the naval postgraduate school

Abstract

Metal aircraft fuel tanks that are subjected to ballistic impact and penetration by small arms fire and missile fragments can be severly damaged, with large petalling of the tank walls occurring at the entrance and exit points of the projectile. The damage mechanism, called hydraulic ram, is a very high pressure wave in the fuel caused by the passage of a ballistic penetrator through the fuel. Fluid pressures and wall strains have been experimentally measured and analytically predicted at NPS for rectangular tanks with aluminum walls. A summary of this work is given in Ref. [I]. Due to the fact that aircraft fuel tanks made of composite materials are now being seriously considered, the effect of hydraulic ram on composite material tank walls has been investigated. In Ref. 121, the various effects of hydraulic ram on a clamped I l-in. square, 0.067-in. thick, graphite/epoxy wall due to penetration by a .22 caliber projectile were examined. Shots at 2fXlOfps caused only light damage to the plate. At 2800 fps, the hydraulic ram caused considerable damage, including total severance of the plate from its clamped support over much of the outer perimeter. The objectives of the research in Ref. [3] were to show the relative importance of the transverse shear@ forces produced by hydraulic ram loading on military aircraft fuel tank joint designs for composite mater& and to propose fuel tank test section designs based upon specific composite material fuel tank design concepts for the F-16, F-18 and a Navy V/STOL delta wing. With the use of a finite element analysis, the transverse shearing force at a metal fastener was shown to be a major cause of failure at the attachment, primarily by an out-of-plane push-out mode of failure. This type of failure could have a significant effect on the structural integrity of a major load crrrying member of the aircraft, such as the wing box beam. In this situation, a large portion of the wing skin over the fuel tank may become detached from the spars, ribs and stringers, causing a serious degradation in the strength and stiffness of the wing. Future research will be devoted to the study of the amount of resistance to out-of-plane push-out shown by various composite attachment designs and to the determination of the amount of area over which push-out may occur.