Fracture Control in Ballistic-Damaged Graphite/Epoxy Wing Structure

Abstract

This paper describes the development of a graphite/epoxy wing skin configuration capable of sustaining limit load following damage from a 23-mm high-explosive (HE) projectile impact. The skin configuration incorporates information learned in sawcut and ballistic fracture testing, and consists of a ±45 graphite/epoxy laminate with integral spanwise and chordwise crack-arresting pads formed by adding 0-deg glass/epoxy between the plies of graphite/epoxy. The ±45-ply orientation provides enhanced battle damage tolerance because of its higher residual strain-to-fracture capability relative to quasi-isotropic 0/±45/90 laminates. This permits developing the full load-carrying capability of the spar chords before unstable crack propagation can occur in the damaged skin. The 0-deg glass/epoxy was added to further enhance this capability, and concentrated into pads to avoid undesirable ballistic damage augmentation caused by the glass fibers, and to provide a controlled failure mode under blast pressures. The optimum graphite-to-glass ratio and the required width of the crack-arresting pads were determined using a new analysis method for predicting fracture in laminates containing high-strength 0-deg fibers. The effectiveness of the damage tolerance concept was demonstrated by firing a 23-mm HEI projectile into the tension surface of a full-scale wing-box test component loaded in combined bending and torison. Following damage, limit load was achieved as a result of successful crack arrestment.