Internal Impact Damage to Lightweight Composite Sandwich Panels

Abstract

Impact damage developing within lightweight Nomex® honeycomb core sandwich panels were investigated when subjected to impacts by low velocity metaltips, and high speed ice spheres representing hail ice. The metal tips had radius ranging from 12.7 to 76.2 mm, whereas all ice spheres were 50.8 mm. Of interest is the observation of damage modes and severity formed by impacts of varying radius for metal tips, and of varying glancing angle for ice spheres, with particular focus on the severity of internal damage modes to the core vs. external visibility. For the low velocity metal tip impacts, four modes of internal core damage were identified (initial slight wrinkling of core walls, visible wrinkling, buckling/kinking, and core wall fracture). Damage depth within the core was radius-dependent, but stabilized at constant depth independent of impact energy. Internal damage span increased with increasing impact energy, but not with increasing tip radius. Fractured core typically corresponded with readily-visible dents, when viewed from the external surface. For the high speed glancing ice impacts, the four modes of core damage were also observed. However, impact damage tended to either cause face sheet rupture (readily visible), or leave a very shallow surface dent (often non-visible) while also causing significant internal core crushing and even core fracture. Cases of core fracture with no external visibility present a safety concern, as subsequent separation of the facesheet can occur under specific loading conditions (e.g., facesheet in compression or in-plane shear, core cell pressurization as aircraft climbs to high altitude).