Relation between Interfacial Fracture Toughness and Mode-mixity in Honeycomb Core Sandwich Composites

Abstract

A fracture mechanics approach is proposed to determine the residual strength of debonded honeycomb core composite sandwich panels. Asymmetric double cantilever beam sandwich specimens are tested in order to measure the core/ face sheet interfacial fracture toughness. Graphite/epoxy composite laminates are used as face sheets, and the core is an aramid fiber/phenolic resin honeycomb structure. The experimentally measured critical loads, disbond lengths, and specimen dimensions are used in a finite element analysis to estimate the fracture (FE) toughness and the mode-mixity at the crack tip. The finite element results for the critical energy release rate agreed well with the experimentally measured values. It is found that the interfacial fracture toughness increases as the mode II component increases, and the difference can be as much as 70%. In order to demonstrate the importance of mode-mixity on damage tolerance, the design of a sandwich pressure vessel is considered. The allowable pressure for various size disbonds are calculated using the average fracture toughness as well as mode-mixity dependent fracture toughness. It is found that ignoring mode-mixity can lead to overestimation of the load carrying capacity of debonded sandwich panels by as much as 40%.