Assessment of fracture toughness in sandwich structures using high speed infra-red thermography

A critical damage type in sandwich structures is the debonding between the face sheet and core. Debonds often initiate as the result of manufacturing defects as well as by in service overload or impact. Propagation of the debonded area can cause catastrophic failure of sandwich structures as their ultimate strength and stiffness are drastically reduced, with the effective removal of one face sheet. An important parameter that controls the propagation of debonds is the interfacial fracture toughness. In the paper a methodology is described that enables the characterization of the interfacial fracture toughness by directly measuring increases in temperature at the crack front during crack propagation. One of the challenges associated with this temperature measurement is that as the crack increments there is a rapid heat dissipation at the newly created surfaces. Thus, high speed infra-red (IR) thermography is employed to capture the temperature evolution at the crack front. When the fracture occurs in brittle or semi-brittle materials (e.g. cross linked PVC foam), two assumptions can be made: 1) the plastic deformation zone at the crack tip is highly localised; 2) the heat generated in the area below the crack surfaces is small and negligible. Based on these assumptions, a simple linear relationship exists between interfacial fracture toughness and the temperature change. The linear relationship is validated through a series of test on sandwich beams loaded in a mixed mode bending (MMB) rig, for which there is a theoretical solution for interfacial fracture toughness.