Thermal transport across delaminations in aerospace composite laminate materials

Abstract

A promising new technique for detecting aircraft structural cracks is called thermal diffusivity imaging. A heat flux is applied to one surface of a structural area while the opposite surface temperature field is scanned with an infrared television camera. In theory, layer delamination or other material non-uniformities will produce changes in the material thermal resistance between source and detector. These changes will produce regions of lower temperature than the surrounding material at the rear surface of the laminate. The technique has several advantages over the other methods available for detecting structural problems in aerospace applications: it is capable of scanning large areas in a short time; it avoids the use of potentially hazardous radiation, and it does not require forcing the material to the level of failure. In the present paper, an effective thermal diffusivity model is used to provide estimates of the sensitivity of the technique. A solution is determined for the differential energy equation through the composite laminate which is subjected to flux type boundary conditions with a homogeneous initial condition. For a single material, this results in a compact mathematical solution which can easily be applied to the determination of both transient and steady-state thermal transport properties. For composites, the problem is more difficult but the application of the effective thermal diffusivity model greatly reduces the mathematical complexity. Thermal transport across a gap (delamination) is modeled as an optically-thin process and it is shown that thermal conduction is likely to be the dominant mode of heat transfer.