Transient thermographic technique for NDI of aerospace composite structures

Transient thermography is well established as a capable tool for non-destructive testing (NDT) in aerospace composite structures (see for example Favro, et. al 1995). The basic process involves altering the steady state thermal condition of a structure by adding or removing thermal energy (heat) and then observing the transient temperature patterns on a surface by means of a sensitive infrared imaging system. Many techniques for heat addition have been tried including most commonly convective and radiative transfer to external surfaces. As an NDT technique it is especially appealing for composite structures whose constituent thermal properties may vary considerably leading to interesting and illuminating transient patterns (Favro, et. al. 1993). We have developed a novel application of transient thermography with an application to detecting bonding flaws in boron/epoxy skinned aluminum honeycomb composite structures as found for example in the F14 and F15 aircraft. The technique described below uses induction to selectively heat the structure near the flaw region and has potential benefits in a range of applications.