Quantitative Themographic Characterization of Composites

Abstract

Thermography has become a widely accepted method for NDT of composite structures. The low thermal effusivity of polymer and ceramic matrix components makes the time scale and signal strength of their response to thermal excitation nearly ideal for thermographic inspection methods. At present, composite NDT applications are primarily qualitative, based on identification of discrete flaws such as delaminations, voids, inclusions or unbonds. Quantification is limited to flaw sizing, and to a lesser extent, measurement of flaw depth or thermal diffusivity. Using the Thermographic Signal Reconstruction (TSR) method to interrogate each pixel time history individually, it is possible to accurately measure flaw size and depth, and also to characterize the thermophysical properties of the constituent layers of a multilayer sample. This type of material characterization task can be performed by quantitative analysis of the logarithmic time derivatives and their attributes, obtained from the TSR analysis. However, a significant amount of information can be inferred from the basic shape characteristics of the derivatives. As an example, we present a set of primitives that represent 2nd derivative responses for various one and two layer material configurations.