Improving the probing depth of thermographic inspections of polymer composite materials

Abstract

The maximum depth for which a defect of a given size can be detected in thermographic inspections is known as the 'probing depth'. For materials such as polymer composites, with low through-thickness thermal diffusivity, inspections are limited to thin materials or near surface inspection. With the aim of improving probing depth, the paper describes how established signal processing techniques are adapted for thermographic inspections. The procedures are implemented for both pulse thermography and pulse phase thermography inspections of laminated composite materials and sandwich structures. It is demonstrated that the adaptation significantly improves the probing depth, identifying defects that could not be identified using existing procedures. The applicability of the new approaches is discussed, paying particular attention to systematic and random errors resulting from equipment setup. Simple and efficient compensation methods to reduce the effect these errors are presented.