Passive Defect Detection and Imaging in Structures by Cross-correlations of Infrared Thermography Measurements

Abstract

Infrared Thermography (IT) is one of the most common techniques employed for Non-Destructive Evaluation (NDE) of structures, particularly due to its wide area coverage and imaging potentials. Its drawbacks, as the need of a powerful heat source and set-up, could be overcome if applying the technique in a passive mode. In this work, the feasibility for passive Green’s function retrieval from diffuse thermal field is investigated. Relying on some wave propagation aspects, it is proven that a thermal Green’s function can be reconstructed using long-time cross-correlations of temperature time histories, recorded from an IR camera. The random thermal fluctuations of the structure would still contain information related to the heat propagation path and hence could be exploited to detect anomalies. A successful application of this concept would allow defect detection and imaging, avoiding external sources. This technique has been already applied in many other environments, as seismology, oceanography and structural health monitoring, relying on acoustic wave fields. In the thermal case, the exploitation of the already available 2D pixel grid resolution as a large temperature array would also allow beamforming, due to its large aperture. This paper will discuss the feasibility of passive thermography and present ongoing studies in aluminum and composite plates with engineered defects