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Pulsed Thermography Signal Processing Techniques Based on the 1D Solution of the Heat Equation Applied to the Inspection of Laminated Composites

Pulsed thermography has become a widely accepted technique for the nondestructive testing of materials. In its simplest form, an operator views the infrared image of the sample surface as it cools in response to a brief heat pulse. Subsurface discontinuities, such as voids or delaminations, appear as transient hot spots in the image sequence. However, to improve sensitivity to smaller, deeper features, and to compensate for common issues including diffusion blurring, anisotropy and non-uniform heating, additional signal processing is often applied. Three of the most popular processing techniques, thermographic signal reconstruction, differential absolute contrast and pulsed phase thermography, are based on the 1D solution of the heat conduction equation. These techniques were applied to experimental pulsed thermography data from laminated composite samples with simulated discontinuities (fluorocarbon resin inserts) and were compared to unprocessed results based on signal-to-noise ratio at maximum signal contrast. Results indicate that anomalies with large diameter/depth ratio do not require additional processing, but processing does become essential as the diameter/depth ratio becomes smaller. The threshold at which processing becomes necessary depends on both material properties (for example, anisotropy, thermal conductivity and diffusivity) and experimental parameters (for example, energy input, camera frame rate and spatial resolution).

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