Second harmonic passive thermography generated by cyclic loading in composites

Previous passive thermography efforts to monitor the progression of damage during cyclic fatigue testing have focused on the heat generated at the cyclic load frequency. A closer examination of the acquired data has shown that at some locations on a test panel, the passive thermal response has significant content at twice the cyclic frequency. The locations of responses, with the largest harmonic contributions, are typically at the edge of subsurface delaminations based on ultrasonic scans of the panel. A physics-based model is presented that describes how a harmonic signal is expected if there is a friction based heat source. From the model, the expected phase of the heat source is calculated. The source is inserted in one-dimensional and three-dimensional simulations for heat diffusion to calculate the surface response for a subsurface friction source. The phase of the harmonic is found to agree reasonably well with a source at a delamination depth in the same location on the test panel as determined from ultrasonic scans of the damage.