We present a methodology to characterize the width of infinite vertical cracks using flying-spot thermography. We present the calculation of the evolution of the surface temperature distribution when a continuous wave laser spot scans at constant velocity the surface of a specimen containing an infinite vertical crack. The presence of the crack is revealed by a discontinuity on the surface temperature. By analyzing this temperature contrast between both sides of the crack, we determine the optimum experimental conditions to size width of the crack. We propose to fit the temperature profile perpendicular to the crack through the center of the laser spot to assess the thermal resistance associated to the crack. In order to check the validity of the method, we present experimental flying-spot data obtained on samples containing artificial and calibrated vertical cracks. The results confirm that, although detecting wide cracks is easy, it is not always possible to assess the width of wide cracks. The technique is better adapted to evaluate narrow cracks, which is the most challenging situation for other nondestructive evaluation techniques.
We present a methodology to measure the in-plane thermal diffusivity of (an)isotropic samples using flying spot thermography. We obtain an analytical expression for the surface temperature distribution when a continuous wave laser spot scans the sample surface at constant velocity. By analyzing this expression, we propose three simple methods to measure the thermal diffusivity in the directions parallel and perpendicular to the motion. The methodology can also be applied in the case where the laser spot is at rest, and the specimen moves at constant velocity. This configuration is interesting for in-line evaluation of industrial products. Finally, we present a set-up allowing the inspection of large and complex parts, by means of a robotic arm used to displace the part and orient the region of interest perpendicular to the optical axis of the camera.
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