Flying-spot thermography: sizing the thermal resistance of infinite vertical cracks

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.