Imaging real cracks: evaluation of the depth and width of narrow fatigue cracks in and Al-alloys using laser-spot lock-in thermography

Thermographic nondestructive techniques with focused laser excitation have proven as very efficient tools for the detection of narrow cracks. Moreover, it has been shown that in the ideal case of infinite cracks, the width of the crack can be assessed quantitatively using laser spot thermography, both in lock-in and pulsed regimes. In this ideal case, the surface temperature of the cracked material can be obtained analytically. However, real cracks feature finite penetration and length and, in these conditions, the calculation of the surface temperature needs to be performed numerically. In this work, we combine laser-spot lock-in thermography with finite elements modelling (FEM) to perform a full characterization of the local values of the width and depth of narrow cracks along the whole crack length in two Alalloys plates after fatigue test. First, in order to locate and image the crack, we combine the squares of the spatial derivatives of the amplitude thermograms along two perpendicular directions for different positions of the laser spot. Then, we place the laser close to the crack and we fit the numerical model to the amplitude data, so as to obtain the values of the width and depth of the crack at the current position of the laser. By displacing the laser spot at different positions along the crack length, we fully characterize the width and depth of the crack, whose resulting values are of the order of 1 µm and 0.5 mm, respectively.