Numerical method of active thermography for the reconstruction of back wall geometry

The paper presents a numerical method to detect and characterise defects and inhomogeneities by means of active thermography. The objective was to determine the wall thickness of structure elements with an inaccessible back wall, e.g., elements of pipes or containers. As test specimens we used PVC samples with the thickness of about 2 cm that had spatial variations in the back wall geometry. Flash lamps provided the heating. To measure the thickness of the wall, we employed the Levenberg–Marquardt method, which we applied here to experimental thermographic data for non-destructive testing. We started the inversion procedure by making a rough first estimation of the back wall geometry following the echo defect shape method, and then we calculated the thickness of the back wall. We found reasonable reconstruction results which differed from the real value significantly below 1 mm at the defect centre, whereas the error wais increased at the edge of the defect, depending on its shape and depth.

► Wall thickness was detected by thermography considering the lateral heat flow.
► Defects and reference areas were easily reconstructed–apart from one challenging aspect.
► The reference area (of a 2 cm thick PVC) close to sharp edges of defects is challenging.
► Back wall geometries were reconstructed using a classical tool of inverse problem solvers.