Detection of subsurface defects in aluminium with thermo-inductive inspection

摘要

Active thermography can be well used to detect subsurface defects like buried cavities in materials. For metallic materials induction heating is the most efficient technique, because the heat is generated directly in the material and therefore the usually low emissivity and absorption coefficient of the metallic surface does not affect the heating process. Short inductive heating pulses (0.5-2 s) have been used to detect holes with a diameter of 2 mm in a depth of 2-4 mm below the surface in aluminum samples. Some of the defects were generated during the production process; other ones were created artificially. The size and the depth of these defects were determined with the help of computer tomography. Additionally to the experimental data, also finite element simulations and analytical calculations have been carried out in order to model the heat distribution for different defect sizes and defect depths. The calculations have been used to optimize the heating pulse duration. Based on the modelling results, an evaluation algorithm has been developed, which allows an automatically localization of the defects with help of image processing techniques. In order to test the stability of the automated evaluation, noise has been added to the calculated temperature distribution. The same processing technique has been used for the evaluation of the experimental data to localize subsurface defects and very good detection results could be achieved.