Determining the dimension of subsurface defects by active infrared thermography – experimental research

摘要

This paper presents research into a method of processing thermalimages aimed at detecting and characterizing material defects, ornon-uniformities, of the internal structure of materials. Active thermographywas chosen as the NDT method. Hidden defects were revealed by analysing thetemperature field of the tested material's front surface which was externallyexcited with heating lamps. Background removal and image segmentation wereapplied to the last thermogram in the sequence recorded at the end of theheating phase. The paper focuses on the quality of determining lateraldimensions of subsurface flaws in a polymethylmethacrylate slab with bottom holes drilled to imitate flaws. The followingaccuracy-affecting factors were taken into account: defect depth, emissivityof the inspected surface as an input, user-set parameter for the IR camera,type of filtering used to eliminate the effect of non-uniformity when heatingthe object surface with an external source, and global and local thresholdingas a segmentation method used for defect detection and sizing.