Automated surface crack detection with inductive thermography

Abstract

In recent years, inductive thermography has become established as a non-destructive inspection technique for detecting surface cracks in metallic parts. A short inductive heating pulse (0.1 – 1 s) is applied to the sample and an infrared camera records the surface temperature during and after the heating pulse. Since a surface crack obstructs both the eddy current flow and the heat flow, the defect becomes well visible in the infrared images. The technique has the advantage of being contact-free, fast and it can be fully automated. Additionally, it also provides information about the crack depth, because the deeper the crack, the greater the obstacle to the eddy current and to the heat flow. In order to detect the cracks in the infrared images of castings and forgings with complex geometry, a comparison method has been developed. The images of the inspected parts are compared with images of reference parts, and in this way artefacts caused by edges and corners of the parts are filtered out and not marked as defects. This greatly increases the robustness of the fully automated inspection, which can test parts 24 hours a day, and 7 days a week, without any human interaction, and the results of each part are well documented and stored.