Calibration of thermographic spot weld testing with X-ray computed tomography

Abstract

The paper presents an attempt for the calibration of an active thermography method that is suitable for the non-destructive evaluation of spot welds. Nowadays, the quality of spot welds is commonly characterised by the application of random chisel tests, which are time consuming, expensive and destructive. Recently a non-destructive testing method by means of active thermography was proposed that relies on the fact that the mechanical connection formed by the spot weld also serves as a thermal bridge between the two steel sheets joined in the welding process. It is shown in this paper that this thermal bridge can be thermographically characterised by extracting a measure for the spot weld diameter and hence the quality of the spot weld. The determination of the absolute value of the diameter hereby relies on a calibration of the testing system, which is performed by means of X-ray computed tomography in this study. The experiments were carried out using different experimental approaches, namely transmission as well as reflection geometry wSetup in reflectionith laser illumination. A comprehensive evaluation of samples produced using different welding currents, hence different quality, was carried out in order to validate the thermographic results.

Keywords:

• Thermographic testing
• spot weld
• spot weld testing
• computed tomography
• non-destructive testing

Acknowledgement

The IGF-project 17686 N of the Research Association for Steel Application – FOSTA, Sohnstraße 65, 40237 Düsseldorf, was funded via AiF by the Federal Ministry for Economic Affairs and Energy (BMWi) within the framework of the program Industrial Collective Research (IGF) on the basis of a decision of the German Bundestag.