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.
Spot welding is one of the most important technologies for joining sheet metal. While there are lot of approaches to non-destructive testing, quality assurance still mainly relies on welding parameter monitoring and destructive testing, leading to significant failure rates. In this paper an approach to spot weld testing using flash thermography is presented. The main focus of attention is on the identification of two typical error classes: stick welds and welds at the splash limit. Besides investigating the principal feasibility of thermography for zinc plated samples the results of a series test of spot welds joining 1 mm thick TRIP steel are shown. Based upon these results a statistical criterion is developed which allows a reliable classification of the named error classes.
A method is reported with which heat affected zone (HAZ) cracks in a specific region of spot welded joints, made of advanced high strength steels, could be realised. The influence of HAZ cracks on the fatigue behaviour, which is the focus of the contribution, was studied by evaluating the stiffness characteristics as a function of the number of cycles N in comparison with crack-free specimens, i.e. by analysing HAZ and fatigue cracks on the specimen surface and the surface of fatigue fractures containing beach marks. It was found that the fatigue cracks are not initiated at the HAZ cracks. The number of cycles to failure Nf is not significantly reduced for specimens with HAZ cracks. Overall, the examinations did not reveal any influence of HAZ cracks on the fatigue strength of resistance spot welded joints.