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- Non-destructive testing (2) (entfernen)
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Crack detection in steel bars at high surface temperatures is a critical problem in any manufacturing industry. Surface breaking cracks are the major problems during the billet casting. Many NDT techniques are proven its capability in crack detection at room temperature. Here, we are demonstrating the possibility of exposure of cracks using laser line thermography at higher surface temperatures (up to 600 °C). A continuous-wave (CW) laser is used to excite the sample kept at higher surface temperatures. The temperature distribution over the sample due to the laser line scanning is captured using a temperature calibrated infrared (IR) thermal camera. The response of the sample temperature in crack detection is investigated using a validated FE model. The impact of the oxide layer in crack detection is investigated by using two types of samples; one without any oxide layer and the second is with the oxide layer. The influence of laser power in the detection of defects at high temperatures is studied. 3D numerical models were developed for the cases; when the sample is with oxide layer and without any oxide layer for a better understanding of physics. The surface temperature rise due to laser heating is higher for the scaled sample compared to the no-scale sample. The presence of the oxide layer above the parent metal will reduce the reflectivity of the surface. Lower reflectivity will lead to increased absorption of incident energy so that the surface temperature rise will be higher than the surface with no scale. Thermal contrast linearly depends on laser power, which means higher laser power will increase the defect detectability even at a higher surface temperature.
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.