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The detection and characterization of surface cracks in steel specimens prior to damage is a technologically and economically highly significant task and is of utmost importance when it comes to safety-relevant structures. In steel production where steel billets at high temperatures have to be inspected while moving a number of well-established NDT methods cannot be applied. Laser thermography however is a promising candidate to serve as a fast, non-contact and remote tool in this case. We present a study that shows that the crack detection capabilities of laser thermography can be extended also to specimens at high temperature. A combination of inductive and laser heating allows to systematically study the contrast formation as well as the optimization of the important measurement parameters. The experiments are accompanied by FEM simulations that provide a better insight of the physical correlations and support the experimental developments. The aim of these studies is to develop a system with high inspection speed and detection performance to be in-line operated under the hostile environment of steel production lines.
The detection and characterization of surface breaking cracks in steel prior to damage is a technologically as well as economically important task especially for safety-relevant structures. Detection of small cracks already during the steel production process might significantly reduce the risk of failure and reduce production costs due to an obsolete post-processing.
However, the hostile environmental conditions (high temperature specimens) together with very strict requirements in current steel production (production speeds, in-line testing and evaluation) are challenging and render many well-established NDE techniques hardly applicable.
We present an approach to use laser thermographic testing as a fast, remote and contactless NDE method, that addresses these challenges and might ultimately allow for online crack detection.
The basic idea of laser thermographic testing, introduced by Kubiak in 1968, is the monitoring of the heat flow as induced by local heating. Disturbances within the heat flow generated by the presence of surface cracks can then be analyzed by image processing algorithms, as we have shown in previous work.
The aim of the presented work is to advance laser thermographic testing to be applicable to the specific conditions of steel production environments. This purpose was met by the development of a laboratory setup that allows us to simulate production conditions, as rolling speed, specimen temperature, laser heating power and study their influence on crack detection performance. This parametric study enabled us to develop and improve data processing and crack detection algorithms with the final goal of providing optimized in-line crack detection. The studies were accompanied by comprehensive FEM simulations to intensify the understanding of the contrast formation as well as the crucial parameters influencing the performance of the method.
Crack detection in metallic samples at high surface temperature, hostile and hazardous environments, etc. is challenging situation in any manufacturing industries. Most of the present NDE methods are suitable only for lower surface temperatures, especially room temperature. In this situation, we need a fast and non-contact NDT method which can be applied even in high sample surface temperature. Laser thermography is one of the techniques having a high potential in non-contact inspection. As a preliminary investigation, in this article, we have studied the potentiality of laser line thermography in crack detection at room temperature. In laser line thermography, a continuous wave (CW) laser is used to generate a laser line, which in turn is used to scan the metal surface. The heat distribution over the sample surface is recorded by an infrared thermal (IR) camera. Two different approaches are reported in this work. Firstly, a stationary laser line source and its interaction with cracks; secondly, moving laser line source scanning over a surface with crack. When the distance between crack centre to laser line centre increases, crack detectability will decrease; and when laser power increases, crack detectability will increase. A dedicated image processing algorithm was developed to improve the detectability of the cracks. To understand the heat transfer phenomenon, a simplified 3D model for laser thermography was developed for the heat distribution during laser heating and was validated with experimental results. Defects were incorporated as a thermally thin resistive layer (TTRL) in numerical modeling, and the effect of TTRL in heat conduction is compared with experimental results.