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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.
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.
The detection of cracks before the failure is highly significant when it comes to safety-relevant structures. Crack detection in metallic samples at high surface temperature is one of the challenging situation in manufacturing industries.
Laser thermography has already proved its detection capability of surface cracks in metallic samples at room temperature. In this work a continuous wave (CW) laser use to generate a laser, which is using to scan the metal surface with notch.
The corresponding heat distribution on the surface monitored using infrared thermal (IR) camera. A simplified 3D model for laser thermography is developed and validated with experimental results. A dedicated image processing algorithm developed to improve the detectability of the cracks. To understand the dependency of surface temperature, laser power, laser scanning speed etc. in defect detection, we carried out parametric studies with our validated model. Here we Report the capability of laser thermography in crack detection at elevated 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.
Laser thermography has its roots in the late-1960s when Kubiak first used a focused light source to raster-scan a specimen containing surface breaking cracks and employed an infrared detector to monitor the blocking effect of the cracks on the heat flow. Nowadays, this technique has evolved into the flying-spot laser thermography. In the mid-1970s, when Rosencwaig and Gersho worked out the theory for the photoacoustic effect, they laid the foundation for the development of the photothermal testing methodology. The decisive difference to the flying-spot technique is that in the photothermal methodology, a high-frequency modulated laser beam is used, which explicitly uses the concept of the thermal wave.
Starting in the 1980s, infrared cameras came into play and enabled full-field thermal imaging, but this time, slow thermal light sources were used. Today, flash lamps (pulsed planar heating) and halogen lamps (modulated planar heating) have been established as the standard light sources for the specific regimes of pulsed and lock-in thermography.
Only recently, the availability of novel technologies – fast and high-resolution infrared cameras, innovative brilliant laser sources and high-performance data acquisition and processing technology – has enabled a paradigm shift from the separated photothermal and thermographic methodologies to a versatile tool for NDE that combines both approaches. In addition, the heating can also be spatially structured using laser arrays. This new degree of freedom allows the development of completely new thermography NDE methods.
In the talk, we present current activities with kilowatt-class high-power laser sources for advanced flying-spot, pulsed, lock-in and laser-projected thermography.
Die Detektion bzw. Charakterisierung von Oberflächenrissen in Stahl ist eine herausfordernde Aufgabe für die zerstörungsfreie Prüfung. Dies gilt insbesondere, wenn eine solche Prüfung bereits während der Produktion selbst erfolgen soll. Die hier vorgefundenen Randbedingungen, wie z.B. die hohen Probentemperaturen (>700°C) oder Vorschubgeschwindigkeiten von bis zu 0,9 m/min, machen die Anwendung etablierter und vor allem berührender ZfP-Methoden praktisch meist unmöglich. Laser-thermografische Verfahren können hier eine Alternative bieten, da sie berührungslos und schnell arbeiten. Wir stellen die Ergebnisse einer Weiterentwicklung der Laser-thermografischen Prüfung vor, bei der die Anwendbarkeit bzw. die Übertragbarkeit der Methode auf die Bedingungen der Stahlproduktion untersucht wurde.
Neben der Simulation der Produktionsbedingungen im Labor und der experimentellen Untersuchung der entscheidenden Einflussgrößen, wurden umfangreiche FEM-Simulationen durchgeführt. Mit Hilfe dieser theoretischen und experimentellen Ergebnisse wurde das Verfahren auch hinsichtlich der Analyse-Algorithmen weiterentwickelt, um eine zuverlässige und schnelle In-Line Prüfung während des Produktionsprozesses zu ermöglichen. Um zudem die Fähigkeiten der Thermografiekamera für schnelle online-Messungen und ihre Integrierbarkeit in verschiedene Anwendungssysteme zu verbessern, wurde die für die Verarbeitung der Detektordaten erforderliche Non-Uniformity Correction der Pixeldaten auf schneller Spezialhardware innerhalb der Kamera implementiert.
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.
The aim of this work is to illustrate the contribution of signal processing techniques in the field of Non-Destructive Evaluation. A component’s life evaluation is inevitably related to the presence of flaws in it. The detection and characterization of cracks prior to damage is a technologically and economically significant task and is of very importance when it comes to safety-relevant measures. The Laser Thermography is the most effective and advanced thermography method for Non-Destructive Evaluation. High capability for the detection of surface cracks and for the characterization of the geometry of artificial surface flaws in metallic samples of laser thermography is particularly encouraging. This is one of the non- contacting, fast and real time detection method. The presence of a vertical surface breaking crack will disturb the thermal footprint. The data processing method plays vital role in fast detection of the surface and sub-surface cracks.
Currently in laser thermographic inspection lacks a compromising data processing algorithm which is necessary for the fast crack detection and also the analysis of data is done as part of post processing. In this work we introduced a raw data based image processing algorithm which results precise, better and fast crack detection. The algorithm we developed gives better results in both experimental and modeling data. By applying this algorithm we carried out a detailed investigation Variation of thermal contrast with crack parameters like depth and width. The algorithm we developed is applied for various surface temperature data from the 2D scanning model and also validated credibility of algorithm with experimental data.
Comparison of quantitative defect characterization using pulse-phase and lock-in thermography
(2016)
Using optical excitation sources for active thermography enables a contactless, remote, and non-destructive testing of materials and structures. Currently, two kinds of temporal excitation techniques have been established: pulse or flash excitation, usingmostly flash lamps; and periodic or lock-in excitation, using halogen lamps, LED, or laser arrays. From the experimental point of view, both techniques have their advantages and disadvantages. Concerning the comparison of the testing results of both techniques, only very few studies have been performed in the past. In this contribution, the phase values obtained at flat bottom holes in steel and CFRP and the spatial resolution measured at crossed notches in steel using flash and lock-in excitation are compared quantitatively.
In diesem Beitrag werden Verfahren und Strategien basierend auf optischen und thermografischen Methoden zur Lokalisierung, Erfassung und Bewertung von Putzablösungen, Ablösungen von Fassadenelementen und Rissen vorgestellt. Abgelöste Fassadenbereiche und Risse zeigen dabei sowohl geometrische als auch thermische Auffälligkeiten, die im Rahmen eines Monitoring auch über größere Zeiträume verfolgt werden können. Als optische Verfahren werden die trackingbasierte taktile 3D‐Erfassung von Oberflächenmerkmalen und die Stereophotogrammetrie eingesetzt. Zur Charakterisierung verdeckter Ablösungen und Bauteile wird die aktive Thermografie mit natürlicher und künstlicher Erwärmung der Bauteiloberfläche verwendet. Systematische Untersuchungen an Probekörpern und Fallstudien (Magdeburger Dom, Wandbild in Cobbelsdorf, Giebichensteinbrücke) zeigen, wie u. a. mit der Verfahrenskombination die Schäden charakterisiert werden können.