Filtern
Dokumenttyp
Schlagworte
- Thermography (7)
- Active thermography (4)
- Crack sizing (4)
- Laser excitation (4)
- Defekte (3)
- Inverse Probleme (3)
- Rekonstruktion (3)
- Aktive Thermografie (2)
- Cracks (2)
- FEM (2)
Active thermography with lock-in excitation is a non-destructive testing method that is also feasible for testing of carbon fiber reinforced polymers (CFRP). For validating the method extensive investigations were done during a research project in order to advance a standardization process.
The most important parameters of optical and ultrasonic excitation thermography were investigated. For example, the appropriate selection of spectral sensitivity of the used infrared camera systems is important for recording undisturbed thermal signals. Regarding excitation,
influences of excitation power and ultrasonic frequency were studied. Furthermore, material parameters such as CFRP layup are known for strongly affecting the results of
measurement.
We present a systematic study on the performance of laser-thermography for the detection of surface cracks in metallic components. Scanning a metallic surface with laser causes local heating that is mapped simultaneously by an IR-camera and allows identifying cracks with sub-µm openings. The detectability, however, depends on a number of acquisition parameters (e.g. scanning speed, laser power, IR-camera resolution) that typically relate on each other. Most importantly, the detection-sensitivity of surface breaking cracks is given by a particular combination for the acquisition parameter values. As a result, this sensitivity is adaptable within wide ranges allowing the detection of cracks with openings ranging from 200 to 0.1 µm at testing speeds of 100 to 0.05 cm²/s. By examining artificial as well as fatigue cracks, we demonstrate that the method can be even applied to shiny surfaces with no need of pretreatments, which makes it an entirely contactless, remote and automatable NDT technique. A comparison with magnetic particle testing shows that laser-thermography has the potential to become a strong competitor to conventional surface inspection methods in the future.
Der Large Hadron Collider (LHC) des europäischen Kernforschungszentrums CERN ist einer der weltweit größten und leistungsfähigsten Teilchenbeschleuniger. Um den Teilchenstrahl auf seiner Kreisbahn zu halten, werden mit Hilfe supraleitender Spulen sehr starke Magnetfelder erzeugt. Die hierzu benötigten hohen Ströme werden über Verbunddrähte aus Niob-Titan und Kupfer geleitet.
Die Drähte werden u.a. durch Ultraschallschweißungen miteinander verbunden. Eine mangelhafte Verbindung kann zur Erwärmung der Kontaktstelle und in Folge davon zum Zusammenbruch der Supraleitung führen. Auf Anfrage des CERN wurden an der BAM Untersuchungen zur Prüfbarkeit solcher Verbindungen bei Umgebungstemperatur durchgeführt. Es gelang, für einen Verbindungstypen einen Wirbelstromsensor für eine manuelle Prüfung zu entwickeln. Ein Prototyp des Sensors befindet sich beim CERN in der praktischen Erprobung. Das Sensorprinzip sowie Prüfergebnisse aus Labor und Praxis werden vorgestellt.
Thermografische ZfP-Verfahren erlauben die Erkennung einer Vielzahl von Defekten in unterschiedlichsten Werkstoffen. Neue Ansätze unter Verwendung von Lasern ermöglichen darüber hinaus die Prüfung auf Oberflächenrisse. Wir stellen die neuesten Entwicklungen dieser vielversprechenden Technik vor und zeigen warum deren Einsatz potentiell die Standardprüfverfahren Magnetpulver- bzw. Eindringprüfung für eine Reihe von Prüfproblemen beerben könnte. Hierzu stellen wir die Prüfergebnisse zu unterschiedlichen metallischen Werkstoffen, Rissgrößen und -typen in Abhängigkeit der Prüfparameter vor. Die aktuelle Grenze der Nachweisempfindlichkeit für die mittels klassischer Thermografie schwer prüfbaren hochreflektierenden Metalloberflächen liegt mit Rissbreiten und -tiefen von weni-gen Mikrometern im Bereich der fluoreszierenden Magnetpulver- und Eindringprüfung. Dabei sind weder Verbrauchsmittel noch eine Oberflächenpräparation notwendig und eine Prüfung kann berührungslos und automatisiert über Entfernungen im Meter-Bereich erfolgen.
The paper presents a numerical method to detect and characterise defects and inhomogeneities by means of active thermography. The objective was to determine the wall thickness of structure elements with an inaccessible back wall, e.g., elements of pipes or containers. As test specimens we used PVC samples with the thickness of about 2 cm that had spatial variations in the back wall geometry. Flash lamps provided the heating. To measure the thickness of the wall, we employed the LevenbergMarquardt method, which we applied here to experimental thermographic data for non-destructive testing. We started the inversion procedure by making a rough first estimation of the back wall geometry following the echo defect shape method, and then we calculated the thickness of the back wall. We found reasonable reconstruction results which differed from the real value significantly below 1 mm at the defect centre, whereas the error wais increased at the edge of the defect, depending on its shape and depth.
We report on recent developments in the detection of surface breaking cracks using
flying laser spot thermography. Application of an infrared camera for mapping the
thermal radiation after excitation with a diode laser equipped with an optical scanner
allows us to examine a surface containing cracks in an entirely non-destructive,
contactless and fast way, without even moving the camera. We developed an efficient
and robust algorithm that can be applied directly to the recorded thermal sequences,
and that derives a single image containing all crack signatures. For this crack detection
technique, no specific synchronisation between laser and camera is required. Hence,
our approach is suitable for an upgrade of existing thermographic systems. The
feasibility of the proposed procedure is proven by testing an artificial test sample and a
piece of rail that comprises roll contact fatigue cracks and by comparing the results
with magnetic particle testing.
Active thermography is a nowadays widely used NDT method making use of thermal material properties for defect detection. Basically, the sample is heated and the resulting surface temperature is recorded by an IR camera. For laser thermography a laser is used to heat the sample locally. The resulting spherical heat flow allows the detection of voids in arbitrary orientation. In this work, a method is presented which is suitable for the quantitative characterization of depth and angle of surface cracks. The main idea is to evaluate the crack-caused asymmetries of the laser's thermal footprint. The heat is introduced at fixed reference positions relative to the crack. In this paper a data analysis procedure is presented which allows the crack depth and angle to be described by only two characteristic scalar parameters. By investigating artificial test specimens with spark eroded notches, the feasibility of this method is validated. Furthermore, the behavior of the characteristic parameters with variations of crack angle, depth and experimental conditions is studied systematically by FEM simulations, showing that these parameters are well behaved.
Impulse and lock-in thermography have been applied to detect delaminations of prototype solder joints, similar to those to be produced between Cu shunts and Cu busbar stabilisers at the Large Hadron Collider (LHC) at CERN. Two infrared cameras with different detector materials and with different spectral ranges and two excitation techniques have been tested and compared for their ability to detect delaminations behind 2 and 3 mm thick Cu shunts. We have analyzed the signal to noise ratio (SNR) for each detected defect and are able to detect defects down to a nominal edge length of 4 mm behind 2 mm thick Cu shunts by using fast impulse thermography and a camera with a microbolometer array. For the 3 mm thick Cu shunt, on the other hand, the nominal 4 mm defect is only visible in the lock-in thermography phase images and the highest SNR has been achieved with a cooled InSb-based camera. In addition, numerical simulations show the influence of the minimum detectable defect size on the shunt thickness and that the developed on-site testing technique is sufficient to find all defects that are detectable theoretically.
Untersuchungen zur 2D- und 3D-Rekonstruktion von Rückwandgeometrien in der Impuls-Thermografie
(2012)
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.