Günter Walle

Bio: Günter Walle is an academic researcher from Fraunhofer Society. The author has contributed to research in topics: Thermography & Round robin test. The author has an hindex of 4, co-authored 11 publications receiving 124 citations.

Induction thermography: principle, applications and first steps towards standardisation

Abstract: A survey on theory, characteristic quantities and the experimental technique of induction thermography is given. Induction thermography is used for surface defect detection in forged parts of ferromagnetic steel at typical frequencies of 100–300 kHz. The sensitivity for crack detection is comparable to magnetic particle inspection. A hidden defect in ferritic steel with a coverage of 140 μm was detected by lowering the induction frequency down to 1500 Hz. Defects of fibres were detected in carbon fibre reinforced polymer (CFRP). Inductive excitation is complementary to flash excitation. By increasing the induction frequency up to 52 MHz, surface heating of CFRP can be realised. Cracks in silicon solar cells were detected. The crack tip is revealed with very high contrast. A new field is crack detection in railway components like rails and wheels. In rails, a larger defect could be detected from a test car moving at a speed of up to 15 km/h. A fully automated wheel testing system was built up as a ...

Induction Thermography as a Tool for Reliable Detection of Surface Defects in Forged Components

Abstract: Dynamic thermography with inductive excitation is analysed as an alternative to magnetic particle inspection or to eddy current testing. Given by the relation of the electromagnetic skin depth, the thermal penetration depth and the crack dimensions to be detected, different regimes for defect detection are identified. The effect of the crack parameters length, depth and inclination angle are discussed. In ferritic steel, at induction frequencies of 100-200 kHz, perpendicular open cracks with a length of 7.5 mm were detectable when their depth was minimum 0.15 mm. For inclined cracks, the sensitivity is even higher. Experiments were performed on cold and warm forged steel components. The signal-to noise ratio obtained from defects was usually high, the critical limitation on technical surfaces is the background due to surface roughness and due to surface contamination. A series investigation on forged components showed a good probability of detection and a low false alarm rate compared to magnetic particle testing. The short testing times of a few 100 ms per object view will allow short cycle times for mass products.

NDT of railway components using induction thermography

Abstract: Induction or eddy current thermography is used to detect surface cracks in ferritic steel. The technique is applied to detect surface cracks in rails from a moving test car. Cracks were detected at a train speed between 2 and 15 km/h. An automated demonstrator system for testing railway wheels after production is described. While the wheel is rotated, a robot guides the detection unit consisting of inductor and infrared camera over the surface.

Probability of detection analysis of round robin test results performed by flash thermography

Abstract: Within the scope of a standardisation research project, a flash thermography round robin test that evaluates reliability, comparability and efficiency of different testing situations was performed....

Eine thermische Prüftechnik zur Oberflächenrissprüfung leitfähiger Materialien

Abstract: Kurzfassung Die induktiv angeregte Thermografie stellt eine Alternative zu klassischen Oberflachenrisspruftechniken wie der Magnetpulverprufung oder auch der Farbeindringprufung bei leitfahigen Materialien, insbesondere ferromagnetischen und austenitischen Stahlen dar. Im vorliegenden Beitrag sollen die Grundlagen des Verfahrens, einige Anwendungsbeispiele sowie die Nachweisgrenzen und besonderen Vorteile der Pruftechnik dargestellt werden. Die thermische Pruftechnik hat sich als ein Verfahren erwiesen, das eine objektive Oberflachenrissprufung mit hoher Nachweisempfindlichkeit, Prufgeschwindigkeit und Zuverlassigkeit ermoglicht. Da die Pruftechnik vollkommen beruhrungslos und schnell arbeitet und eine relativ einfache Automatisierung ermoglicht, eignet sie sich insbesondere fur vollautomatische Prufanlagen zur 100%-Prufung von Bauteilen im Rahmen der industriellen Qualitatssicherung.

Infrared thermography for temperature measurement and non-destructive testing.

Abstract: The intensity of the infrared radiation emitted by objects is mainly a function of their temperature. In infrared thermography, this feature is used for multiple purposes: as a health indicator in medical applications, as a sign of malfunction in mechanical and electrical maintenance or as an indicator of heat loss in buildings. This paper presents a review of infrared thermography especially focused on two applications: temperature measurement and non-destructive testing, two of the main fields where infrared thermography-based sensors are used. A general introduction to infrared thermography and the common procedures for temperature measurement and non-destructive testing are presented. Furthermore, developments in these fields and recent advances are reviewed.

Defect characterisation using pulsed eddy current thermography under transmission mode and NDT applications

Abstract: Pulsed eddy current (PEC) thermography has been successfully applied to detect cracks in metal alloy and carbon fibre reinforced plastic (CFRP) in previous works. In these applications, an inductor and infrared camera are on the same side of components, named reflection mode. In this work, the transmission mode, where the inductor and infrared camera are on opposite sides of component is investigated for defect characterisation through the analytical analysis and experimental studies. The studies show that the detection mechanisms for impact and delamination in CFRP are totally different. Carbon structure can be observed on the early stage of heating phase and impact leading to decreasing conductivity can be also detected in heating phase. However, delamination can be characterised using late stage of cooling phase. Combing the detection mechanisms, principal components analysis and independent components analysis, image reconstruction method is used to improve the sensitivity.

Eddy-current induced thermography—probability of detection study of small fatigue cracks in steel, titanium and nickel-based superalloy

Abstract: Eddy-current induced thermography (induction thermography, hereon referred to as eddytherm) is an active thermographic method which is capable of rapid and non-contacting detection of out-of-plane cracks in electrically conductive parts. In an eddytherm inspection, the part is induction heated; cracks cause localised changes in the induced eddy-current flow and the associated Joule heating is imaged at the surface of the part with an infrared camera. In this study the detectability of fatigue cracks in steel, titanium and Waspaloy is quantified by novel but simple image processing routines which are specifically applicable to eddytherm inspection. The quantitative detection data is then input into a cumulative log-normal probability of detection model to estimate the probability of detecting the fatigue cracks as a function of crack length. a90,95 (i.e., the crack length which can be detected 90% of the time with 95% confidence) is found to be 0.60 mm for steel, 0.78 mm for titanium and 1.50 mm for Waspaloy (a nickel-based superalloy), showing eddytherm to be an extremely sensitive method.

Crack sizing by laser excited thermography

Abstract: 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.