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Mit Laserlicht kann man eine Materialoberfläche berührungslos und schnell moduliert aufheizen. Dabei entsteht eine stark gedämpfte Wärmewelle, die tief ins Material eindringen kann. Erzeugt und überlagert man solche thermischen Wellen auf kohärente Weise, dann kann man damit versteckte Materialfehler zerstörungsfrei und sehr präzise aufspüren. Sogar eine bildgebende Tomografie ist denkbar.
Optical lock-in thermography is a completely contactless and very sensitive NDE technique. As an optical source of energy, incandescent (i.e. halogen) lamps are most commonly used because they are relatively inexpensive, do not need any work safety measures and offer high irradiances at the test site. However, they are strongly restricted by their low modulation bandwidth with a maximum modulation frequency of only about 1 Hz. The use of high-power kilowatt-class laser sources, e.g. diode laser arrays, pushes this constraint beyond 100 Hz. This allows for the exploration of the near-surface region of metals and layer systems with better and more accurate penetration depth and depth resolution. Moreover, these lasers are virtually free of any additional thermal radiation that could interfere with the “true” thermal response emitted from the heated sample. In turn, they can be easily used in a one-sided test configuration. Altogether using lasers considerably increases the application range of lock-in thermography, since especially for metals with a high reflectance and high thermal diffusivity a high irradiance is vitally important to allow for lock-in testing [1, 2]. We report on the mentioned benefits of using such high-power lasers and analyze the range of materials to be tested using lock-in thermography in dependence on the laser irradiance, the modulation frequency, the infrared camera as well as the optical and thermal material parameters. In this context, we also address a number of systematic errors caused by the use of ideal and non-ideal heat sources. For example, the measured phase angle in lock-in thermography depends on the irradiance and the modulation bandwidth of the source. This in turn has a decisive influence on the uncertainty in the quantification of, e.g. layer thicknesses.
Due to their high irradiance and wide modulation bandwidth, high-power lasers open up a wide field of application. For example, the classical methods of pulse and lock-in thermography can be realized in high quality. In addition, structured heating is also possible by using arrays of such lasers. This makes it possible to implement new thermographic methods, such as interference-based detection of cracks or super resolution.
Thermographic non-destructive testing is based on the interaction of thermal waves with inhomogeneities. The propagation of thermal waves from the heat source to the inhomogeneity and to the detection surface according to the thermal diffusion equation leads to the fact that two closely spaced defects can be incorrectly detected as one defect in the measured thermogram. In order to break this spatial resolution limit (super resolution), the combination of spatially structured heating and numerical methods of compressed sensing can be used. The improvement of the spatial resolution for defect detection then depends in the classical sense directly on the number of measurements. Current practical implementations of this super resolution detection still suffer from long measurement times, since not only the achievable resolution depends on performing multiple measurements, but due to the use of single spot laser sources or laser arrays with low pixel count, also the scanning process itself is quite slow. With the application of most recent high-power digital micromirror device (DMD) based laser projector technology this issue can now be overcome.
Data-driven research is considered the new paradigm in science. In this field, data is the new resource from which knowledge is extracted that is too complex for traditional methods. Several factors such as national funding and advances in information technology, are driving the development. In particular, the creation of databases and the analysis of data with artifical intelligence are playing an important role in establishing the new paradigm. However, there are numerous challenges that must be overcome to realize the full potential of data-driven methods. This talk sets the stage for the upcoming workshop by reviewing some of the historical developments and the current state of data-driven science in NDT and materials science.
In the field of non-destructive testing (NDT) in civil engineering, a large number of measurement data are collected. Although they serve as a basis for scientific analyses, there is still no uniform representation of the data. An analysis of various distributed data sets across different test objects is therefore only possible with high manual effort.
We present a system architecture for an integrated data management of distributed data sets based on Semantic Web technologies. The approach is essentially based on a mathematical model - the so-called ontology - which represents the knowledge of our domain NDT. The ontology developed by us is linked to data sources and thus describes the semantic meaning of the data. Furthermore, the ontology acts as a central concept for database access. Non-domain data sources can be easily integrated by linking them to the NDT construction ontology and are directly available for generic use in the sense of digitization. Based on an extensive literature research, we outline the possibilities that this offers for NDT in civil engineering, such as computer-aided sorting, analysis, recognition and explanation of relationships (explainable AI) for several million measurement data.
The expected benefits of this approach of knowledge representation and data access for the NDT community are an expansion of knowledge through data exchange in research (interoperability), the scientific exploitation of large existing data sources with data-based methods (such as image recognition, measurement uncertainty calculations, factor analysis, material characterization) and finally a simplified exchange of NDT data with engineering models and thus with the construction industry.
Ontologies are already the core of numerous intelligent systems such as building information modeling or research databases. This contribution gives an overview of the range of tools we are currently creating to communicate with them.
We report on photothermal detection of subsurface defects by coherent superposition of thermal wave fields. This is made possible by structured heating using high-power VCSEL laser arrays whose individual emitter groups can be arbitrarily controlled. In order to locate the defects, we have developed a scanning method based on the continuous wavelet transformation with complex Morlet wavelet using the destructive interference effect of thermal waves. This approach can also be used for thermally very fast and highly reflective materials such as uncoated aluminum. We show that subsurface defects at an aspect ratio of defect width to defect depth down to 1/3 are still detectable in this material.
Die tiefgreifende Reparatur von Faserverbundwerkstoffen stellt durch deren inneren Aufbau eine besondere Herausforderung dar, die unter anderem bei der Instandhaltung von Rotorblättern von Windkraftanlagen zum Tragen kommt. Um die Reparaturen von Rotorblattschalen zu untersuchen und zu optimieren, wurden an der BAM Sandwich-Schalenprüfkörper aus Glasfaserkunststoff (GFK) entwickelt. Nachdem in diese Schalenprüfkörper Modell-Reparaturen eingebracht wurden, werden zyklische mechanische Ermüdungsversuche bis zum Bruch durchgeführt. Vorrangiges Ziel ist es, den Einfluss verschiedener Reparaturvariablen auf die Ermüdungslebensdauer der Struktur zu verstehen, sowie die stabilste Reparaturmethode zu bestimmen.
Während der zyklischen Lastversuche wurden regelmäßig in-situ Messungen mit einer Thermografie Kamera und einem optischen Felddehnungsmesssystem durchgeführt. Durch beide Messmethoden lassen sich Eigenschaften der Probekörper zeitlich und räumlich aufgelöst darstellen.
Während die Felddehnungsmessungen Aufschluss über die Verformung an der Oberfläche geben, erfasst die thermische Inspektion Temperaturänderungen an der Oberfläche. Besonders die thermografischen Messwerte bilden hier das oberflächennahe Volumen ab, in dem sich Temperaturänderungen an der Oberfläche abzeichnen.
Die Messergebnisse beider Systeme stellen also verschiede aber korrelierte Parameter dar. Denn die zyklische Belastung (Bewegung) ruft eine periodische Temperaturänderung im Bauteil hervor. Dieser so genannte Thermoeleastische Effekt kann durch eine Lockin-Analyse der Temperaturtransienten (Temperaturverläufe) nachgewiesen werden. Die räumliche Darstellung der thermischen Effekte lässt Rückschlüsse auf die tatsächliche Lastverteilung im Probekörpervolumen zu.
Darüber hinaus führen nichtelastische Verformungen zu einer zusätzlichen Wärmefreisetzung, die durch geeignete Auswerteverfahren ebenfalls nachgewiesen werden können. Entsprechende Hot-Spots in der räumlichen Verteilung erlauben eine frühzeitige Erkennung besonders belasteter Bereiche lange vor dem eigentlichen Bauteil-Versagen. Das gestattet die Lokalisierung des Initial-Schadens, was mit der numerischen Spannungsanalyse korreliert werden kann und die Entwicklung und Bewertung verschiedener Reparaturstrategien.
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.