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Non-destructive testing for surface crack detection and head check depth quantification at the gauge cor-ner of railway tracks can be achieved using eddy current methods. With the extension of the tested zone to the running surface, rail defect signal types other than head checks can be measured. Due to their most-ly irregular shape, a quantitation based on a calibration against regular test cracks of varying depth may not be linear. Estimates of the expected influence of more complex crack patterns may be obtained by a finite element simulation of sufficiently simple limiting cases, like two displaced or intersecting cracks or a simply branched or flexed crack. As a first step, a 3D finite element model of the HC10 eddy current probe distributed by Prüftechnik Linke und Rühe (PLR), Germany was built and verified against meas-ured results from an (easily fabricated) reference block with isolated long cracks.
Bei der mechanisierten Prüfung von verlegten Eisenbahnschienen mit Schienenprüfzügen werden große Prüfdatenmengen mit einer komplexen Prüfkopfanordnung in kurzer Zeit bei hohen Prüfgeschwindigkeiten aufgenommen. Der Typ und der Zustand des Gleises kann während der Prüfung in schnellem Wechsel streckenabhängig variieren, z.B. unterschiedliche Schienentypen oder Abnutzung sowie beim Durchfahren von Weichenstraßen. Der logistische Aufwand diese Prüfung insgesamt durchzuführen ist dabei sehr hoch und eine Wiederholung der Prüfung in einem kurzen Zeitintervall nicht möglich.
Es werden während der Prüfdatenaufnahme bereits Datenreduktionsverfahren, Rekonstruktionsverfahren und Online-Parametervariationen eingesetzt, um die Ergebnisdaten zum einen in Ihrer Menge zu begrenzen zum anderen auf die lokalen Geometrie- und Zustandsänderungen des Prüfobjektes Schiene zu reagieren.
Im Regelwerk EN 16729-1 „Zerstörungsfreie Prüfung an Schienen im Gleis -Teil 1: Anforderungen an Ultraschallprüfungen und Bewertungsgrundlagen“ werden Bezugsreflektoren definiert, um die Leistung automatisierter Ultraschallsysteme für die Schienenprüfung zu verifizieren.
Um die Einflüsse der komplexen Randbedingungen der mechanisierten Schienenprüfung auf das Prüfergebnis evaluieren zu können, wurde ein Softwarewerkzeug erarbeitet, mit dem eine Prüffahrt auf einer Schiene simuliert werden kann. Basierend auf der Datenaufnahme mit dem Glassy-Rail-Diagramm können alle relevanten Prüfparameter und Artefakte in Schienen sowie Störeinflüsse bei der Simulation berücksichtigt werden.
For more than 60 years ultrasonic rail inspection is used as non-destructive testing method to ensure the safe operation of rail tracks. Constantly increasing traffic density and heavy loads have been the motor for the development of new test equipment from handheld devices to rail inspection trains. (Krull 2003)Up to the present most of the system solutions feature conventional ultrasonic transducers housed in wheel-type and slide-type probes. Different tasks have to be carried out during an in-service inspection for flaws in the rail head, rail web and rail foot as well as rolling contact fatigue (Heckel 2018). The more tasks the inspection system has to perform, the more probes are needed. Compared against standard ultrasonic testing methods the application of array probes offers advantages and flexibility by the electronic steering possibilities to control the transmitted and received sound fields. This allows to increase functionality by software while decreasing the number of probes needed in hardware in parallel. One drawback in application of phased array probes is that the repetition frequency of the subsequent measurements will be reduced by the number of virtual probe functions each phased array probe has to perform. This may limit the range of use for phased array probes in high speed applications. To overcome these limits special designs for array probes and signal processing are necessary.
Damage Quantification in Aluminium-CFRP Composite Structures using Guided Wave Wavenumber Mapping
(2019)
The use of composite materials is associated not only with the advantages of weight reduction and improved structural performance but also with the risk of barely visible impacts or manufacturing damages. One of the promising techniques for the detection and characterisation of such damages is based on ultrasonic guided wave propagation and analysis. However, the multimodal nature and dispersive behaviour of these waves make their analysis difficult. Various signal processing techniques have been proposed for easier interpretation of guided wave signals and extraction of the necessary information about the damage. One of them is the wavenumber mapping which consists of creating a cartography of the wavenumber of a propagating mode over an inspected area, using a dense wavefield acquisition measured for example with a scanning laser Doppler vibrometer. This technique allows both the quantification of the in-plane size and the depth of damage, for example, impact-induced delamination in composite laminates.
In this contribution, wavenumber mapping is applied to a delaminated aluminium-CFRP composite structure which corresponds to composite-overwrapped pressure vessels used for storing gases in aerospace and automotive industries. The analysis of experimental data obtained from measurements of guided waves propagating in an aluminium-CFRP composite plate with impact-induced damage is performed. The output of the imaging is a three-dimensional representation of the delamination induced by the impact. Good agreement between conventional ultrasonic testing and guided wave damage mapping can be found.
Aluminum alloys are extensively used in the automotive industry. Particularly, squeeze casting production of Al-Si alloys is employed in the conception of metal matrix composites (MMC) for combustion engines. Such materials are of a high interest since they allow combining improved mechanical properties and reduced weight and hence improve efficiency. We investigate two types of MMCs, which can be potentially used for production of combustion engine pistons: 1) a near-eutectic cast AlSi12CuMgNi alloy reinforced with 15%vol. Al2O3 random planar oriented short fibers and 2) the same alloy reinforced with 7%vol. Al2O3 random planar oriented short fibers + 15%vol. SiC particles.
Complex 3D microstructure of the samples in as-cast condition, consisting of four and five phases (Al matrix, eutectic Si, intermetallics, Al2O3 fibers and SiC particles) was investigated by synchrotron computed tomography (CT). Advanced methods based on machine learning were applied for segmentation of all phases. This allowed extracting quantitative information such as volume fraction, shape and interconnectivity of every phase.
In-situ compression tests during neutron diffraction experiments were used to track the load transfer among phases, while CT on pre-strained samples to monitor and quantify damage. A micromechanical model was developed to simulate the evolution of the internal stress of each phase during uniaxial compression. We showed that in composites with the Al2O3 fiber mat plane perpendicular to the load axis the Al-alloy matrix presents a large hydrostatic stress component, i.e. undergoes compression also in the direction transverse to the external load. This feature holds with and without the addition of SiC ceramic reinforcement and is absent in the case when the Al2O3 fiber mat plane is parallel to the load axis. We show, that the intermetallics play a decisive role at very high loads, when all other reinforcement phases suffer (extensive) damage. The addition of SiC particles does alleviate the load on the Al2O3 fibers, on the eutectic Si, and on the intermetallic phases in both cases of parallel and orthogonal (to the load axis) Al2O3 fiber orientation. Apart from the beneficial addition of ceramic reinforcement, the presence of intermetallic phase and eutectic silicon, forming an interconnected network even at high loads, when single particles break, confers peculiar properties to these multi-phase composites.
Die Varroose gilt weltweit als eine der bedeutendsten Erkrankungen der westlichen Honigbiene, Apis mellifera. Verursacht wird sie durch den Ektoparasiten Varroa destructor, der durch Saugen der Hämolymphe und Virenübertragung sowohl die Bienenbrut als auch die adulten Individuen schädigt.
Ohne imkerliche Unterstützung gehen betroffene Bienenvölker in der Regel innerhalb von drei Jahren ein. Als bewährte, häufige Behandlung wird flächendeckend 60%ige Ameisensäure (AS) - im Bienenstock appliziert über verschiedene Verdampfungssysteme - eingesetzt. Sie wirkt als einziger zugelassener Stoff gegen die Varroamilbe auch in die verdeckelte Brut hinein und birgt kein Risiko der Resistenzentwicklung.
Allerdings ist das therapeutische Fenster, der Bereich zwischen Milbenabtötung und Bienenschädigung, relativ schmal und die tatsächliche AS-Konzentration im Bienenstock stark abhängig von äußeren Einflussfaktoren, wie Temperatur, Luftfeuchte oder Applikationsart.
Eine Bestimmung der exakten AS-Konzentration im Bienenstock ist also notwendig, um einerseits das Wirkungsoptimum zu bestimmen und andererseits bei Übersteigen der Maximalkonzentrationen mit Folge-Schädigung der Bienen, ein rechtzeitiges Eingreifen der Imker zu ermöglichen.
Reinforced concrete (RC) is used as structural material in most diverse civil engineering applications. For the variability of its physical properties it is still an engineering challenge to meet all necessary requirements for the prediction of dynamic effects under impact loading. In this paper, investigations are shown within the scope of quantifying and evaluating the damage caused by an impact. The experimental investigations are performed in the field of low- and medium-velocity impact. The chosen flat nose shape results in small penetrations on the top side and scabbing on the bottom side. The plate is scanned with an adapted planar tomographic examination after the impact, and the damage is analysed, afterwards. Cracks and spalling are made visible with a reconstruction. The numerical model validated on the tomographic results justifies the application for further predictions of the damage description.
The application of phased arrays in either linear or matrix arrangement in combination with signal processing opens the door for the mechanized inspection of different thin wall materials itself as well as their joining methods.
Modern lightweight components are typically manufactured by a composition of different types of e.g. steels, metals, fiber reinforced plastics and glass. Dependent on the material combinations e.g. welding, brazing, bonding, cladding or coating may be applied during the manufacturing process to join different semi-finished products to form the component.
The resulting complex material composition and different damage mechanisms pose new challenges to non-destructive testing with ultrasound due to the different material properties and the overall arrangement of the materials employed.
The sound field variation capabilities of array probes may be helpful to overcome some of these challenges if adapted to the specific inspection task.
Examples for the non-destructive mechanized phased array inspection of different types of materials and joining methods will be presented.
Despite of the significant advances in AM process optimization there is still a lack of experimental results and understanding regarding the mechanical behavior and microstructural evolution of AMparts, especially in loading conditions typical for safety-relevant applications e.g. in the aerospace or power engineering. Within the scope of the presented investigations, a characterization of the fatigue behavior of additively manufactured Ti-6Al-4V in the low cycle fatigue regime was carried out in the range of 0.3 to 1.0 % strain amplitude at room temperature, 250°C and 400°C. The Ti-6Al-4V specimens are machined out of lean cylindrical rods, which were fabricated using powder laser metal deposition (LMD) with an improved build-up strategy. The improved strategy incorporates variable track overlap ratios to achieve a constant growth in the shell and core area. The low-cycle-fatigue behavior is described based on cyclic deformation curves and strain-based fatigue life curves. The lifetimes are fitted based on the Manson-Coffin-Basquin relationship. A characterization of the microstructure and the Lack-of-Fusion (LoF)-defect-structure in the as-built state is performed using optical light microscopy and high-resolution computed tomography (CT) respectively. The failure mechanism under loading is described in terms of LoF-defects-evolution and crack growth mechanism based on an interrupted LCF test with selected test parameters. After failure, scanning electron microscopy, digital and optical light microscopy and CT are used to describe the failure mechanisms both in the longitudinal direction and in the cross section of the specimens. The fatigue lives obtained are comparable with results from previous related studies and are shorter than those of traditionally manufactured (wrought) Ti-6Al-4V. In this study new experimental data and understanding of the mechanical behavior under application-relevant loading conditions (high temperature, cyclic plasticity) is gained. Furthermore, a better understanding of the role of LoFdefects and AM-typical microstructural features on the failure mechanism of LMD Ti-6Al-4V is achieved.
An essential task in many industries, e.g. food, petrol or chemical industry, is the precise and accurate characterization of liquids. Therefore, the development of innovative in-line sensors is of great interest. New concepts based on periodic structures, so-called phononic crystals (PnCs), are an interesting idea for the design of innovative sensors.
A PnC-based sensor can be designed by introducing a resonance inside a bandgap, a frequency region where no wave propagation is allowed. High-Q measurement systems using PnCs are already reported in the literature. However, existing designs cannot be implemented into a piping system directly, but need special fittings, openings or by-passes to be in contact with the liquid.
To circumvent this issue, we develop a new sensor based on PnCs, which can be directly implemented as part of the piping system. For this purpose, we use a PnC consisting of hollow cylinders with a periodic change of the outer diameter.
A bandgap could be found for the described geometry without fluid in simulation and measurement. However, simulations show, that a bandgap for fluid-filled cylinders can only be obtained for quasi-longitudinal modes. Hence, we propose a mode selective excitation for the sensor.
Residual stresses count among the most limiting factors in the application of additively manufactured materials in safety relevant components subject to cyclic loading. The source of such stresses is inherent in the manufacturing Laser Beam Melted (LBM) process due to rapid cooling and solidification and their distribution in the specimen or component is not homogeneous, but it is usually characterized by high gradients. Moreover, the magnitude of the residual stress field depends very much on the orientation, being higher in the build direction.
Many works in the literature advise to carry out a post-LBM treatment to relieve residual stresses, even though particular attention should be paid in choosing the parameters for the thermal treatment in order to avoid microstructural/phase transformations, which would affect greatly the mechanical properties of the material.
Particularly in the case of the austenitic stainless steel 316L, it has been shown that annealing at high temperatures (above 900°C), besides relieving the residual stresses, may cause recrystallization, grain growth and even phase transformation. In contrast, if a too low annealing temperature is chosen in order to preserve the microstructure, the residual stresses cannot be completely relieved.
This work aims to address the effect of residual stresses on short and long crack propagation for SEN(B) specimens made of 316L fabricated by LBM, in which notches have been machined by electro-discharge machining (EDM) on the mid-plane, perpendicular to the build direction. The specimens underwent different annealing treatments in inert atmosphere, in which the maximum temperature has been varied up to 900°C. The amount of residual stresses and their distribution has been measured by X-ray and neutron diffraction and the specimens have been subject to cyclic loading in a resonant testing machine. The tests show a massive influence of the residual stresses in the build direction on the resistance to fatigue crack propagation of additively manufactured 316L. Finally, a comparison with conventionally manufactured 316L is presented.
The present study deals with the experimental characterization of short crack propagation in SLM (selective-laser-melting) manufactured stainless steel. More specifically, the determination of cyclic R-curves is discussed. This describes the dependency of the crack propagation threshold on crack growth during the short crack propagation stage. For metals, the threshold, starting at a material-intrinsic value, increases until it reaches a value independent of the crack length due to crack closure phenomena which build up at that stage. The cyclic R-curve, when used in the frame of a cyclic R curve analysis, characterizes the resistance of a material to fatigue crack growth and the ability to arrest a physically short crack. Thus, it is the link between classical fatigue and fracture mechanics. In the high-cycle-fatigue range, the short crack propagation stage dominates the overall lifetime, i.e., the number of cycles until failure. Below the fatigue limit crack arrest of hitherto propagable micro-cracks will occur. The effort for the experimental characterization of the short fatigue crack propagation behavior and the cyclic R-curve is very high compared to experiments on long crack propagation. A very exact measurement of crack extension is required, since small increments need to be depicted. Pre-cracking must leave a closure free initial crack, since closure must be build up only by the cyclic R-curve. The closure-free status is achieved by compression pre-cracking. The aim of the present study is an insight into the influence of an AM process on the short crack propagation threshold. Cyclic R-curves are experimentally determined at different load-ratios for 316L austenitic steel specimens produced by SLM and conventional manufacturing. Residual stresses are measured in the crack plane and their influence on the cyclic R-curve is discussed.
Die Verwendung von Verbundwerkstoffen ist nicht nur mit den Vorteilen einer Gewichtsreduzierung und einer verbesserten strukturellen Leistung verbunden, sondern auch mit der Gefahr von kaum sichtbaren Impakt- oder Herstellungsschäden. Eine der vielversprechenden Methoden zur Erkennung und Charakterisierung solcher Schäden basiert auf der Ausbreitung und Analyse geführter Ultraschallwellen. Der multimodale und dispersive Charakter dieser Wellen erschweren jedoch die Analyse. Verschiedene Signalverarbeitungsmethoden wurden vorgeschlagen, um die Interpretation von Signalen und die Extraktion der notwendigen Informationen über den Schaden zu erleichtern. Eine davon ist die Erstellung einer Wellenzahlkarte. Die Wellenzahlkarte erlaubt es jeden Punkt in einer hochaufgelöste Wellenfeldaufnahme eine Wellenzahl zuzuordnen. Diese Methode ermöglicht sowohl die Quantifizierung der Größe als auch der Tiefe des Schadens.
In diesem Beitrag wird diese Bildgebungsmethode auf delaminierte Aluminium-CFK-Verbundstrukturen angewendet. Solche Strukturen entsprechen den umwickelten Druckbehältern, die zur Speicherung von Gasen in der Luft- und Raumfahrt sowie in der Automobilindustrie verwendet werden. Zunächst werden die numerischen Untersuchungen zur einfachen Delamination in unterschiedlicher Tiefe vorgestellt. Als nächstes wird die Analyse von experimentellen Ergebnissen von einer geschädigten Aluminium-CFK-Platte präsentiert. Das Ergebnis der Bildgebung ist eine dreidimensionale Darstellung, die sowohl die Größe als auch die Tiefe des Impakt-Schadens liefert.
Laser powder bed fusion (L-PBF) is one of the most promising additive manufacturing (AM) technologies for the production of complex metallic real part components. Due to the multitude of factors influencing process conditions and part quality and due to the layer-wise characteristic of the process, monitoring of process signatures seems to be mandatory in case of the production of safety critical components. Here, the iterative process nature enables unique access for in-situ monitoring during part manufacture. In this talk, the successful test of the synchronous use of a high-frequency infrared camera and a camera for long time exposure, working in the visible spectrum (VIS) and equipped with a near infrared filter (NIR), will be introduced as a machine manufacturer independent thermal detection monitoring set-up. Thereby, the synchronous use of an infrared camera and a VIS NIR camera combines the advantages of high framerate and high spatial resolution. The manufacture of a 316L stainless steel specimen, containing purposely seeded defects and volumes with forced changes of energy inputs, was monitored during the build. The measured thermal responses are analysed and compared with a defect mapping obtained by micro X-ray computed tomography (CT).
The first results regarding methods for data analysis, derived correlations between measured signals and detected defects as well as sources of possible data misinterpretation are presented in this talk.
Residual stresses in Laser Beam Melting (LBM) – Critical Review and outlook of activities at BAM
(2019)
Additive manufacturing (AM) technologies have experienced an exceedingly rapid growth, which is coupled with the knowledge about the resulting material properties and performance. In particular, residual stress (RS) was soon recognized as an important issue in AM parts, such that parts are usually subjected to a post build-heat-treated. Significant effort has been spent on simulations of RS in AM, especially using finite element methods. As a consequence, the experimental determination of RS has thereby become increasingly important as a validation tool for simulations, as well as a method for assessing the influence of process parameters. In particular, diffraction methods, which are fundamentally non-destructive, offer enormous possibilities to gain knowledge on the residual stress state in real components, since synchrotron radiation and neutrons can penetrate even heavy metals up to several millimeters or centimeters, respectively. Indeed, significant progress has been achieved, in the understanding of the origins of the RS fields as a function of process parameters, as well as their stability under thermal and/or mechanical exposure.
In this paper, a few success stories will be outlined. It will be shown how the determination of RS in metallic parts (with the focus on those produced by laser powder bed fusion) has even revealed that process parameters that were previously considered unimportant (e.g. the position and orientation on the base plate) play a major role in the onset of residual stress accumulation.
However, while RS characterization is starting to be considered in the component design, deposition strategy (e.g. build plate temperature), and even in the definition of the relevant metric to assess the quality of a part, much is still to be investigates about the hypotheses underlying its experimental determination. Therefore, some aspects to be aware of, or even those which to date are unclear, will also be discussed. These include the determination of the stress-free reference and of the principal axes of stress. All of these aspects will lead towards a comprehensive understanding of the process-structure-performance relationships in AM materials and parts.
Issues that prevent Structural Health Monitoring (SHM) based on Guided Waves (GW) from being a part of today’s monitoring solutions in industry are not all obvious to the scientific community. To uncover and overcome these issues, scientists working on SHM and GW problems joined in an expert committee under the patronage of the German Society for Non-Destructive Testing. An initiated online survey among more than 700 experts and users reveals the hurdles hindering the practical application of GWbased SHM. Firstly, methods for proof of reliability of SHM approaches are missing.
Secondly, detailed understanding of phenomenological described wave-damage interactions is needed. Additionally, there are significant unsolved implementation issues and unsolved problems of signal processing including handling of environmental influences.
To enable substantial proof of reliability without unaffordable experimental effort also efficient simulation tools including realistic damage interaction are needed, enabling the joint use of experimental and simulated data to predict the capabilities of the Monitoring system. Considering these issues, the committee focusses on simulation, signal processing, as well as probability of detection and standardization. In the presented work, recent activities of the expert committee starting with survey results are summarized. An open access data basis of life-like measurements is presented to allow testing and comparison of signal processing and simulation algorithms. Finally, a strategy for efficient proof of reliability increasing the acceptance of SHM in industry and for successful Integration of SHM into real-world engineering structures is proposed.
3D images such as those produces by X-ray tomography can provide a wealth of information on the internal structure of materials, but quantification of specific geometrical or topological characteristics linked to some bulk physical property is far from being straightforward. This study focuses on methods to quantify the differences in physical properties as a function of direction, i.e. their anisotropy, and how it can be linked to measures of anisotropy of the internal structure of the material. The auto-correlation function gives a similarity measure in the volume as a function of distance and direction. This is a cross-correlation of the image with itself fast to compute and relatively insensitive to noise. It is why we focus on this method to compare with the physical property of our DPF material. Diesel Particulate Filter (DPF) materials are porous ceramics that; a) can be used at very high temperatures; b) have very good thermal shock resistance; c) are inert; d) can be manufactured with tailored porosity. Their usual way of production consists of the extrusion of a slurry into the desired filter shape, with successive ceramming at high temperature. This process causes anisotropy at both microscopic and macroscopic levels.
Der Beitrag beschreibt den Aufbau und die Entwicklung einer Heißluftquelle als Wärmequelle für die aktive Thermografie. Als Ausgangspunkt wird zunächst die Quellenlage betrachtet. In der Literatur über Thermografie spielt Heißluft als Wärmequelle nur in weniger als 0,1% eine Rolle, die Ursache dafür ist nicht bekannt. Es wird vermutet, dass dieser Erwärmungsmethode unterstellt wird, sie hätte Nachteile bezüglich Homogenität der Erwärmung sowie Leistungseintrag. In einer der raren Publikationen über einen Verfahrensvergleich schnitt die Heißluft-Thermografie jedoch nicht schlechter als die Vergleichsverfahren mit optischer Anregung ab, bei gleichzeitig deutlichem Zeitgewinn und geringerem technischen Aufwand.
Danach wird der fertige Versuchsaufbau mit seinen wesentlichen Komponenten dargestellt. Der Einfluss der Geometrie der Luftaustrittsdüse auf die erreichbare Wärmeverteilung an einem flachen Blech wurde an mehreren Düsen untersucht. Hierbei erwies sich eine langgestreckte Umlenkdüse mit einer Strahlumlenkung von etwa 20° als optimal zur Erzeugung einer möglichst homogenen Erwärmung.
Ein weiterer zu optimierender Parameter war die erreichbare Schaltdynamik des Heißgasstromes. Beim schnellen Schalten großer Volumenströme treten Druckstöße mit entsprechenden Temperaturänderungen auf. Hinzu kommt, dass der verwendete MFC (mass flow controller) bei großen Volumenströmen zum Einschwingen bei Schaltprozessen neigt.
Daher enthält der Aufbau einen offenen Bypass zur Stabilisierung des Volumenstroms. Mit dem so realisierten Messplatz wurde die Reproduzierbarkeit der erzielten Erwärmung durch mehrfache Wiederholungsmessungen über einen längeren Zeitraum untersucht. Hierzu wurde ein auch zu Validierungszwecken genutzter PVC-Block mit 13 mm Dicke jeweils für 60 s erwärmt und die Oberflächentemperatur des PVC-Blocks mit einer IR-Kamera aufgezeichnet. Bei jeweiligem Neuaufbau ohne Justierhilfe ergab sich eine Reproduzierbarkeit von etwa 22% bei 5 Versuchen. Unter Nutzung einer Justierhilfe (Schablone für Abstand in der Kamerasoftware) ließ sich die Reproduzierbarkeit auf 5% bei 7 Versuchen reduzieren.
Ein weiterer zu klärender Punkt war der erzielbare Wärmeeintrag im Vergleich zu einer Anregung mit Blitzlampen. Hierzu wurde eine Messung mit 60 s Heißluftheizung direkt mit einer Evaluierungsmessung mit Blitzlampen am gleichen PVC-Probekörper mit 4 FBB an der Rückseite verglichen. Die Inhomogenität der Erwärmung wurde durch eine Phasenauswertung unterdrückt. Die Phasenauswertung gestattete den Nachweis aller 4 FBB, wobei das Signal zu Rausch-Verhältnis bei der Heißlufterwärmung deutlich besser war. Der direkte Vergleich der Temperaturkurven zeigt, dass bei 60 s Heizlufterwärmung ca. 4,5 mal mehr Energie als bei der Blitzanregung vom Probekörper absorbiert wurde. Hieraus lässt sich die eingekoppelte Heizleistung mit 100 mW/cm² abschätzen. Das entspricht in etwa dem Niveau, wie es auch mit bewegten Halogenstrahlern, bewegten IR-Strahlern oder der Sonne erreicht wird.
Diese Ergebnisse belegen, dass eventuell vorhandene Vorbehalte gegenüber einer Heißluft als Wärmequelle für die aktive Thermografie zumindest für thermisch langsame Materialien wie Kunststoffe oder mineralische Baustoffe unbegründet sind. Die Einsatzgrenzen bei der Untersuchung thermisch schnellerer Materialien wie Metalle steht noch aus, hier ist mit Abstrichen zu rechnen. Auf der anderen Seite lässt eine mögliche Düsenoptimierung noch deutliche Verbesserungen erwarten.
Pulsed thermography is a well-known non-destructive testing technique and has proven to be a valuable tool for examination of material defects, to determine thermal material parameters, and the thickness of test specimens through calibration or mathematical models. However, the application to semitransparent materials is quite new and demanding, especially for semitransparent materials like epoxy, polyamide 12, or glass fiber reinforced polymers with epoxy or polyamide matrix.
In order to describe the temporal temperature evolution in such materials, which are recorded with an infrared camera during pulse thermography experiments, much more influences have to be considered, compared to opaque materials:
- The wavelength of the excitation source and the spectral range of the infrared camera
- The angles between the specimen, the excitation source and the infrared camera
- The area behind the specimen
- The roughness of the material surface
- The scattering mechanism within the material
Here, we will consider all these influences and describe how they can be treated mathematically in analytical or numerical models (using COMSOL Multiphysics software). These models describe the temperature development during the pulse thermography experiment in reflection and transmission configuration. By fitting the results of the mathematical models to experimental data it is possible to determine the thickness or the optical and thermal properties of the specimen.
The detection of delamination, rot and cracks causing a decrease of strength in wooden construction elements is a key task for nondestructive testing (NDT). Air-coupled ultrasound (ACU) is used to detect flaws without having to provide contact to the surface or otherwise affect the object. Novel ferroelectric transducers with a high signal-to-noise ratio enable an accurate flaw detection. Transducers made of cellular polypropylene (PP) are suitable for characterizing wood-based materials (WBM) because their extremely low Young’s modulus and low density mean a favorable acoustic impedance for the transmission of ultrasonic waves between the transducer and air. The transducers can be applied during the production of WBM and during its service life. The device enables a fast in-situ recognition of defects with frequencies between 90 kHz and 250 kHz. Measurements can be performed in through transmission for a high resolution or in pulse-echo technique in case of one-sided access at the expense of reduced signal strength. Ultrasonic quality assurance for wood is an important attempt to increase the acceptance of wooden structures and towards sustainability in civil engineering in general.
Explosive spalling is caused by, among others, the thermohydraulic spalling mechanism. During this process, vaporization, dehydration, moisture-transport and condensation processes interact. As a result, a drying and dehydration zone as well as a saturated zone, known as a moisture clog, are observed inside the unilaterally-heated concrete. The presented research is focused on the experimental investigation of the underlying thermohydraulic processes.
To investigate these, a test methodology based on X-ray computed tomography (CT) and nuclear magnetic resonance (NMR) was developed. Thereby, the X-ray CT scans are carried out simultaneously during the application of a defined unilateral-heating regime on a specially-constructed specimen. This miniaturized specimen, equipped with a double-layer casing, reproduces the condition within a planar, unilaterally-heated building component.
A preliminary test methodology and the first experimental results were presented at the 5th International Workshop on Concrete Spalling in Borås, Sweden (2017). The contribution for the upcoming workshop presents an improved version of this test methodology and new results for a high-performance concrete (HPC) mixture exposed to temperatures up to 500 °C. Regarding the CT measurements, a higher time-resolution of 15 min was achieved and a quantification of the moisture changes was implemented. Due to an increase in signal quality of the NMR measurements, a pore-size specific moisture distribution can now be resolved. This allows to conclude about the moisture reconfiguration between small gel pores and larger interhydrate pores. Additionally, the NMR measurement are no longer limited to first 2.5 cm below the heated surface but a one-dimensional moisture distribution can now be estimated over the whole 10 cm long specimen.
The presented results demonstrate that the combination of X-ray CT and NMR measurements enables to image and quantify the thermally-induced moisture transport and reconfiguration from small gel pores up to macro pores. This provides important insights into the thermohydraulic damage mechanism and leads to a better understanding of spalling avoidance strategies, like the addition of polypropylene fibres.
The applicability of 1D-quadrupole-model on the depth estimation of overload-induced delaminations is tested in this contribution. While classical 1D-methods like Pulsed Phase Transformation and Thermographic Signal Reconstruction determine the depth of a defect by one parameter, a two-layer quadrupole model provides two parameters: depth and thermal resistance. In addition, the convectional losses at the surfaces may be considered. The defect investigated is a large-scale delamination in glass fibre reinforced polymer generated by tension overload.
Im Rahmen des Themenfeld Projektes „Process Monitoring of AM“ (ProMoAM) evaluiert die BAM gegenwärtig die Anwendbarkeit verschiedenster ZfP-Verfahren, darunter die Thermografie, zur Prozessüberwachung in der additiven Fertigung von Metallen in Hinblick auf die Qualitätssicherung.
In diesem Beitrag werden SWIR-Thermografiemessungen während des Bauprozesses mittels Laser Pulver Auftragsschweißen (LPA) vorgestellt. Eine Herausforderung im Rahmen dieser Messungen liegt in der Positionierung der Kamera, welche entweder fixiert am Schweißarm, also mitbewegt, oder fixiert in der Baukammer, also stationär, erfolgen kann, wobei beide Varianten mit individuellen Vor- und Nachteilen verbunden sind. Eine stationäre Befestigung der Kamera ermöglicht zwar eine einfachere Zuordnung der Messdaten zu der jeweiligen Position im Bauteil, führt jedoch bei komplexeren Geometrien zwangsläufig zu Problemen durch Abschattungen und zu defokussierten Bereichen. Zur Auswertung von Thermogrammen, welche durch eine mit dem Schweißarm mitbewegte Kamera aufgenommen wurden, sind hingegen für jedes Bild akkurate Positionsdaten der Kamera nötig um die Messdaten einer Position im Bauteil zuzuordnen. Da die Positionsdaten des Schweißarmes im allgemeinen Fall durch die Anlagensoftware nicht zur Verfügung gestellt werden, muss diese Information durch zusätzliche Messungen aufgezeichnet werden. Hierzu verwenden wir einen an der Kamera befestigten Beschleunigungssensor. Dieser ermöglicht einen zeitlichen Abgleich mit dem vorprogrammierten Verfahrweg des Schweißarmes, welcher im Allgemeinen noch Unsicherheiten bezüglich genauer Geschwindigkeiten und Beschleunigungen offenlässt. Weiterhin untersuchen wir den Einfluss des empfindlichen Spektralbereiches der IR-Kamera durch den Vergleich von Messungen mit verschiedenen schmalbandigen Bandpassfiltern (25 nm FWHM) in einem Bereich von 1150 nm bis 1550 nm.
By allowing economic on demand manufacturing of highly customized and complex workpieces, metal based additive manufacturing (AM) has the prospect to revolutionize many industrial areas. Since AM is prone to the formation of defects during the building process, a fundamental requirement for AM to become applicable in most fields is the ability to guarantee the adherence to strict quality and safety standards. A possible solution for this problem lies in the deployment of various in-situ monitoring techniques. For most of these techniques, the application to AM is still very poorly understood. Therefore, the BAM in its mission to provide safety in technology has initiated the project “Process Monitoring of AM” (ProMoAM). In this project, a wide range of in-situ process monitoring techniques, including active and passive thermography, optical tomography, optical emission and absorption spectroscopy, eddy current testing, laminography, X-ray backscattering and photoacoustic methods, are applied to laser metal deposition (LMD), laser powder bed fusion and wire arc AM. Since it is still unclear which measured quantities are relevant for the detection of defects, these measurements are performed very thoroughly. In successive steps, the data acquired by all these methods is fused and compared to the results of reference methods such as computer tomography and ultrasonic immersion testing. The goal is to find reliable methods to detect the formation of defects during the building process. The detailed acquired data sets may also be used for comparison with simulations.
Here, we show first results of high speed (> 300 Hz) thermographic measurements of the LMD process in the SWIR range using 316L as building material. For these experiments, the camera was mounted fixed to the welding arm of the LMD machine to keep the molten pool in focus, regardless of the shape of the specimen. As the thermograms do not contain any information about the current spatial position during the building process, we use an acceleration sensor to track the movement and synchronize the measured data with the predefined welding path. This allows us to reconstruct the geometry of the workpieces and assign the thermographic data to spatial positions. Furthermore, we investigate the influence of the acquisition wavelength on the thermographic data by comparing measurements acquired with different narrow bandpass filters (50 nm FWHM) in a spectral range from 1150 nm to 1550 nm.
This research was funded by BAM within the Focus Area Materials.
Polymerbeschichtungen auf Beton bieten einerseits mechanischen Schutz vor Beschädigungen und verhindern das Eindringen von Wasser, gelösten Salzen und CO2 und können andererseits der Oberfläche ein gewünschtes Aussehen geben. Daher werden sie in der Bauindustrie insbesondere bei Brücken und Parkhäusern häufig verwendet. Für die beabsichtigte Wirkung der Beschichtung ist deren Schichtdicke ein entscheidender Faktor und muss daher nach Aufbringung der Schicht und im Betrieb regelmäßig überprüft werden. Üblich sind hier zerstörende Verfahren mit nachträglich erforderlicher Ausbesserung. Im Rahmen eines Vorgängerprojektes wurde an der BAM in Kooperation mit der IBOS GmbH ein zerstörungsfreies Verfahren entwickelt, welches auf aktiver Thermografie basiert. Ein Prototyp mit Halogenlampe und Infrarot-Kamera wurde erprobt.
Das hier beschriebene Folgeprojekt hat das Ziel, dieses Verfahren zu standardisieren, damit es sich in der Industrie als zerstörungsfreies Alternativverfahren zur Schichtdickenbestimmung durchsetzt. Im ersten Schritt wird gezeigt, bei welchen Oberflächenschutzsystemen das Verfahren bereits zuverlässige Schichtdickenmessungen ermöglicht und praxistauglich ist. Anschließend werden weitere Forschungsthemen definiert und bearbeitet, die eine Anwendung für alle Oberflächenschutzsysteme ermöglichen sollen. Besondere Herausforderungen stellen etwa Sandeinstreuungen in der Polymerschicht dar. Auch muss der Frage nach der Anwendbarkeit auf Mehrschichtsysteme nachgegangen werden. Des Weiteren stellt sich die Frage nach dem Einfluss der Beschaffenheit des Betonuntergrunds auf die Anwendbarkeit des Verfahrens. Im zu erarbeitenden Normentwurf sollen diese Untersuchungen die Möglichkeiten, aber auch die Grenzen der aktiven Thermografie zur Schichtdickenmessung von Oberflächenschutzsystemen aufzeigen. Im Beitrag werden die systematischen Untersuchungen bezüglich dieser Fragestellungen dargelegt.
Increasing requirements on the infrastructure caused by more and heavier traffic coincide with both the aging and the progressive degradation of bridge constructions. These exemplary issues lead to a non-negligible need for reassessments of existing bridges. Compared to the new building design, safety reserves can be used in the evaluation of such existing structures, since uncertainties that may arise during the design, construction and previous Service time are better known or no longer present. Uncertainties can be evaiuated more precisely by measurements. In the meantime, the application of advanced NDT-methods on concrete structures have become so capable, that crucial structure Parameters like certain material properties or geometrical dimensions can be quantified without significant destructive interventions. The measured data can be explicitly incorporated into the probabilistic computation models used for the bridge reassessment in form of random variables. Therewith, a previously insufficient model can be refined to achieve a higher level of approximation. This contribution emphasises two fundamental steps of the measured data based probabilistic reassessment: The sensitivity analyses to identify the cruciai Parameters and the needed accuracy of the related information as well as the comparison with expected uncertainties of non-destructively measured Parameters.
Usability of the written procedures: an eye tracking study Non-destructive evaluation (NDE) is regarded as one of the key elements in ensuring quality of engineering systems and their safe use. A failure of NDE to detect critical defects in safety-relevant components, such as those in the nuclear industry, may lead to catastrophic consequences for the environment and the people. NDE inspection procedure is one of the most important tools in the everyday life of an NDE inspector. Experience and research studies have shown that NDE procedures are not always used as foreseen and may need to be optimized. Both the content and format play an important role in producing reliable procedures. Whereas the content aids in the understanding of the task and in its subsequent correct execution, the format determines the efficiency and frequency of the procedure’s use.
For the purposes of this study, an inspection procedure - designed to be used in the inspection of components used in the final disposal of spent nuclear fuel - was further developed by adopting a user-centered design. This was achieved by following the eye movements during a data evaluation task whilst using the procedure, by interviews and group discussions, accompanied by a review from a human factors’ perspective, the suggestions from relevant literature and by reiterating the entire process once again. This process resulted in identifying shortcomings and in suggesting alterations to the existing procedure. The newly created procedure was empirically tested for its usability using eye tracking. By comparing the old instruction format to the newly developed one, the results showed that appropriately highlighting warnings, exceptions, or reminders, and placing them at a logical position assures that they will be read and identified with ease rather than overlooked. The presentation of the information in a stepwise manner, with one action per step, allows the user to follow more easily the steps that must be carried out to save time and effort.
This study - the first of its kind in the field of non-destructive evaluation - showed that the procedure can be improved by applying human factors principles to the design and by involving the users into the process of procedure development and quality assurance.
Introduction of the speaker – Marija Bertovic
Marija Bertovic has a degree in psychology and a PhD in human factors. Since 2006 she has been working on topics related to human factors in non-destructive testing at the BAM Federal Institute for Materials Research and Testing, German Society for Non-Destructive Testing and as an independent consultant. The focus of her research has been on identifying and studying human factors’ related risks during NDT inspections and suggesting ways of optimizing the inspections and inspection procedures. She is currently a research staff member at BAM responsible for probabilistic safety and reliability analyses AND human factors analyses. She is the chairwoman of the German Society for Non-Destructive Testing’s (DGZfP) subcommittee on human machine interaction and the winner of the DGZfP Science Award in 2018.
The closure of underground nuclear waste disposal facilities requires reliable gas- and watertight engineered barriers. In Germany, barriers made from salt concrete have been evaluated in full scale. While the barriers seem to fulfill the requirements regarding permeability, some unexpected cracks have been detected at the surface and at depth. In cooperation between the Federal Company for Radioactive Waste Disposal (BGE) and the Federal Institute for Materials Research and Testing (BAM), several experiments have been carried out to evaluate the applicability for ultrasonic measurements in crack detection and general quality assurance. Both commercial instruments and specially developed devices have been tested on site. Using commercial ultrasonic echo devices designed for concrete inspection it was possible to detect cracks and object in salt concrete up to a depth of 2 m. The check for delamination in shotcrete is another field of application. A unique device available at BAM, the wide aperture, deep penetration instrument LAUS, was able to locate cracks and objects up to a depth of 8 m so far, which is thought to be a record for ultrasonic echo measurements in concrete. Adapted imaging procedures, partly adopted from geophysics, helped to reveal 3D structure at depth. In addition, we have developed ultrasonic probes to be deployed in boreholes, currently at up to 20 m depth. They can collect information on cracks and other features in a radius of about 1.5 m around the borehole in the current version and might be used in echo or transmission mode. Evaluation experiments have been performed at an experimental barrier at the ERAM site in Morsleben, Germany. The results showed several empty and injected cracks as well as built in instrumentation. The results have been verified using borehole endoscopy as well as core examination and will be used to set up a reliable quality assurance system for engineered barriers. All instruments are based on ultrasonic shear wave transducers with a frequency range between 25 kHz and 100 kHz. Current research focuses on the improvement of the hardware (e. g. optimization of array characteristics) and imaging techniques as Reverse Time Migration, both aiming at the improvement of depth of penetration, resolution and probability of detection.
Conventional ultrasonic testing involves the application of a fluid couplant for impedance matching. Increasing use of lightweight structures, composite materials and adhesive joints mostly in aerospace and automotive industry created an increasing interest in air-coupled ultrasonic testing methods, to protect sensitive surfaces and simplify maintenance. Commercially available air-coupled transducers consist of a piezocomposite material and matching layers to reduce the impedance mismatch between the transducer and air. This contribution is an overview of innovative approaches using new piezoelectric materials and other physical principles to transmit and receive an ultrasonic pulse in air.
Capacitive and piezoelectric micromachined ultrasonic transducers (CMUTs and PMUTs) produce high pressure levels, but they exhibit a very narrow bandwidth. Optical laser-based methods for transmitting and receiving ultrasound promise a higher bandwidth, but do not achieve the same sensitivity as conventional air-coupled transducers. Ferroelectrets are charged cellular polymers exhibiting piezoelectric properties, having a very small acoustic impedance well matched to air. Ferroelectret transducers achieve about the same bandwidth as the most broadband conventional air-coupled transducers, having a higher sensitivity. Thermoacoustic transducers use heat to initiate an acoustic wave, acting as transmitters in ultrasonic range. Thermoacoustic transducers enable excitation of extremely broadband pulses while producing high pressure levels, which opens new possibilities for advanced signal processing. The newest member of the family of air-coupled ultrasonic transmitters is the plasma-based transducer, using both the thermoacoustic effect and the movements of the ions (so called ionic wind) to create acoustic waves.
An der BAM gibt es seit vielen Jahren Erfahrungen in der Entwicklung und im Bau speziell angepasster Wirbelstromsonden. Mit der Zielstellung, eine möglichst hohe Ortsauflösung zu erreichen, wurde vor einigen Jahren die sogenannte Eindrahtsonde (Patentanmeldung Nr. WO2015177341A1) entwickelt. Diese besteht aus der Kombination eines einzelnen dünnen Drahtes als Sender und GMR-Sensoren (giant magneto-resistance, Riesenmagnetwiderstand) als Empfänger. Beim Aufbau solcher Wirbelstromsonden hat sich der Einsatz sogenannter Starr-Flex-Leiterplatten bewährt. Der Aufbau und die Funktionsweise der Sensoren werden erläutert. Anhand spezieller Muster kleiner Vergleichsfehler werden Ortsauflösung, Richtungsabhängigkeit, Tiefenwirkung und Randeffekt hochauflösender Wirbelstromsonden aus Spulensystemen mit der Eindrahtsonde gegenübergestellt.
PileInspect
(2019)
The EC funded project PileInspect (2013-2016) has tried to improve the traditional low strain pile integrity test by using a shaker instead of a hamm and sophisticated higher order spectra analysis methods to provide automated, reliable interpretation. However, the results, even if promising, have not seen full validation yet. A second module, providing depth information by regularized deconvolution, was developed and tested successfully, but doesn't deliver additional information compared tomteh traditioanl test.
Within polycrystalline porous ceramics used in automotive applications as diesel particulate filters, it is evidenced that during cooling from firing temperature micro cracks are gradually formed. The cracks are formed as a consequence of strong thermal anisotropy of grains. Typically these micro cracks are granting better thermal shock resistance, with respect to dense materials, but reduce stiffness. The reduction can be quantified by measuring the drop in elastic properties of bulk material which, depending on the level of porosity, can decrease even by 50% with respect to its value at high temperature. It is further observed that upon subsequent heating these cracks are closing and partially or totally healing at very high temperatures. Such peculiar behavior results in partial or complete recovery of the elastic properties of bulk material upon completing one thermal cycle. Despite its evident practical application, still there is no constitutive description of this phenomenon, capable of predicting the evolution of Young's modulus as a function of temperature history. For reliable numerical simulation of this phenomenon, it is required to model fracture. To model inter-crystalline fracture, an effective strategy is to use cohesive elements, since crack patterns are a priori known. Major limitation of this approach is that the cohesive elements already implemented within commercial codes cannot take into account crack healing upon subsequent heating. In this study new cohesive element is developed and numerically implemented within ABAQUS commercial finite element code, capable to model crack opening, closing and healing. Further on, a computer code is generated to build numerical model of porous ceramic specimens that takes into account experimentally measured crystallographic orientation and porosity, and models the microstructure by using Voronoi polygons. The developed numerical tools serve as a framework for more realistic simulations, required to study the hysteresis in elastic properties within porous ceramics provoked by thermal cyclic. In a subsequent phase, an inverse analysis procedure is developed, in which macroscopic properties are used to calibrate parameters entering into micro crack model. The approach is centered on a minimization of a discrepancy function designed to quantify the difference between experimentally measured quantities and their computed counterpart. The model is calibrated on the basis of experimental data regarding the drop of bulk Young's modulus with decrease of temperature. Developed procedure is tested with porous cordierite sample, and obtained results are quit promising despite the current limitation of using only two-dimensional model.
Lamb waves are widely used for non-destructive evaluation of material parameters as well as for detection of defects. Another application of Lamb waves is quality control of adhesive joints.
Researchers are currently investigating shear horizontal and zero-group velocity modes for characterisation of the adhesive bonding strength. In a new approach, Lamb wave mode repulsion is used to obtain the coupling strength between different layers to characterise the adhesive bonding strength. The modes of the individual layers become coupled in the multilayered systems forming particular regions, the so-called mode repulsion regions. This study investigates these modes and their interaction in two-layered plate-like structures with varying coupling strength both numerically, with the Scaled Boundary FEM, and experimentally.
Acoustic-structure interaction in the scaled boundary finite element method for primsatic geometries
(2019)
Due to the short wavelength compared to the dimensions of the structure, the simulation of ultrasonic waves is still a challenging task. A numerical method well suited for this purpose is the semi-analytical Scaled Boundary Finite Element Method (SBFEM). When applying this method, only the boundary of a computational domain is discretized using finite elements, while the interior is described by an analytical ansatz. Hence, the number of degrees of freedom is reduced significantly compared to the classical Finite Element Method (FEM).
In recent years, a particular formulation of the SBFEM for the simulation of ultrasonic guided waves was developed. The method constitutes an efficient algorithm for prismatic structures of arbitrary length, such as plates, pipes, or beams. Wave propagation phenomena in such structures can be modeled for isotropic and anisotropic inhomogeneous waveguides. Even though the method is an efficient tool for the simulation of guided waves in solid media, a reliable model for the simulation of acoustic wave propagation in fluids as well as acoustic-structure interaction in terms of SBFEM is still missing. In principle, the fluid can be described by a displacement-based formulation and thus be implemented in existing SBFEM algorithms for solid bodies. However, due to the discretization with classical finite elements, spurious modes occur, which cannot be separated from the physical modes straightforwardly. The spurious modes can be suppressed using a penalty parameter. Although very accurate results were achieved for some problems, this procedure has been proven unreliable for certain cases.
For this reason, we propose a different approach in this contribution. We employ a pressure model to simulate the acoustic behavior of fluids. The implementation of the pressure model results in a higher effort due to the necessity of incorporating coupling terms, but it presents a stable alternative without spurious modes. The accuracy of the method is demonstrated in comparison with analytical solutions and results obtained using the FEM.
DICONDE für ET
(2019)
Against the background of sustainable resource management and efficiency, wood-based materials are currently experiencing a revival and, among others, plywood, Laminated Veneer Lumber and glued laminated timber are becoming increasingly more important in the building sector. Even though these materials are so-called engineered products, the element wood is naturally grown with intrinsic variability in mechanical properties and requires professional handling on-site. Otherwise, load-bearing structures made of wood materials may entail certain risks. Critical situations can, in principle, be avoided by implementing a structural health monitoring system into components or structures made from wood material. The aim is to indicate accumulation of mechanical damage and to eliminate or at least significantly reduce the risk of unexpected failure. Toward this purpose, the failure behavior of several layered wood materials under quasi-static tension was investigated in laboratory-scale experiments by means of acoustic emission (AE) measurement. Based on spectral analysis and pattern recognition, two classes of AE signals are identified for each investigated lay-up that are characterized by either low or high frequency contents in the respective power spectra. AE activity and intensity of both signal classes are analyzed, striving for predictors appropriate for AE monitoring concepts.
In the present study, the impact of static traffic loading on the slight deflection effects in the concrete structure of an existing bridge has been investigated using distributed fiber optic sensors. In the face of increasing traffic density and severe traffic loading, the results of the load tests on the Amsterdam bridge 705 make an important contribution to the understanding of its structural behavior. The concept of the static loading was based on the use of two 36-ton trucks stopped on the bridge at multiple pre-determined locations. The load applied in this way led to location-dependent small deflection effects recorded as longitudinal strain of the sensing fiber embedded at the underside of the bridge. The measurements were performed with a commercially-available solution based on Tunable Wavelength Coherent Optical Time Domain Reflectometry with the measurement accuracy in the range of 0.5 µm/m.
Wavelength-scanning distributed acoustic sensing for structural monitoring and seismic applications
(2019)
We introduce wavelength-scanning coherent optical time domain reflectometry (WS-COTDR) for dynamic vibration sensing along optical fibers. The method is based on spectral shift computation from Rayleigh backscatter spectra. Artificial neural networks (ANNs) are used for fast and high-resolution strain computation from raw measurement data. The applicability of the method is demonstrated for vibration monitoring of a reinforced concrete bridge. We demonstrate another application example for quasi-static and dynamic measurement of ground deformation and surface wave propagation along a dark fiber in a telecommunication cable.
The propagation of ultrasonic waves in concrete is affected by its micro- and macro-structure, geometry and properties as well as external influences as stress, temperature or moisture. In addition, age and degradation have a strong influence. Therefore, Ultrasound has been used to monitor concrete samples and structures since decades. However, early applications using conventional techniques as time-of flight or changes in amplitudes have been limited to detect changes in a late stage close to serviceability or ultimate load states.
Around 2000, several new, more sensitive techniques adopted from geophysics or other field of material sciences have been introduced to research in ultrasonic monitoring of concrete. The most discussed methodologies are coda wave interferometry, a technique which allows to detect very subtle changes from repeated ultrasonic measurements. Nonlinear acoustic techniques help to identify e. g. cracks even in an inhomogeneous background. Both techniques can be combined.
This paper reviews methods and results achieved so far on the laboratory scale and with full scale models the directions for future research and application is given as well.
Ultrasonic non-destructive testing methods have found various applications in quality assurance and condition assessment of nuclear concrete structures. This includes but is not limited to the localization of construction features (thickness measurements, reinforcement, tendon ducts, and others) and damage detection (e. g. corrosion or cracks). However, there are still limitations, e. g. limited penetration depth and resolution, issues when testing hybrid steel/concrete structures as well as absence of standards and regulations.
This presentation will show progress in some of these issues and will focus on advanced instrumentation and validation of ultrasonic NDT for concrete structures.
Ultrasonic echo testing has been limited to a penetration depth of less than one meter in reinforced concrete. The development of the LAUS (Large Aperture Ultrasonic System), using a large array of ultrasonic transducers and wider offsets between transmitter and receiver, allows to reach up to 5 m in reinforced concrete and 9 m in unreinforced concrete. This capability has been demonstrated by collecting data from an engineered salt concrete barrier in an underground nuclear waste storage facility. Due to an increasingly competitive environment, commercial manufacturers will continue to develop equipment with enhanced penetration and/or increased resolution.
To quantify the capabilities and reliability of ultrasonic tests, validation mockups with engineered flaws are required. EPRI and BAM have built a large-scale mockup at a test site close to Berlin (BAM-TTS, Horstwalde) using a design which is repeated at other places. This presentation will also cover the methods deployed on engineered flaws and the repeatability of the test results.
The 3D microstructure of an Al alloy matrix composite with two ceramic reinforcements was investigated by synchrotron X-ray tomography. A deep learning algorithm was used for the segmentation of four different phases. We show that convolutional networks with the U-Net architecture are able to solve complex segmentation tasks with small amount of training data.
The separation of two closely spaced defects in fields of Thermographic NDE is very challenging. The diffusive nature of thermal waves leads to a fundamental limitation in spatial resolution. Therefore, super resolution image reconstruction can be used. A new concerted ansatz based on spatially structured heating and joint sparsity of the signal ensemble allows for an improved reconstruction of closely spaced defects. This new technique has been studied using a 1D laser array with randomly chosen illumination pattern.
This paper presents the results after applying super resolution algorithms, such as the iterative joint sparsity (IJOSP) algorithm, to our processed measurement data. Different data processing techniques before applying the IJOSP algorithm as well as the influence of regularization parameters in the data processing techniques are discussed. Moreover, the degradation of super resolution reconstruction goodness by the choice of experimental parameters such as laser line width or number of measurements is shown.
The application of the super resolution results in a spatial resolution enhancement of approximately a factor of four which leads to a better separation of two closely spaced defects.
Thermographic nondestructive evaluation (NDE) is based on the interaction of thermal waves with inhomogeneities. These inhomogeneities are related to sample geometry or material composition. Although thermography is suitable for a wide range of inhomogeneities and materials, the fundamental limitation is the diffusive nature of thermal waves and the need to measure their effect radiometrically at the sample surface only. The propagation of the thermal waves from the heat source to the inhomogeneity and to the detection surface results in a degradation in the spatial resolution of the technique. A new concerted ansatz based on a spatially structured heating and a joint sparsity of the signal ensemble allows an improved reconstruction of inhomogeneities. As a first step to establish an improved thermographic NDE method, an experimental setup was built based on structured 1D illumination using a flash lamp behind a mechanical aperture. As a follow-up to this approach, we now use direct structured illumination using a 1D laser array. The individual emitter cells are driven by a pseudo-random binary pattern and are additionally shifted by fractions of the cell period. The repeated measurement of these different configurations enables to illuminate each spot of the sample surface in lateral direction. This allows for a reconstruction that makes use of joint sparsity.
The measured data set is processed using super resolution image reconstruction algorithms such as the iterative joint sparsity (IJOSP) algorithm. Using this reconstruction technique and 150 different illumination patterns results in a spatial resolution enhancement of approximately a factor of four compared to the resolution of 5.9 mm for homogenously illuminated thermographic reconstruction.
Further, new data processing techniques have been studied before applying the IJOSP algorithm that are more performant or less prone to errors regarding image reconstruction. The choice of regularization parameters in data processing as well as experimental parameters such as the illumination pattern as a variable heat flux density (i.e., the Neumann boundary condition for convolution with the constant Green's function) have a big influence on the reconstruction goodness. With analytical-numerical modelling and numerical FEM simulations, we studied the influence of the experimental parameters on the result of the non-linear IJOSP reconstruction. This has also been investigated experimentally e.g. using different laser line widths or more measurements per position. These studies are used to derive optimal conditions for a certain measurement image reconstruction technique.
The simulator aRTist combines analytical and Monte Carlo methods to efficiently model the radiographic process of industrial radiology. In this contribution we focus on virtual computer tomography, the simulation of tomographic scans. The possibilities to simulate complex scanning trajectories are shown in combination with the tomographic reconstruction using individual projection matrices.
Non-destructive testing for surface crack detection and head check depth quantification at the gauge corner of railway tracks can be achieved using eddy current methods. With the extension of the tested zone to the running surface, rail defect signal types other than head checks can be measured. Due to their mostly irregular shape, a quantitation based on a calibration against regular test cracks of varying depth may not be linear. Estimates of the expected influence of more complex crack patterns may be obtained by a finite element simulation of sufficiently simple limiting cases, like two displaced or intersecting cracks or a simply branched or flexed crack. As a first step, a 3D finite element model of the HC10 eddy current probe distributed by Prüftechnik Linke und Rühe (PLR), Germany was built and verified against measured results from an (easily fabricated) reference block with isolated long cracks.
Um die Gültigkeit der Vorhersagen des aktuellen Solid-State-Transformation-Creep (SSTC)-Modells zu verifizieren, wurde die Schadensakkumulation während eines Kriecheversuchs an einer Al-3,85Mg Legierung mittels Synchrotron Röntgenrefraktionstechnik untersucht. Das Röntgenrefraktionsverfahren erfasst die innere spezifische Oberfläche (d.h. Oberfläche pro Volumeneinheit) im durchstrahlten Volumen. Dabei liegt die Detektierbarkeit von Rissen und Poren in der Größenordnung von wenigen Nano- bis Mikrometern. Im Vergleich zu CT-Messungen mit vergleichbarer Sensitivität ist das untersuchte Probenvolumen jedoch um den Faktor 10 größer. Ein signifikanter Anstieg der inneren spezifischen Oberfläche mit zunehmender Kriechzeit wurde beobachtet, was den Nachweis für die Entstehung einer feinkörnigen Substruktur liefert, wie sie im SSTC-Modell vorhergesagt wird. Diese feinkörnige Substruktur wurde auch mittels Rasterelektronenmikroskopie beobachtet.
New concepts for the measurement of focal spot parameters of nano- and microfocus X-ray tubes
(2019)
Accurate measurement of focal spot sizes of X-ray tubes, betatrons and linear accelerators is still an open problem. Several standards are currently active, which provide different results depending on the focal spot shape. CEN published the standard series EN 12543 (five parts) in 1999, which describes the measurement of focal spot sizes of X-ray tubes for NDT applications. The measurement procedure and its tolerances deviate from the measurement standards for medical X-ray tubes, e.g. EN 60336:2006 / IEC 60336:2005. EN 12543 describes different measurement methods for X-ray tubes with focal spots in several size ranges, defined as macrofocus, minifocus and microfocus. CEN TC 138 WG 1 und ASTM E 07.01 are revising (CEN) or have just revised (ASTM) the standards for focal spot size measurements, basing them on pin hole or edge measurements only. Slit cameras are applied for medical tube evaluation only. Tomographic methods for focal spot size shape reconstruction were proposed and will be discussed, but have not yet been used for standardization. The currently available standards cover the range of spot sizes from 5 µm up to several millimetres. During the last years, more and more nanofocus tubes have been developed for high-resolution applications and microscopy. Manufacturers and users apply different methods for measurement of nanofocus spot sizes and image resolution, which differ by a factor of up to two. These well-known methods and new alternative methods for the measurement of nanofocus spots (size and shape) will be evaluated and improved in the EMPIR project “NanoXSpot” in order to develop a related measurement standard.
Modern wind turbine rotor blades consist of sandwich shell segments made from glass fiber reinforced polymers. During manufacturing, defects can arise which could lead to failure of the whole component under dynamic mechanical and thermal loads. Hence during operation defects can arise which, if detected, can be repaired locally and in-situ by applying repair patches instead of taking the whole rotor blade down and repair it remotely. This method is much more time and cost effective, since the shut-down time of the energy converter is limited to a minimum. These repair patches can, however, also lead to new defects if not applied optimally. Therefore, it is necessary to control the quality of the repair patches to ensure the best possible restoration of structural integrity of the component. As a rotor blade is an object with a large aspect ratio, X-ray laminography is predestined to provide 3D information of the objective volume. To enhance the amount of information gained from laminographic reconstruction, we use in this study a photon counting and energy discriminating X-ray detector and apply a material decomposition algorithm to the data. By inherently separating the incident spectra within the detection process into two distinct energy bins, the basis material decomposition can provide material resolved images. Choosing glass and epoxy resin as basis materials and numerically solving the inverse dual-energy equation system, the reconstructed laminographic datasets contain highly valuable information about the distribution of the basis materials within the structure. Furthermore, cross- artifacts arising from the limited angle of the projection data can be reduced by this method which allows to investigate structures that were hidden underneath the artefacts.
Die Effizienz von Gassensoren zur sicheren Verwendung von Wasserstoff hängt wesentlich von der Reaktion des Sensors auf eine Änderung der Wasserstoffkonzentration ab. Daher wurde die Ansprechzeit von fünf im Handel erhältlichen Wasserstoffsensoren unterschiedlichen Messprinzips bestimmt. Experimente haben gezeigt, dass alle Sensoren die Anforderungen der Norm ISO 26142 übertreffen; es wurden Ansprechzeiten im Bereich (t90-Zeiten) von 2 s bis 16 s ermittelt. Die Ergebnisse können mit Exponential- oder Sigmoidalfunktionen approximiert werden. Es kann gezeigt werden, dass die Ergebnisse zum transienten Verhalten sowohl von den Betriebsparametern der Sensoren und Prüfverfahren als auch von den experimentellen Bedingungen wie Gaswechselrate sowie Konzentrationssprung abhängen.
In this talk, the evolution of the demand for engineering applications of neutrons at the ILL in the last 10 years is presented. It is shown that small angle neutron scattering and diffraction are highly demanded by the materials science community. Further development in the field is foreseen, with the use of in-situ (furnace, tension/compression rig, humidity cells, etc.) techniques to associate to the neutron techniques.
Der praktische Einsatz von SHM-Verfahren mit geführten Ultraschallwellen basiert auf einem fundierten Verständnis der speziellen physikalischen Zusammenhänge der Wellenausbreitung. Das umfasst nicht nur das multimodale Verhalten, welches von den Dispersionsdiagrammen repräsentiert wird, sondern auch die Vorhersage der Interaktion zwischen verschiedenen Wellenmoden und den zu erwartenden Fehlstellen.
Ziel des Workshops ist es, das Verständnis der Ausbreitung geführter Wellen zu verbessern und die vielfältigen Anwendungsmöglichkeiten, die sich aus der multimodalen Wellenausbreitung ergeben, an praktischen Beispielen zu demonstrieren. Die vorgestellten Szenarien beschränken sich dabei nicht nur auf eine Anregung grundlegender Dehnungs- und Biegewellen, sondern beziehen auch geführte Wellenmoden höherer Ordnung zur Verbesserung der Aussagekraft des Prüfergebnisses mit ein.
Nach einer theoretischen Einführung werden ausgewählte Demonstratoren, vorrangig mit Bezug zum Automobilbau, vorgestellt. Diese schließen Fragestellungen der Integration der Sensorik in gekrümmte Faserverbundbauteile ebenso mit ein, wie die Überwachung von Metall-Faserverbund-Komposit-Materialien, die in Drucktanks zum Einsatz kommen.
Lamb waves are widely used for monitoring the health of structures made of laminated materials (metals, composites). Piezoelectric ultrasonic transducers transmit and receive acoustic guided waves that interact with the elastic properties and the defects of the material under test. In recent years, DINFO developed piezopolymer Interdigital Transducers (IDTs) proven to be suitable to build SHM systems for aerospace applications. Their base material is a Cr/Au-coated PVDF-TrFE copolymer sheet, having a thickness of 100μm and a
metallization of 0.1μm on both sides.
IDTs are attractive for SHM systems for their capability to select specific Lamb wave modes when designed with appropriate electrode pattern. For a reliable application, it is crucial to characterize the in-plane and out-of-plane displacement of the transducer and to correlate the ultrasonic propagating signal with the Lamb waves dispersion curves in laminates. For example, for a 2 mm aluminum plate A0 mode has dominant out-of-plane displacement, whereas S0 mode has dominant in-plane displacement at the plate's surface.
In collaboration with BAM the characterization of free vibrational modes using a 3D laser doppler vibrometer was performed. The analysis in the frequency domain was performed using the following parameters: a chirp excitation with frequencies 50-500 kHz and 0.5-1 MHz with 195 Hz and 390 Hz frequency resolution respectively at 100 Vpp for a total of 90 averaged measurements per point. Also propagating modes were characterized by bonding the IDT on a 2 mm aluminum plate and analyzing the in-plane and the out-of-plane components. A0-mode was successfully excited having the main lobe aligned with the main axis of the IDT, confirming the dominant presence of this propagating mode.
The results could be used as a basis for design and optimization of the IDTs in terms of directivity and mode selection, and to facilitate interpretation of the ultrasonic guided waves propagating in laminated structures.
Ultrasonic guided waves (UGW) have been shown to be suitable for non-destructive testing (NDT) and structural health monitoring (SHM) of many engineering structures. Development of a technique based on UGWs requires careful understanding obtained through modelling and analysis of wave propagation and mode-damage interaction due to their dispersion and multimodal character. This presentation will provide insights into the Scaled Boundary Finite Element Method and its applicability for tackling wave propagation problems. Features and limitations of the SBFEM will be presented on an example of a multi-layered plate structure consisting of isotropic and anisotropic materials bonded together. You will be guided through the process of picking up the wave modes for your application. Starting from the calculation of dispersion curves and mode shapes to the analysis of wave propagation and mode-damage interaction. The main highlight of the presentation lies in the ability to detect damage in a certain layer depending on the mode used. The resulting deeper understanding of the wave propagation in multi-layered structures is the key to further developments of NDT and SHM for engineering structures consisting of multiple layers.
The material and damage behaviour of additively manufactured polyamide 12 (PA12) under dynamic loading was characterized by cyclic tests and microstructure analysis. The results were used to develop a numerical material and damage model. In a recent study, it was shown that the material and damage behaviour of 3D printed PA12 under quasistatic loading is simulated in a realistic way by coupling the
material model by Chaboche and the damage model by Gurson-Tvergaard-Needleman (GTN).
Using microscopy, X-ray refraction, and computed tomography, a porosity of about 5% was evaluated. These results served as a starting point for the present work. For the dynamic load, both the previously used Chaboche model and the GTN model were extended. Furthermore, the temperature was measured during the experiment and the self-heating effect was observed. Therefore, a temperaturedependent material parameters for the simulation were introduced. Considering the results of mechanical experiments, microstructural investigations, and self-heating effects, a good agreement between Experiment and numerical simulation could be achieved.
Behälter aus hochdichtem Polyethylen (PE-HD) werden zur Lagerung und für den Transport von Kraftstoffen eingesetzt. Beim Kontakt beider Medien diffundiert der Kraftstoff in das Polymer, erhöht dessen Volumen und Duktilität und ändert damit das Fließverhalten des Polymers unter Zugbelastung. Zudem weist PE-HD in Luft im verstreckten Bereich das sogenannte strain-whitening (Lichtstreuung) auf, während in Diesel gelagertes PE-HD optisch transparent wird. Zur Untersuchung dieses Phänomens wurden ortsaufgelöste Röntgenrefraktion und –diffraktion (Topographie) eingesetzt. Aus Sicht der molekularen Struktur (Diffraktion) weisen die beiden Probentypen keine Unterschiede auf: im verstreckten Bereich liegt eine „Fasertextur“ der Polymerketten in Belastungsrichtung vor, im unverstreckten Bereich nahezu ideale Isotropie. Die für mikroskopische Strukturen empfindliche Röntgenrefraktion zeigt außerdem für die Probe in Luft im verstreckten Bereich eine ausgeprägte Orientierung von Grenzflächen in Zugrichtung, während die in Diesel gelagerte Probe keine Grenzflächen zeigt. In Analogie zum optischen Verhalten bewirken die ähnlichen Brechungsindizes beider Medien im Röntgenbereich, dass eindiffundierter Diesel als Immersionsflüssigkeit die Brechungseigenschaften herabsetzt.
Es wurden mittels Fused Layer Modeling (FLM) Probekörper aus ABS und mittels Laser Sintering (LS) Probekörper aus PA 12 hergestellt, die anschließend für drei Monate einer künstlichen Bewitterung unterzogen wurden. Die dabei erzeugte definierte Alterung wurde nach drei Zeitabschnitten jeweils mit zerstörungsfreien Methoden charakterisiert. Dabei wurde neben den spektroskopischen Standardmethoden auch die aktive Thermografie eingesetzt. Die Ergebnisse zeigen, dass sich mittels der Thermografie sowohl chemische Veränderungen als auch mechanische Schädigungen (Fehlstellen) darstellen lassen. Die Bewitterung beeinflusst die Teiltransparenz der Proben zumindest im sichtbaren Spektralbereich, was bei der Charakterisierung der Fehlstellen, die mittels optisch angeregter aktiver Thermografie detektiert wurden, berücksichtigt werden muss.
Die Erkenntnisse dienen für die Entwicklung einer Strategie für eine Qualitätskontrolle von additiv gefertigten Kunststoffteilen.
Additiv gefertigte Prüfkörper aus Polyamid 12 (Laser Sinter Verfahren) und Acrylnitril-Butadien-Styrol (Fused Layer Modeling Verfahren) wurden über 2000 Stunden künstlich bewittert und ihr Alterungsverhalten untersucht. Die Ergebnisse wurden anschließend mit denen von Prüfkörpern verglichen, welche auf dieselbe Weise künstlich bewittert, aber mittels konventionellem Kunststoff-Spritzguss hergestellt wurden.
Die Erkenntnisse dienen für die Entwicklung einer Strategie für eine Qualitätskontrolle von additiv gefertigten Kunststoffteilen.
Entwicklung eines luftgekoppelten Ultraschall-Echo-Prüfverfahrens mittels fluidischer Anregung
(2019)
In vielen technischen Bereichen werden Ultraschallverfahren zur zerstörungsfreien Werkstoffprüfung eingesetzt. Dabei wird ein Schallpuls von einem Prüfkopf in ein Prüfobjekt eingebracht. Der Puls kann unter anderem durch Membranen oder Piezoelemente erzeugt werden und wird in der Regel durch direkten Kontakt oder über ein Koppelmittel an das Objekt übertragen. Luftgekoppelter Ultraschall spielt in kommerziellen Anwendungen bisher eine untergeordnete Rolle, da die Differenz der akustischen Impedanzen von Luft und Festkörpern immense Verluste beim Übergang des Schallsignals hervorruft.
In diesem Beitrag soll ein neuartiges Anregungsprinzip vorgestellt werden, mit dem ein Großteil dieser Verluste vermieden wird. Anstelle eines Festkörpers soll mit Hilfe einer fluidischen Düse Druckluft zur Signalerzeugung eingesetzt werden. In dieser Düse wird eine selbsterhaltende Strömungsinstabilität erzeugt, die einen Schallpuls von bis zu 100 kHz hervorruft, sodass der Impedanzverlust in die Umgebungsluft entfällt. Da die charakteristische Frequenz eines fluidisch generierten Pulses maßgeblich von der Bauform der Düse und dem anliegenden Druck abhängt, lässt sich mittels einer geeigneten Strömungsregelung ein breiter Frequenzbereich zur Abtastung nutzen. Die so emittierten Pulse werden auf das Prüfobjekt gerichtet und die reflektierten Signale mit einem Laservibrometer an der Oberfläche des Objekts im Echo-Verfahren abgetastet. Von drei signalmindernden Materialübergängen bei gewöhnlichem luftgekoppeltem Ultraschall bleibt in dem hier vorgeschlagenen Messystem lediglich die Grenzfläche von Luft zu Prüfkörper, sodass eine höhere Signalausbeute als bisher erwartet werden kann.
Безопасность движения на железнодорожном транспорте обеспечивается широким применением современных компьютеризированных средств неразрушающего контроля (НК), которые требуют квалифицированного персонала для их обслуживания и расшифровки результатов контроля. Персонал, занятый НК элементов инфраструктуры и подвижного состава при их эксплуатации и техническом обслуживании (ремонте), а также в предприятиях, изготавливающих продукцию железнодорожного назначения, включает: инженерные кадры - работников, в функциональные обязанности которых входит организация или организация и выполнение НК (в том числе и разработка технологий и технологической документации по НК), а также расшифровка результатов НК, и рабочих – дефектоскопистов или операторов дефектоскопных тележек.
Система подготовки персонала по НК, действующая на железнодорожном транспорте и реализуемая в ПГУПС кафедрой «Методы и приборы неразрушающего контроля», созданной А.К.Гурвичем в 1977 году, включает профессиональнее обучение, высшее профессиональное образование по направлению «Приборостроение», дополнительное профессиональное образование - повышение квалификации и переподготовку (в том числе, для персонала железнодорожных специальностей).
В соответствии с требованиями действующих нормативных документов подтверждение квалификации производится путем сертификации. Уполномоченный орган по сертификации персонала по НК, создан и функционирует в ПГУПС в рамках системы добровольной сертификации персонала по НК (Руководящий орган – Регистр систем сертификации персонала), деятельность которой в области железнодорожного транспорта управляется и контролируется Технической комиссией по железнодорожному транспорту, включающей представителей Росжелдора, ОАО «РЖД», промышленных предприятий - работодателей, Уполномоченных органов и Экзаменационных центров системы сертификации.
В докладе представлены и проанализированы результаты деятельности кафедры МПНК и Квалификационного центра по НК в ПГУПС.
Neue Konzepte zur Messung von Parametern der Brennflecke von Nano- und Mikrofokus-Röntgenröhren
(2019)
Die exakte Messung der Brennfleckgrößen von Röntgenröhren, Betatrons und Linear-Beschleunigern ist immer noch ein offenes Problem. Es existieren diverse Standards, die alle zu unterschiedlichen Resultaten führen in Abhängigkeit von der Brennfleckform. CEN hat 1999 die Serie EN 12543 (5 Teile) veröffentlicht, die die Messung der Brennfleckgrößen von Röntgenröhren für ZfP-Anwendungen beschreibt. Die Messprozeduren und Toleranzen unterscheiden sich von den Messstandards für medizinische Röntgenröhren, wie z. B. DIN EN 60336:2006 / IEC 60336:2005. EN 12543 beschreibt verschiedene Messmethoden für Standard-, Minifokus- und Mikrofokus-Röntgenröhren. CEN TC 138 WG 1 und ASTM E 07 revidieren bzw. revidierten daher die Standards zur Brennfleckmessung, wobei alle Messungen auf Pin-Hole- oder Kantenabbildungen zurückgeführt werden sollen. Die Schlitzblenden-Messung wird dann nur noch in der Medizin Anwendung finden. Es wurden auch tomographische Verfahren zur Rekonstruktion von Brennflecken vorgeschlagen. Die derzeit verfügbaren Standards decken den Bereich von 5 μm bis zu mehreren Millimetern ab. Mit der Entwicklung von Nanofokus-Röhren haben Hersteller unterschiedliche Verfahren zur Auflösungs- oder Brennfleckmessung eingeführt, deren Werte teilweise um den Faktor 2 unterschiedlich sind. Vorrangig wird hierbei auf ein JIMA-Target zurückgegriffen, dessen Auswertung die Detailerkennbarkeit widerspiegelt. Verschiedene Europäische Partner diskutieren derzeit Konzepte zur Messung von Brennflecken im Bereich von 100 nm bis 5 μm. Dabei werden hauptsächlich Messprozeduren zur Auswertung der Aufnahmen von Strichgruppenkörpern und kleinen Lochplatten untersucht. Als Ergebnis sollen Daten zur Brennflecklänge, -breite und -form bestimmt und standardisiert werden, um Produkte vergleichbar zu klassifizieren. Erste Konzepte hierzu und Ergebnisse früherer Untersuchungen werden vorgestellt.
The quantification of experimental and data analysis errors is most of the times jeopardised by the presence of surface roughness, waviness, as well as by typical measurement artefacts of the XCT technique (for instance refraction at sharp edges). One way to at least estimate the influence of those factors on dimensional measurements and on tolerances is to simulate them using Monte-Carlo method. An alternative strategy is to use independent knowledge (e.g. CMM data) to smoothen/calibrate/correct XCT data.
The characterisation of AM structures is an important aspect of the AM process, required in order to:
1. optimise the AM printing process
2. assess the quality of produced parts
A wide range of characterisation techniques are available, and the selection can be complex, based on multiple factors.
One output from the MetAMMi project is a good practice guide on the correct choice of characterisation technique.
Remote gas sensors like those based on the Tunable Diode Laser Absorption Spectroscopy (TDLAS) enable mobile robots to scan huge areas for gas concentrations in reasonable time and are therefore well suited for tasks such as gas emission surveillance and environmental monitoring.
A further advantage of remote sensors is that the gas distribution is not disturbed by the sensing platform itself if the measurements are carried out from a sufficient distance, which is particularly interesting when a rotary-wing platform is used.
Since there is no possibility to obtain ground truth measurements of gas distributions, simulations are used to develop and evaluate suitable olfaction algorithms.
For this purpose several models of in-situ gas sensors have been developed, but models of remote gas sensors are missing.
In this paper we present two novel 3D ray-tracer-based TDLAS sensor models. While the first model simplifies the laser beam as a line, the second model takes the conical shape of the beam into account.
Using a simulated gas plume, we compare the line model with the cone model in terms of accuracy and computational cost and show that the results generated by the cone model can differ significantly from those of the line model.
For metal-based additive manufacturing, sensors and measuring systems for monitoring of the energy source, the build volume, the melt pool and the component geometry are already commercially available. Further methods of optics, spectroscopy and non-destructive testing are described in the literature as suitable for in-situ application, but there are only a few reports on practical implementations.
Therefore, a new BAM project aims to develop process monitoring methods for the in-situ evaluation of the quality of additively manufactured metal components. In addition to passive and active thermography, this includes optical tomography, optical emission and absorption spectroscopy, eddy current testing, laminography, X-ray backscattering and photoacoustic methods. These methods are used in additive manufacturing systems for selective laser melting, laser metal deposition and wire arc additive manufacturing. To handle the sometimes huge amounts of data, algorithms for efficient preprocessing are developed and characteristics of the in-situ data are extracted and correlated to defects and inhomogeneities, which are determined using reference methods such as computer tomography and metallography. This process monitoring and fusion of data of different measurement techniques should result in a significant reduction of costly and time-consuming, destructive or non-destructive tests after the production of the component and at the same time reduce the production of scrap.
Here, first results of simultaneous measurements of optical emission spectroscopy and thermography during the laser metal deposition process using 316L as building material are presented. Temperature values are extracted from spectroscopic data by fitting of blackbody emission spectra to the experimental data and compared with results from a thermographic camera. Measurements with and without powder flow reveal significant differences between welding at a pristine metal surface and previously melted positions on the build plate, illustrating the significant influence of the partial oxidation of the surface during the first welding process on subsequent welding. The measurement equipment can either be mounted stationary or following the laser path. While first results were obtained in the stationary mode, future applications for online monitoring of the build of whole parts in the mobile mode are planned.
This research was funded by BAM within the focus area Material.
This talk presents a new application for high-spatial resolution eddy current testing (ET) with magneto resistive (MR) sensor arrays. With rising popularity and availability of additive manufacturing (AM), companies mainly in the aerospace sector, set high requirements on quality control of AM parts, especially produced with selective laser melting (SLM). Since it was shown that those parts are prawn to flaws like pores or cracks, every part needs to be tested. Therefore, NDT Methods, like ET, could help to characterize SLM parts. Research on ET has shown, that offline ET with high spatial resolution MR sensor arrays is possible and that flaws as small as 50 µm could be detected while significantly reducing testing time. In this talk a first approach on automated online ET method for testing SLM parts is proposed. An approach with hundreds of MR sensor is made to maintain spatial resolution and short testing times. Classic signal conditioning methods are used to reduce cost and complexity while maintaining high testing bandwidths. The proposed idea enables further research on automated generation of testing reports, process control or automated flaw curing.
Leitungswasserschäden nahmen im Jahr 2016 mit rund 2,6 Milliarden Euro den größten Posten bei Gebäudeversicherungen ein. Die Bestimmung und Lokalisierung des Schadensausmaßes ist dabei ein erster wichtiger Schritt, um die erforderlichen Sanierungsarbeiten abschätzen und effizient vornehmen zu können. Hierfür sind Radar und Neutronensonden bereits etablierte Verfahren. Für verlässliche Aussagen zum quantitativen Feuchtegehalt eines Baustoffes werden jedoch zerstörende Sondierungsbohrungen zur Kalibrierung der aufgenommenen Daten benötigt. Dieser hohe zeitliche und finanzielle Aufwand soll zukünftig durch den parallelen Einsatz der genannten Messverfahren verhindert werden.
In der durchgeführten Studie wurden Estrichprobekörper auf Zement- und Anhydritbasis mit variablen Dicken und Festigkeiten hergestellt und deren Austrocknung mit Radar und Neutronensonde über einen Zeitraum von 90 Tagen beobachtet. Als Referenzverfahren zur Ermittlung des Feuchteverlaufs dienten die gravimetrische Darr- und chemische Calcium-Carbid-Methode, welche an kleineren Schwesterproben der gleichen Charge angewendet wurden. Kontinuierliche Wägungen der größeren Probekörper gaben hierbei Aufschluss über den durch Austrocknung schwindende Wasseranteil.
Aus dem aufgenommen Radardatensatz konnten anschließend markante Signalmerkmale im Zeit- und Frequenzbereich extrahiert und deren Zusammenhang zum Feuchtegehalt der Estrichtypen untersucht werden. Hierbei wurden gängige Methoden aus der Literatur zur Feuchtemessung mit Radar angewandt, wobei die Kombination dieser und das Hinzuziehen der Neutronensonde mittels Datenfusion zu einem höheren Informationsgehalt beiträgt. Somit kann eine zerstörungsfreie Quantifizierung des Feuchteschadens an schwimmenden Fußbodenaufbauten vorgenommen werden
In this paper, we introduce a nano aerial robot swarm for Indoor Air Quality (IAQ) monitoring applications such as occupational health and safety of (industrial) workplaces. The robotic swarm is composed of nano Unmanned Aerial Vehicles (UAVs), based on the Crazyflie 2.0 quadrocopter, and small lightweight Metal Oxide (MOX) gas sensors for measuring the Total Volatile Organic Compound (TVOC), which is a measure for IAQ. An indoor localization and positioning system is used to estimate the absolute 3D position of the swarm similar to GPS. A test scenario was built up to validate and optimize the swarm for the intended applications. Besides calibration of the IAQ sensors, we performed experiments to investigate the influence of the rotor downwash on the gas measurements at different altitudes and compared them with stationary measurements. Moreover, we did a first evaluation of the gas distribution mapping performance. Based on this novel IAQ monitoring concept, new algorithms in the field of Mobile Robot Olfaction (MRO) are planned to be developed exploiting the abilities of an aerial robotic swarm.
Composite-Druckgefäße für den Gefahrguttransport sowie für Wasserstoff- und Erdgasfahrzeuge bestehen aus einem lasttragenden Composite und einer gasdichten, metallischen oder polymeren Barriereschicht (Liner). Zur Untersuchung des Alterungsverhaltens solcher Composite-Druckgefäße wurde in der BAM das interdisziplinäre Themenfeldprojekt COD-AGE durchgeführt. Ziel des Projektes war die Entwicklung von Methoden und Modellen zur Beschreibung und Bestimmung des Alterungsverhaltens von Carbonfaser-Compositen am Beispiel von Druckgefäßen, um eine bessere Vorhersage der Alterung und der sicheren Gebrauchsdauer zu ermöglichen. Ein Schwerpunkt in diesem Projekt war die Bereitstellung geeigneter ZfP-Methoden. Dabei ging es sowohl um versuchsbegleitende Prüfungen, als auch um die mögliche Entwicklung von Prüfeinrichtungen für den späteren praktischen Gebrauch.
In dem Vortrag werden Prüfergebnisse der alterungsbegleitenden Wirbelstromprüfung an Composite-Druckgefäßen vorgestellt.
In Alterungsversuchen wurden Druckgefäße aus einem ca. 4 mm dicken Aluminiumliner und ca. 8 mm dicker CFK-Ummantelung untersucht. Typisches Einsatzgebiet solcher Druckgefäße sind Atemschutzgeräte der Feuerwehr. Die Druckgefäße wurden sowohl mit konventioneller Wirbelstromtechnik von außen und von innen als auch mit hochfrequenter Wirbelstromtechnik von außen geprüft. Es konnten sowohl Beschädigungen des metallischen Liners, als auch Strukturen des CFK-Geflechts nachgewiesen werden. Eine besondere mechanische Herausforderung bestand in der Prüfung der Innenseite der Liner, da hier eine zylindrische Fläche mit einem Innendurchmesser von 150 mm durch einen Zugang von nur 15 mm Durchmesser geprüft werden muss.
The project ProMoAM is presented. The goal of the project is to evaluate which NDT techniques or combination of techniques is suited for in-situ quality assurance in additive manufacturing of metals. To this end, also 3d-data fusion and visualization techniques are applied. Additional ex-situ NDT-techniques are used as references for defect detection and quantification. Feasability studies for NDT-techniques that are presently not applicable for in-situ use are performed as well.
The presentation gives a brief overview of the whole project and the different involved NDT-techniques.
Die Varroose gilt weltweit als eine der bedeutendsten Erkrankungen der westlichen Honigbiene, Apis mellifera. Verursacht wird sie durch den Ektoparasiten Varroa destructor, der durch Saugen der Hämolymphe und Virenübertragung sowohl die Bienenbrut als auch die adulten Individuen schädigt.
Ohne imkerliche Unterstützung gehen betroffene Bienenvölker in der Regel innerhalb von drei Jahren ein. Als bewährte, häufige Behandlung wird flächendeckend 60%ige Ameisensäure (AS) -im Bienenstock appliziert über verschiedene Verdampfungssysteme- eingesetzt. Sie wirkt als einziger zugelassener Wirkstoff gegen die Varroamilbe auch in die verdeckelte Brut hinein und birgt kein Risiko der Resistenzentwicklung.
Allerdings ist das therapeutische Fenster, der Bereich zwischen Milbenabtötung und Bienenschädigung, relativ schmal und die tatsächliche AS-Konzentration im Bienenstock stark abhängig von äußeren Einflussfaktoren, wie Temperatur oder Applikationsart.
Eine Bestimmung der exakten AS-Konzentration im Bienenstock ist also notwendig, um einerseits das Wirkungsoptimum zu bestimmen und andererseits bei Übersteigen der Maximalkonzentrationen mit Folge-Schädigung der Bienen, ein rechtzeitiges Eingreifen der Imker zu ermöglichen.
Die Entwicklung eines Detektors zur Bestimmung der tatsächlichen AS-Konzentration in der Bienenstockluft würde einen großen Fortschritt in den Bereichen der Bienenforschung, der Varroa-Behandlung in der Imkerei, sowie einer gesundheitlich bedenklichen AS-Exposition für den Imker bedeuten.
Langfristiges Ziel dieses Projekts ist es, ein mobiles, einfach handhabbares Gerät zur Detektion der AS-Konzentration in der Bienenstock-Luft während einer AS-Behandlung zu entwickeln.
Grundlage hierfür bietet ein kommerziell verfügbares Multigassensorsystem (PEN3, AIRSENSE Analytics GmbH, Schwerin).
Ziel der beschriebenen Vorversuche ist die generelle Eignungsprüfung des Multigassensorsystems PEN3 zur Detektion der hohen AS-Konzentration in Gasgemischen unter Bedingungen wie sie im Bienenstock während einer AS-Behandlung herrschen.
Als Messplatz zur Detektion von AS in Luft wurden zwei Systeme vorbereitet:
A) Eine Plexiglasbox mit einem Volumen von ca.1,9 l diente als Prüfkammer, wurde von einem Gasstrom befeuchteter synthetischer Luft durchströmt (50 mL min-1, 100 mL min-1 oder 300 mL min-1) und zur Temperierung auf 25 °C auf einer Heizmatte bzw. auf 35 °C in einem Ofen platziert. AS-Wasser-Gemische verschiedener Volumenverhältnisse (0-100%) wurden in einer Glasschale (Oberfläche Flüssigphase 9,6 cm2) in der Prüfkammer vorgelegt.
B) In einer durch eine Heizmatte auf 25 °C temperierten, zwei-zargigen Segeberger Beute wurden vier Messpunkte angebracht (davon drei in der oberen und einer in der unteren Etage) und 60%ige AS über einen Nassenheider-Verdampfer vertikal vorgelegt.
Zur Detektion des AS-Luft-Gemischs wurde das PEN3 Multigassensorsystem genutzt. Das zu untersuchende Gasgemisch wurde über die internen zehn Metalloxid-Halbleiterdetektoren (1-10) der PEN3 geleitet. Das so generierte Messsignal beschreibt ein Widerstandsverhältnis zwischen dem Widerstandwert zum Zeitpunkt der Messung (R) und dem elektrischen Widerstand bei der Nullluft-Messung (R0), wobei hier befeuchtete synthetische Luft mit einer reltativen Feuchte von 49 % ± 4 % oder Raumluft genutzt wurde. Gase unterschiedlicher Zusammensetzung können anhand der dargestellten Signalmuster verschiedener Sensoren voneinander unterschieden werden.
In beiden Messsystemen wiesen die Sensorelemente 1, 6, 7, 8 und 9 gesteigerte Sensitivitäten beim Vorliegen von Ameisensäure im Gasgemisch auf. Die größten Widerstandsverhältnisse zeigten dabei die Sensorelemente 6, 8 und 9, sodass hier von der höchsten Empfindlichkeit für AS auszugehen ist.
Basierend auf den erhobenen Messdatensätzen wurden Hauptkomponenten- (PCA) und lineare Diskriminanzanalysen (LDA) zur Dimensionsreduktion durchgeführt, um charakteristische Muster aus den Messsignalen zu extrahieren. Die PCA- und LDA-Plots für die jeweils bedeutsamsten Komponenten PC 1 und PC 2 bzw. LD 1 und LD 2 eines Versuchslaufs in der Plexiglasbox (A) bei einer Temperatur von 35°C, einem Volumenstrom von 300 mL min-1 und variablen Ameisensäure-Wasser-Volumenverhältnissen von 0 % bis 100 %. Deutlich erkennbar sind die Abhängigkeiten der Komponenten PC 1 und LD 1 (Abszissen) vom vorgelegten AS-Wasser-Volumenverhältnis. Unsere Ergebnisse zeigen, dass das Multigassensorsystem zur Bestimmung von AS in Luft unter ähnlichen Bedingungen wie bei einer AS-Behandlung im Bienenstock angewandt werden kann.
Die Versuche zeigen, dass die im PEN3 eingesetzten Metalloxid-Sensoren bzw. Metalloxidsensor-Arrays zur Detektion von AS in Luft auch bei höheren Konzentrationen als beispielsweise der MAK-Wert von 5 µmol/mol zur Überwachung des AS-Gehaltes in Bienenstöcken angewandt werden können. Aufgrund der Möglichkeit der quasi Echtzeit-Messdatengenerierung, der geringen Kosten kommerziell verfügbarer Sensorelemente, der Portabilität und der Möglichkeit zur Miniaturisierung zeichnen sich solche Messsysteme zukünftig durch eine besondere Praxisnähe aus. Diese Erkenntnisse sind Grundlage für eine Verbesserung des Detektionsverfahrens und zur Vorhersage über die Effizienz und Effektivität der Varroha-Milbenbehandlung.
Kurzfassung. Die meisten Faserverbundbehälter, die in Europa für den Transport gefährlicher Stoffe oder als Speicherbehälter für Wasserstoff oder Erdgas in Fahrzeugen Verwendung finden, werden für eine nicht begrenzte Lebensdauer ausgelegt. In regelmäßigen Abständen sind für diese Druckbehälter wiederkehrende Prüfungen vorgeschrieben, die in der Regel aus visuellen Inspektionen und hydraulischen Innendruckprüfungen bestehen [1]. Die wiederkehrenden Prüfungen sind jedoch weder geeignet, die sichere Betriebsdauer festzulegen noch den Alterungszustand der Druckbehälter zu beurteilen. Aufgrund des weiter steigenden Drucks zur Kosten- und Gewichtsreduktion von Faserverbundbehältern und der damit einhergehenden Ausnutzung der existierenden Sicherheitsmargen gewinnen Methoden zur Beurteilung des Alterungsverhaltens dieser Druckbehälter immer mehr an Bedeutung.
Vor diesem Hintergrund wurde in der BAM das orhaben „Alterungsverhalten von Composite-Druckgefäßen mit Carbonfaser-Compositen unter gekoppelten Beanspruchungen (COD-AGE)“ gestartet, um die Alterung von Faserverbundbehältern mittels verschiedener zerstörungsfreier Prüfverfahren zu untersuchen. Die hier vorgestellten Ergebnisse wurden an einem Typ 4 Hybridbehälter mit Kunststoffliner gewonnen, in den im Rahmen des Herstellungsprozesses optische Glasfasern in verschiedenen Schichten integriert wurden. Der Behälter wurde mittels hydraulischem Innendruck und erhöhter Umgebungstemperatur zyklisch bis zum Lebensende belastet. Zwischen den einzelnen Lastsequenzen erfolgte eine an die wiederkehrende Prüfung im Betrieb angelehnte hydraulische Druckbelastung.
Sowohl die hydraulische Alterung als auch die Druckrampen wurden mittels Schallemissionsprüfung (SEP) und verteilter faseroptischer Sensorik mit dem Ziel einer frühzeitigen Versagensvorhersage überwacht. Durch die Schallemissionsanalyse sollte untersucht werden, ob zum Lebensende hin ein signifikanter Anstieg der Schallemissionsaktivität und -intensität, unterteilt in verschiedene Lastbereiche, beobachtet werden kann. Zusätzlich zum kontinuierlichen Monitoring der Zyklierung sollten die Druckrampen separat untersucht werden, um hier Potenziale für die wiederkehrende Prüfung zu erkennen.
We present the results of distributed fibre optic strain sensing for condition monitoring of a hybrid type IV composite fully wrapped pressure vessel using multilayer integrated optical fibres. During load cycle tests material fatigue could be localised and monitored 17,000 load cycles before burst. Results have been validated by acoustic emission analysis.
Das Projekt befasst sich mit einem neuen Ansatz, den pH-Wert im Beton zu bestimmen.
Der pH-Wert ist vor allem für Stahlbeton-Bauwerke von Bedeutung, da dieser maßgeblich die Korrosion des Stahls beeinflusst. Im frischen Beton liegt der pH-Wert im basischen Bereich.
Der Stahl ist in diesem Bereich passiviert, also vor schädlicher Korrosion geschützt. Durch die sogenannte Karbonatisierung sinkt der pH-Wert und die Korrosionswahrscheinlichkeit steigt deutlich an. Die Stabilität von Bauwerken in denen Stahlbeton verbaut ist, wird durch diese Korrosion langfristig beeinträchtigt. Allein in deutscher Infrastruktur rechnet man mit circa 5 Milliarden Euro Schaden jährlich.
Die entwickelte Methode verwendet Sonden, welche die Korrosion Monitoren sollen. Im Sondeninneren befindet sich eine Schicht pH-Indikator (Thymolblau) und eine Schicht mit Quantenpunkten. Die Quantenpunkte fluoreszieren, nach Anregung durch zum Beispiel einen Laser, bei circa 440 nm (blau) beziehungsweise 610 nm (gelb-orange). Das Thymolblau ist im basischen Milieu (pH > 9,6) blau, im neutralen Milieu (um pH 7) gelb-orange. Der Indikator wirkt wie ein Farbfilter und lässt, je nach pH-Wert, unterschiedliche Wellenlängen zur Glasfaser durch. Aus der gemessenen Intensität bei 440 nm und 610 nm kann ein Verhältnis ermittelt werden. Dieses Verhältnis lässt erkennen, welchen pH-Wert das Milieu besitzt, in dem sich die Sonde befindet.
Die entwickelte Methodik ist zerstörungsfrei, das heißt kein Material muss aus den Bauwerken entnommen werden. Vielmehr sollen die Sonden beim Betonieren in das Bauwerk eingebettet werden. Über Glasfasern können die Sonden jederzeit angesprochen werden. Dies ermöglicht permanentes pH-Monitoring, was die Früherkennung von Korrosionsgefahr verbessert und die Sanierungskosten verringert.
The lecture gives an overview of the core competencies of the department 8.5 Micro NDT (CT equipment, X-Ray Refraction, WAXS - Texture analysis, Data correction, Reconstruction algorithms, Image analysis, Dimensional control, Multi-Source Data Fusion and Combination, Micromechanical Properties by Neutron Diffraction) with regard to the characterization of lightweight materials (fibre composites, aluminum and titanium MMCs) used in aviation. The focus is on characterization by synchrotron and X-ray refraction techniques as well as computed tomography.
The lecture about gas sensor characterization and calibration contains the content of 3S: Sensitivity, selectivity and stability, sensor drift, aging and poisoning as well as gas mixing systems for laboratory and field calibration, measurement uncertainty for sensor calibration and traceable calibration of gas sensor systems.
Nominal-actual comparisons are routinely performed to compare a manufactured specimen to a reference specimen. X-ray Computed Tomography (CT) has brought a profound change in the way that tolerance verifications are performed in industry, by allowing the inner and outer geometries of an object to be measured, without the need for external access or destructive testing. As a results, CT is increasingly used in additive manufacturing applications, where a nominal-actual comparison performed between the digital model (CAD file), used as an input for the 3D printer, and the CT volume from the printed part, can provide invaluable information as to the accuracy of the printing process. However, the nominal-actual comparison process is somewhat different when applied to additively manufactured specimens by comparison to conventionally manufactured specimens.
One of the great strengths of X-ray computed tomography over conventional inspection methods (ultrasound, thermography, radiography) is that it can image damage in 3D. However for curved or deformed composite panels, it can be difficult to automatically ascribe the damage to specific plies or inter-ply interfaces. An X-ray computed tomography (CT) data processing methodology is developed to extract the through-thickness distribution of damage in curved or deformed composite panels. The method is applied to [(0°/90°)2]s carbon fibre reinforced polymer (CFRP) panels subjected low velocity impact damage (5 J up to 20 J) providing 3D ply-by-ply damage visualisation and analysis. Our distance transform approach allows slices to be taken that approximately follow the composite curvature allowing the impact damage to be separated, visualised and quantified in 3D on a ply-by-ply basis. In this way the interply delaminations have been mapped, showing characteristic peanut shaped delaminations with the major axis oriented with the fibres in the ply below the interface. This registry to the profile of the panel constitutes a significant improvement in our ability to characterise impact damage in composite laminates and extract relevant measurements from X-ray CT datasets.
Fibreoptic sensors (FOS) represent sensing technology with small footprint, low invasiveness, electromagnetic passivity and immunity, plus potential for remote and real-time monitoring. Modern FOS techniques allow truly temporally- and spatially-continuous monitoring over extended distances; a feature not attainable with any other sensing technology. Moreover, depending on their particular material composition and design, optical fibres can be made resistant to high temperatures, chemicals and ionizing radiation. Due to this unique combination of advantageous properties, ever since their emergence, FOS have been attracting considerable attention for monitoring tasks in harsh, hazardous and difficult-to-access locations. The potential of FOS has been recognized also in the field of radioactive waste management and fibreoptic sensors belong to the most promising technologies for nuclear waste repositories (NWR) monitoring.
Vast majority of distributed fibreoptic sensor applications rely on use of silica-based optical fibres as sensing elements. At the same time, distributed measurement of local temperature and strain along the fibre are the most common monitoring tasks addressed by fibreoptic sensors. Nevertheless, FOS offer much larger flexibility both in terms of utilized sensing fibre as well as targeted measurand. In this contribution, we will review some of more alternative implementations of FOS that are being explored at “Fibre Optic Sensors” division of Federal Institute for Material Research and Testing (BAM), in Berlin. The main focus will be twofold. On one side, we will address FOS applications with polymer optical fibres (POF), that may enable monitoring of large strains (>100%) and high-sensitivity radiation detection. On the other side, we will present our activities in the area of distributed acoustic sensing (DAS); one of the most recent developments in the fibreoptic sensing field enabling highly-dynamic vibration sensing with nanostrain sensitivity. We will introduce the principles of the addressed FOS technologies, present application examples from our case studies, discuss advantages and limitations of the techniques and highlight their potential for NWR monitoring.
The closure of underground nuclear waste disposal facilities requires reliable gas- and water-tight engineered barriers. In Germany, barriers made from salt concrete have been evaluated in full scale. While the barriers seem to fulfill the requirements regarding permeability, some unexpected cracks have been detected at the surface and at depth. In cooperation between the Federal Company for Radioactive Waste Disposal (BGE) and the Federal Institute for Materials Research and Testing (BAM), several experiments have been carried out to evaluate the applicability for ultrasonic measurements in crack detection and general quality assurance. Both commercial instruments and specially developed devices have been tested on site.
Using commercial ultrasonic echo devices designed for concrete inspection it was possible to detect cracks and objects in salt concrete up to a depth of 2 m. The check for delamination in shotcrete is another field of application. A unique device available at BAM, the wide aperture, deep penetration instrument LAUS, was able to locate cracks and objects up to a depth of 8 m so far, which is thought to be a record for ultrasonic echo measurements in concrete. Adapted imaging procedures, partly adopted from geophysics, helped to reveal 3D structure at depth.
In addition, we have developed ultrasonic probes to be deployed in boreholes, currently at up to 20 m depth. They can collect information on cracks and other features in a radius of about 1.5 m around the borehole in the current version and might be used in echo or transmission mode. Evaluation experiments have been performed at an experimental barrier at the ERAM site in Morsleben, Germany. The results showed several empty and injected cracks as well as built-in instrumentation. The results have been verified using borehole endoscopy as well as core examination and will be used to set up a reliable quality assurance system for engineered barriers. All instruments are based on ultrasonic shear wave transducers with a frequency range between 25 kHz and 100 kHz. Current research focuses on the improvement of the hardware (e. g. optimization of array characteristics) and imaging techniques such as Reverse Time Migration, both aiming at the improvement of depth of penetration, resolution and probability of detection.
Die Brücken im Netz der Bundesverkehrswege sind überwiegend in einem ausreichenden bis guten Zustand. Allerdings steigt der Unterhalts- und Sanierungsaufwand aufgrund des inzwischen hohen Alters vieler Brücken sowie des ständig wachsenden Schwerlastverkehrs. Techniken zur Einschätzung der verbleibenden Lebensdauer von Brücken sowie zur dauerhaften Beobachtung des Tragverhaltens bzw. des Erfolges von Sanierungsmaßnahmen werden daher für den sicheren und wirtschaftlichen Betrieb dringend benötigt. Zur Evaluierung dafür
geeigneter holistischer Ansätze wurde in der BAM das Projekt BLEIB - Bewertung, Lebensdauerprognose und Instandsetzung von Brückenbauwerken - ins Leben gerufen.
Ein zentrales Ergebnis des Projektes ist eine extern vorgespannte Stahlbetonbrücke als Zweifeldträger mit einer Gesamtlänge von 24 m, die für den Test verschiedenster Sensorsysteme, zur Validierung numerischer Modelle und zur Erprobung von Sanierungs- und Verstärkungsmaßnahmen entwickelt wurde. Für die Simulation unterschiedlicher Schädigungsgrade kann die Vorspannung der Brücke variiert werden. Die Brücke wird mit beweglichen Gewichten belastet und über einen Shaker zum Schwingen angeregt.
Das Brückenmodell wurde bewusst geschädigt, indem die Vorspannung der Struktur erstmalig schrittweise bis auf null reduziert wurde. Unter der Eigenlast verformte sich die Brücke, wodurch eine Rissbildung im Beton einsetzte. Die Zugspannung, die zuvor durch die Vorspannung aufgenommen wurde, übernahm Schritt für Schritt der Beton. Als die Zugspannungen die relativ geringe Zugfestigkeit des Betons überstiegen, begann dieser zu reißen und die schlaffe Bewehrung der Struktur nahm die Spannungen auf. Dieser Versuch wurde unter anderem von Schallemissionsmessungen begleitet. Der Rissbildungsprozess konnte damit, bei gleichzeitiger Aufzeichnung der Vorspannung, früh detektiert und die Risse geortet werden. Die Ergebnisse korrelieren gut mit den Ergebnissen der stereophotogrammetrischen Verformungsmessungen der Struktur.
Die zeitaufgelöste THz-Spektroskopie (THz-TDS) operiert mit elektromagnetischen Impulsen, die ein Amplitudenspektrum im unteren THz-Bereich aufweisen. Die THz-TDS ist in der Lage, dielektrische Volumina zerstörungs- und kontaktfreie zu durchstrahlen. Um eine 3D-THz Bildung zu ermöglichen, wurde für die THz-TDS ein rechnergestütztes Mess- und Rekonstruktionsverfahren (THz-TD-SAFT) entwickelt, mit dem es gelingt, unter Verwendung der im Zeitbereich registrierten THz-TDS-Daten das Volumeninnere in Form eines dreidimensionalen Tomogramms zu rekonstruieren. Die THz-TD-SAFT basiert auf die für die Ultraschalltechnik entwickelte Synthetische Apertur Fokussierung (SAFT) bzw. des heuristisch begründeten Algorithmus des Synthetischen Apertur Radars (SAR). Anhand von Beispielen wird die Möglichkeiten der THz-TD-SAFT demonstriert und nachgewiesen, dass mit dem Rekonstruktionsansatz die künstlich eingebauten Fehlstellen bezüglich der Position und Größe geometrisch richtig im Tomogramm abgebildet werden können.
The Spectral Element Method (SEM) has been proven to be an efficient numerical method for solving the wave equation in linear elastic bodies. This efficiency is reduced if a stress singularity is present in the body. For example, re-entrant corners, material interfaces, fixed boundaries, and especially crack tips can cause stress singularities. To preserve the efficiency of the SEM, special solution strategies are required. There are many approaches which consider stress singularities, but comparisons are rare for dynamic problems. Finding an efficient model is an important step for many applications. In particular, applications for structural health monitoring and non-destructive evaluation rely on accurate and efficient crack models.
In this contribution, we present several models to capture singularities in combination with the SEM. The theory behind the models are shortly summarized and we show results for different benchmark problems in two dimensions. The first model deploys a new class of singular elements. For these singular elements, the crack tip approximation is computed based on the static Scaled Boundary Finite Element Method (SBFEM). The second model uses the continued‐fraction‐based SBFEM, while the third model uses an hp-refinement near the singularity. The models are compared among each other and evaluated in terms of their respective efficiency and accuracy.
We compare four different sensing solutions suitable for distributed fiber optic humidity sensing in per uorinated graded-index polymer optical fibers (PFGI-POFs). Compared to silica fbers, polymer optical fibers over advantageous beneffits including signifficantly higher break down strain, fracture toughness and humidity sensitivity. Various humidity-related effects in PFGI-POFs have been reported in the last years including measured attenuation and length changes as well as Brillouin frequency and Bragg wavelength shifts. The four aforementioned methods could serve as a basis for distributed and quasi-distributed humidity sensing and are described here closely with an emphasis on plausible cross effects to temperature and strain. The main focus of this paper lies on the comparison of four approaches with regard to method complexity, sensitivity to humidity, spatial resolution, real-time capability and effort to compensate for cross effects.
The reassessment of bridges is becoming increasingly important. The basic requirement for analyses of structural safety is reliable knowledge about individual structures. This paper introduces the new approach to evaluate the quality of measured data gained from non-destructive testing (NDT) to provide reliable, objective, and relevant information about existing bridges. The purpose is to relate this validated knowledge to probabilistic analyses. Bridging the gap between NDT and numerical reassessments indicates reduced numerical uncertainties and residual service time extensions. This paper deals with an application of this approach using measurement data collected by ultrasonic technique at a prestressed concrete bridge.
Ein Endlager für radioaktive Abfälle muss auf Dauer vor dem Eindringen von Wasser geschützt werden. Aus diesem Grund errichtete die Bundesgesellschaft für Endlagerung (BGE) ein Verschlussbauwerk als InSitu Versuch im Endlager Morsleben. Hierfür wurde eine bergmännische Auffahrung mit Salzbeton verfüllt.
Zusätzliche in regelmäßigen Abständen eingebaute Trennbleche sollen der Ausbreitung eventueller Risse entgegenwirken.
Das Versuchsobjekt wurde mit Ultraschallmessungen bezüglich der Lage und Orientierung möglicher Risse untersucht. Diese Messungen wurden an der Stirnfläche des Salzbetonkörpers durchgeführt. Für die Datenakquisition fand ein neuartiges Messsystem Anwendung, mit welchem ein seismisches Akquisitionsschema realisiert werden kann. Dieses setzt sich aus mehreren Prüfköpfen zusammen, welche sowohl als Sender als auch als Empfänger fungieren. Somit lassen sich auch größere Sender-Empfänger-Abstände realisieren.
Vor der Bearbeitung der realen Messdaten wurde ein realitätsnahes synthetisches Modell erstellt, mit welchem die Abbildung eines Risses, eines eingebauten Monitoring-Instruments sowie des ersten Trennbleches in einer Tiefe von 8 m untersucht wurden. Die generierten synthetischen Daten wurden nach minimaler Preprozessierung mittels Kirchhoff-Prestack-Tiefenmigration sowie Reverse Time Migration (RTM) in Tiefensektionen umgewandelt. Insbesondere bei der Abbildung steilstehender Reflektoren lieferte die RTM hier ein besseres Ergebnis.
Im Anschluss wurden reale Messdaten in der gleichen Weise bearbeitet. Zusätzlich wurden die Ergebnisse mit denen der Synthetic-Aperture-Focusing-Technique (SAFT) verglichen. Dieses Verfahren ist der Kirchhoff-Migration sehr ähnlich und findet in der zerstörungsfreien Werkstoffprüfung routinemäßig Anwendung. Es stellte sich heraus, dass der RTM Algorithmus in einigen Fällen ein realitätsnäheres Abbild liefert. Die Resultate von Kirchhoff-Migrationund SAFT-Rekonstruktion sind hingegen sehr ähnlich. Alle Verfahren lokalisierten in gleicher Weise Risse in verschiedenen Tiefenlagen, welche hauptsächlich oberflächenparallel orientiert sind. Diese werden auch von Bohrungen bestätigt. Besonders herauszustellen ist die deutliche Abbildung des Trennbleches in großer Tiefe, was bisher noch nicht eindeutig gelungen war.