8.6 Faseroptische Sensorik
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Organisationseinheit der BAM
- 2 Prozess- und Anlagensicherheit (16) (entfernen)
Protection against terroristic or accidental scenarios in industrial settings requires suitable designs of structures to resist blast loads. Field testing as well as finite element simulations are among the techniques available to engineers in the understanding of the structural behavior against blast loading.
As blast testing of complex scenarios can be very resource intensive, tests are generally performed for simplified scenarios. Numerical tools can be used to model these scenarios to get better insight into blast loading, structural response, and the resulting damage to the structure. During the next steps, the simplified scenario is successively modified in numerical simulations to incorporate complexities that cannot be covered in blast testing experiments. One of the conditions for this approach to work is that the original simplified numerical simulation is valid. The scopes and challenges encountered in such a validation are the focus of this presentation/article.
A relatively ‘simple’ field test of a horizontal reinforced concrete (RC) slab subjected to blast loading is taken as an example for validation of the performance of numerical tools. The blast test incorporated various measurement techniques to quantify the blast load as well as the behavior of the RC slab. Blast load was measured using flush mounted piezoelectric pressure gauges, whereas acceleration sensors and fiber-optic sensor cables were used to characterize the dynamic behavior of the slab under blast loading. Additionally, damage characteristics were ascertained also using fiber optic sensing. The application of such measurement techniques, along with different numerical software available for the analysis of the scenario in question, demonstrate the scope of our contribution.
When it comes to the challenges, it begins with proper instrumentation of the test specimen followed by the data processing. For numerical modelling, geometric conditions with appropriate boundary constraints, physical conditions such as the configuration of the rebars, as well as material parameters add to this challenge. The issues of choosing appropriate material models and comparison of results with multiple software tools will be discussed. This discussion forms the basis for a coherent approach to technical-safety assessment of blast effects on structures in its broader sense.
Current capabilities for full-scale field testing are highly resource intensive. Reliable small-scale experiments are an effective alternative. Characterization of the dynamic response and damage of RC elements to scaled blast loads was investigated in scaled-down field experiments. Spatially resolved information on the dynamic structural response using distributed fiber optic acoustic sensing (DAS) and acceleration as well as blast loading by piezoelectric pressure sensors.
A safety or security related assessment of explosions, accidental and intentional scenarios alike, often necessitate performance of replication-tests. Such test results are necessary to clarify the causes within the scope of forensic investigations. To gain important insights into the behavior of structures and materials under such loading, field tests may also be performed in accordance with different test standards. To determine the resistance of building-structures after explosions, estimation of the residual load-bearing capacity in addition to the assessment of dynamic structural response and damage to the building components is important. In most cases an evaluation of structural integrity is based only on the visual damage, resulting in an overestimation of the residual capacity.
The Bundesanstalt für Materialforschung und -prüfung (BAM) operates the Test site for Technical Safety (TTS) on an area measuring about 12 km2 in the Federal State of Brandenburg for execution of true-to-scale explosion tests. At the TTS, building component testing was performed to assess the suitability of different non-destructive testing methods to characterize the dynamic structural response and damage resulting from the detonation of high explosives.
Different blast-loading scenarios were realized by varying the net explosive mass and the standoff distance with all scenarios representing a near-field detonation. The test object was a reinforced concrete wall 2 m high, 2.5 m wide and 20 cm thick, fixed at both vertical edges. The dynamic loading of the wall was characterized with 8 piezoelectric pressure sensors flush-mounted on the front surface, thus measuring the reflected pressures from the shock wave. The tests were conducted with the aim of characterizing the global behavior of the wall under dynamic shock loading and the resulting local damage pattern, respectively. High speed digital image correlation was implemented in combination with multiple acceleration sensors to observe the rear surface of the wall to chart the dynamic deflection during the loading and to determine the residual deformation after the loading had ceased. In addition, one test specimen was instrumented with fiber optic sensor cables, both fixed to the rebars and embedded in the concrete-matrix, respectively. Firstly, these sensors were interrogated during the blast test by a distributed acoustic sensing (DAS) device using a particularly high sampling rate to measure the shock-induced vibrations in the structure with high temporal resolution. This delivers information on dynamics of compression and tension cycles from within the structure. Secondly, the local damage-pattern emerging during the series of blasts was determined via distributed fiber optic strain sensing (DSS) by interrogating the embedded fiber optic sensors with a high spatial resolution DSS device after each blast. This enabled the characterization of non-visual damage to the structure, in particular with regard to the formation of localized cracks in the concrete matrix. The DSS was further complimented by a structure-scanner based on ultrasonic measurements.
Our contribution describes this new test approach in detail. Results of the three datasets, namely dynamic shock loading, global behavior of the test object and the local damage pattern will be presented. The suitability of the implemented measurement methods will be discussed in combination with the challenges in their application for technical safety evaluation of building components under explosive loading.
Zur Bewertung der Widerstandsfähigkeit von Schutzobjekten und Bauteilen gegen Explosionsbeanspruchung führt die Bundesanstalt für Materialforschung und -prüfung (BAM) regelmäßig Sprengversuche im Realmaßstab auf einem Testgelände durch.
Solche Versuche sind planungs- und ressourcenintensiv, sodass sie zunehmend durch numerische Simulationen ergänzt bzw. ersetzt werden.
Aktuelle Forschungsarbeiten zielen darauf ab, neue Berechnungsmethoden und Materialmodelle zu entwickeln, mit denen die Bestimmung der Bauteilantwort sowie die daraus resultierende Schädigung nach einem Explosionsereignis möglich sind.
Zur Validierung solcher Ansätze ist es notwendig, systematische Versuche mit unterschiedlichen Szenarien durchzuführen, bei denen die relevanten Parameter mit Hilfe verschiedener Messmethoden aufgezeichnet werden.
Sprengversuche sind ein wesentliches Werkzeug, um die Eignung von Schutzobjekten und Bauteilen zum Schutz der Bevölkerung vor Anschlägen mit explosionsgefährlichen Stoffen wie Explosivstoffen, sogenannten „home-made-explosives“ (HME) und unkonventionellen Spreng- und Brandvorrichtungen (USBV) zu bewerten. Für forensische Zwecke werden oft Explosionsszenarien nachgebildet, um die Auswirkung von Anschlägen oder Unfällen genau untersuchen zu können. Feldversuche in Kombination mit numerischen Simulationen ermöglichen eine Charakterisierung der Belastung und Schädigung von Bauteilen und Strukturen unter hochdynamischen Explosionsbelastungen.
Auf dem Testgelände Technische Sicherheit (TTS) der BAM in Horstwalde, Brandenburg werden Sprengversuche im Real-Maßstab durchgeführt. Dabei werden Bauteile mit Messtechnik ausgestattet, um eine dynamische Strukturcharakterisierung mittels sensorbasierten Monitorings zu ermöglichen.
In diesem Vortrag werden am Beispiel einer Versuchskampagne mit einer Stahlbetonwand als Prüfkörper die Eignung verschiedener Prüfmethoden bei Sprengversuchen evaluiert. Es werden Messmethoden zur Charakterisierung der Explosionsereignisse, des globalen Strukturverhaltens sowie der lokalen Schädigung vorgestellt.
Die direkte Charakterisierung der Explosionsbelastung erfolgt mit Hilfe von piezo-elektrischen Druckaufnehmern (bündig mit der Oberfläche im Prüfkörper montiert) und High-Speed-Kameras. Die Beobachtung der explosionsinduzierten Reaktion des Bauteils während und nach dem Eintreffen der Stoßwelle erfolgt mittels Beschleunigungssensoren, Stereo-Fotogrammetrie sowie mit in den Prüfkörper eingebetteter verteilter faseroptischer Sensoren, mit Hilfe derer die Dehnungsraten im Bauteil zeit- und ortsaufgelöst gemessen werden können. Die faseroptischen Sensoren erlauben außerdem die örtlich hochaufgelöste Bestimmung statischer Dehnungsänderungen im Bauteil nach einem Versuch, sodass auch nicht sichtbare Schädigungen wie kleinste Risse detektiert und charakterisiert werden können. Vergleichende Messungen mit Ultraschall komplettieren die Schadenscharakterisierung. In unserem Beitrag diskutieren wir die Herausforderungen dieser Messungen sowie die Ergebnisse und Möglichkeiten der genannten komplementären Messmethoden.
Pipe integrity is a central concern regarding technical safety, availability, and environmental compliance of industrial plants and pipelines. A condition monitoring system that detects and localizes threats in pipes prior to occurrence of actual structural failure, e.g., leakages, especially needs to target transient events such as impacts on the pipe wall or pressure waves travelling through the medium. In the present work, it is shown that fiber-optic distributed acoustic sensing (DAS) in conjunction with a suitable application geometry of the optical fiber sensor allows to track propagating acoustic waves in the pipeline wall on a fast time-scale. Therefore, short impacts on the pipe may be localized with high fidelity. Moreover, different acoustic modes are identified, and their respective group velocities are in good agreement with theoretical predications. In another set of experiments modeling realistic damage scenarios, we demonstrate that pressure waves following explosions of different gas mixtures in pipes can be observed. Velocities are verified by local piezoelectric pressure transducers. Due to the fully distributed nature of the fiber-optic sensing system, it is possible to record accelerated motions in detail. Therefore, in addition to detection and localization of threatening events for infrastructure monitoring, DAS may provide a powerful tool to study the development of gas explosions in pipes, e.g., investigation of deflagration-to-detonation-transitions (DDT).
The feasibility study „AGIFAMOR. Ageing infrastructures – distributed acoustic monitoring of pipes” is an interdisciplinary research project at BAM internally financed from 2015 to 2018. Therefore, the quite young fibre optic sensing technology of distributed acoustic sensing (DAS) was investigated to possibly be extended towards a global condition monitoring system for pipelines operating in real time.
DAS is a highly dynamic fibre optic sensing technology based on the method of coherent optical time domain reflectometry (C-OTDR). DAS allows capturing strain changes in the range of kHz. For the experimental work, the most suitable application yielding an optimum sensitivity was proven by wrapping a standard single-mode silica fibre around the pipe.
The DAS sensitivity was investigated regarding the detection of 1) incidents that initiate propagation of acoustic waves in the pipe wall, 2) changes inside the pipeline causing altered flow and 3) damage development in the pipe wall. Therefore, several testing setups in laboratory as well as in real scale were realized. For comparison purposes, experiments were accompanied by acoustic emission analyses and by measurements with accelerometers.
DAS was found to be very sensitive to gas ignition and its propagation across the pipe. Furthermore, the ability of DAS to detect and localize acoustic signals associated with pipeline leakage was demonstrated. The detection of crack formation and propagation within the pipe wall by means of DAS was studied during bending tests on several pipe segments, but was not proven so far with certainty. As expected, these studies turned out as the most difficult challenge due to the random occurrence and transient nature of microscopic damage phenomena.
Feasibility study: Continuous monitoring of pipes using distributed acoustic and fibre optic sensors
(2018)
The feasibility study „AGIFAMOR. Ageing infrastructures – distributed acoustic monitoring of pipes” is an interdisciplinary research project at BAM internally financed from 2015 to 2018. Therefore, the quite young fibre optic sensing technology of distributed acoustic sensing (DAS) was investigated to possibly be extended towards a global condition monitoring system for pipelines operating in real time.
DAS is a highly dynamic fibre optic sensing technology based on the method of coherent optical time domain reflectometry (C-OTDR). DAS allows capturing strain changes in the range of kHz. For the experimental work, the most suitable application yielding an optimum sensitivity was proven by wrapping a standard single-mode silica fibre around the pipe.
The DAS sensitivity was investigated regarding the detection of 1) incidents that initiate propagation of acoustic waves in the pipe wall, 2) changes inside the pipeline causing altered flow and 3) damage development in the pipe wall. Therefore, several testing setups in laboratory as well as in real scale were realized. For comparison purposes, experiments were accompanied by acoustic emission analyses and by measurements with accelerometers.
DAS was found to be very sensitive to gas ignition and its propagation across the pipe. Furthermore, the ability of DAS to detect and localize acoustic signals associated with pipeline leakage was demonstrated. The detection of crack formation and propagation within the pipe wall by means of DAS was studied during bending tests on several pipe segments, but was not proven so far with certainty. As expected, these studies turned out as the most difficult challenge due to the random occurrence and transient nature of microscopic damage phenomena.