8.6 Faseroptische Sensorik
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- Acceleration sensors (2)
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- DAS (2)
- Digital image correlation (2)
- Distributed acoustic sensing (2)
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- Embedded sensors (2)
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Organisationseinheit der BAM
- 7.4 Baustofftechnologie (14) (entfernen)
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.
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.
Acoustic emission testing
(2018)
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.
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.
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.
Bending beams and slabs are typical examples for structural elements used for reinforced concrete structures such as bridge girders, T-beams and bridge decks. Their strength related failure modes at maximum loading can be divided into bending and shear failure. The failure of beams loaded in bending can occur with or without indication. Therefore, conventional design concepts aim on failure modes with sufficient indication (e.g. large deflections or cracks), as it occurs in the case of secondary flexural compression failure. These indicating factors can also be used for Structural Health Monitoring (SHM) of civil infrastructure systems (e.g. bridges) to identify structural changes. In this context, non-destructive testing (NDT) methods offer different techniques for measuring deflections or crack formation and opening. However, profound knowledge on the determining failure modes of bending beams and their detection by NDT methods is required for the reliable application of SHM. Different NDT methods have been used in this study for analysing the load-bearing behaviour of a reinforced concrete beam in bending. The different measuring techniques are briefly described and their applicability is discussed by means of experimental results. For this purpose, the load-bearing behaviour of a reinforced concrete beam having a span of 2.75 m was investigated in a four-point bending flexural test at laboratory scale. The focus is on the characterization of determining failure modes by optical NDT and the comparison with classical measuring techniques (e.g. deformation measurements by displacement transducers). The bending beam was equipped with two single-mode (SM) sensor fibres. One fibre served as Distributed Optical Fibre Sensor (DOFS), whereas the other fibre contained Fibre Bragg Grating (FBG) sensors. In addition, optical deformation measurements using Digital Image Correlation (DIC) and Stereophotogrammetry (SP) were conducted.
Grouted connections are intensively used in offshore rigs, platforms as well as jacket and monopile offshore wind turbine structures. Being located in remote offshore conditions, these connections can experience considerable adverse loading during their lifetimes. Degradation was reported inside similar connections, which were installed in the last three decades. Grouting in the offshore sites may often be proven difficult, which eventually leads to reduced load-bearing capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs. In this study, scaled grouted connections were manufactured using a novel mould, whose integrity were monitored using digital image correlation (DIC). The connections were loaded under static load to visualize the main failure pattern using distributed fibre optic sensors and acoustic emission (AE) analysis. Grouted connections were then repaired using a cementitious injectable grout. The effectiveness of the grout injection was monitored using dye penetration technique. Finally, specimens are reloaded to identify the potential of such repair for grouted connections.