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Fibre Optic Sensing @BAM
(2024)
In industrial applications, the effects of explosions must be considered as important part of safety assessments. This is particularly crucial in applications involving explosives or pressurized containers. The evaluation of the effects of shock waves on the environment becomes essential in such scenarios and requires comprehensive experimental test series.
This article therefore presents a test bench that can generate and record reproducible, adjustable shock waves with short set-up times. The presented data proves the quality and validity of this set-up to generate case relevant data, like overpressure in comparison to explosives and shock tubes, with load cases of high relevance.
The presented free-field tests are carried out at BAM's Test Site Technical Safety (TTS) with a gas-operated shock wave generator. This shock wave generator consists of a pressure vessel (autoclave) that is operated with a detonable gas mixture or compressed air. The pressure is released through an orifice by the instantaneous bursting of a diaphragm.
The aim is to develop further the novel test bench that generates shock waves on models in a generic scenario and offers several advantages to create a valid database for the effects of shock waves.
In contrast to classic shock tubes, this test bench offers the advantage of realistic hemispherical shock wave propagation and scalability, both for the investigated model size and for the generation of load cases, so that different model variables can be investigated under different boundary conditions (mainly overpressure); at the same time, typical interference of a shock tube experiment due to reflections, blockage effects and the following fluid flow can be minimized. These negative influences are not realistic compared to the ideal event of a detonation.
Protection against terrorist 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 in order to get a better insight into blast loading, structural response and the resulting damage to the structure. In 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. 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.
In the approval process of dangerous goods packagings, drop tests onto a flat, essentially unyielding surface are used to assess resistance against mechanical damage. International adopted regulations like ADR and RID define filling good dependent drop heights and filling degrees whilst the user needs to define the maximum gross mass to be tested and approved. Maximum packaging gross mass is defined conservatively and not reached in practice. To meet the defined gross mass in testing, using additives is permitted. However, in some cases, additives are not desirable due to packaging design or filling substance properties. This leads to deviations from the initial gross mass definition. Hence, a certain drop height adjustment is necessary to achieve the required impact loading. Laboratories frequently adjust drop height assuming a perfectly elastic collision which is inaccurate. Appropriate adjustment is not trivial due to energy conversion processes, e.g., plastic deformation. In this work, a test stand is developed for measuring the change in kinetic energy of different packaging designs and filling substances in regulative drop tests. The experimental results are used to validate finite-element (FE) models so that packaging properties can be varied in simulated drop test scenarios. The findings intend to describe the appropriate drop height adjustment of the respective packaging with same design but deviating gross mass to produce comparable mechanical response. The results are highly beneficial for testing laboratories, approval issuing authorities and packaging institutes confronted with the problem of testing packages with gross masses other than those to be approved.
Guaranteeing safety and security of citizens requires a significant effort and innovative tools from national and international agencies and governments, especially when it comes to the field of explosives detection. The need to detect Improvised Explosive Devices (IEDs) and Home-made Explosives (HMEs) at a point of suspicion, has grown rapidly due to the ease with which the precursors can be obtained and the reagents synthesised. The limited availability of immunoanalytical tools for HME detection presents an opportunity for the development of new devices, which enable a rapid detection and recognise the target analyte with high specificity and sensitivity. In this work, we introduce an optical biosensor for highly specific and sensitive HME detection. The immunoassay system is placed in a hydrogel environment permeable to the analyte and transparent to light interrogating the fluorescently labelled antibodies. The readout of the immunoanalytical system is realized with Supercritical Angle Fluorescence (SAF), an advanced microscopy technique. To accomplish this, we made use of recent, commercial high resolution (< 22 µm) Liquid Crystal Display 3D printers to fabricate a parabolic optical element with high refractive index (RI>1.5) and transmission values (>90%) from photo-resin. Aiming at a new generation of sensors, which not only can meet the requirements of trace detection, but can also be used for substance identification, the combination of immunoanalytical recognition with SAF detection offers a modularity and versatility that is principally well suitable for the measurements of target analytes at trace levels.
Distributed fiber optic sensing (DFOS) technique is a promising and robust non-destructive testing tool that can seamlessly acquire environmental conditions over large scales. Therefore, it has found extensive applications in structural health monitoring. Its appeal for monitoring underground facilities lies in the intrinsic properties of the optical fiber, such as immunity to magnetic interference, small size, chemical inertia, etc. This paper provides a concise overview of DFOS applications in underground facility monitoring. Following a brief introduction to the workingbnprinciple of the DFOS technique, various examples are provided to demonstrate how distributed fiber sensors contribute to monitoring underground facilities. The paper presents unpublished field test results with an emphasis in the energy sector, including monitoring gas storage facilities, geothermal reservoir exploration, and ground movement detection. Furthermore, the paper identifies several directions for enhancing the DFOS system.
Distributed fiber optic sensing (DFOS) technique is a promising and robust non-destructive testing tool that can seamlessly acquire environmental conditions over large scales. Therefore, it has found extensive applications in structural health monitoring. Its appeal for monitoring underground facilities lies in the intrinsic properties of the optical fiber, such as immunity to magnetic interference, small size, chemical inertia, etc. This paper provides a concise overview of DFOS applications in underground facility monitoring. Following a brief introduction to the working principle of the DFOS technique, various examples are provided to demonstrate how distributed fiber sensors contribute to monitoring underground facilities. The paper presents unpublished field test results with an emphasis in the energy sector, including monitoring gas storage facilities, geothermal reservoir exploration, and ground movement detection. Furthermore, the paper identifies several directions for enhancing the DFOS system.
Blast injuries are among the most common injuries in military operations. Also, in civilian environments, more explosive threats are expected in the future due to emerging conflicts and threats. While the effect of fragments, which is classified as secondary blast injury, could be minimized by police and military personnel’s modern ballistic body protection systems, the effects of shock wave propagation in the body as part of the primary explosion trauma still remain a serious threat needing further research.
The detonation-physical processes of highly dynamic pressure changes within the human body, the reflection-related amplification of shock waves at organ-dermis interfaces, and the consequences of injury mechanisms have become more prominent in international research. Various approaches have been used to investigate these aspects. Animal experiments on free field test sites or shock tube setups combined with a subsequent biological evaluation and numerical simulations provided promising results and allowed the discussion of different biomechanical aspects. However, due to poor reproducibility and a lack of short-term dynamic material properties, most research approaches have significant limitations. Laboratory test setups do not represent real-scale high explosive detonation parameters with regards to pressure characteristics, impulse duration and blockage problems. Measured values are interpreted with partly outdated, selective and not validated limit values for overpressures from field tests with animals. This is due to the lack of a validated and comprehensive data set covering a variation of the crucial parameter. Injury mechanisms and their effects have not yet been sufficiently elucidated for the torso and extremities.
In order to contribute to the research efforts on the primary explosion effects, the German Federal Armed Forces established an interdisciplinary military medical research project in cooperation of the Bundeswehr Hospital Berlin and the German Federal Institute for Materials Research and Testing (BAM). The aim of the planned investigation is the development of a multidisciplinary method to investigate shock wave behavior in various generic tissue simulants under the most reproducible conditions possible with realistic loads in an experimental test series with short set-up times.
Following directly from the information presented at IFBIC 2023 on the research project 'Investigation of intracorporeal shock wave propagation using a simplified torso model and a shock wave generator', the latest results are presented below [1]. In order to contribute to the research efforts on the primary explosion effects, the aim of the investigation is the development of a multidisciplinary method to investigate shock wave behavior in various generic tissue simulants under the most reproducible conditions possible with realistic loads in an experimental series with short set-up times.
To achieve this, an autoclave with a volume of 0.065 m3 is used to generate reproducible pressure waves (Fig. 1). The autoclave will be referred to as Shockwave Generator (SWG). The pressure wave is produced by the detonation of a stoichiometric mixture of acetylene and oxygen under atmospheric conditions. The SWG-outlet is sealed by a rupture disc, which can be adjusted in thickness to vary the initial pressure. Additionally, modifying the composition of the acetylene-oxygen mixture can result in different load cases. The SWG requires approximately 30 minutes of set-up time between trials. The SWG was adjusted to match the characteristics of a real and typical explosive through free-field measurements. The experiments demonstrated that the pressure wave propagates in a hemispherical shape and has sufficient reproducibility.
To represent soft tissue, a basic model made of homogeneous ballistic gelatin is used in a geometrically simplified torso model (STM) (Fig. 2). The STM was enlarged to outer dimensions of 400 x 250 x 240 mm. Viscoelastic behavior of the ballistic gelatin can be assumed due to the collagen structure [2]. The density of the ballistic gelatin, and therefore the sound velocity, can be adjusted by changing the mixing ratio. Simplified simulants, such as hollow and solid material (bone simulant), were embedded in the STM. Additionally, the STM has been tested as a carrier material for biological substances, such as indicator species. The next step is to test the STM as a carrier material for larger organic tissue structures such as lungs or kidneys. The organic tissue samples will undergo histological examination to analyze their structural changes afterwards. The STM is equipped with embedded piezoelectric pressure sensors, an accelerometer and a temperature sensor. Visual documentation is captured using a high-speed camera. This measurement setup allows for the tracking of the coupled pressure wave and its behavior within the model and at the media transitions. Pressure values can be recorded after certain media changes, such as from soft tissue to a solid material (Figure 3).
The following is a review of the optimizations made to the entire experimental setup. The reproducibility of the SWG was increased. This was achieved by extending the evacuation process to the entire feed system of the SWG. Nitrogen residues in the SWG system can thus be avoided. As a result, the double peaks that occur at the first maximum can be avoided, as combustion runs more evenly without the nitrogen residues. In addition, partially varying peak overpressures can be avoided, as the acytelene-oxygen mixture is not too lean due to the elimination of the nitrogen residues. The workflow has been streamlined, resulting in an average set-up time of 25 minutes for the SWG. In the case of the STM, the manufacturing process in particular was optimized by examining and defining the manufacturing process based on scientific publications, which increases the reproducibility of the STM base bodies. As already mentioned in the previous paragraph, the STM was equipped with various additives in order to investigate the transition behavior of the pressure wave.
The following trail serves as an example: A comparison is made between the STM in the basic structure (Fig. 2 l.s.) and with an embedded solid material (bone plate) (Fig. 3 l.s.). An external overpressure of 100 kPa is generated by the SWG at a distance of 1 m, where the STM is placed. For the basic STM structure, the sensor distance is consistently 100 mm. The internal overpressure is visible in Fig. 2 r.s.. A typical ideal pressure curve can be seen. The characteristic points are easily distinguishable from the reflection peaks. In the STM with an embedded solid material, the sensor distance remains consistently at 66.6 mm
Im Rahmen des Vortrags wurden die Ergebnisse und die Methodik zur Erhebung von Forschungsbedarfen und Fähigkeitslücken bei den Behörden und Organisationen mit Sicherheitsaufgaben, welche im Rahmen des Projektes InnoBOSK durchgeführt wurden, vorgestellt. Den Schwerpunkt bildete dabei die Umsetzung der Workshops gemeinsam mit den Einsatzkräften und die Umsetzung der problemorientierten Erhebungsmethodik.