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Organisationseinheit der BAM
Eingeladener Vortrag
- nein (5)
Fiber optic sensors have gained increasing importance in recent years and are well established in many areas of industrial applications. In this paper, we introduce a concept of a self-diagnostic fiber optic sensor. The presented sensor is to resolve the problems of embedded fiber optic sensors in complex structures and to enable the validation under operational conditions. For this purpose, different magnetostrictive coated fiber optic sensors were developed and various experiments were performed to verify their mode of Operation and to determine the respective reproducibility. The measuring principle is illustrated by obtained experimental results, which showed a change in wavelength from 1 pm at a magnetic field strength change of 0.25 mT.
In addition, the temperature characteristics of the implemented magnetostrictive sensor were analyzed and an experimental factor of 1.5 compared to a reference fiber optic sensor was determined.
A high-performance fiber Bragg grating-based (FBG) sensor device for the detection of small magnetic fields has been developed. Based on a smart multilayer coating exposed over the physical length of the FBG, magnetic fields exhibited by rotating machine parts, power generators or current cable can easily be detected, analysed and evaluated. Consequently, this innovative, in-process and non-contact inspection method leads to an increase in quality and reliability of high-performing machine parts, devices and cables. The basic physical concept is based on a magnetostrictive multilayer coating system that strains the high-resolution FBG element. Subsequently, a fixed relationship between induced magnetic field and wavelength change of the FBG element forms the characteristic sensitivity curve. Intensive tests regarding the characterisation of the magnetic field FBG sensor have been carried out and its performance has been evaluated.
Air-coupled ultrasound has been applied increasingly as a non-destructive testing method for lightweight construction in recent years. It is particularly appropriate for composite materials being used in automotive and aviation industry. Air-coupled ultrasound transducers mostly consist of piezoelectric materials and matching layers. However, their fabrication is challenging and their signal-to-noise ratio often not sufficient for many testing requirements. To enhance the efficiency, air-coupled ultrasound transducers made of cellular polypropylene have been developed. Because of its small density and sound velocity, this piezoelectric ferroelectret matches the small acoustic impedance of air much better than matching layers applied in conventional transducers. In our contribution, we present two different methods of spherical focusing of ferroelectret transducers for the further enhancement of their performance in NDT applications. Measurements on carbon-fiber-reinforced polymer (CFRP) samples and on metal adhesive joints performed with commercially available focused air-coupled ultrasound transducers are compared to measurements executed with self-developed focused ferroelectret transducers.
Ultrasonic testing of adhesively bonded joints using air-coupled cellular polypropylene transducers
(2014)
In air-coupled ultrasonic testing, the impedance mismatch between the transducer and the air is commonly being solved by adding matching layers to composite transducers. To avoid the difficult technological procedure regarding matching layers, some new piezoelectric materials have been proposed. Most promising are ferroelectrets, which are charged cellular polymers, having ferroelectric and consequently piezoelectric properties. In particular, the extreme softness of cellular polypropylene (cPP) leads to a high piezoelectric constant and to a good impedance match with the air, making matching layers redundant. Its elasticity modulus below 1 MPa causes an additional effect not observed with common piezoelectric materials: that is the electrostrictive effect, here defined as the thickness change due to the attractive force between the transducer electrodes. This effect exceeds the piezoelectric effect at excitation voltages over 1 kV. The extreme softness of cPP leads also to high flexibility, enabling easy focusing by bending the transducer.
We have developed air-coupled ultrasonic transducers based on cPP. This includes the electrical matching networks for the transmitter and for the receiver. The transmitter is excited with voltages up to 2.5 kV, so that the electrostrictive effect dominates, leading to sound pressure around 145dB at the transducer surface. These transducers have been applied for testing carbon-fiber-reinforced polymer plates, adhesive joints and other composite structures. Here we report about ultrasonic transmission of two types of adhesive joints. The first one is multi-layer aluminium components with some artificial disbonds, which are common in aerospace industry, and the second one is an aluminium-steel joint with polyurethane adhesive, which is used in automotive industry.
The absolute detection of a gap distance of loose and ftxed extrinsic Fabry-Perot interferometer (EFPI) sensors with conventional measurement Systems is not possible. Due to this EFPI-sensors were deemed to be unfit for distance determination in comparision with FBG sensors. With the conventional measurement method a continous measurement of the EFPIsensors was necessary. Otherwise data about the distance change was lost. Additional it is necessary to detect the tumaround points in the interference signal with 100%. This made it worse for the analysis and the use of the sensor. At BAM Federal Institute for Materials Research and Testing a measurement System called “EFPI-Scan” is developed which can measure the absolute gap distance of four connected EFPI-sensors synchronous. Every measurement channel is temperature independetn and long term stable. Test measurements showed a deviation of maximum ±1 gm per hour. Furthermore it is possible to measure with different measurement modes (abolute, relative and a combination of both). Special emphasis should be placed to the absolute measurement accuracy of ± 5 pm at a gap distance determination ftom 40 up to 4000 pm.
Zur Untersuchung der Sicherheit von Behältern für den Transport und die Lagerung von Gefahrstoffen und -gütern werden Fallprüfungen durchgeführt. Die Aufpralldynamik und die strukturmechanischen Einwirkungen auf den Behälter werden mittels Beschleunigungsaufnehmern und Dehnungsmessstreifen erfasst. Dabei kommen derzeit Vielkanalmesssysteme zum Einsatz, die eine Verkabelung jeder einzelnen Messstelle und somit das Mitfallen eines Kabelbaums erfordern, wodurch Probleme bei der Versuchsvorbereitung und Durchführung entstehen. Die Verwendung eines Messsystems mit Datenbus und dezentraler Datenspeicherung bietet diesbezüglich einen vielversprechenden Lösungsansatz.
A high-performance fiber Bragg grating-based (FBG) sensor device has been developed for the detection of small magnetic fields. Based on a smart multilayer jacket around the fibre over the physical length of the FBG, magnetic fields generated by rotating machine parts, power generators or power cable can be easily detected, analysed and evaluated. Consequently, this innovative, on-line and non-contact inspection method results in an increase in quality and reliability of high-performing machine parts, devices and cables. The basic physical principle is based on a magnetostrictive multilayer system that strains the high-resolution FBG element in presence of magnetic fields. Subsequently, a fixed relationship between induced magnetic field and wavelength change of the FBG element describes the characteristic sensitivity curve. Intensive tests regarding characterisation of this magnetic field FBG sensor have been carried out and its performance has been evaluated.
Sensors integrated into devices and structures provide essential data to control, optimize or manage machines and structural components. In the last years, processes, applications and machine parts became more and more intelligent. Consequently, the need for sensor validation increased significantly in order to rely on sensor data and measurement results.
Structure-integrated fibre optic strain sensors, such as fiber Bragg gratings (FBG), are of special interest in the composite manufacturing industry. This type of sensor makes it possible to gain information and to collect measurement data about entire production processes and whole life-time cycles of composite-made machine parts and structures. However, validation concepts and approaches for this type of sensors are barely reported in literature. Furthermore, all the reported activities have the drawback that the sensor diagnosis strategy is implemented as limited mathematical models and/or complex time-consuming spectral analysis tools which are applied in data post-processing loops. However, to automate sensor validation and to apply real-time and in-field sensor fault detection, it is necessary to acquire measurement data and information about the measurement reliability at the same time.
Die Langzeitüberwachung von Bauteilen (Structural Health Monitoring, SHM) ging in den letzten Jahren zunehmend von verkabelten Sensoren zu kabellosen Sensornetzwerken (WSN) über. Frühwarnsysteme sollen rechtzeitige Signale senden, wenn Schädigungsprozesse einsetzen, wenn ein Bauteil z. B. zu feucht wird (Deiche, Staumauern), oder wenn Feuchteprozesse in Bauteilen auftreten, die den Transport von schädlichen Stoffen ermöglichen. Mittels handelsüblicher multifunktionaler Sensoren wie einem Bluetooth Low Energy (BLE) wurden erste Machbarkeitsuntersuchungen zur Massenfeuchtemessung durchgeführt. Die untersuchten Parameter sind der sogenannte Received Signal Strength Indicator (RSSI) eines Bluetooth-Low-Energy(BLE)-Signals und die abgestrahlte Sendeleistung eines eingebetteten BLE-Moduls. Das BLE-Modul kommuniziert mit einem mobilen smarten Empfänger wie einem Tablet oder einem Smartphone als SmartHub. Die Veränderung des RSS-Indikators ist ein Maß für die Veränderung der Feuchte im Bauteil, während die ausgesandte Signalstärke konstant bleibt. Wenn das BLE-Signal einen feuchten Bereich im Baumaterial durchläuft, verändert sich der RSS-Indikator. Nach erfolgreicher Machbarkeitsstudie wurde nun ein aus dem Netzwerk MI4G (Modern Industry for Germany) zwischen der Bundesanstalt für Materialprüfung und dem Klein- und mittelständigen Unternehmen LinTech GmbH heraus beantragtes ZIM-Projekt bewilligt. Der Bericht zeigt erste Ergebnisse von Voruntersuchungen und gibt einen Ausblick auf weitere Forschungsaktivitäten.
Structural health monitoring contributes to early damage detection in the built infrastructure. During the last two decades, the sensor networks transferred from wired to wireless sensor networks.
Several methods exist to measure moisture in building materials. Most of the introduced commercial humidity measurement methods as provide information about the local or near surface moisture. A feasibility study is presented to demonstrate, how the Received Signal Strength Indicator (RSSI) of a BlueTooth® Low Energy (BLE) signal, transmitted from the BLE-module embedded in building materials with changing moisture content. The BLE-module communicates with a mobile Smart Device as tablet or mobile phone. The RSSI indicates to what extend the received signal strength is changed due to moisture, while the transmitted signal strength remains constant.
The stress vs. strain curve of materials is affected the rate of imposed straining. Among the methods for dynamic testing the technique known as 'split Hopkinson pressure bar' (SHPB) has evolved into the most widely used one to exert high-speed straining. The theory behind it comprises simple equations to compute stress and strain. The reliability of the strain analysis can be assessed by digital image correlation (DIC). The present results indicate that the visually observed strain is smaller than predicted by theory.
In diesem Paper wird ein absolut messendes faseroptisches Messsystem zur Bestimmung des Gap-Abstandes von niedrig reflektierenden losen und festen extrinsischer Fabry-Pérot Interferometer (EFPI) vorgestellt. Die häufigste Methode zur Bestimmung von Dehnungen oder Bewegungen durch EFPI-Sensoren ist eine kontinuierliche relative Ermittlung einer Veränderung mit einer nachträglichen Auswertung. Unterbrechungen in der Datenerfassung oder Fehler bei der Interferenzstreifen-Auswertung führen zu einer nicht abschätzbaren Messunsicherheit. Damit war es bis dato nicht möglich, eine absolute Veränderung von eingesetzten EFPI-Sensoren zu bestimmen. Mit dem an der BAM entwickelten Verfahren, ist dies jetzt möglich, die absolute Veränderung zu bestimmen. Dieses System namens 'EFPI-Scan' wird in dem Papier dargestellt werden.