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- 8.6 Faseroptische Sensorik (2)
Eingeladener Vortrag
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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. 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 Vorhaben „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.
Compressed natural gas (CNG) is a widely used automotive fuel in a variety of countries. In case of a vehicle fire where the safety device also malfunctions, a failure of the CNG automotive cylinder could occur. Such a cylinder failure is associated with severe hazards for the surrounding environment. Firstly, a comprehensive analysis is given below, summarizing various accidents involving CNG automotive cylinders and their consequences. In an extensive experimental program, 21 CNG automotive cylinders with no safety device were tested. Of the 21, burst tests were carried out on 5 Type III and 5 Type IV cylinders. Furthermore, fire tests with 8 Type III and 3 Type IV cylinders were conducted. Apart from cylinder pressure, inner temperature and cylinder mantle temperature, the periphery consequences, such as nearfield blast pressure and fragmentation are documented. The maximum measured overpressure due to a Type III cylinder failure was p = 0.41 bar. Each traceable fragment was georeferenced. All-in-all, fragment throw distances of d > 300 m could be observed. As one key result, it can be stated that the tested Type IV CNG cylinders showed less critical failure behavior then the Type III cylinders under fire impingement.
In case of a vehicle fire, an installed LPG (liquefied petroleum gas) tank with a malfunctioning safety device poses severe hazards. To investigate the consequences in case of tank failure, we conducted 16 tests with toroidal shaped LPG vehicle tanks. Three tanks were used for a Hydraulic Burst Test under standard conditions. Another three tanks were equipped with a statutory safety device and were subjected to a gasoline pool fire. The safety device prevented tank failure, as intended. To generate a statistically valid dataset on tank failure, ten tanks without safety devices were exposed to a gasoline pool fire. Five tanks were filled to a level of 20 %; the re-maining five were filled to a level of 100 %. In order to gain information on the heating process, three tem-perature readings at the tank surface, and three nearby flame temperatures were recorded. At distances of l = (7; 9; 11) m to the tank, the overpressure of the shock wave induced by the tank failure and the unsteady tem-peratures were measured. All ten tanks failed within a time of t < 5 min in a BLEVE (boiling liquid expanding vapor explosion). Seven of these resulted directly in a catastrophic failure. The other three resulted in partial failure followed by catastrophic failure. A near field overpressure at a distance of l = 7 m of up to p = 0.27 bar was measured. All ten tests showed massive fragmentation of the tank mantle. In total, 50 fragments were found. These 50 fragments make-up 88.6 % of the original tank mass. Each fragment was georeferenced and weighed. Fragment throwing distances of l > 250 m occurred. For the tanks with a fill level of 20 %, the average number of fragments was twice as high as it was for the tanks that were filled completely.
Fibre-reinforced plastics (FRP) especially carbon-fibre-reinforced polymer (CFRP) and glass-fibre-reinforced polymer (GFRP) are commonly used materials in high pressure vessels and storage units for automotive and aerospace purposes. Optical fibres are suitable to be integrated or directly applied to the surface of FRP components. Using optical fibres it is possible to monitor the distributed strain profiles and changes within the fatigue life of a pressure vessel to ensure the operational safety. Within artificial ageing experiments we used swept wavelength interferometry (SWI) based distributed strain sensing for the monitoring of commercial high-pressure composite cylinder. This artificial ageing was performed using test conditions of 503bar pressure load (service pressure 300 bar) and 89 °C for 100 h. The polyimide coated optical fibres were glued to the surface externally in circumferential and axial direction. Using distributed strain sensing (DSS) material expansion of over 0.5% were monitored with sub-centimetre spatial resolution. Within the circumferential direction we observed up to 10 % local fluctuation compared to the median strain caused by inhomogeneous material expansion, which could cause local material fatigue. In addition, we determined material degradation manifested itself as localized remaining material expansion and/or contraction. Results have been validated by other non-destructive methods like digital strip projection.
Within a current research project at the Federal Institute for Materials Testing and Research (BAM), the degradation process of composite pressure vessels is studied to be able to give more accurate lifetime predictions in future. The presented research is based on type III pressure vessels consisting of an aluminium tank which is fully wrapped with carbon fibre reinforced plastics. Focus is set on the analysis of residual stresses which are induced into the pressure vessel during manufacturing process in order to increase high cycle fatigue. However, with increasing lifetime residual stress conditions do change. To be able to measure and monitor stress conditions, the application of a non-destructive measurement method is aspired.
In this paper, potential of an experimental modal analysis is worked out to capture and monitor aging and degradation effects in pressure vessels. With the presented method, information about changes in residual stress can be obtained via an analysis of the modal parameters. To realize an application, first, a finite element simulation is used to prove and evaluate potential capability and validity. In the following, a test bench is set up and successively optimized in its accuracy and efficiency. Sensitivity of the applied measurement technique is experimentally ascertained trough the measurement of several prestress modified pressure vessels. Finally, experimental results are interpreted and evaluated with the help of numerically gained findings.