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Eingeladener Vortrag
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Emergency response units increasingly use pressure cylinders made from fibre composites for breathing air as elements of personal protective equipment (PPE). Such applications expose the composite cylinders to harsh temperature and handling conditions. At least in Europe, standards have been used for certifying PPE, which are designed and mandatory for the approval of pressure receptacles for the transport of dangerous goods. Therefore, service conditions specific to PPEs are not accounted for in these standards. In this paper, BAM (Federal Institute for Materials Research and Testing) investigates the residual safety of a composite cylinder design at the end of their designated service life of 15 years. The cylinders (test pressure PH 450 bars; 6.8 Litres) are of one design type with aluminium liner and fully wrapped with carbon fibres, which is commonly considered a “Type III” cylinder. All cylinders were used as PPE by the Berlin fire department and randomly picked in three samples of 25 cylinders each before tested.
At BAM, the cylinder samples underwent hydraulic load cycle tests (LCT), conventional burst tests (BT) and so called slow burst tests (SBT). A concept for quantification of strength degradation already introduced by BAM was applied. This concept is based on a probabilistic assessment of the average strength and scatter of each sample of cylinders. The strength distributions of the used PPEcylinders is shown and analysed. Some unexpected effects are shown and a refinement of the statistical assessment is introduced.
Gas cylinders made from composite materials receive growing popularity in applications where light weight is of advantage. At the same time manufacturers are interested in cutting cost and weight through material reduction for better acceptance of a product, e.g. H2-vehicles. This requires a better understanding of safety relevant properties.
The safety determination of current standards is mainly based on the minimum burst pressure of a few specimens. Various research projects were conducted aiming at reducing minimum burst pressure requirements without compromising safety. No satisfying results were found. While looking at reliability aspects it could be concluded, that the minimum burst pressure of unused specimens is not a satisfying criterion for safety during service life.
This paper introduces first ideas for a method to determine one aspect of reliability of composite gas cylinders, employing probabilistic analysis of burst pressures of a sample of specimens. This can create potential for saving material cost while granting a higher safety level than the current method. Additionally, degradation over service time can be assessed.
ISO standards for gas cylinders made from aluminum alloys generally require heat treatment after the forming process. Such heat treatment is a costly part of the manufacturing process. Particularly due to increasing international trade, market-surveillance authorities need simple and reliable methods to field-test cylinders for proper heat treating. By the present contribution the difference in strength of heat-treated and non-heat-treated gas cylinders is exemplarily shown and several material analysis methods are investigated for their suitability as field tests.
Faserverbundwerkstoffe (Komposite) zeigen außergewöhnliche Veränderungen ihrer Eigenschaften in Abhängigkeit von der Betriebsdauer. Daraus leitet sich auch die Notwendigkeit ab, eine Methode zu finden, die in der Lage ist, Festigkeiten und Restfestigkeiten von Komposit-Druckbehältern zu ermitteln. Dies kann mittels Lastwechselprüfung oder in anderen Fällen vorzugsweise mittels der langsamen Berstprüfung erfolgen. Für eine aussagekräftige Beschreibung der Festigkeit bedürfen die Ergebnisse beider Prüfverfahren einer statistischen Auswertung. Aber insbesondere die statistische Auswertung der berstbezogenen Festigkeitseigenschaften unterliegt neben der Frage der Stichprobengröße einer großen Unsicherheit mit enormen Folgen. Dies ist die Frage, welche Verteilungsfunktion der Streuung der Festigkeitseigenschaft konservativ zugrundegelegt werden darf. So ist zu untersuchen, ob die Gaußsche Normalverteilung (NV) die wahre Verteilung richtig beschreibt. Es könnten aber auch andere Funktionen, wie z. B. die Weibull-Verteilung notwendig sein, um eine Überschätzung der Überlebenswahrscheinlichkeit bzw. Zuverlässigkeit in dem interessanten Bereich von einem Ausfall pro 104 bis 108 Druckbehältern zu vermeiden.
There is evidence that composite materials degrade under static loads. Consequently there is no way to design composite gas cylinders for unlimited service life. Yet this fact is not reflected by design standards for composite receptacles approved for the transport of dangerous goods or for stationary application. Unnecessary cost and weight result from this.
Due to the degradation aspects of composites it is necessary to provide appropriate retesting procedures, which allow rejection of used vessels as is common practice for metallic receptacles. The current procedure of hydraulic tests appears to be an expensive procedure with serious doubts regarding its usefulness for composite cylinders.
Consequently, an efficient method is needed, which allows limiting the life time to its safe maximum. It is thus recommended to combine retesting with the assessment of residual life time. Since relevant non-destructive methods are not yet sufficiently matured and standardized enough, a concept featuring destructive tests on composite cylinders parallel to operation should be considered. An alternative concept introduced here, proposes to substitute the hydraulic testing of 100% of a population by a probabilistic evaluation of destructive tests on samples parallel to operation. Intensified pre-fill inspections should be an additional component of such a concept.
Schrankenlos im Seeverkehr
(2014)
Aufgrund der wachsenden Internationalisierung des Flaschenparks stieg das Volumen im internationalen Gefahrguttransport in den letzten Jahren stark an. Gleichzeitig traten besonders im Mittelmeerraum Unwägbarkeiten im Landtransport durch politische Krisen auf. Das auf See gestiegene Transportvolumen und die verstärkte Beachtung der Rechtslage führte auch zu Fällen, bei denen deutlich wurde, dass ohne IMDG-Code-Zulassung kein Beförderungsanspruch ableitbar ist. Die zuständige Behörde - in Deutschland die BAM - bietet Lösungen mit erweitertem Nutzen an. Hier kann für Bestandsflaschen mit Landzulassung eine Verwendungserweiterung für den Seeverkehr beantragt werden.
Slow burst testing of samples as a method for quantification of composite cylinder degradation
(2014)
The current practise to focus periodic retesting of composite cylinders primarily on the hydraulic pressure test has to be evaluated as critical. The test itself always causes a certain amount of micro damage to the cylinders but does not necessarily deliver sufficient and evaluable information. Thus BAM Federal Institute of Materials Research and Testing (Germany) moves the focal point to a new approach for validation of composite cylinders, based on destructive sample tests parallel to operation. Statistical assessment of results of these destructive tests is employed for the estimation of remaining safe service life, based on reliability demands. The estimated service life is also used for the determination of re-test periods of the examined population of composite cylinders.
An essential aspect of this approach is the validation of current residual strength and its prediction at any point of service life. In cases of gas cylinders with very high cycle strength, residual strength cannot be quantified statistically by means of hydraulic load cycles. As a replacement, creep tests or burst tests may be employed. BAM suggests the 'slow burst test SBT' as a combination of these two test procedures. This is a compromise between the practicability of the (conventional) burst test and the practical relevance of sustained loads during service, to be tested in creep rupture tests. In this paper, a variety of 99 burst results of a cylinder design type used for breathing apparatus (CFRP with PE-liner) is evaluated. The influence of test procedure parameters and nature and intensity of artificial ageing on the test sample strength are analysed statistically. This leads to an evaluation of different procedures of artificial ageing and the recommendation to substitute conventional burst tests by slow burst tests for the assessment of composite pressure receptacles.
Composite-Druckgefäße kommen auch in Atemschutzgeräten als persönliche Schutzausrüstung (PSA) zunehmend zum Einsatz und sind dabei außergewöhnlich rauen Betriebsbedingungen ausgesetzt. Da diese durch die angewendeten Normen nominell nicht umfassend abgedeckt sind, wurden die End-of-Life-Restfestigkeiten untersucht. Die untersuchten drei Stichproben bestehen aus je 25 im Feuerwehrbetrieb über 15 Jahre gealterten Composite-Atemluftflaschen eines Baumusters. Mit diesen Restfestigkeitsuntersuchungen wird am Beispiel den Fragen nachgegangen, ob dieses Baumuster statistisch eine ausreichende Sicherheit am Lebensende aufweist und welche Verteilungsfunktionen die Ergebnisse am besten beschreiben. Methodisch kommen für die Prüfung eine konventionelle Berstprüfung, das Konzept der BAM für eine langsamen Berstprüfung und die hydraulische Lastwechselprüfung zur Anwendung. Das Ergebnis zeigt für das Baumuster hinreichende Sicherheit; aber auch die besondere Bedeutung der sog. Frühausfälle für die Statistik und den Bedarf an einer entsprechend auf die Frühausfälle ausgerichteten, zerstörungsfreien Prüfmethode.