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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.
Conventional approval requirements exclusively ask for minimum strength values, which have to be met. The probabilistic approach estimates how likely none of the comparatively manufactured units fails during operation.
Both questions are juxtaposed and compared here with respect to the load cycle tests. The influence of the sample sizes is discussed additionally.
Hydrogen is an attractive energy carrier that requires high effort for safe storage. For ensuring safety they have to undergo a challenging approval process. Relevant standards and regulations for composite cylinders used for the transport of for on-board storage of hydrogen are currently based on deterministic (e.g. ISO 11119-3) or semi-probabilistic (UN GTR No. 13) criteria. This paper analysis the properties of such methods in regards to the evaluation of load cycle strength. Their characteristics are compared with the probabilistic approach of the BAM. Based on Monte-Carlo simulations, the available design range (mean value and scatter of strength criteria) of current concepts were exemplarily estimated. The aspect of small sample sizes is analysed and discussed with respect to the evaluation procedures.
Composite materials show a degradation of properties depending on service life. This creates the necessity to find tailored methods to determine strength and residual strength of composite cylinders. The determination can be done e.g. by load cycles tests. The result needs a statistical assessment for the precise description of strength. Especially the statistical assessment of load cycle strength properties has a high uncertainty. It is unclear if a Log-Normal distribution, a WEIBULL distribution or others, describe the scatter behaviour of residual strength properties correctly.
Distribution functions aim at approximating the frequency of occurrence of residual load cycle strength for high survival rates. An assumption has to be found and confirmed to prevent over-estimation of reliability.
Gas cylinders made of composite materials receive growing popularity in light-weight applications. Current standards are mostly based on safety determination relying on minimum amounts of endured load cycles and a minimum burst pressure of a small number of specimens. This paper investigates the possibilities of a probabilistic strength assessment for safety improvements as well as cost and weight savings. The probabilistic assessment is based on destructive testing of small sized samples. The influence of sample size on uncertainty of the assessment is analysed. Furthermore, methods for the assessment of in-service ageing (degradation) are discussed and displayed in performance charts.
Existing standards for the approval of composite cylinders for the transport of compressed hydrogen are currently based on deterministic (ISO 11119-3) and semi-probabilistic (UN GTR No. 13) criteria. This paper analysis the capabilities of these procedures with respect to the evaluation of burst strength. Their characteristics are compared with the probabilistic approach developed at BAM. Based on Monte-Carlo simulations the available design range (mean value and scatter of burst strength) of all concepts are assessed. In addition, the probability of acceptance for potentially unsafe design types is determined.
The results show large areas of burst properties with a sufficient reliability which cannot be used for the design of composite cylinders, due to current approval criteria. On the other hand, existing standards allow the approval of potentially unsafe designs in case of a very high scatter regarding their burst strength.
It is also shown that existing design types are already designed to the limits of the available design area. A further weight and cost reduction of composite cylinders is therefore closely related to the approval criteria. An example based on UN GTR No. 13 shows how an approval criterion can be optimized by using statistical methods. The example shows that a reduced minimum burst pressure can be combined with a lower probability of acceptance for potentially unsafe design types.
Existing regulations and standards for the approval of composite cylinders in hydrogen service are currently based on deterministic criteria (ISO 11119-3, UN GTR No. 13). This paper provides a systematic analysis of the load cycle properties resulting from these regulations and standards. Their characteristics are compared with the probabilistic approach of the BAM. Based on Monte-Carlo simulations the available design range of all concepts is compared. In addition, the probability of acceptance for potentially unsafe design types is determined.
Existing regulations and standards for the approval of composite cylinders in hydrogen service are currently based on deterministic criteria (ISO 11119-3, UN GTR No. 13). This paper provides a systematic analysis of the load cycle properties resulting from these regulations and standards. Their characteristics are compared with the probabilistic approach of the BAM. Based on Monte-Carlo simulations the available design range of all concepts is compared. In addition, the probability of acceptance for potentially unsafe design types is determined.
Composite materials show a degradation of properties depending on Service life. This creates the necessity to find tailored methods to determine strength and residual strength of composite cylinders. The determination can be done e.g. by load cycles tests. The result needs a Statistical assessment for the precise description of strength. Especially the Statistical assessment of load cycle strength properties has a high uncertainty. It is unclear if a Log-Normal distribution, a WEIBULL distribution or others, describe the scatter behaviour of residual strength properties correctly. Distribution functions aim at approximating the frequency of occurrence of residual load cycle strength for high survival rates. An assumption has to be found and confirmed to prevent overestimation of reliability.
The increasing amount of composite transport systems for hydrogen leads to new and therefore unknown potential hazards for general public. Due to lack of experience, risks of new technologies tend to be rated higher than existing familiar applications. An approach for probabilistic safety assessment of technologies or products requires the definition of minimal acceptable reliability levels. This ensures that the probability of a critical failure with a certain consequence is limited to an acceptable risk. But what is the acceptable risk and which risk results from specific probabilities of occurrence and consequences? This is always a very complex question. The following example is based on a probabilistic approach for safety assessment of composite cylinders developed at the BAM (Federal Institute for Materials Research and Testing).