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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.
Viele Konzepte zur Sicherheitsbewertungen beruhen auf der Untersuchung weniger, oder einzelner Prüfergebnisse. Aber auch im Fall umfangreicherer statistischer Untersuchungen können nur begrenzte Stichproben untersucht werden. Mittelwerte und Streuung kleiner Stichproben weichen mehr oder weniger von den wahren Eigenschaften des untersuchten Baumusters ab. Solche Abweichungen werden derzeit nicht oder nur sehr vage berücksichtigt. In diesem Aufsatz wird der Einfluss der Unsicherheiten aus Stichproben anhand der Bewertung von Composite-Druckgefäßen durch Berst- und Lastwechselprüfungen dargestellt.
The increasing amount of composite transport systems for hydrogen leads to new and there-fore unknown potential hazards for general public. Due to lack of experience, risks of new technologies tend to be rated as 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 as-sessment of composite cylinders developed at the BAM (Federal Institute for Materials Re-search and Testing).
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 as 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).
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.
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).