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Eingeladener Vortrag
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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.
Hydrogen is an attractive energy carrier that requires high effort for safe storage. For ensuring safety, they must undergo a challenging approval process. Relevant standards and regulations for composite cylinders used for the transport of hydrogen and for its onboard storage 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 with respect 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 was exemplarily estimated. The aspect of small sample sizes is analysed and discussed with respect to the evaluation procedures.
Hydrogen is an attractive energy carrier that requires high effort for safe storage. Forensuring safety, they must undergo a challenging approval process. Relevant standardsand regulations for composite cylinders used for the transport of hydrogen and for its on-board storage 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 withrespect to the evaluation of load cycle strength. Their characteristics are compared withthe probabilistic approach of the BAM. Based on Monte-Carlo simulations, the availabledesign range (mean value and scatter of strength criteria) of current concepts was exem-plarily estimated. The aspect of small sample sizes is analysed and discussed with respectto the evaluation procedures.
Full-scale bonfire test
(2016)
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.
The simulation of strength experiments by the Monte-Carlo method enables the numerical generation of data representing a complete populations of composite pressure vessels. In the case of composite pressure vessels used for hydrogen storage, properties like burst strength or fatigue cycle strength are of interest. This paper provides comprehensive information on how populations are generated and how samples can be taken and evaluated; it also explains how to determine the acceptance rate of random samples from simulated populations for passing the approval test "minimum burst pressure". A word of caution is also expressed regarding the evaluation of acceptance rates from a small sample.
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.