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The reduced volume method has found a particular zone to be used for evaluating the strength of unidirectional composite materials. To validate the method, modelling an experimental test where it has the same basic assumption of the model had been carried out. The result was favourable as the model can predict the average value from the experiment and also the time dependent effect.
The developed model has certain limitations of the element size to be used in the simulation to characterise the strength of composite materials. A reduced volume method is proposed in order to reduce the number of degree of freedom of the finite element simulation.This study has revealed certain configuration to be followed to speed up the computation time.
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.
Safety aspects of the storage containments of hydrogen are independent from the kind of usage: The main criteria is the resistance against the pressure load. On the 2nd level there are aspects specific to the usage as e.g. crash loads etc.
Safety is achieved when the probability (frequency) of a failure during the service life is acceptable!
But an aspect common for all usages is the scatter of properties resulting from the material and the production. Therefore, it is worth to have a look on the current approach of minimum burst ratios from an unknown point of view: the basic populations of a product – based on exemplarily assessments.
According to this approach, several diagrams with Monte-Carlo generated basic populations in new and aged conditions have been shown. Thus, it has been demonstrated that not the conditions at the begin of life are the critical ones. No, the end of life properties are the critical ones.
This has been combined with the acceptance rate of borderline basic populations. The approach of minimum burst ratios leads to the acceptance rate of 40 % and higher for the approval of critical basic populations – critical due to their production scatter.
This problem becomes much more critical when taking into account the unknown effects of degradation and in-service ageing.
Therefore, we cannot support the proposed reduction of minimum burst ratio without having a stronger assessment of production scatter.
Defining a Reduced Volume Zone for the Simulation of Burst Test on a Composite Pressure Vessels
(2018)
The random nature of fibre break in composite materials has to be modelled in all part of the structure as it may contain millions of fibres. The reduced volume method was introduced to determine a smaller zone where we could characterise the strength based on the accumulation of fibre break, hence increasing the computation time
It is pertinent to have a correct description of fibre strength described by the twp parameter Weibull distribution when evaluating a type Iv pressure vessel using the fibre break multiscale model developed at Mines ParisTech. Earlier studies have shown a positive comparison result when T600S fibre strength were used. This study has discovered what would be the effect on the predicted strength when T700S strength is used.
Safety criteria for the transport of hydrogen in permanently mounted composite pressure vessels
(2019)
The recent growing of the net of hydrogen fuelling stations increases the demands to transport compressed hydrogen on road by tube-trailers in composite pressure vessels. As transport regulation the ADR is applicable in Europe and adjoined regions and used for national transport in EU. This regulation provides requirements based on the burst pressure of each individual pressure vessel, regardless the capabilities of the transported hydrogen and relevant consequences resulting from worst case scenarios. In 2012, BAM (German Federal Institute for Materials Research and Testing) introduced consequence-dependent requirements and established them in national requirements concerning the “UN service life checks” etc.) to consider the transported volume and pressure of gases. However, this results in a stringent requirement in case of using large pressure vessels (tubes) on tube-trailers. In the studies presented here, the key safety factors for using hydrogen trailers are identified and reviewed through some safety measures from some countries like Japan, USA and China. Subsequently, the risk, chance, failure consequences of using trailers are evaluated, in addition, the difficulties for approving huge pressure vessels (large tubes) are addressed. There, a maximum acceptable pressure-volume product is defined. Finally, a performance-based criterion for the balance of chance and risk of hydrogen trailers is suggested to add into regulations and consequently into standards for hydrogen trailers.
The presentation summarizes all the safety concerns that have led DIN to reject an ISO project about extremely large gas/hydrogen transport units.
The core point is the consequence potential, which has increased considerably with the scope of the standardization project in comparison with state of the art systems. At the same time, there is a lack of an approach to take into account the reliability requirements that depend on the consequences, at least up to the level of a catastrophe.
Safety criteria for the transport of hydrogen in permanently mounted composite pressure vessels
(2019)
The recent growing of the net of hydrogen fuelling stations increases the demands to transport compressed hydrogen on road by batterie vehicles or tube-trailers, both in composite pressure vessels. As transport regulation the ADR is applicable in Europe and adjoined regions and used for national transport in EU. This regulation provides requirements based on the burst pressure of each individual pressure vessel, regardless the capabilities of the transported hydrogen and relevant consequences resulting from worst case scenarios. In 2012, BAM (German Federal Institute for Materials Research and Testing) introduced consequence-dependent requirements and established them in national requirements concerning the “UN service life checks” etc. to consider the transported volume and pressure of gases. However, this results in a stringent requirement in case of using large pressure vessels (so called “tubes”) on vehicles. In the studies presented here, the safety measures for hydrogen road transport are identified and reviewed through some safety measures from some countries like Japan, USA and China. Subsequently, the failure consequences of using trailers, the related risks and chances are evaluated. Finally, a chance-related risk criterion is suggested to add into regulations and consequently to be defined as safety goal in standards for hydrogen transport vehicles and consequently for mounted pressure vessels.
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.