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Due to high specific stiffness and strength properties, fibre reinforced plastics are used more and more often for the construction of pressure vessels. Within a recent research project run by the Federal Institute for Materials Research and Testing (BAM), aging process of composite pressure vessels is investigated in order to be able to give more accurate lifetime predictions in the future. Focus is set on type III pressure vessels consisting of an aluminium tank which is fully wrapped with carbon fibre reinforced plastics. In order to increase high-cycle fatigue, residual stresses are induced into the pressure vessel during manufacturing process. In particular, residual compressive stresses within the inner aluminium layer have been defined as a main parameter affecting fatigue strength. The aim is to identify and evaluate residual stresses of the pressure vessel by analysing its modal parameters. Through the set-up of a finite-element model potential capability and validity for the use of modal analysis is proven and evaluated, considering influences resulting from manufacturing deviations, too. In the following, a number of stress sensitive modes are defined. Based on these preliminary numerical investigations, a test bench is set up in order to measure pressure vessels via an experimental modal analysis. A final critical evaluation regarding the accuracy of the modal analysis is made by comparing experimental results with data obtained through simulations.
At the Bundesanstalt für Materialforschung und –prüfung (BAM), within a recent research project, the aging process of composite pressure vessels is investigated in order to be able to give more accurate lifetime predictions in future. All investigations are primarily based on type III pressure vessels consisting of an aluminium tank which is fully wrapped with carbon fibre reinforced plastics. Research is focused on residual stresses which are induced into the pressure vessel during manufacturing process in order to increase high cycle fatigue. However, with increasing lifetime residual stress conditions of type III pressure vessels change. For measuring and monitoring inner stress conditions, the application of a non-destructive measurement method is aspired.
Within this paper, the potential of an experimental modal analysis is tested to determine aging and degradation effects of pressure vessels. Based on this method, information about changes of the residual stresses can be obtained via an analysis of the modal parameters. Realizing this, first a finite element model is used to prove and evaluate potential capability and validity for the application of an experimental modal analysis. Based on this, a test bench is set up and successively optimized in its accuracy and efficiency. The sensitivity of the applied measurement technique is experimentally evaluated via measuring multiple prestress modified pressure vessels. Furthermore, a selection of prestress modified pressure vessels is monitored via the presented method. Finally, experimentally obtained results are interpreted and evaluated with the help of numerically gained finding.
Due to high specific stiffness, fibre reinforced plastics are the dominant material group for the design of mobile pressure vessels. At the Federal Institute of Materials Research and Testing (BAM) aging process of composite pressure vessels is studied to be able to give more accurate lifetime predictions in future. Investigations are based on type III breathing air cylinders consisting of an aluminium tank which is fully wrapped with carbon fibre reinforced plastics. Goal is to detect changes of residual stresses over life time which directly affect fatigue strength. Within this paper an approach is presented to monitor residual stresses via an experimental modal analysis (EMA). First, the influence of changed stress conditions on modal parameters is analysed via a numerical study. Secondly, a test bench for an EMA is set up. To be able to analyse cylinders of different prestress condition, several specimens are prestress modified via high-temperature and high-pressure treatment. During modification processes, specimens are monitored via optical fibres to control prestress modifications. Through experimental measurements of the modified specimens via EMA changes in prestressing can be detected. Finally, validity and accuracy of the EMA is evaluated critically by comparing all numerically and experimentally obtained data.
Degradation and damage analysis of composite pressure vessels via experimental modal analysis
(2023)
For mobile gas storage systems, the application of type IV pressure vessels is state of the art. Type IV tanks consist of an inner polymer liner fully wrapped with fibre-reinforced plastic (FRP). Because of the complex fabric of the FRP as well as a difficulty estimable interaction behaviour between the single components under load, there are still no satisfying non-destructive testing methods to assess the current state of failure nor to estimate the level of degradation accurately and economically. At BAM division 3.5, analysing the ageing process of mobile composite pressure vessels is a major task to ensure safe usage over the whole lifetime. In this context, key aspects of our ongoing research activities are the invention of new test procedures and the development of accurate lifetime prediction models. In order to determine the level of degradation or damage, one meaningful non-destructive approach is to analyse the structural dynamic behaviour via an experimental modal analysis (EMA). Over the last few years, different types and sizes of composite pressure vessels have been tested in several research projects. The presented paper gives an insight into how to extract and interpret modal parameters and how to fit them to the results of residual strength tests.