Application of an Experimental Modal Analysis on Composite Pressure Vessels for Monitoring Prestress Conditions

Due to high specific stiffness, fibre reinforced plastics are the dominant material group for the design of mobile pressure vessels. At the Federal Institute for 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. The 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 the 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, the validity and accuracy of the EMA is evaluated critically by comparing all numerically and experimentally obtained data.