Process Analysis & Simulation of Composite Pressure Vessels

Abstract

The automotive industry has been in an increasing drive toward finding more sustainable transportation alternatives to abandon the era of Internal Combustion Engines (ICE). This was manifested in the large market share of Battery Electric Vehicles (BEVs) that have been established in recent years. As BEVs are not an efficient option in long-haul transportation, Fuel-Cell Electric Vehicles (FCEVs) have presented themselves as a prominent choice. To maximize the attainable driving range, gaseous hydrogen is compressed to 70 MPa per recent regulations. As a result, hydrogen storage pressure vessels need to provide high structural integrity, whilst being as lightweight as possible. This has led to the introduction of Carbon Fiber Reinforced Polymer (CFRP), where type IV Composite Pressure Vessels (CPVs) with CFRP overwrap and plastic liner are gaining popularity. The mass implementation of FCEVs is however directly dependent on the cost and reliability of CPVs. Increasing the reliability levels inherently lead to minimizing the materials used inducing cost reduction possibilities, and presenting FCEVs as a more feasible solution. To attain such high levels of reliability, a thorough understanding of the manufacturing process of CPVs is required. This research project aims at contributing to an increased level of understanding of CPVs by investigating the influence manufacturing parameters have on the final product quality. This study is done in collaboration with Plastic Omnium, which allowed for the possibility of obtaining experimental data for validation purposes. The approach followed in this research project is by developing an analytical model that is capable of representing the manufacturing process of CPVs. The objective is to be able to link the manufacturing parameters to the vessel’s quality for a given vessel configuration. The analytical model takes the vessel configuration as an input, alongside the definition of the considered manufactured parameters, winding tension force and internal liner pressure, and outputs quantitative predictions of changes in dimensions, compaction and burst performance. The model was found on the basis of adopting a common analysis method for composite materials, Classical Laminate Theory (CLT) and adapting it to be capable of representing the manufacturing process. A major challenge in developing such a model is dealing with uncured composite materials, where different assumptions were imposed for the model to be physically sound. Hypotheses were generated using the analytical model for eight different vessel configurations, where either the vessel configuration was changed or the magnitude of manufacturing parameters. The proposed experimental campaign consists of microscope inspection and burst testing of 22 sub-scale vessels in total, where the obtained data were used to prove or disprove the hypotheses. The analytical model and experimental results showcased the significant influence varying tension force and internal pressure have on the end-product quality where at different parameter settings, differences up to 4 % in Fiber Volume Fraction (FVF), 4.5 % in porosity content and 8 % in burst performance were observed with respect to a baseline configuration. Changes in vessel configuration, by varying stacking sequence grouping or liner diameter, led to notable differences in compaction and burst performance. The work presented in this report highlights the significance manufacturing parameters have on the scatter and reliability of CPVs and showcases the possibility of improving mechanical performance for a fixed design. Additionally, a framework is proposed in this work to help in defining the optimum process parameters for different vessel configurations. This work serves as a basis for future investigations into further optimizing the process parameters definition as it has been shown that there is still room for further improvements.