Investigation of Sealing Methods for Tubular CFRP Specimens to Analyze Permeation Relevant Manufacturing Parameters for Liquid Hydrogen Tanks in Future Aviation Applications

Abstract

Hydrogen is one of the most promising alternatives for aviation fuel, with carbon fibrereinforced polymer (CFRP) tanks offering significant weight savings over traditional metal options. Given the global imperative to reduce CO2 emissions from fossil fuels, the aviation industry must contribute to these efforts. While synthetic fuels can be integrated into current aircraft with minimal modifications, only hydrogen offers a net-zero carbon solution when produced sustainably. However, hydrogen as a long-term aviation fuel presents several challenges, particularly in developing safe, low-loss, lightweight storage solutions. This work contributes to addressing these challenges, focusing on the development of leak-tight Type V tanks for cryogenic hydrogen storage. One of the main issues is the permeation and leakage of hydrogen molecules through the composite wall structure. To streamline development and achieve an optimal cost-to-insight ratio, cryogenic permeation tests are conducted at element-level on tubular specimens simulating the cylindrical section of the final tank structure. These tests provide initial insights and preliminary assessments, especially regarding the material manufacturing process and design. However, there is a concern that leakage could alter the measurements. This thesis examined how to seal a CFRP tube at room temperature, ensuring sufficient leak-tightness for hydrogen applications under cryogenic conditions and internal pressure. A numerical and experimental approach was chosen to define critical parameters for designing an effective sealing concept using bonded flanges at the tube ends. A parametric study was conducted using Abaqus software to identify advantageous trends that lead to reduced stress and strain in the assembly. The influence of material and adhesive choice was found to have a significant impact, partly due to the differing coefficients of thermal expansion (CTE). Geometric design adaptation also shows a substantial impact through various bonding improvement strategies to minimize stiffness discontinuities. On the other hand, overlap lengths and adhesive thicknesses showed less influence on the outcome. Experimental investigations were conducted to validate these findings. Manufacturing tests of bonded cylindrical specimens and single lap shear tests highlighted the critical role of adhesive viscosity, ductility, and temperature resistance. Surface treatment of the bonding areas was also identified as a crucial factor, emphasizing the necessity of experimental testing for obtaining accurate data. These findings provide valuable insights into developing an effective leak-tight solution for tubular specimens, with the potential for scaling to full-size cryogenic tank structures.