Long term material properties of pressure vessels made of composite material

Abstract

Pressure vessels are currently the limiting factor in the hydrogen distribution chain. The pressure vessels are preferably made out of composite materials to achieve practical weight to strength ratios and avoid hydrogen cracking in steels, which is the alternative material. While composites are more environmentally stable and stronger, they are inherently more complex mechanically. To allow full use of the composite materials potential, better modelling and structural integrity monitoring methods needs to be developed. In this work, methods have been developed to better model and monitor damage from mechanical loads on composite materials, with particular application to filament wound materials such as used in pressure vessels. The goal of the work is to allow better use of composite material’s potential in hydrogen pressure vessels. The methods may achieve this by allowing better lifetime estimates and structural integrity monitoring along with increased understanding for the failure mechanisms. The failure mechanisms and progressive fatigue damage was studied using digital image correlation (DIC) applied on split disk fatigue tests of pressure vessel cut outs. As part of this work a novel method for monitoring damage progression was used and suggested on the experimental level. The DIC data was used to estimate S-N curves for the fiber direction on the local level using a novel method that may also be used as a damage monitoring method. The S-N curves were used as input for a fatigue damage model formulated as a user material subroutine (UMAT) in the finite element software Abaqus capable of modelling fatigue damage in composite materials. The UMAT and the experiment matched in terms of damage development.