• Ferdinand, Pierre
• Maurin, Laurent
• Villalonga, Stéphane
• Bertin, Maxime
• Langlois, Christophe
• Nony, Fabien
• Devilliers, Clémence

Abstract

This document presents the results of mechanical tests performed on carbon fibre epoxy composite samples of high pressure storage hydrogen vessels, equipped with embedded Optical Fibre Sensors devoted to their Structural Health Monitoring (SHM). Early measurements on flat samples have proved the Fibre Bragg Gratings (FBG) transducers ability to measure strains into such anisotropic materials, and pointed out the optical fibre location importance relative to the structural layers orientations for strain measurements of the structure. Comparative tests on flat samples and NOL rings, with and without embedded optical fibres, did not permit to conclude about any significant intrusive effect of optical fibres on their ultimate tensile strength. Moreover, interpretation in energy of fatigue tests performed on NOL rings, known to be more representative of pressure vessels, have first demonstrated the weakness of the sole interpretation of force measurements which can lead to wrong interpretations, and second the need to accurately control the energy during the first fatigue cycles, since it has a significant effect on the sample fatigue strength. Mechanical tests on vessels first performed to acquire some knowledge about their behaviour, and also to early detect the occurrence of “critical” structural defects, have demonstrated the essential contribution in this application of the Optical Frequency Domain Reflectometry (OFDR) based on Rayleigh backscattering compared to other “traditional” optical techniques, e.g. Brillouin reflectometry. Based on the fact that pressure vessels are subjected to an homogeneous hydrostatic stress (their internal pressure), we demonstrate, according to OFDR measurements on first composite vessels, the ability to detect some structural defects without the need to inflate them at pressures greater than their working pressure, thus reducing the risk of creation or evolution of new or existing internal defects. Besides the fact that these optical measurements provide information of a mechanical nature (i.e. the distributed strain profile all along the optical fibre), they are performed by the same sensor –the optical fibre– embedded into the composite structure throughout its lifetime, eliminating by design any measurement dispersion between different sensors.

Topics

• dispersion
• Carbon
• strength
• anisotropic
• fatigue
• composite
• Hydrogen
• defect
• tensile strength
• reflectometry