Embedded fiber optic sensing of composite-overwrapped pressure vessels
Luna Innovations will be presenting a paper at the upcoming CAMX show titled “In-Situ SHM of Composite-Overwrapped Pressure Vessels”. The paper is a collaboration with Structural Composites Industries (SCI), a leading commercial and military supplier of composite overwrapped pressure vessels (COPV), and demonstrates a novel approach to structural health monitoring (SHM) of air flasks using embedded high definition fiber optic sensing (HD-FOS) technology. Luna’s presentation will outline how Luna and SCI engineers embedded a fiber optic strain sensor during the fabrication of a composite overwrapped pressure vessel (COPV) and used a Luna ODiSI system with HD-FOS technology to record strain readings at maximum pressure conditions. This high definition strain data was used to calculate the volumetric expansion of the COPV and this data was in turn compared to the volumetric expansion recorded with the standard hydrostatic test protocol. Currently, most composite pressure vessels must be recertified every 2-5 years. Normal recertification requirements require pressurization in a fluid filled chamber with the global volumetric expansion compared to acceptance criteria. This requirement poses significant cost and time out-of-service issues across many industries. The approach demonstrated in the In-Situ strain sensing method offers the potential for significant cost savings by deferring or eliminating the periodic requirement to recertify through hydrostatic testing. Luna’s ODiSI product platform with HD-FOS offers unique capabilities for this application. The sensor is constructed of unaltered fiber optic cable and at only 155 microns in diameter can be embedded within composite structures without influencing the parameters under test. The high definition data, with a strain measurement for each millimeter of sensor, is needed to ensure that the calculated volumetric expansion is suitably accurate. Figure 1 shows how the embedded fiber optic sensor was wound helically during the fabrication of the COPV. The COPV consists of an aluminum liner with layers of high strength carbon fiber/epoxy applied via filament winding over the aluminum liner. Glass fiber and epoxy layers are added to improve impact damage resistance. Figure 2 shows the various stages of COPV manufacture.
The volumetric expansion calculated using strain data was 101cc. This value compared favorably to an average volumetric expansion of 137 cc averaged over four tests using standard hydrostatic test protocols. These results are reasonable considering that the calculated strain did not consider the longitudinal expansion of the COPV. If desired, sensors could similarly be embedded in the longitudinal direction and over the end regions of the flask. If you’d like to learn more about this specific application, or HD-FOS in general, then stop by Luna’s booth at the upcoming CAMX show Dallas. Luna’s booth is number F84. If you can’t make the CAMX show then please request additional information through our website or by responding to this blog. Reference: Sandie M. Klute, Daniel R. Metrey, Naman Garg, Nur Aida Abdul Rahim “In-Situ SHM Composite-Overwrapped Pressure Vessels” Luna Innovations Incorporated 3155 State Street Blacksburg VA 24060