Barely visible impact damage detection and location on composite materials by surface mounted and embedded aerospace-compatible optical fibre Bragg grating sensors

Composite materials are finding their way into aerospace applications thanks to their high stiffness-to-mass ratio. Nevertheless, composite components require frequent inspections because of their sensitivity to critical damage. Damage as small as barely visible impact damage (BVID) can grow as result of structural loading, with component failure as possible outcome. Optical fibre sensors (OFS) are considered excellent candidates for permanently installed structural health monitoring (SHM) systems, owing to their many advantages over electrical sensors. Current state-of-the-art BVID detection with OFS has so far however been limited to proof-of-concept demonstrations at low technology readiness levels. In this work, we equipped a total of 16 coupons, made of 5 different state-of-the-art composite materials, with aerospace compatible embedded or surface mounted optical fibre Bragg gratings (FBGs). We impacted each coupon at two locations and acquired the FBG reflection spectra before and after each impact. We first demonstrate how changes in the Bragg wavelength and in the Bragg peak shape can be quantified when the FBGs are exposed to the (non-)uniform strain field of BVIDs. Second, we show that this method was able to successfully detect the BVID in all considered scenarios and that in most cases, it was able to also locate the damage within an uncertainty of +/- 1 FBG location. Finally, we show the reliability of this method in terms of repeatability and considering the effects of temperature changes and on-ground airplane vibration. To the best of our knowledge, we are the first to use in-flight-compatible embedded and surface mounted FBG-sensors for the detection and location of BVIDs on aerospace-grade composite materials. These results motivate the use of FBG sensors as a permanent sensor network for cost-efficient damage detection in composite aerospace components for locally monitoring damage-prone locations.