Damage mapping using strain distribution of an optical fiber embedded in a composite cylinder after low-velocity impacts

Impact damage was mapped using the strain distributed in an optical fiber that was embedded in a composite cylinder after low-velocity impact. Three-dimensional strain images were obtained to allow damage inspection using the optical fiber as a distributed sensor for the first time. An acrylate-coated standard optical fiber was embedded in the hoop layer of the composite cylinder, [901/OF/901/+-201/903/+-201/903/+-202/EPDM]T, as a sensor. This sensing fiber was wound around the cylinder at intervals of 12 mm in the longitudinal direction. Impacts of varied energies that yield barely visible impact damage (BVID) were applied to the cylinder by a drop-weight impact machine with a hemispherical tip. The residual strain caused by external impact of the composite cylinder was measured by the sensing fiber with a Brillouin optical correlation domain analysis (BOCDA) sensor system using phase code modulation. To determine the impact damage location and severity, the strains of the sensing fiber were mapped to positions on the cylinder surface. In the first impact test, eight impacts were applied to the cylinder at energy levels of 10 and 20 J. After impact, the strain measurement using the BOCDA system gave well damage information via cylinder surface strain mapping. In the second impact test, five impacts were applied at four points with the energy of 40 J and at one point with that of 30 J. The strain mapping clearly identified the additional impact locations and severities. These results verified that impact damage to the composite cylinder could be accurately analyzed as mapping images by the buried sensing fiber and the BOCDA sensing system.