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The strain pattern distributed on composite cylinders after impacts was detected using optical fibers for the first time. The sensing optical fiber was implemented on the composite cylinders using aluminum (Al)-coated optical fiber, polyimide-coated optical fiber, or standard single mode fiber (SSMF) of polymer-coating. The residual strain of this sensing fiber was measured by a Brillouin optical correlation domain analysis (BOCDA) sensor system, using phase modulation and single side band modulation methods. Impact events of 10, 20, and 40 J energies as barely visible impact damages (BVID) were applied on the cylinders, and these fibers were deployed on the cylinder surface, or were embedded in the cylinders. For the surface deployment, Al-coated fiber exhibited the highest residual strain due to Al plastic deformation, and the strain was three times higher than the lowest SSMF value. For the embedment deployment, these fibers were individually embedded in the cylinder, which was composed of sixteen carbon fiber reinforced polymer layers using a filament winding process. In contrast to the surface deployment, the embedded SSMF gave as high residual strain as that of the Al-coated fiber. These similar residual strains of the three embedded optical fibers suggested that they came from permanent material damage inside the material. Impact events with fiber embedment caused the optical signal in the Al-coated fiber to suffer serious additional insertion loss, but the SSMF did not show loss. So the general fiber for optics communications was successfully demonstrated as a distributed residual strain sensor to detect BVID impacts of composite materials. The strain values were maintained for at least 15 months after impact events. This residual strain sensor is economical, because special fibers like Al-coated fiber were not needed, and the sensor is efficient, because it could sense the strain for long distance, without additional impact-driven insertion loss. [ABSTRACT FROM AUTHOR]

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, [90 1 /OF/90 1 /+-20 1 /90 3 /+-20 1 /90 3 /+-20 2 /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. [ABSTRACT FROM AUTHOR]

Impact traces on a composite pressure vessel were detected by measuring residual strain in aluminum (Al)-coating optical fiber for the first time. The residual strain was obtained by a Brillouin optical correlation domain analysis (BOCDA) sensor system, where a continuous optical signal was simultaneously phase-modulated to choose the sensing position, and single-side-band modulated to find the Brillouin frequency. This sensor system successfully measured the Brillouin frequency, which can be converted to the strain, along a 500 m long optical fiber, with distance resolution of less than 1.5 cm. The cantilever beam test using a bonded optical fiber presented the strain conversion coefficient, which was determined by the comparison between the measured Brillouin frequency and the strain of an electric strain gauge. Al-coating optical fiber was attached around the surface of a composite pressure vessel, and consecutive impacts at six separate positions were applied using hemisphere and wedge impactors with energies of 10, 20, and 40 J. The measured Brillouin frequencies were the unchanged intrinsic frequency along the whole fiber length, except for the six peak positions, which exactly coincided with the impact positions. The fine measurement at these peak positions showed the frequency peaks to be composed of two split peaks, which were compared with photographs taken while the impacted surfaces were deformed. This confirmed that a dent temporarily formed at an impact position caused the optical fiber to bend twice, and the strain remained at two adjacent positions. Hydraulic pressure test of the composite pressure vessel after the impacts showed that this measurement system could detect small impact traces that do not cause critical structural degradation, as well as large impact damages. [ABSTRACT FROM AUTHOR]