Detecting impact traces on a composite pressure vessel with aluminum-coating optical fiber using a phase-modulated BOCDA sensor

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.