Optical fiber acoustic emission system for monitoring molten salt attack

We developed an advanced optical-fiber acoustic-emission (AE) monitoring system with a phase-compensation feedback circuit, and detected AE signals from molten-salt attack of Type 304 stainless steel pipe. The system is a Mach-Zender type laser interferometer, consisting of a diode laser, two single-mode fibers of sensing and reference arms and two photodiodes as detectors of interfered laser beams. Outputs of the photodiodes are combined in a differential amplifier, which can extract error signal (noise) and significantly improve the S/N ratio by utilizing a specially designed feedback circuit. An optical fiber coated by UV-cured polymer can detect AE signals at 673 K and be utilized to monitor the cylinder wave from oxidation at elevated temperatures. The sensor monitored the weak longitudinal (L-) mode as well as the flexural (F-) mode of the cylinder wave AE signals when it was wound on the pipe surface by several turns. Source locations of AE signals in the axial direction were determined from the arrival time differences of the L- and F-mode at a selected frequency. The system measured AE signals from frequent fractures of the oxide film produced by molten salt attack (85 mol% V2O5+Na2SO4) at 1073 K. AE sources, detected during cooling of the pipe, were located in the zone severely attacked by the molten salt.