Development of heat resistant AE monitoring system using optical fiber sensor

We developed an advanced optical fiber Acoustic Emission (AE) monitoring system with a phase compensation feed back circuit. Two lasers transmitted through the reference and sensing arms are combined by a 2x2 coupler and detected by two photodiodes as the intensity change of the interfered laser. Outputs of the photodiodes are then combined in a differential amplifier which extracts error signal and improves the SIN ratio. The developed system was demonstrated to detect weak guided wave AE signals such as zero-th order longitudinal cylindrical wave (Lo-mode) with the SIN ratio higher than that of PZT sensor. This system was utilized to monitor the oxidation resistance of various alloys during thermal cycles from 300 K to 1273 K. We detected weak AE signals from micro fractures of growing oxide films of boiler tube and austenitic stainless steel, but no AE from Inconel 600 tubes. Here the optical fiber sensor was wound over the tube at 600 K. Both the AE counts and amplitudes during thermal cycles showed clear tendency depending on the oxidation resistances of the alloys.