OPTICAL FIBER SYSTEM FOR AE MONITORING OF HIGH TEMPERA- TURE DAMAGE OF STAINLESS STEEL TUBING

We utilized optical fibers as heat-resistant AE sensor elements, and monitored acoustic emission (AE) from molten salt (85 mol% V2O5 + Na2SO4) attack of 304 stainless steel tubing at elevated temperatures. AE signals were monitored by a homodyne Mach-Zender-type laser interferometer developed in our laboratory. Using optical fibers coated by UV-cured polymer or by copper, we monitored cylindrical-wave AE signals below 450oC and 750oC, respectively. Multiple winding of the sensing fiber on the tubing significantly improved the sensor sensitivity and allowed the detection of L- and F-mode cylindrical waves. Source location in the longitudinal direction of the tubing was obtained accurately utilizing the wavelet-based arrival-time difference of L(0,2)- and F(1,1)-mode waves at selected frequencies.