2007 Volume 56 Issue 12 Pages 560-567
We developed a new heat-resistant optical fiber AE sensor and a homodyne Mach-Zender type laser interferometer to monitor the acoustic emission (AE) from hot members. The optical fiber coated with copper was used as a heat resistant sensor, and AEs from hot pipes were measured up to 600°C for 108 ks. The sensor was found to monitor the longitudinal mode cylinder wave which conventional PZT sensor can not monitor, when the sensing fiber was wound on the pipe. The system was utilized for monitoring AEs from crack and exfoliation of non-protective scales produced by catastrophic oxidation of Type 304 stainless steel pipe at 700°C. The cylindrical AEs were classified into two fracture types, i.e., Mode-I fracture with a crack opening vector in the circumferential direction (scale fracture) and in the radial directions (exfoliation). A number of AEs detected during cooling was found to be produced by Mode-I crack of the scales. Their sources were located using arrival time differences of L(0,2) and F(1,1) modes at 80 kHz and found to be in the zone with non-protective scales.