Baseline-free pipeline monitoring using optical fiber-guided laser ultrasonics

This study develops an optical fiber-guided laser ultrasonic system and baseline-free damage detection method that enables structural health monitoring of pipelines employed under a high-temperature environment of nuclear power plants. First, this study designs embeddable optical fibers and fixing devices, so that the laser beams used for ultrasonic excitation and sensing could be transmitted between laser sources and target pipe surfaces through the optical fibers. All devices are specially designed for a high-temperature environment up to 300 degrees C. Then, a damage detection method is developed to identify typical pipe defects without any previous baseline data. This technique is based on the premise that multiple identical wave paths exist in an intact axisymmetric pipe, but the similarity among the ultrasonic signals obtained from these identical wave paths breaks down when there is a nonaxisymmetric defect. The feasibility of the proposed technique has been experimentally verified using a stainless steel pipe specimen under a high-temperature environment.