Takuma Matsuo

Evaluation of the Thickness and Bond Quality of Three-Layered Media using Zero-Group-Velocity Lamb Waves

Bonding conditions such as jointing agent thickness and bond strength (quality) at the interface between two components is a critical concern in an assembled product because the jointing area is often the weakest component in products. Using a laser ultrasonic technique, zero-group-velocity (ZGV) Lamb waves were utilized to characterize the thickness of an adhesive layer for an epoxy-bonded sample and the bond quality for brazed samples. Using two modes of the ZGV Lamb waves, the thickness of the adhesive layer ranging from 0.2 to 0.8 mm can be estimated with high accuracy. In bond quality measurements, poor bond quality lowered the frequency of ZGV Lamb waves. The decrease in the frequency depended on the amount of shear stress along the interface caused by ZGV Lamb waves.

Optical fiber acoustic emission system for monitoring molten salt attack

Abstract 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% V2O5CNa2SO4) at 1073 K. AE sources, detected during cooling of the pipe, were located in the zone severely attacked by the molten salt.

Development of Novel Optical Fiber AE Sensor with Multi-Sensing Function

A novel optical fiber acoustic emission (AE) system with multi-sensing function in single long fiber was developed and utilized for the estimation of AE sources of model steel plate and jointed pipes. Multi-sensing function was achieved by dividing the single sensing fiber into several sensor portions with different resonance frequencies. The resonance frequencies were provided by winding the sensing fiber around the solid rods (sensor holders) with different diameters. The monitoring system with three sensors in a 10 m long fiber was demonstrated to detect three wave packets with different frequencies and correctly estimate the source locations of AEs from artificial (Nelson-Sue) sources on a 0.9 wide x 1.8 m long steel plate. Here the arrival times of AEs for the source location were determined by the continuous wavelet transform. Source locations on the steel plate were determined within a distance error of 53 mm. The system also makes the location of the pipe with damage possible.