Graduate School of Engineering
Department of Materials Processing
Microsystems Design and Processing
Materials Evaluation and Sensing (Prof. Mihara)
In manufacturing industries, reliable inspections of highly precise and functional products are needed, because their strength and functional deficiencies sometimes prove fatal to a company. Though infrastructure constructed during rapid growth periods are simultaneously being degraded, low-cost safety insurance is needed through pinpoint detection and repair of serious damages because Japan’s economy does not allow for their total reconstruction. Our laboratory aims to develop mainly ultrasonic nondestructive measurement techniques to evaluate automobiles, aerospace and railroad products, and to monitor chemical, power-generation and civil infrastructures.
(1) Detection of micro creep voids by water immersion testing
Because of the massive earthquake took place in Japan in 2011, most nuclear power plants have been on hold. Instead of that, aging thermal power plants have been operated to ensure sufficient electric power. For their reliable and efficient maintenance, a means for detecting early-stage creep voids is strongly desired. In this study, we have developed a novel water immersion method with a large-aperture aspherical focused transducer and a mechanical scanning with a very fine pitch (~1 μm) to realize the detection of early-stage micro voids in actual power plants. Furthermore, the detection mechanism has been investigated by a numerical simulation.
(2) Subharmonic phased array for crack evaluation (SPACE)
Ultrasonic testing has been widely used for the inspection of internal defects such as cracks (Fig. 1), because ultrasound is very sensitive to cracks. However, in actual inspection, the signal-to-noise ratio (SNR) are sometimes low due to the other linear scatterers, e.g. coarse grains, welds, geometric change. As a means for achieving crack imaging with high SNR, a novel imaging method, subharmonic phased array for crack evaluation (SPACE), has been developed on the basis of large-amplitude incidence and the subharmonic generation due to the contact vibrations of crack faces. In addition to its demonstration in various crack specimens, we have worked on the development of larger-amplitude ultrasonic transmission system of using e.g. laminated transducer and multiple special pulsers to extend the application fields of SPACE.
(3) Fabrication of novel ultrasonic transducers
Ultrasonic testing is generally carried out in room temperature, because the polarization of piezoelectric materials in transducers is lost at a high temperature above a Curie temperature. On the other hand, if in-service inspection can be conducted at a high temperature, the maintenance cost will be effectively reduced. In our lab, a special transducer that can endure a high temperature has been developed by incorporating not only piezoelectric material with a high Curie temperature but also the other components, such as cable, adhesive, couplant, case, etc. In addition, dry-couplant transducers for aging concrete structures, air-coupled transducer for fuel cell, and a high-output transmitter, laminated transducers, for SPACE have been also developed (Fig. 2).