Efficient transduction of a circumferential (C-) Lamb wave by a pair of noncontact air-coupled ultrasonic transducers (NAUTs) was presented. A line focus type (LFT-) of the NAUT was employed for the efficient transduction of the C-Lamb wave, which was designed so as to take an almost same angle of the incident longitudinal beam on the circular pipe surface. Outstanding characteristics of the LFT-NAUT for the C-Lamb wave transduction were theoretically and experimentally shown in comparison to those of the conventional plane type (CPT-) NAUT. The efficiency of the C-Lamb wave generated and detected by the LFT-NAUT took about 20 times higher than that by the CPT-NAUT. As for an application of the C-Lamb wave generated by the LFT-NAUT, a novel method of an accurate estimation of pipe wall thickness was introduced and evaluated with a theoretical model. It was confirmed that the maximum error between the experiments and the theoretical model was at around 10 πm.

We have developed a fiber-optic distributed sensor that can measure strain or temperature distributions at an arbitrary position along long-length fiber Bragg gratings (FBGs) with the high spatial resolution of less than 1 mm. In order to determine the position of a grating in FBGs, at which the light of the particular wavelength is reflected, optical frequency domain reflectometry (OFDR) and a signal processing technique are used. Although the length of an FBG is usually about 10 mm, that of the long-length FBG is more than 100 mm. We applied this sensing technique to strain measurements and defect detection in a welded joint. We monitored the strain distributions along the weld lines of the tubular joints of steel pipes subjected to tensile loads. We could successfully measure the development of the strain distributions along the weld lines, where the sharply fluctuating strain distributions resulting from some defects were observed. In this paper, we describe the sensing principle and then show the results of strain monitoring in the welded joints.

We have developed an X-ray source using portable X-band (9.3-12 GHz) linear accelerator (linac) for bearing real-time inspection. This system can generate pulsed-X-ray, therefore if X-ray repetition rate synchronizes with bearing rotation rate, we can get still images without stopping rotation. We succeeded in static imaging of rotating equipment. We start to develop more advanced X-ray generators using portable linac and multi-beam klystron (MBK). The advanced systems use higher electron energy linac and MBK as more stable radio-frequency (RF) source. We used 950 keV linac and 250 kW magnetron for X-ray imaging. However oscillation is unstable because magnetron is self-excited oscillator. Moreover, its temperature and rotation influence its output RF (Radio-Frequency) power, electron beam current and X-ray intensity. For the purpose of more stable operation and compactness of the system, we propose a multi-beam klystron (MBK) as its RF power source. In this paper, we show the basic parameters of the system which stability is tolerable for bearing condition based maintenance (CBM) in near future.