Hisashi Yamawaki

Ultrasonic Imaging of Welded Metals Using Simplified Ultrasonic Computerized Tomography.

In this paper, as a new measurement method to estimate the infinitesimal change of material condition, the simplified ultrasonic computerized tomography (CT) system, which uses the information of three directions, i.e., the 90°, +45° and -45° directions about the inspection plane, is proposed. The use of the simplified ultrasonic CT system has two merits: First, the measurement time is very short compared with that of the conventional CT method. Second, it can detect sensitively infinitesimal defects in vertical or oblique directions about the inspection plane because the obtained image is not a C scan image but a CT image calculated from the three directions. Due to these merits, this method can be considered to be a very effective method for the evaluation of material condition. In order to determine the applicability of actual NDT, several kinds of welded specimens are investigated. The results showed that the CT images obtained were very similar to actual defects of specimens. Also, the same CT images were obtained by computer simulation and the obtained CT images by computer simulation were almost the same as the experimental images.

Mid IR pulsed light source for laser ultrasonic testing of carbon-fiber-reinforced plastic

A quasi-phase-matched (QPM) optical parametric oscillator (OPO) was developed using a periodically poled Mg-doped stoichiometric LiTaO3 crystal to generate mid-IR light for excitation of laser ultrasound in carbon fiber reinforced plastic (CFRP). The ultrasound generation efficiency was measured at the three different wavelengths that emanate from the OPO: 1.064 μm, 1.59/1.57 μm, and 3.23/3.30 μm. The measurements indicate that mid-IR 3.2–3.3 μm light generates the most efficient ultrasonic waves in CFRP with the least laser damage. We used mid-IR light in conjunction with a laser interferometer to demonstrate the detection of flaws/defects in CFRP such as the existence of air gaps that mimic delamination and voids in CFRP, and the inhomogeneous adhesion of CFRP material to a metal plate was also clearly detected.

Noncontact ultrasonic imaging of subsurface defects using a laser-ultrasonic technique

Noncontact ultrasonic imaging of subsurface defects using a laser-ultrasonic technique was demonstrated. Pulsed laser irradiation was used for ultrasonic generation, and optical heterodyne interferometry was used for detection of ultrasonic vibration on the specimen surface. Bonded thin plates with character-shaped hallmarks with 1.5 mm width on bonding layer were used as specimens. Imagings in ultrasonic transmission and reflection modes were performed, and imagings using not only XY-scanning of specimens but also a laser beam scanner were demonstrated. Ultrasonic images of hallmarks located 0.2 mm below the specimen surface were obtained through the experiments.

Detection of ultrasonic longitudinal and shear waves using optical heterodyne interferometer

An optical heterodyne interferometer was developed to enable noncontact detection of ultrasonic motions parallel and perpendicular to the specimen surface. Two optical setups, one for the parallel direction and another for the perpendicular direction, were included in the interferometer. The interferometer enabled simultaneous ultrasonic detection in both directions at the same point on the specimen surface without changing the optical layout of the interferometer. For verification of the technique, longitudinal and shear ultrasonic waves generated by piezoelectric transducers were detected by the interferometer.

The development of simplified ultrasonic CT system and its application to the evaluation of weld metal

In this paper, as a new measurement method to estimate the change of material condition, the simplified ultrasonic CT system, which uses the information of three directions, that is, 90°, + 45° and −45° about inspection plane is proposed. Use of simplified CT system has two merits: Firstly, the measurement time is very short compared with general CT. Secondly, it can detect sensitively small defect in vertical or slant direction about inspection plane because the obtained image is CT image calculated from three directions. From these merits, this method can be considered as an effective method to evaluate material conditions. The basic performance of the proposed method was confirmed through several specimens with several simple defects. In order to confirm the applicability of actual NDT, several kinds of welded specimens are investigated. The result showed that the CT image obtained had good agreement with actual defect of specimens.

Development of Three-Dimensional Ultrasonic Simulation and Its Application.

A computer simulation technique for 3-dimensional ultrasonic waves was developed for visualization and investigation of ultrasonic propagation in isotropic and anisotropic solids with defects. The technique is similar to a finite-difference method (FDM) and a mass-particle model method, but uses a new nodal calculation method based on fundamental consideration of an elastic wave equation. The new method simplifies calculations on boundary nodes, so that it requires no pseudonodes as in the FDM, and is applicable to not only anisotropic but also complex materials including defects. A program has been developed for 3-dimensional calculation, but it requires the huge memory and high speed of supercomputers. Due to the limitation in computer performance, we present here conventional applications under the 2-dimensional condition including 3-dimensional motion. Some simple applications are shown for ultrasonic propagation in anisotropic (hexagonal) materials and layered materials, and wave scattering at a crack.

In-Situ Observation and Acoustic Emission Analysis for Corrosion Pitting of MgCl2 Droplet in SUS304 Stainless Steel

Acoustic emission with near-field detection and optical video microscope monitoring was proposed to investigate the pitting corrosion caused by the small magnesium chloride droplet. For the corrosion testing of work-hardened SUS304 stainless steel, four types of the detected waveforms were characterized by the wavelet analysis. After the high activity AE signals, the corrosion product sheet was extended without or with low activity AE signals. With the cross-section observations, the detected AE signals were mainly attributed to the following three sources, the longitudinal and transverse cracking around the covered pitting, cracking of the oxidation products, and the early stage of the corrosion potential fluctuation.

Computer simulation of ultrasonic testing for aerospace vehicle

Non-destructive testing techniques are developed to secure reliability of aerospace vehicles used repetitively. In the case of cracks caused by thermal stress on walls in combustion chambers of liquid-fuel rockets, it is examined by ultrasonic waves visualization technique developed in AIST. The technique is composed with non-contact ultrasonic generation by pulsed-laser scanning, piezoelectric transducer for the ultrasonic detection, and image reconstruction processing. It enables detection of defects by visualization of ultrasonic waves scattered by the defects. In NIMS, the condition of the detection by the visualization is investigated using computer simulation for ultrasonic propagation that has capability of fast 3-D calculation. The simulation technique is based on finite-difference method and two-step elastic wave equations. It is reported about the investigation by the calculation, and shows availability of the simulation for the ultrasonic testing technique of the wall cracks.

Electromagnetic properties of plasma sprayed thermal barrier coatings in terahertz range

Dielectric properties of yttria partially stabilized zirconia thermal barrier coatings with various pore density were investigated by terahertz time domain spectroscopy in the continuous frequency range of 0.3 ~ 1.3 THz. The real part of the refractive index was 4.2 ~ 4.8 and the value increases as the porosity decreases. It indicates the potential application of terahertz spectroscopy to inspect the microstructure of plasma sprayed coatings nondestructively.

Image enhancement of simplified ultrasonic CT using frequency analysis method

In this paper, a simplified ultrasonic CT system, which uses the information in three directions, that is, 90°, +45° and —45° about the inspection plane, is applied to the high strength steel, and the frequency analysis method for enhancing the C scan or CT image is developed. This frequency analysis method is based on the frequency response property of the material. By comparing the magnitudes in the frequency domain, the special frequency which shows a significant difference between the welded joint and base material was found and used to obtain a C scan or CT image. Experimental results for several kinds of specimens, having a welded joint by electron beam welding, a weld joint by arc welding, on a fatigue crack, showed that the obtained C scan or CT image has better resolution than the results of previous experiments using the maximum value of the received waveform.