Soon Woo Han

Torsional wave experiments with a new magnetostrictive transducer configuration

For the efficient long-range nondestructive structural health inspection of pipes, guided waves have become widely used. Among the various guided wave modes, the torsional wave is most preferred since its first branch is nondispersive. Our objective in this work is to develop a new magnetostrictive transducer configuration to transmit and receive torsional waves in cylindrical waveguides. The conventional magnetostrictive transducer for the generation and measurement of torsional waves consists of solenoid coils and a nickel strip bonded circumferentially to test pipes. The strip must be premagnetized by a permanent magnet before actual measurements. Because of the premagnetization, the transducer is not suitable for the long-term on-line monitoring of pipes buried underground. To avoid the cumbersome premagnetization and to improve the transduction efficiency, we propose a new transducer configuration using several pieces of nickel strips installed at 45 degrees with respect to the pipe axis. If a static bias magnetic field is also applied, the transducer output can be substantially increased. Several experiments were conducted to study the performance of the proposed transducer configuration. The proposed transducer configuration was also applied for damage detection in an aluminum pipe.

Noncontact torsional wave transduction in a rotating shaft using oblique magnetostrictive strips

A noncontact method to generate and measure torsional waves in rotating shafts by a set of magnetostrictive patches and a solenoid is proposed. The main issues in the present wave experiment are how to generate torsional waves having sufficient power and how not to interfere with shaft rotational motion. Magnetostrictive patches are bonded obliquely to the shaft axis for pure torsional wave generation and an arrayed patch configuration is employed for sufficient power generation and frequency localization. For uninterrupted shaft rotation, a solenoid encircling a rotating shaft wirelessly supplies or detects the ac magnetic field in the patches. The validity and performance of the proposed wave transduction method were checked by several experiments. An application of this method to the detection of a circumferential crack in a rotating shaft is also presented.

High-frequency torsional modal testing of a long cylinder by magnetostriction

The torsional modal testing of cylindrical bodies still remains formidable. This is because pure torsional eigenfrequencies typically lying in the kilohertz frequency range are nearly impossible to excite by popular excitation methods. In this study, a torsional modal testing method using a set of magnetostrictive patches, specially arranged permanent magnets, and solenoids is investigated to cover a frequency range of a few kilohertz. The use of magnetostrictive patches for the testing was motivated by torsional wave experiments using the patches, but the use of special magnet configurations was a key to successful modal testing. The developed method was validated with experiments.

Noncontact Damage Detection of a Rotating Shaft Using the Magnetostrictive Effect

The objective of this investigation is to develop a cost-effective noncontact diagnostic method for rotating shafts in operation. Unlike most existing diagnostic methods developed for rotating shafts, longitudinal stress waves are used and processed for damage assessment. For the non-contact measurement of stress waves in rotating shafts, we propose to use magnetostrictive effect. Shaft rotations inevitably accompany lateral vibrations; thus the effects of the vibrations on the measurement accuracy of the sensor are studied to verify the validity of the magnetostrictive effect application to rotating shafts. For damage location estimation, we use the continuous wavelet transform of the measured wave signals. In particular, we propose to adopt the real-valued Gaussian wavelets as the mother wavelet in order to pinpoint accurately the arrival time of the reflected wave from a crack. Several case studies are considered to show the effectiveness of the present diagnostic method.The objective of this investigation is to develop a cost-effective noncontact diagnostic method for rotating shafts in operation. Unlike most existing diagnostic methods developed for rotating shafts, longitudinal stress waves are used and processed for damage assessment. For the non-contact measurement of stress waves in rotating shafts, we propose to use magnetostrictive effect. Shaft rotations inevitably accompany lateral vibrations; thus the effects of the vibrations on the measurement accuracy of the sensor are studied to verify the validity of the magnetostrictive effect application to rotating shafts. For damage location estimation, we use the continuous wavelet transform of the measured wave signals. In particular, we propose to adopt the real-valued Gaussian wavelets as the mother wavelet in order to pinpoint accurately the arrival time of the reflected wave from a crack. Several case studies are considered to show the effectiveness of the present diagnostic method.

Investigation of the phase separations and the local electronic structures of Zn1−xTxO (T=Mn, Fe, Co) magnetic semiconductors using synchrotron radiation

The spatial chemical distributions of doped T ions and their local electronic structures in Zn1−xTxO (T=Mn, Fe, Co; x⩽0.1) have been investigated simultaneously by using scanning photoelectron microscopy (SPEM), photoemission spectroscopy (PES), and soft-x-ray absorption spectroscopy (XAS). The measured SPEM images for Zn1−xTxO single crystals reveal the uniform chemical distributions of T ions, indicating the homogeneous substitution of T ions for Zn sites. The T2p XAS spectra of Zn1−xTxO show that Mn and Co ions are in the divalent states, while Fe ions are in the Fe2+–Fe3+ mixed-valent states. The valence-band PES study of Zn1−xTxO (T=Mn, Fe, Co) shows that T3d states lie near the top of the O 2p valence bands. Only ferromagnetic Zn1−xTxO shows a significantly large Fe 3d photoemission intensity near EF, in contrast to the negligible Mn and Co 3d photoemission intensities near EF in nonferromagnetic Zn1−xTxO (T=Mn, Co), suggesting that the high carrier density is important in determining the ferromagne...

Valence values of the cations in selenospinel Cu(Cr,Ti)2Se4

A long-standing issue about the Cu valency in selenospinel CuCr2Se4 was investigated by soft X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). Using the sensitivity of XAS and XMCD to the valence value of transition metal ion and its local symmetry, we checked the valence value of each cation in selenospinel CuCrxTi 2−xSe4 (x = 1.0, 1.1, 1.5, and 2.0) and obtained spectroscopic evidence that a small amount of the Cu cation changes the valency from Cu(I) to Cu(II) as the Cr concentration increases from 1.0 to 2.0. Dependence of the Cu(II) concentration and the mean field magnetic exchange energy on the Cr concentration suggests the Cu d-hole could play a crucial role in the intriguing magnetic/electrical properties of CuCr2Se4.

Magnetic sensor for the noncontact measurement of flexural vibrations of a nonferromagnetic metallic hollow cylinder

A solenoid-type magnetic sensor is developed for noncontact measurement of flexural vibrations of a nonferromagnetic metallic cylinder. The sensor measures the vibration by means of the reversed Lorentz force mechanism but flexural vibrations are measured only when antisymmetrically polarized static bias magnetic field is applied to the cylinder. In this respect, the effect of the bias magnetic field distribution on vibration sensing was investigated in the first place. Because the sensor uses a solenoid, it can measure vibrations of a cylinder even if it is covered with insulation; no insulation removal is needed with this sensor. Fundamental experiments underlying the sensor characteristics were conducted and the developed sensor was applied for experimental modal testing of pipes with and without insulation.

Synchrotron-radiation spectroscopy of electron- and hole-doped collossal magneto-resistance double perovskites: BxA2−xFeMoO6(A=Ba,Sr;B=La,K)

The electronic structures of BxA2−xFeMoO6 (A=Ba,Sr;B=La,K) double perovskites have been investigated by using photoemission spectroscopy and soft-x-ray absorption spectroscopy (XAS) near the Fe 2p and O 1s absorption edges. The Fe 2p XAS spectra of undoped A2FeMoO6 (A=Ba,Sr) provide evidence for the mixed-valent Fe2+–Fe3+ states. The substitution of La3+ and K+ ions for A2+ ions causes the increasing Fe3+ component and increasing Fe2+ component, respectively. The states close to the Fermi level have mainly the Mo–Fe t2g(↓) character, consistent with the Fe2+–Fe3+ mixed-valent states. As the amount of the K+ substitution increases in KxSr2−xFeMoO6, the photoemission intensity near the Fermi level decreases, reflecting the decreasing carriers and the decreasing bandwidth of the itinerant t2g(↓) states.

Damage detection on composite structures with standing wave excitation and wavenumber analysis

This paper describes the use of wavenumber filtering for damage detection with a signal-frequency standing wave excitation on composite structures. Using a single, fixed frequency excitation from a mounted piezoelectric transducer, the full steady-state wavefield could be obtained using a Laser Doppler Vibrometer with a mirror-tilting device. After completing the scanning, a wavenumber filtering is applied to determine dominant wavenumber components of the measured wavefield, which could be used for indicative of structural damage. Mapping processes based on local wavenumber filtering is then carried out for damaged area visualization. Also introduced are the comparison of two methods for damage identification and visualization: the local wavenumber mapping and acoustic wavenumber spectroscopy. To demonstrate the proposed techniques, several experiments are performed on composite structures with different types of damage, including debonding and delamination on composite plates. The results demonstrate that the techniques are very effective in localizing damage with the potential for the quick inspection of a variety of composite structural components.

Synchrotron radiation spectroscopy study of FeCr2X4 (X=S and Se)

Electronic structures of FeCr2X4 (X=S and Se) have been investigated by employing soft-x-ray absorption spectroscopy and soft-x-ray magnetic circular dichroism (XMCD). It is found that FeCr2S4 and FeCr2Se4 have similar electronic structures. The valence states of Cr and Fe ions are nearly trivalent (Cr3+) and divalent (Fe2+), respectively. The Fe 3d states are strongly hybridized to the X p states. The Fe and Cr 2p XMCD study provides evidence for the antiparallel alignment between Cr and Fe spins and the strong hybridization between the Fe 3d and X p electrons.