Bong Young Ahn

Microstructural Characterization of Creep Damaged 11Cr-3.5W-3Co Steel

The effects of the precipitate and martensite lath on the softening behavior have been investigated for 11Cr-3.5W-3Co steel during creep at 700. During creep, the precipitate on the PAG (prior austenite grain) boundaries and martensite lath boundaries coarsened. The recovery of dislocation density and an increase of martensite lath width took place. It is shown that the inverse of the size of the precipitates and the inverse of the square root of the martensite lath width have a linear relation with the Vickers hardness, which corresponds to the Hall-Petch relation and particle looping mechanism.

THICKNESS MEASUREMENT OF ALUMINUM PLATES WITH A PULSED EDDY CURRENT METHOD

In this study, coil sensors that are used with a pulsed eddy current (PEC) were designed and fabricated. The proposed sensor is a differential send-receive type that eliminates the voltage induced by the direct field from the exciting coil in a PEC probe. The signal from the sensing coil is generated only on a metal specimen. For the experiment, various metal plates of different conductivities and aluminum plates of different thicknesses from 1 mm to 25 mm were prepared. A voltage square pulse was applied to an exciting coil, and the output from the sensing coil was captured by a digital oscilloscope. The applied voltage was approximately 20 volts at its greatest with a current peak of approximately 15 amperes. A 10% variation of the thickness of an aluminum plate of 20 millimeter thick was measured.

Multi-Coil Eddy Current Probe for the Detection of Circumferential Cracks in Tubing

Conventional eddy current bobbin probes, multi-pancake and/or rotating pancake probes, and transmit-receive eddy current probes are currently utilized in testing metal tubing. Each method has respective strengths and weaknesses. This paper proposes another eddy current probe with new features. The structure is designed to be sensitive to circumferential cracks, which are not easily detected with ordinary bobbin coil probes. The directions of the magnetic field and the eddy current around the coil were considered in design of the probe structure. Signals of these probes from the artificial defects were acquired and analyzed. Experimental results show that the developed probes are more sensitive to circumferential defects than comparable ordinary bobbin probes. In addition, the new probes are insensitive to axial defects. By employing both the new probes and ordinary bobbin probes, more reliable ECT can be performed.

Simultaneous Detection of Size and Depth of Embedded Reinforcement in Concrete by Arraying Sensing Coils in a Probe

This study suggests an electromagnetic method for simultaneous detection of the size and depth of embedded reinforcement in concrete by using a developed probe with multiple sensing coils. The probe is made so that the sensing coils are located at the inside and the outside of an exciting coil. The signal characteristics of the sensing coils are investigated with the aid of an LCR meter. By using a commercial eddy current test system, with the developed probe, we simultaneously evaluated the size and cover depth of reinforcement. The voltage amplitudes of the sensing coils in contrast to the concrete cover depths show a unique value for each depth and size of reinforcement.

Elastic Constants Determination of the Thin Cold-Rolled Stainless Steel by Acoustic Resonance Method

Stainless steel has a good corrosion resistance and a good high temperature mechanical properties. In this study, temperature dependence of elastic constants of thin cold rolled stainless steel has been measured by using the acoustic resonance method[1]. Identification of the vibration mode has been examined numerically and experimentally. The elastic constants at room temperature have been also measured by Pulse Echo method. In addition, the texture effect on the elastic constants has been analysed by assuming the specimen as orthorhombic structure.

Manipulation of the behavior of SiC particles and oil bubbles using ultrasonic standing wave field

Using ultrasound, particles submerged or flowing in fluid can be manipulated since ultrasound has an effect on the behavior of particles. Specifically, in standing wave field, particles generally move to pressure nodes or pressure antinodes due to acoustic radiation force. In this work, the behavior of SiC particles and oil bubbles in flowing water by standing wave field was investigated. Standing wave field in frequencies between 2 and 2.5 MHz was formed in a few mm narrow flow channel using a water coupled ultrasonic transducer and a steel reflector. We observed the effect of the standing wave parameters such as frequency, flow channel width, or sound intensity on the behavior of the particles. Various interesting results were obtained through some experiments. We separated SiC particles and oil bubbles. It was shown that the operating frequency of standing wave can control the particle moving location. Sound intensity increase also leads to the entrapment of moving particles. The resulted observations ...

Design and Characterization of Capacitive Micromachined Ultrasonic Transducer

As cMUTs (capacitive Micromachined Ultrasonic Transducer) offer numerous advantages over traditional transducers in terms of efficiency, bandwidth, and cost, they are expected to replace piezoelectric transducers in many applications. In particular, 2D-array cMUTs have aroused great interest in the medical engineering society because of their ability to materialize a true volumetric ultrasonic image. In this study, single element cMUTs with 32 x 32 and 64 x 64 cells were successfully fabricated. The diameter and thickness of the membrane are 35 and 1000 nm, respectively, with a sacrificial layer thickness of 600 nm. The electric characteristics of the fabricated cMUT were measured. Tests on the efficiencies of the cMUT in terms of wave generation and in terms of detection according to the bias and pulse voltage were performed in an air atmosphere.

Degradation Evaluation of 2.25Cr-1Mo Steel Using Nondestructive Method

Several nondestructive evaluation methods were attempted for the estimation of the creep damage of degraded 2.25Cr-1Mo steel. The specimens of three different aging periods were prepared by an isothermal heat treatment at 430°C, 482°C, and 515°C. The effect of probe configuration on the electrical resistivity was studied. Single configuration method and dual-configuration method were utilized for measuring electrical resistivity. The electrical resistivity was determined by a standard DC four-point probe method at 24±0.5°C. Indentation test, magnetic characteristics test and ultrasonic test were carried out to investigate the correlation between the major characteristics and aging parameter. Unlike the electrical resistivity characteristics, ultrasonic attenuation coefficient and indentation characteristics did not show a relation to Larson-Miller parameter. However, a correlation between the electrical resistivity and aging parameter was identified, which allows one to estimate the extent of material degradation.

Applications of a Capacitive Probe to Detect Defects on Dielectric Materials

This paper suggests a capacitive probe method as a nondestructive test technique for detecting flaws on dielectric materials. Presented includes a theoretical explanation of a mechanism whereby the phase of a detected flaw signal changes with relation to a particular lift-off distance. This method uses absolute and differential modes of arrayed capacitive probes with a commercial eddy current test system. This method can detect cylindrical surface flaws of 1.5 mm diameter and depths of more than 0.05 mm in a GFRP plate. Introduction For any given material, its typical constants, such as electric conductivity, magnetic permeability, and permittivity are used to represent the electromagnetic properties of that material. Materials can be classified into two groups by their electromagnetic properties: conductors and dielectrics. We can employ the eddy current test (ECT) method to inspect the defects of conductors nondestructively. Dielectric materials, however, can not be tested with the ECT technique, owing to the very low conductivities of such materials. The ECT, an excellent technique for the detection of surface or sub-surface flaws in conductive materials, is an inductive method based on the magnetic induction among coils and conductors. Reflecting upon the electrical duality between inductance and capacitance, a capacitive probe method can be derived for dielectric materials, as the counterpart of the ECT for metals. This method uses the capacitive coupling between the metal plates (electrodes) and the dielectric medium of a capacitor. This nondestructive test method for dielectric materials, by using the measurement of the variation of capacitance, was suggested by P. J. Shull et al.[1-2]. We modified the structure of the probe so that it can be used in real time. Using a fabricated capacitive probe, we investigated the signal response to a dielectric material and evaluated its characteristics.. V. K. Tewary [3] studied the noncontact characterization of dielectric properties of materials. Tewary’s study showed that admittance of the probe increased exponentially as the dielectric material approached the probe. We have observed some interesting properties in our experiments. When we plot the current variation of the probe caused by presence of the dielectric material as a function of lift off distance (the distance between the probe plane and the surface of the dielectric material), the curve momentarily decreases near the dielectric and then slowly increases again. This phenomenon also causes the detected flaw signal to change its phase, depending on the lift off distance. We will explain the phenomenon theoretically, using a computational electromagnetic package, which is based on the Boundary Element Method (BEM). The theoretical investigation confirmed our experimental results. Principle of the Capacitive Probe In Fig. 1(a), when we supply AC voltage to the exciting (or source) electrode, the electric field generated from the exciting electrodes is received by the sensing (receiver) electrode(s). Then a displacement current flows between the electrodes. As Eq. 1 and Eq. 2 indicate, the intensity of the electric field and the displacement current are influenced by the medium placed between the Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 606-611 doi:10.4028/www.scientific.net/KEM.270-273.606

Flaw Detection of Railroad Wheel Tread Using EMAT

Since railroad wheels undergo high stresses due to dynamic or static loads continuously, flaws may be produced on the tread surfaces and they can lead to catastrophic failures of the wheels. In this study an EMAT system for inspecting every wheel of a train in a roll-by mode will be introduced. Two EMATs were installed in two shielding boxes separately for generating and receiving surface wave respectively. The boxes were designed for the EMAT to keep up the lift-off of 0.2 mm over the entire surface. The system was able to observe at least 10 round-trips with the EMAT on an unflawed railroad wheel. Four flaws were made on a wheel surface to evaluate the flaw detectability of the system and the signal from the 3 mm deep notch showed good signal to noise ratio. Since the variations in the lift-off due to some factors such as dirt can be caused in rail yard usages, flaw signals were divided by the round-trip signal for the normalization. And a new algorithm to distinguish the flaw signal from abrupt noise was suggested. Introduction While the railroad can transport much population and freight very fast, an abrupt railroad accident can cause a catastrophic disaster. The derailment occurred in Germany at 1998 was recorded as worst rail disaster in half a century. In Korea a project to construct the high-speed rail has been progressed for about 11 years. In the long run a line between Seoul and Daejeon will be completed at December 2003 and an interim revenue service of Seoul-Busan line will be started at April 2004. Therefore in this country the importance of the safety in the high-speed rail increases more and more. Since the railroad wheels always undergo stresses due to dynamic or static loads and due to heating during braking, the possibility of crack occurrence on the surface is very high. The surface crack can cause the serious wheel failure resulting in considerable equipment damage and possible derailment. Therefore the flaw detection test of the wheel has been performed periodically in a maintenance shop and also by a trackman during staying in the railway station. But there was no way to test every wheel of a train in roll-by mode although it is very important in a high-speed train. To solve that problem a noncontact ultrasonic test method using EMAT was developed by IzfP in Germany [1][2] and it was installed in ICE line. Although their system could detect the flaws in new wheels, there are some problems in detecting the flaws in aged wheels where the risk of cracks is increased. It was recognized that the flaw detectability of EMAT was lowered because of the large scattering of Rayleigh wave at shattered surfaces. In this study, we were focused on the EMAT design to maximize the amplitude of the ultrasonic wave and minimize the noise for effective railyard usage. New design concept was adopted to fit the transducer surface to curved and tapered tread surface. And an algorithm to discriminate the flaw signal from abrupt noise was suggested. EMAT & System Construction Wave Generation. EMAT consists of a coil and a magnet array as shown in Fig. 1 [3][4]. When the alternating current flows in the coil above the surface of electrically conductive materials, eddy Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 619-624 doi:10.4028/www.scientific.net/KEM.270-273.619