Youn Ho Cho

A Wall Thinning Detection and Quantification Based on Guided Wave Mode Conversion Features

This study presents a feasibility of using guided waves for a long-range inspection of pipe through investigation of mode conversion and scattering pattern from edge and wall-thinning in a steel pipe. Phase and group velocity dispersion curves for reference modes of pipes are illustrated for theoretical analyses. Predicted modes could be successfully generated by controlling frequency, receiver angle and wavelength. The dispersive characteristics of the modes from and edge wall-thinning are compared and analyzed respectively. The mode conversion characteristics are distinct depending on dispersive pattern of modes. Experimental feasibility study on the guided waves was carried out to explore wall thinning part in pipe for data calibration of a long range pipe monitoring by comb transducer and laser.

A Characteristic of Scattering Patterns from Defect in a Rail

Rail represents one of the most important means of transportation. Many nondestructive testing methods have been used to find defects in rail. The guided wave technique is the most efficient because of its long propagation characteristic along the rail. The wave structure of the rail cross-section for a particular loading condition of modes and frequencies is an important feature. The wave structures are examined at different modes and frequencies. The propagation scattering patterns of guided waves from various multiple defects in a rail are also studied using finite element method.

A Computational Tool for Defect Analysis in Rail with Ultrasonic Guided Waves

Rail represents one of the most important means of transportation. Many nondestructive testing methods have been used to find defects in rail. The guided wave technique is the most efficient because of its long propagation characteristic along the rail. Potential for detecting transverse cracks exists whereas standard bulk wave technique could miss the cracks. The wave structure of the rail cross-section for a particular loading condition of modes and frequencies is an important feature. In this paper, the propagation and scattering patterns of guided waves in a rail are studied using finite element methods. The wave structures are also examined. Various multiple defect situations and rail boundary conditions can also be studied.

SH Guided Wave Scattering in a Structure with Thickness Variation

Presented in this paper is a numerical approach to extract useful features of SH guided wave scattering in a Plate-like structure with thickness variation. The problem statement is chosen to establish a physically based guideline for mode selection and data analyses in power industry application. The hybrid BEM technique with absorbing boundaries on the structure cross-sections is proposed to calculate reflection and transmission from thickness variation. Sample numerical data is presented as a function of incident mode, frequency and defect geometry. Mode optimization is proposed to enhance penetration power and quantify geometry change based on various interesting features including reverberation pattern of scattering fields.

A Study on the Relationship of Frequency and Deformation for Viscoelastic Material

Viscoelastic materials are used in automobiles and other products. However, because of theoretical complexity, it has not been easy to put all the energy exhausted into the automotive viscoelastic materials. Since time is the most important factor in the study of viscoelastic material, creep and stress relaxation functions are very important. In this study, a bushing was selected for special viscoelastic material. A bushing is a device used in automotive suspension systems to cushion the force transmitted from the wheel to the frame of the vehicle. A bushing is essentially a hollow cylinder which is bonded to a solid metal shaft at its inner surface and a metal sleeve at its outer surface. The shaft is connected to the suspension and the sleeve is connected to the frame. The cylinder provides the cushion when it deforms due to relative motion between the shaft and sleeve. The relation between the force applied to the shaft or sleeve and their deformation is nonlinear and exhibits features of viscoelasticity. A force-displacement relation for a bushing is important for multi-body dynamics numerical simulations. Hence, an explicit force-displacement relation has been introduced. The relation is expressed in terms of a force relaxation function. With Pipkin-Rogers model, the direct relation of force and displacement that has been derived from experiment and numerical simulation, the sinusoidal displacement was chosen and the relation of frequency and deformation for the viscoelastic material was studied.

Sub-Surface Crack Detection by Using Laser Induced Transient Stress Wave Propagation

In this study we attempt to investigate the possibility of detecting sub-surface crack and the understanding of the propagating phenomena of transient stress waves due to impact in thick aluminum plate by the simultaneous measurement of longitudinal and shear creeping and Rayleigh wave resulted from the mode conversion of laser induced transient stress wave impact. The propagation of the transient stress wave generated by laser irradiation is affected by the sub-surface crack and the result is analyzed. It was observed that the longitudinal and shear creeping wave velocities are varied depending on the depth of sub-surface crack. In addition, the variation of amplitude ratio generated by propagating the stress wave is investigated. The longitudinal creeping wave velocity in the presence of the sub-surface crack is somewhat faster than in case of non-crack. And the shear creeping wave velocities represent large variations which are shown nearly 2nd order quadratic curve shape as the sub-surface crack depth increase under the same experimental condition. The results of this study are very useful for the nondestructive evaluation of the surface layer in thick structures by non-contact method and the opposing and the structures difficult to access.

A Study on the Behavior of Ultrasonic Guided Wave Mode in a Pipe Using a Comb Transducer

A preliminary study of the behavior of ultrasonic guided wave mode in a pipe using a comb transducer for maintenance inspection of power plant facilities has been verified experimentally. Guided wave mode identification is carried out in a pipe using time-frequency analysis methods such as wavelet transform (WT) and short time Fourier transform (STFT), compared with theoretically calculated group velocity dispersion curves for longitudinal and flexural mode. The results are in good agreement with analytical predictions and show the effectiveness of using the time-frequency analysis method to identify the individual guided wave modes. And, It was found out that longitudinal mode (0, 1) is affected by mode conversion less than the other modes. Therefore, L (0, 1) is selected as a optimal mode for evaluating location of the surface defect in a pipe.

Ultrasonic Monitoring Technique for Icing and Flow in a Duct

Transient freezing of laminar water flow in a square duct was investigated experimentally while varying the temperature and flow rate of the water and the temperature of the brine. In addition, the use of ultrasonic waves to monitor the freezing and flow in the duct was explored. There was excellent agreement between the ultrasonic data and data obtained using a conventional flow measurement technique in determining the thickness of the ice layer and flow velocity. Ultrasonic waves can be used to measure the growth of an ice layer and flow velocity in a duct. Introduction In this paper, experiments are conducted to examine the freezing process of water on the chilled plate. The work was related to the cold energy storage using a phase change material. Although the tracking of interface is required to design and control of the thermal storage system, it’s not easy to monitor the interface of phase change material in most cases. Numerous studies have examined solidification in cylinders and spheres and between two flat plates. Zhang et al. (1) investigated the effect of natural convection on solidification in a closed vessel with a cool side wall. Bennon et al. (2) investigated the effects of natural and forced convection on the solidification of high and low Pr fluids between two flat plates numerically. Cheng et al. (3) also studied the solidification of a fluid between flat plates. Seki et al. (4) studied the freezing of water flowing in cooled channels. This study considered the freezing of water flowing in a duct, which is related to continuous casting, to consider the freezing of living cells. Since freezing in a pipe may result in blockage of the flow, it is very important to predict and measure the thickness of the freezing layer and the flow velocity along the pipe to prevent blockage. With an opaque pipe, it is difficult to measure the thickness of the ice layer or to install a device to measure flow velocity. Unlike conventional measuring devices, which have measurement limits, methods using ultrasonic waves have the advantage of being able to obtain a signal simply by placing a sensor in contact with the surface of the pipe. There are two ways to measure flow velocity using ultrasonic waves: the ultrasound Doppler measurement (5,6) and transient-time method (7) . Commercial LDV(Laser Doppler Velocimetry) devices use the first method. In biomechanics, flow velocity has been measured using ultrasonic waves. (8) This study used the transient-time method, because the results are easier to analyze than those of the ultrasound Doppler measurement. The study observed the transient freezing of water flowing in a rectangular duct and used an ultrasonic wave to measure the thickness of the frozen layer and the flow velocity. Experimental setup Fig. 1 shows the experimental device, which consisted of a circuit for circulating water and brine, cold bath, flow meter, pump, and pipe. The test section had a rectangular acryl duct with a copper bottom measuring 7 mm thick and 15 cm long. In order to cool the copper plate, a flow path of brine was formed below it. At three points along the copper plate, 30-gauge K-type thermocouples were installed. The transient freezing process was recorded with a camcorder, which took several images at a constant time interval. The contours of the ice layer were overlapped in order to investigate the characteristics of the frozen layer. The ultrasonic wave measuring-device consisted of a tone-burstKey Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 396-401 doi:10.4028/www.scientific.net/KEM.270-273.396

Investigation of Guided Wave Scattering in Curved Waveguides

This paper aims at investigating guided wave scattering phenomena in curved waveguides. The use of hybrid numerical technique combining interior boundary element solutions with the finite element method for a semi-infinite waveguide is proposed for investigating guided wave scattering from a doubly bent section. Some sample results of numerical parametric study are also given with a physical interpretation of the variation of guided wave scattering profiles with respect to the bend geometry and the corresponding modes. Introduction Recently, the use of ultrasonic guided wave has been well known as a promising alternative in various NDE applications especially for a long range inspection [1-5]. This is mainly because it has a unique acoustic character that its propagation path is guided by a certain geometry of structures to be tested while bulk waves can interact only with local zone [1, 2]. As an example, corrosion in pipework is a major problem in the oil, chemical and other industries. Many pipes are insulated which means that quite often, approaching to desired inspection area is not even possible without removing the insulation. A significant advance has established in modeling of various guided wave scattering problems though the vigorous efforts in recent years [6]. The more practical implementation of the guided waves is attempted for NDE applications [3-5], modeling studies take the more crucial role for optimization of inspection scheme and quantification of signal interpretation. In 80’s, the guided waves modeling works have been carried out mainly by either pure analytical or semi-analytical approaches with the assumption to simplify their complicate “mode conversion” [6, 7]. In 90’s, the breakthrough to employ the various hybrid techniques in conjunction with either FEM [8, 9] or BEM [10, 11] allows us to handle more realistic models with sophisticate geometry and consequently, the various mode conversions turned to be rather unique promising features for the guided wave technique which can be physically predicted and readily used for tuning experimental schemes. Although a number of earlier works have been reported in the modeling of guided wave scatterings from various defects [6], the studies on waveguide shape changes such as the curved ones are still relatively rare. A lack of physical interpretation for the data is also a room for improvement and some pioneering efforts has just recently begun to focus on the issue. Consequently, the establishment of more detailed data library with physical explanation on guided wave scattering profiles is still highly demanded. In this study, the hybrid modeling technique combining the semi-analytical finite element method and the boundary element method is proposed and the detailed data library with some physical explanation on the S0 Lamb mode interaction with a doubly bent waveguide joint is illustrated varying incident modes, frequency and joint configuration. Descriptions on Numerical Scheme and Models Our goal is to study the feasibility of transmitting the Lamb waves with satisfactory penetration efficiency through an S bending in a plate. Therefore, the motivation of this study is to investigate if Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 447-452 doi:10.4028/www.scientific.net/KEM.270-273.447

Ultrasonic Sensitivity Enhancement for Material Characterization Using Guided Wave Energy Loss Parameters

The feasibility of Lamb waves for monitoring thermally degraded materials is explored. It turns out that the use of Lamb waves leads to a promising nondestructive technique for the purpose of microstructure evaluation and material characterization of such materials. This is because Lamb modes can interact with entire area of a plate-like specimen while a conventional point-by-point technique is confined to just local investigations. 2.25Cr-1Mo steel specimens for various degradation levels were prepared by isothermal aging heat treatment at 650°and evaluated by the present technique to investigate the influence of the thermal damage to the Lamb wave feature based on the modal energy loss ratio. Introduction Recently, the typical material degradation found in the atomic or turbine power plant is due to high temperature creep and aging [1,2]. However, it is not always possible or practical to evaluate wellprepared specimens of identical condition to ones in use for laboratory test. In this sense, development of an efficient and reliable technique to monitor material degradation condition has been of great concern [3]. It is well known that various ultrasonic waves have been used for material inspection and condition monitoring. In general, the term, “attenuation” represents the decay of wave energy in terms of amplitude change. However, this definition becomes valid only in the case of a nondispersive waveform with a consistent pulse duration time. Consequently, a technique based on the use of a dispersive waveform such as guided wave modes needs to involve the changes in amplitude as well as pulse duration time for more precise material condition monitoring with wave attenuation parameters. In this sense, the guided wave modal energy loss parameter defined as time integration of amplitude variation inside a wave packet is introduced in this study to alleviate the experimental deviation induced by the dispersion effect. The experimental data based on the Lamb wave technique is correlated with the ones of conventional, visual material characterization by TEM (Transmission Electron Microscopy) so that the feasibility of the present approach is verified. Experimental Details Specimens. The test material is 2.25Cr-1Mo steel used as a turbine rotor material for a hightemperature and high-pressure power plant. The reason why this material was chosen for this study is that there is much demand for this alloy in various industries because of its unique characteristics like corrosion resistance and the suitability for use in a high temperature environment. Experimental Setup. Fig.1 is a schematic diagram of the experimental setup for measuring attenuation by the immersion test. Broadband immersion type transducers of 0.5 inch diameter with Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 442-446 doi:10.4028/www.scientific.net/KEM.270-273.442