Papers by Author: Youn Ho Cho

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.