Younho Cho

A boundary element solution for a mode conversion study on the edge reflection of Lamb waves

The boundary element method, well known for bulk wave scattering, is extended to study the mode conversion phenomena of Lamb waves from a free edge. The elastodynamic interior boundary value problem is formulated as a hybrid boundary integral equation in conjunction with the normal mode expansion technique based on the Lamb wave dispersion equation. The present approach has the potential of easily handling the geometrical complexity of general guided wave scattering with improved computational efficiency due to the advantage of the boundary‐type integral method. To check the accuracy of the boundary element program, vertical shear wave diffraction, due to a circular hole, is solved and compared with previous analytical solutions. Edge reflection factors for the multibackscattered modes in a steel plate are satisfied quite well with the principle of energy conservation. In the cases of A0, A1, and S0 incidence, the variations of the multireflection factors show similar tendencies to the existing results fo...

Estimation of ultrasonic guided wave mode conversion in a plate with thickness variation

The hybrid boundary element method aimed at analyzing Lamb wave scattering from defects can provide us with an excellent numerical tool for tackling complicated mode conversion phenomena under waveguide thickness variation. In this paper, utilization of hybrid boundary element modeling for specific Lamb wave mode incidence situations with special energy distributions along the structural cross section is proposed for estimating reflection and transmission from various scatterers, such as a step discontinuity and tapered parts of a waveguide, etc. Interaction of individual Lamb wave modes with scatterers that represent arbitrary thickness variation along the direction of guided wave propagation is investigated by calculating the scattered fields for varying incident modes, frequency, and scatterer shape. The mode conversion phenomena through step discontinuity in a plate are also experimentally explored. The theoretical predictions of reflection and transmission by boundary element methods and the utility of dispersion curves are compared with experiments for specific modes. Results in this paper can be used to improve inspection sensitivity and penetration power for a variety of practical NDE applications, notably those in which thickness variation is found. In addition, the feasibility of inspecting sections located behind a waveguide thickness variation region and subsequent mode control will also be discussed.The hybrid boundary element method aimed at analyzing Lamb wave scattering from defects can provide us with an excellent numerical tool for tackling complicated mode conversion phenomena under waveguide thickness variation. In this paper, utilization of hybrid boundary element modeling for specific Lamb wave mode incidence situations with special energy distributions along the structural cross section is proposed for estimating reflection and transmission from various scatterers, such as a step discontinuity and tapered parts of a waveguide, etc. Interaction of individual Lamb wave modes with scatterers that represent arbitrary thickness variation along the direction of guided wave propagation is investigated by calculating the scattered fields for varying incident modes, frequency, and scatterer shape. The mode conversion phenomena through step discontinuity in a plate are also experimentally explored. The theoretical predictions of reflection and transmission by boundary element methods and the utility of dispersion curves are compared with experiments for specific modes. Results in this paper can be used to improve inspection sensitivity and penetration power for a variety of practical NDE applications, notably those in which thickness variation is found. In addition, the feasibility of inspecting sections located behind a waveguide thickness variation region and subsequent mode control will also be discussed.

Lamb wave scattering analysis for reflector characterization

The potential use of guided waves for defect characterization is studied. The influence of defect shape and size on transmitted and reflected fields is considered. Using the hybrid boundary element technique, the reflection and transmission coefficients for selected guided wave modes are numerically calculated and compared to experimental data. Selecting the aspect ratio as a shape parameter for various defects, the transmission and reflection coefficients are measured for certain guided wave modes input to the defect. The influence of defect size is then studied by monitoring the transmission and reflection coefficients for defects of various shapes and depths. The studies presented indicate that defect characterization is possible if a proper mode selection criteria can be established. The suitable features related to transmission and reflection coefficient data can also be used for algorithm development and implementation purposes of defect characterization.

An elastodynami`c hybrid boundary element study for elastic guided wave interactions with a surface breaking defect

Elastic guided wave interactions with various defects are explored for investigating defect characterization possibilities by using a hybrid boundary element method (BEM) in combination with an elastodynamic boundary integral equation and the Lamb wave normal mode expansion technique. The BEM code accuracy is verified based on energy conservation and available bench marking data for guided wave scattering problems. Through two-dimensional (2-D) parametric studies for an arbitrarily shaped defect, from a surface breaking crack model to a round surface defect, a waveguide cross-section including a defect is locally selected as a model for a given incident mode, frequency and a specific set of material properties. Mode reflection and transmission factors are numerically calculated to evaluate mode sensitivities and to obtain the potentially good classification features. It turns out that the guided wave scattering profiles show quite different behaviors as functions of incident mode, frequency, defect shape and size in providing us with enough rich feature extraction information for defect classification and sizing analysis. The theoretical analysis can be used to establish efficient guidelines for both data acquisition and feature selection in a pattern recognition analysis program of study. Sample results are presented.

Elastic wave analysis for broken rail detection

A technique is presented to identify broken rail. The technique is based on elastic guided wave propagation through the rail. Broken rail detection is possible by either utilizing an impact device or energy propagating from the train wheel in contact with the rail. In principle, elastic guided wave energy propagates along the track, a natural wave-guide, until a broken rail is encountered. Pulse echo reflection is then used to identify broken rail. Considering an exponential decrease in penetration power from a source as a function of distance and natural energy filtering mechanisms of a rail, a simple model of guided wave propagation in rail is discussed. A feasibility demonstration of the concept is presented for a train going 50 miles per hour (90 km/hour) towards a broken rail with sensors fixed on rails close to the break.

Modeling for flaw sizing potential with guided waves

Guided waves have demonstrated their value in flaw detection in a variety of different and unusual circumstances, including inspection over long distances with just 1 or 2 probes and inspection under coatings, fluids, and insulation. Via mode control and phase velocity and frequency tuning, defect detection sensitivity can be superb. The problem of going beyond detection to defect classification and sizing, however, is extremely difficult. By way of boundary element modeling, some new approaches to classification and sizing are introduced. A theoretical presentation illustrates some trends and features that might be useful in sizing analysis. Parametric studies and analysis showing amplitude versus frequency profiles for various mode input and mode conversion output via through transmission are presented. A few basic flaw shapes of different size are studied in an attempt to shed some light onto the difficult classification and sizing process.

Boundary element modeling for guided wave reflection and transmission factor analyses in defect classification

The guided wave scattering process by different defects in plate structures is studied with a boundary element method. The purpose is to separate sharp crack-like defects from smooth volumetric-like ones by analyzing the transmission and reflection factors. The traditional normal mode expansion technique has been combined with the boundary element discretization to form a hybrid BEM scheme. With this method, the scattered near-fields and the transmission and reflection coefficients of the far fields on individual modes can be obtained simultaneously. Several features related to the Lamb mode reflection and transmission process in a plate with these defect types are extracted and compared. The theoretical analysis provides a guideline for data acquisition and feature selection for use in the decision algorithm development program via neural nets.

A Comb Transducer Model for Guided Wave Mode Control

A variety of ultrasonic transducers can be used for quantitative NDE. One commonly used is a comb transducer [1–5]. Design of this probe involves a detailed consideration of its physical and geometrical parameters. These design parameters should provide the best capability for generation and selection of a particular guided wave mode. The comb sizing (element width plus gap) determines the length of the generating wave. Parameter characterization of the comb transducer without time delay features was performed in [1]. A diverse discussion of techniques for transducer design is described in [4] and some practical NDE applications in guided wave generation are described in [5]. Unfortunately, practical considerations make it difficult to alter a comb’s geometry, consequently, restricting the ability to generate an arbitrary point on the dispersion curve. The goal of the current study is to overcome this restriction without adjustment or building another probe, by introducing a time delay profile. This means that each transducer element is excited with some time delay. An arbitrary phase velocity for the excitation of a desired point on the dispersion curve is then selected by determining the appropriate time delay value.

Guided-Wave Tomographic Imaging of Plate Defects by Laser-Based Ultrasonic Techniques

Contact-guided-wave tests are impractical for investigating specimens with limited accessibility and rough surfaces or complex geometric features. A non-contact setup with a laser-ultrasonic transmitter and receiver is quite attractive for guided-wave inspection. In the present work, we developed a non-contact guided-wave tomography technique using the laser-ultrasonic technique in a plate. A method for Lamb-wave generation and detection in an aluminum plate with a pulsed laser-ultrasonic transmitter and Michelson-interferometer receiver was developed. The defect shape and area in the images obtained using laser scanning, showed good agreement with the actual defect. The proposed approach can be used as a non-contact online inspection and monitoring technique.

A study on nondestructive evaluation technique by the use of interface guided waves on shrink fit structure

Guided wave was widely studied for plate and pipe due to the great application area. Guided wave has advantage on long distance inspection for an inaccessible area and apart from transducer. Quite often shrink fit structures were found in nuclear power facilities. In this paper, two pipes were designed with perfect shrink fit condition for Stainless Steel 316. The displacement distribution was calculated with boundary condition. The interface wave propagation pattern was analyzed by the numerical modeling. The experimental results show a possibility of weld delamination and defect detection.