Hak‐Joon Kim

Simulating the Experiments of the 2005 Ultrasonic Benchmark Study

In the first part of the 2005 benchmark study the pulse‐echo response of an artificial crack in a titanium block was obtained experimentally at a number of discrete angles. The second part of the study involved pulse‐echo responses obtained from linear scans of a side‐drilled hole in an aluminum specimen using both planar and focused probes. These experiments were simulated by ultrasonic measurement models and the model‐based and experimental results compared. A multi‐Gaussian beam model was used to model the ultrasonic wave fields present in all cases studied and the Kirchhoff approximation was used to predict the scattering from the crack. The Kirchhoff approximation and method of separation of variables were used to model the wave scattering from the side‐drilled hole. A discussion of the model and experimental comparisons is given.

Simulating Angular Flaw Scattering Responses Using a Hierarchical Triangular Meshing Method

In this paper an ultrasonic measurement model is developed using three main elements: 1) a multi‐Gaussian beam model for simulating the ultrasonic wave fields incident on a flaw 2) a hierarchical triangular meshing (HTM) method for representing the flaw geometry and 3) the Kirchhoff approximation for modeling the waves scattered from the flaw Using this measurement model, we have simulated the angular scattering responses for pores, cracks, and stringer‐like flaws. The possibility of using such results for flaw classification studies is discussed.

Transferring Distance‐Amplitude Correction Curves Using Ultrasonic Modeling

In practice, it is common to manufacture reference blocks containing simple reflectors to obtain distance‐amplitude correction (DAC) curves. However, the construction of DAC curves in this manner requires the use of a large number of specimens with appropriate curvatures and reference reflectors located at various depths. Therefore, less costly and quantitative procedures are strongly needed. To address such a need, in this study, we have developed model‐based transfer curves to relate a DAC curve obtained in a particular reference configuration with that for a completely different configuration an example of transferring DAC curves, using the proposed transfer curves, is given.

Modeling the Response of Ultrasonic Reference Reflectors

Reference reflectors characterized as flat-bottom hole (FBH), side-drilled hole (SDH), and spherical void (SPH) are commonly used in ultrasonic nondestructive evaluation (NDE) for calibration purposes. Here the measurement models currently available to simulate the A-scan response from those types of reference reflectors are examined. Measurement models suitable for both large and small reflectors are described and used to study the effect of beam variations over the surface of the reflector. The adequacy of the Kirchhoff approximation for predicting the waves scattered by these reference reflectors is also studied by comparing the results of that approximation to those obtained from more exact scattering solutions. The waveforms predicted by these various models are compared with experimentally determined responses in a pulse-echo immersion setup, and the accuracy of the models is discussed.

Measurement and Modeling of Ultrasonic Attenuation in Aluminum Rolled Plate

When fabricating a new set of calibration blocks for Aluminum 7075 plate inspections, it is advantageous that the new blocks have similar ultrasonic attenuation to existing block sets. This allows the new set to qualify under the same ASTM procedures used for older sets. In the course of surveying candidate materials for possible use as calibration blocks, some interesting attenuation results were observed. When a candidate block was cut from a thick section of rolled plate, measured back‐wall attenuation values in the rolling or transverse direction were quite sensitive to position in the plate‐normal direction. Such variations are presumably tied to microstructural variations within the plate, as revealed by metallography. Some measured attenuation values were found to be in good agreement with predictions of the Stanke‐Kino model, while others were not. The measurements and modeling work are reviewed, and additional experiments conducted to clarify certain issues are discussed. Those additional experim...

Models of Standard Ultrasonic Reference Reflectors and Their Experimental Validation

In this paper, models of the pulse‐echo ultrasonic immersion responses of three types of reference reflectors are given. The reference reflectors considered are a flat‐bottom hole (FBH), a side‐drilled hole (SDH) and a spherical void (SPH). Reciprocity‐based measurement models suitable for large and small size reflectors are given, where the variations of the incident fields over the surface of the reflector are either considered or neglected, respectively. The scattering responses of these reflectors are modeled with the Kirchhoff approximation and more exact solutions. The received waveforms predicted by these models are compared with experimentally determined responses and the accuracy of the models is discussed.

Generation of the Basis Sets for Multi‐Gaussian Ultrasonic Beam Models

By using a small number of Gaussian basis functions, one can synthesize the wave fields radiated from planar and focused piston transducers in the form of a superposition of Gaussian beams. Since Gaussian beams can be transmitted through complex geometries and media, such multi-Gaussian beam models have become powerful simulation tools. In previous studies the basis function expansion coefficients of multi-Gaussian beam models have been obtained by both spatial domain and k-space domain methods. Here, we will give an overview of these two methods and relate their expansion coefficients. We will demonstrate that the expansion coefficients that have been optimized for circular piston transducers can also be used to generate improved field simulations for rectangular probes. It will also be shown that because Gaussian beams are only approximate (paraxial) solutions to the wave equation, a multi-Gaussian beam model is ultimately limited in the accuracy it can obtain in the very near field.

A Localized Multi‐Gaussian Ultrasonic Beam Model

A multi‐Gaussian beam model is described where the velocity field on the face of an ultrasonic immersion transducer is represented in terms of a large number of localized Gaussians. The versatility of this approach is demonstrated by using this type of model to represent both focused and planar probes, rectangular and circular probe shapes, and piston as well as non‐piston behavior.

Investigation of Time Reversal Techniques to Enhance Flaw Detectability

Phased array transducers are often used to attain high focusing performance. In recent years, time reversal is claimed to be a very robust technique to focus through inhomogeneous medium, using the phased array transducers, as compared to conventional focusing techniques. In this paper, the performance and limitations of the Decomposition of the time Reversal Operator (D.O.R.T.) method, which is one of the time reversal techniques, is considered based on experimental studies. Furthermore, we introduce a variation, pre‐focused D.O.R.T., in order to overcome limitations of the D.O.R.T. method. The results are compared to results obtained by conventional focusing methods.

Measurement Models for Predicting the Ultrasonic Response from Side‐Drilled Holes

We develop measurement models and scattering models that allow us to predict the ultrasonic response from a side‐drilled hole (SDH). The scattering models considered include an explicit model based on the Kirchhoff approximation, as well as an “exact” separation of variables solution. Two measurement models are also derived. One allows for the variation of the incident fields over the cross‐section of the SDH while a simpler model neglects those variations. The predicted responses are compared with experimental signals.