Kazuyuki Nakahata

Elastodynamic inversion for shape reconstruction and type classification of flaws

Two linearized inverse scattering methods are investigated to reconstruct the shape of flaws in the elastic solid. One is based on the Born approximation and the other is based on the Kirchhoff approximation. The Born inversion is sensitive to the volumetric flaw but not to a crack-like flaw. On the other hand, the Kirchhoff inversion reacts to both boundaries of volumetric and crack-like flaws. The combined use of Born and Kirchhoff inversions leads to the classification method of flaw type. The performance of the proposed classification procedure is demonstrated by the numerical simulations and then by the experimental measurements for the two-dimensional models of flaws. An example for the three-dimensional shape reconstruction is also shown by using the numerically calculated backscattering amplitudes.

3‐D IMAGE‐BASED SIMULATION FOR ULTRASONICWAVE PROPAGATION IN HETEROGENEOUS AND ANISOTROPIC MATERIALS

Time domain simulation tools for ultrasonic wave propagation in a target material with a complex outer surface or various inclusions are developed by combining the finite integration technique (FIT) and the finite element method (FEM) with an image‐based modeling. In our simulation, the geometry of the target is determined by a digital image such as X‐ray picture, CAD data, etc. and the processed voxel data is directly fed into the numerical calculation with the FIT or FEM. The accuracy of the two methods is discussed by comparison with the boundary element method. A simulation of the ultrasonic testing for a concrete material is shown with the 3‐D image‐based FIT.

Three Dimensional Born and Kirchhoff Inversions for Shape Reconstruction of Defects

Three dimensional Born and Kirchhoff inverse scattering methods are modified to convenient forms for a cylindrical specimen that includes three dimensional defects. The measurement area in the modified methods is restricted in the plane perpendicular to the axis of cylindrical specimen. The modified methods are applied to the cement paste specimen with an egg‐shaped defect and the performance of the modified methods is confirmed.

3-D Modelings of an Ultrasonic Phased Array Transducer and Its Radiation Properties in Solid

Ultrasonic phased array technology has become popular in the field of industrial nondestructive testing (NDT). The technology is notable for its capability to provide images of the inside of a target in real time. Since the image quality results from rapid steering and focusing of the radiation beam from an array transducer, it is essential to have well understood characteristics of the radiated beam. Even though fundamental concepts of the array transducer in medical fields were introduced over 30 years ago (Macovski, 1979), the beam modeling and optimization of the array transducer for the NDT have only been investigated intensively in the past decade (Azar et al., 2000; Song & Kim, 2002). The characteristics of the radiated beam from the ultrasonic phased array transducer vary according to the transducer parameters such as frequency, aperture size, number of elements, pitch(inter-element spacing), element width, layout dimensions and so forth. If these parameters are not chosen properly, spurious grating lobes or side lobes with high amplitude will exist in the radiation beam field. Consequently, the image quality will be deteriorated significantly.

COMPARATIVE STUDY ON ULTRASONIC IMAGING METHODS WITH ARRAY TRANSDUCERS

The performance of ultrasonic imaging methods developed in time domain and frequency domain algorithms is investigated in this study. The sectorial synthetic aperture focusing technique (S–SAFT) outputs the flaw image by superposing the sector–scan images obtained at various measurement points in the time domain. The inverse scattering imaging method (ISIM) is based on the elastodynamic inversion in the frequency domain and reconstructs the mathematical function which represents the flaw shape. The advantages and usabilities of these methods are discussed, taking into account the practical application to the NDE field.

A classification method of defect type by Born and Kirchhoff inversions

Two linearized inverse scattering methods are investigated to classify the defect type in an elastic body. The methods are based on the Born and Kirchhoff approximations for the unknown displacement in the integral representation of scattered field. From the result of experimental measurement, the Born inversion reconstructs the volume of defects and the Kirchhoff inversion reconstructs the surface of defects. The combination of two inversion methods leads to the classification between volumetric defect and crack.

3‐D Flaw Imaging by Inverse Scattering Analysis Using Ultrasonic Array Transducer

Ultrasonic matrix array transducers have the advantage of receiving flaw echoes simultaneously at various points on a flat surface of the test material. Here we propose 3‐D imaging techniques to reconstruct flaw shapes with the array transducer. These techniques are based on linearized inverse scattering methods in the frequency domain. The principal operation of these methods is the integration of the wave data in the K‐space. In this study, the 3‐D fast Fourier transform is introduced into the inversion algorithm to evaluate the integral in the K‐space. Performance of the 3‐D imaging technique is demonstrated by using the numerically calculated waveforms by the fast multipole BEM.

Nonlinear Ultrasonic Investigation of Concrete with Varying Aggregate Size under Uniaxial Compression Loading and Unloading

AbstractNondestructive testing in concrete is complex due to its nonhomogeneous ingredients. The presence of varying sizes of aggregates in concrete results in nonlinear characteristics. This paper aims to compare mortar and normal concrete with varying aggregate sizes. The specimens are damaged gradually with loading and unloading paths. Fast Fourier transform (FFT) of the recorded time domain waveforms is conducted to show the frequency spectra. The amplitude of the second harmonic frequency is used as a reference to the internal damage. The analysis of the nonlinear ultrasonic parameter second harmonic amplitude is used according to each loading branch with two damage levels: total and incremental. Results showed that the magnitude of the total damage produced by mortar at the last loading branch was relatively lower than all of the concrete mixes with different sizes of aggregates. The magnitude of the incremental damage produced by mortar in every step load consistently increased as the load increase...

Three dimensional image-based simulation of ultrasonic wave propagation in polycrystalline metal using phase-field modeling.

When modeling ultrasonic wave propagation in metals, it is important to introduce mesoscopic crystalline structures because the anisotropy of the crystal structure and the heterogeneity of grains disturb ultrasonic waves. In this paper, a three-dimensional (3D) polycrystalline structure generated by multiphase-field modeling was introduced to ultrasonic simulation for nondestructive testing. 3D finite-element simulations of ultrasonic waves were validated and compared with visualization results obtained from laser Doppler vibrometer measurements. The simulation results and measurements showed good agreement with respect to the velocity and front shape of the pressure wave, as well as multiple scattering due to grains. This paper discussed the applicability of a transversely isotropic approach to ultrasonic wave propagation in a polycrystalline metal with columnar structures.

Numerical Simulation of Scattered Waves from Flaws for Ultrasonic Array Transducer

To enhance the detectability in the phased array UT, it is essential to have well knowledge on the characteristics of ultrasonic waves from array transducers. This paper proposes a mathematical model of the array transducer and a simulation tool to predict the flaw echoes. The modeling of an array transducer is based on the Rayleigh‐Sommerfeld integral and the scattered waves from flaws are calculated with the fast multipole BEM (FMBEM). By using the FMBEM, we can solve large scale scattering problems with relatively low computational cost. Here we focus on the transient wave analysis, in which a pulse‐shaped wave is used for exciting elements of the array transducer.