Diffraction Of Ultrasonic Waves Research Articles

Numerical study on ultrasonic detection of cracks in metallic structures with variable cross-sectional thickness

The metal structures are widely used in aerospace engineering, especially in joints and spars. The safety of these parts is important to ensure the safe service of the whole structures. The ultrasonic wave is a traditional technique to detect cracks in metals. However, it is usually difficult to find all the cracks because of its difficulty in variable directions of the crack and the irregular shapes of the metals. This paper aims to address the challenge of detecting cracks in metallic structures with varying cross-sectional thicknesses using ultrasonic methods. The research investigates the behavior of ultrasonic longitudinal and transverse waves as they propagate through aluminum plates with variable cross sections. Additionally, it examines how cracks in different orientations influence these wave types. The theoretical analyses are performed which explore the reflection of oblique incidence waves and the diffraction of ultrasonic waves when they encounter semi-infinite cracks. Subsequently, numerical simulations are performed to validate the theoretical findings and to detect both horizontal and vertical cracks within metal plates of varying cross-sectional thicknesses. The findings indicate that both longitudinal and transverse waves are effective in identifying the location of damage caused by horizontal cracks. However, when it comes to vertical cracks, only transverse waves are successful in detecting their precise location. From the research of this paper, transverse ultrasonic wave is suggested to detect cracks in metal structures with irregular cross sections.

Acousto-optic method of electronic laser phase control for laser interferometry

The paper describes a method of electronic control of laser phase for laser interferometry. The method utilizes acousto-optic interaction and ultrasonic wave diffraction. The article presents an opto-electronical scheme for phase shift control with high resolution in reference and object beam without using high voltage.

Defect Detection of Aluminium Plates Based on Near-Field Enhancement of Lamb Waves Generated Using an Electromagnetic Acoustic Tranducer

Ultrasonic testing is an important means to detect defacing defects, such as scratches and cracks, but when the size of these defects is smaller than the wavelength of ultrasonic waves, it is difficult to detect them using traditional methods like the pulse-echo method and broadband ultrasound attenuation method for the diffraction of ultrasonic waves at the defects. Based on the non-contact characteristic of electromagnetic acoustic transducers (EMATs), a transducer for scanning inspection was developed in this paper. The transducer was utilized to detect and measure the depth of the defacing defects on an aluminium plate based on the near-field enhancement of ultrasonic Lamb waves. The results show that the amplitude of the S0 Lamb wave experiences a large enhancement when the transducer is passed over the scratch defects and the enhancement has a clearly positive correlation with the depth of the scratch defects. When the depth increases from 0.1 mm to 0.9 mm, the amplitude of S0 Lamb waves increases from 1.13 times to 2.27 times the S0 Lamb waves received on the aluminium plate without defects. The new method can be utilized to detect the defacing defects on the aluminium plate and get better detection effects than the traditional methods without analyzing the relatively small reflection waves.

Osnove ultrazvuka - fizika i artefakti

Ultrasound (US) represents mechanical, longitudinal, acoustic waves with frequencies above 20 000 Hz. Ultrasound waves of frequency from 2 to 20 MHz are used in diagnostics. US waves have the following characteristics: propagation velocity, frequency, wavelength and amplitude. By passing through human bodies, in interaction with different tissues, US loses energy-attenuation. This is due to: absorption, reflection, refraction, dispersion and diffraction of ultrasonic waves. Reflection is responsible for the occurrence of an ultrasound image. In the ultrasonic transducer there are piezoelectric crystals that have the ability to convert electrical impulses into mechanical (ultrasonic) waves and vice versa. By connecting an US with a circuit, piezoelectric crystals convert electrical impulses into US waves. In the interaction with matter, some of them are reflected/rejected. These echoes are again detected by piezoelectric crystals in US transducer and converted into electrical impulses which are displayed on the screen in the form of points. The US transducer is both a generator and a US wave receiver. On the basis of reflected echoes in the form of points, a picture on the screen is formed. The intensity of reflected echoes represents echogenicity, i.e. the ability of a structure to create US waves. Thus, we distinguish anechoic, hypoechoic, hyperechoic and isoechoic structure. Based on the frequency, resolution and penetration of ultrasonic waves, the probe is divided into: convex, linear and sector. The basic rule is that the higher the frequency of the US transducer the better the image resolution, but the smaller the penetration of the US wave. Forms that do not represent anatomical structures appear as a result of US examinations but are the result of specific inspection techniques and are called artefacts. These include: acoustic enhancement, acoustic shadow, reverberation and the phenomenon of 'mirrors'.

Two-dimensional analytic modeling of acoustic diffraction for ultrasonic beam steering by phased array transducers

Phased array ultrasonic transducers enable modulating the focal position of the acoustic waves, and this capability is utilized in many applications, such as medical imaging and non-destructive testing. This type of transducers also provides a mechanism to generate tilted wavefronts in acousto-optic deflectors to deflect laser beams for high precision advanced laser material processing. In this paper, a theoretical model is presented for the diffraction of ultrasonic waves emitted by several phased array transducers into an acousto-optic medium such as TeO2 crystal. A simple analytic expression is obtained for the distribution of the ultrasonic displacement field in the crystal. The model prediction is found to be in good agreement with the results of a numerical model that is based on a non-paraxial multi-Gaussian beam (NMGB) model.

Simulation of Internal Crack Detection in Shape-Distorted Cast Billets Using Time-of-Flight Measurements of Longitudinal Ultrasonic Waves

To detect cracks in shape distorted cast billets, we newly propose a method that is the use of ultrasonic computerized tomography (CT) in combination with measurements of time-of-flight (TOF) of longitudinal ultrasonic waves at multiple planes as a more robust reconstruction method. The TOF is measured by applying the transmission method, and the measured TOF is used to reconstruct the apparent sound velocity profile. Cracks are identified as a decrease in the apparent sound velocity, which is caused by an increase in the TOF after the diffraction of ultrasonic waves at the cracks. The combination makes it possible to remove the effects of shape distortion to be compensated by the TOF measurements at multiple planes. Therefore, cracks are clearly detected using this method. The detection ability of this method is evaluated by numerical simulations of a shape-distorted billet. The results of this study are validated with help of wave propagation simulations using the transmission line matrix (TLM) method. More specifically, the newly proposed method allowed us to detect cracks that could hardly be distinguished with the use of our previously proposed method owing to fluctuations of the apparent sound velocity caused by shape distortions. In addition, using the new method enables the detection of cracks located near the billet’s surface whose dimensional change is as small as 5%. From the results of this study, we believe that the newly proposed method could be useful for the crack detection in shape-distorted billets. [doi:10.2320/matertrans.I-M2011854]

An acoustic multi-touch sensing method using amplitude disturbed ultrasonic wave diffraction patterns

This paper proposes an acoustic multi-touch tactile sensing method. The proposed method is based on an amplitude disturbed ultrasonic wave diffraction pattern. An A 0 Lamb wave transmitted in a thin finite copper plate is processed to provide tactile information, for one or two fingers. A touch event is localized by identifying the diffraction signals among a database of diffracted Lamb wave references. Statistic models are used to improve the localization reliability. An artificial silicone finger is used in the calibration procedure. This touch interface is evaluated as a 2-touch interface.

A217人工欠陥周りの超音波伝搬の可視化

We have developed a laser ultrasonic system for the visualization of elastic waves propagating on a solid surface. Scanning an optical heterodyne probe to measure surface transient displacements, ultrasonic waves in the frequency range up to 50MHz are detected and passed to a personal computer through a digital oscilloscope. The recorded signals are reconstructed to make 3-D displacement images at any propagation time. This system can produce a series of successive images as an animation of wave propagation. Using this visualization technique, we observed the scattering and diffraction of ultrasonic waves around various shapes of simulated defects, and examined its application to nondestructive inspection

Visualization of Elastic Waves by Digital Laser Ultrasonics.

We have developed a laser ultrasonic system for a visualization of elastic waves propagating on a solid surface. Scanning an optical heterodyne probe to measure surface transient displacements, ultrasonic waves in the frequency range up to 50 MHz are detected and passed to a personal computer through a digital oscilloscope. The recorded signals are reconstructed to make 3-D displacement images at any propagation time. This system can produce a series of successive images as an animation of wave propagation. Using this visualization technique, we observed the scattering and diffraction of ultrasonic waves around various shapes of penetrations on aluminum plate, and examined its application to nondestructive inspection.

Diffraction of ultrasonic waves from elastic Galois gratings

A significant attenuation of the specular echo in a wide frequency domain occurs in scattering of waves by Galois gratings. Diffraction of ultrasonic waves by immersed one-dimensional elastic gratings constructed from the finite Galois fields GF(pm) is investigated. We develop a theoretical model taking into account the elasticity of the structure, its finite thickness, the boundary conditions at the fluid–solid and solid–fluid interfaces, and the periodicity of the gratings. In addition to the typical attenuation of the specular echo predicted by discrete Fourier transform, such a model predicts minima in the spectrum of the reflection coefficient corresponding to the excitation of surface waves (generalized Rayleigh and Scholte–Stoneley waves) by mode conversion and to the excitation of Lamb-type modes. The attenuation as well as the minima are experimentally observed and these results are in good agreement with theoretically predicted ones. Galois gratings could constitute an advance in the search for new anechoic devices.

Scattering by immersed two-dimensional elastic galois gratings

Diffraction of ultrasonic waves by immersed elastic bigratings constructed from the Galois fields GF(23) along one direction and GF(32) along the orthogonal one is studied. Such bigratings permit a significant attenuation of the specular echo in a wide frequency domain. This attenuation is experimentally observed and agrees with the theoretical one predicted by a model which takes into account the elasticity of the structure, the boundary conditions at the fluid–solid interface, and the periodicities of the gratings. Furthermore, numerous sharp minima in the spectrum of the reflection coefficient are experimentally observed and theoretically predicted; they correspond to the excitation of surface waves (generalized Rayleigh and Scholte–Stoneley waves) and they contribute significantly to the attenuation of the specular echo.

3-Dimensional Image Reconstruction of Diffraction Tomography from Data Collected with Compound Scanned Pair of Transducers

We extend our 2-dimensional method of ultrasonic diffraction tomography to 3 dimensions. We first derive the theory and the procedure which correspond to 3-D data collection. Then we propose two image-reconstruction methods: interpolation method and noninterpolation method. We also calculate the obtainable resolution of these imaging methods. We numerically simulate the diffraction of ultrasonic waves from the assumed object and calculate the data of diffracted waves. Using the data we finally reconstruct the 3-D images of the object to confirm the feasibility of our theory.

Flaw characterization based on diffraction of ultrasonic waves

A method of flaw characterization based on time domain analysis of the diffracted sound field from a flaw when irradiated by short ultrasonic pulses is described. The echo formation mechanism developed by Freedman has been utilized to predict the return pulse pattern. Experimental results that are in good agreement with theoretical predictions are presented. Using a microcomputer-based scanning and testing system the feasibility of reproducing planar, straight-edged flaw shapes has been successfully demonstrated.

Diffraction of ultrasonic waves from periodically rough liquid–solid surface

Theoretical and experimental results concerning diffraction of ultrasonic waves on periodic liquid–elastic solid interfaces are presented. A general system of linear equations is established and solved numerically for triangular surfaces. Theoretical results are in good agreement with measurements obtained by a broadband pulse-echo system in the region where the wavelength is of the same order of magnitude as the period and much greater than the depth of the grating. In the spectrum of the reflection coefficient frequency minima are identified as the surface waves excited along the periodic surface.

Diffraction of ultrasonic waves by penny-shaped cracks in metals: Theory and experiment

In this paper an analytical solution to the diffraction of elastic waves by penny-shaped cracks in metals is compared with experimental observations. The analysis, which is based on elastodynamic ray theory, is valid for the region of ka ≳ 1. A digitized spectrum analysis system is described which measures the frequency components of the waves diffracted from a 2500-μ-radius crack in diffusion bonded titanium. The amplitude spectra show good agreement between experiment and theory. The theoretically predicted periodicity of the diffracted spectra provides a simple formula for the inverse problem. Application of this formula to the experimental measurements determines the crack size with excellent accuracy.

Influence of fractures on diffraction of ultrasonic waves

Influence of fractures on diffraction of ultrasonic waves

Optical probing of ultrasonic diffraction in single crystal sodium chloride

Results are presented of an experimental study of ultrasonic wave diffraction for the acoustical modes propagating along the [1̄1̄2], [11̄0], and [111] directions in single crystal sodium chloride. Laser beam optical probing in the Raman-Nath regime was used to map the sound intensity distribution in the farfield of circular transducers. Calculations of the diffraction spread based on representation of the acoustic field by an angular spectrum of plane waves are in agreement with the experimental measurements. A simple method for estimating the diffraction spread based on the geometry of the inverse speed (slowness) surfaces is presented.

Secondary Interference in the Fresnel Zone of Gratings

The Fresnel zone of optical gratings—the region behind the grating where secondary interference between the light coming through the different openings produces real images of the grating—can be understood by comparison with other types of wave phenomena. In the Fresnel zone of light diffracted by an optical grating, different types of amplitude modulation of the wave front are observable. Gratings of stationary ultrasonic waves, being phase gratings to the light passing through them, produce similar, though not identical Fresnel patterns. Between the planes in which the secondary interference patterns are true images of the gratings, the modulation changes from amplitude to phase modulation. This change can be demonstrated by an ultrasonic wave analog. Patterns formed by the diffraction of ultrasonic waves by an amplitude grating are given. Similar patterns in surface waves are given for both the phase grating and the amplitude grating analog.