392 research outputs found
Numerical modeling in quantitative ultrasonic tomography of standing trees
International audienceThe aim of this project is to develop an ultrasonic device for parametric imaging of standing trees. The device is designed to perform both transmission and reflection measurements that can be therefore used for quantitative tomographic imaging. It allows various automatic acquisitions, since the angular position of the transducers could be adjusted. This makes possible to scan the wave propagation occurring in all directions inside the medium. The associated electronic set-up allows mainly measuring the slowness (and therefore the velocity) and the attenuation of the ultrasonic waves. Tomograms were computed by fast algebraic algorithms: (1) using the filtered backprojection algorithm with fan beam geometry, (2) using a new algorithm that we are developing based on a "layer-stripping" method. Our first numerical results on an academic and realistic phantom of tree are presented in this paper
Tomography applied to Lamb wave contact scanning nondestructive evaluation
The aging world-wide aviation fleet requires methods for accurately predicting the presence of structural flaws that compromise airworthiness in aircraft structures. Nondestructive Evaluation (NDE) provides the means to assess these structures quickly, quantitatively, and noninvasively. Ultrasonic guided waves, Lamb waves, are useful for evaluating the plate and shell structures common in aerospace applications. The amplitude and time-of-flight of Lamb waves depend on the material properties and thickness of a medium, and so they can be used to detect any areas of differing thickness or material properties which indicate flaws. By scanning sending and receiving transducers over an aircraft, large sections can be evaluated after a single pass. However, while this technique enables the detection of areas of structural deterioration, it does not allow for the quantification of the extent of that deterioration. Tomographic reconstruction with Lamb waves allows for the accurate reconstruction of the variation of quantities of interest, such as thickness, throughout the investigated region, and it presents the data as a quantitative map. The location, shape, and extent of any flaw region can then be easily extracted from this Tomographic image. Two Lamb wave tomography techniques using Parallel Projection tomography (PPT) and Cross Borehole tomography (CBT), are shown to accurately reconstruct flaws of interest to the aircraft industry. A comparison of the quality of reconstruction and practicality is then made between these two methods, and their limitations are discussed and shown experimentally. Higher order plate theory is used to derive analytical solutions for the scattering of the lowest order symmetric Lamb wave from a circular inclusion, and these solutions are used to explain the scattering effects seen in the Tomographic reconstructions. Finally, the means by which this scattering theory can be used to develop Lamb wave Tomographic algorithms that are more generally applicable in-the-field, is presented
Acoustic tomography imaging for atmospheric temperature and wind velocity field reconstruction
Owing to its non-invasive nature, fast imaging speed, low equipment cost,
scalability for a variety of measurement ranges, and ability to simultaneously
monitor both temperature and wind velocity fields, acoustic tomography has
attracted considerable interest in the field of atmospheric imaging. This thesis
aims to improve the reconstruction quality of the acoustic tomography system
for temperature and wind velocity field imaging. Focusing on this goal, the
contribution of the thesis can be summarised from the perspectives of data
collection system development, robust and accurate TOF estimation method,
and high-quality scalar and vector tomographic image reconstruction methods
for temperature and wind velocity fields respectively. Details are given below.
Firstly, in order to facilitate the experimental study of acoustic tomography
imaging, the design and evaluation of the data collection system and TOF
estimation method was presented. The evaluation results indicate that the
presented data acquisition system and TOF estimation method has good
quantitative accuracy in the lab-scale experiments.
The temporal resolution is of great significance for the real-time monitoring of
the fast-changing temperature field. To improve the temporal resolution, a
novel online time-resolved reconstruction (OTRR) method is presented, which
can reconstruct high quality time-resolved images by using fewer TOFs per
frame. Compared to state-of-the-art dynamic reconstruction algorithms such
as the Kalman filter reconstruction, the proposed algorithm demonstrated
superior spatial resolution and preferable quantitative accuracy in the
reconstructed images. These features are necessary for the real-time
monitoring of the fast-changing temperature field.
The forward modelling of most acoustic tomography problems is based on a
straight ray model, which may result in large modelling errors due to the
refraction effect under a large gradient temperature field. In order to reduce
the inaccuracy of using the straight ray model, a bent ray model and nonlinear
reconstruction algorithm is applied, which allows the sound propagation ray
paths and temperature distribution to be reconstructed iteratively from the
TOFs.
Using acoustic tomography to reconstruct large-scale temperature and wind
velocity fields, a fully parallel TOF measurement scheme is necessary. To
achieve this goal, a set of orthogonal acoustic waveforms based on the filtered
and modulated Kasami sequence is designed and a cross-correlation based
TOF estimation method is used for data collection. Besides, to overcome the
invisible field problem and improve the image quality of the wind velocity
reconstruction, a divergence-free regularised vector tomographic
reconstruction algorithm is studied. The proposed method is able to provide
accurate tomographic reconstruction of the 2D horizontal wind velocity field
from the TOF measurements.
In summary, this thesis focuses on the improvement of acoustic tomography
techniques for temperature and wind velocity fields, including the phase
corrected Akaike information criterion (AIC) TOF estimation for accurate and
robust TOF estimation, the online time-resolved reconstruction method for
real-time monitoring of the fast changing temperature field, the nonlinear
reconstruction based on the bent ray model to reconstruct the temperature
field with a large gradient, and the divergence-free regularised reconstruction
method to visualise the 2D horizontal wind velocity field
Physics-Based Imaging Methods for Terahertz Nondestructive Evaluation Applications
Lying between the microwave and far infrared (IR) regions, the terahertz gap is a relatively unexplored frequency band in the electromagnetic spectrum that exhibits a unique combination of properties from its neighbors. Like in IR, many materials have characteristic absorption spectra in the terahertz (THz) band, facilitating the spectroscopic fingerprinting of compounds such as drugs and explosives. In addition, non-polar dielectric materials such as clothing, paper, and plastic are transparent to THz, just as they are to microwaves and millimeter waves. These factors, combined with sub-millimeter wavelengths and non-ionizing energy levels, makes sensing in the THz band uniquely suited for many NDE applications.
In a typical nondestructive test, the objective is to detect a feature of interest within the object and provide an accurate estimate of some geometrical property of the feature. Notable examples include the thickness of a pharmaceutical tablet coating layer or the 3D location, size, and shape of a flaw or defect in an integrated circuit. While the material properties of the object under test are often tightly controlled and are generally known a priori, many objects of interest exhibit irregular surface topographies such as varying degrees of curvature over the extent of their surfaces. Common THz pulsed imaging (TPI) methods originally developed for objects with planar surfaces have been adapted for objects with curved surfaces through use of mechanical scanning procedures in which measurements are taken at normal incidence over the extent of the surface. While effective, these methods often require expensive robotic arm assemblies, the cost and complexity of which would likely be prohibitive should a large volume of tests be needed to be carried out on a production line.
This work presents a robust and efficient physics-based image processing approach based on the mature field of parabolic equation methods, common to undersea acoustics, seismology, and other areas of science and engineering. The method allows the generation of accurate 3D THz tomographic images of objects with irregular, non-planar surfaces using a simple planar scan geometry, thereby facilitating the integration of 3D THz imaging into mainstream NDE use
Applications on Ultrasonic Wave
This book presents applications on the ultrasonic wave for material characterization and nondestructive evaluations. It could be of interest to the researchers and students who are studying on the fields of ultrasonic waves
Imaging with Diffraction Tomography
The problem of cross sectional (tomographic) imaging bf objects with diffracting sources is addressed. Specifically the area of investigation is the effect of multiple scattering and attenuation phenomena in diffraction imaging. This work reviews the theory and limits of first order diffraction tomography and studies iterative techniques that can be used to improve the quality of tomographic imaging with diffracting sources. Conventional (straight-ray) tomographic algorithms are not valid when used with acoustic or microwave energy. Thus more sophisticated algorithms are needed; First order diffraction tomography uses a linearized version of the wave equation and gives an especially simple reconstruction algorithm. This work reviews first order approximations to the scattered field and studies the quality of the reconstructions when the assumptions behind these approximations are violated. It will be shown that the Born approximation is valid when the phase change across the object is less than it and the Rytov approximation is valid when the refractive index changes by less than two or three percent. Better reconstructions will be based on higher order approximations to the scattered field. This work describes two fixed point algorithms (the Born and the Rytov approximations) and an algebraic approach to more accurately calculate the scattered fields. The limits of each of these approaches is discussed and simulated results are shown. Finally a review of higher order inversion techniques is presented. Each of these techniques is reviewed and some of their limitations are discussed
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