Diffuse fields in open systems and the emergence of the Green’s function (L)

2004 ◽  
Vol 116 (5) ◽  
pp. 2731-2734 ◽  
Author(s):  
Richard L. Weaver ◽  
Oleg I. Lobkis
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Open Systems ◽  
Diffuse Fields

Detection of near-surface defects in rails combining Green's function retrieval of ultrasonic diffuse fields and sign coherence factor imaging

2020 ◽  
Vol 62 (4) ◽  
pp. 216-221
Author(s):  
Haiyan Zhang ◽  
Mintao Shao ◽  
Guopeng Fan ◽  
Hui Zhang ◽  
Wenfa Zhu
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Surface Defects ◽  
Signal To Noise Ratio ◽  
Near Surface ◽  
Coherence Factor ◽  
Array Transducer ◽  
Diffuse Fields ◽  
Ultrasonic Phased Array ◽  
Cross Correlations

A method combining Green's function retrieval theory and sign coherence factor (SCF) imaging is presented to detect near-surface defects in rails. The defects are close to the ultrasonic phased array and near-surface acoustic information of defects is obscured by the non-linear effects of the initial wave signal in directly acquired responses. To overcome this problem, cross-correlations of the diffuse field signals captured by the array transducer are performed to reconstruct the Green's function. SCF imaging is used to further improve the spatial resolution and signal-to-noise ratio (SNR) of near-surface defects in rails. Experiments are conducted on two rails containing two and four defects, respectively. The results show that these defects can be clearly identified when using the reconstructed Green's function. However, the images of near-surface defects are masked and cannot be distinguished when using directly captured signals and total focus imaging. The proposed method reduces the background noise and allows for effective imaging of near-surface defects in rails.

Diffuse fields in ultrasonics and seismology

Geophysics ◽  
2006 ◽  
Vol 71 (4) ◽  
pp. SI5-SI9 ◽  
Author(s):  
Richard L. Weaver ◽  
Oleg I. Lobkis
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Ultrasonic Waves ◽  
Passive Imaging ◽  
Basic Principles ◽  
Diffuse Fields ◽  
Recent Developments ◽  
History Of ◽  
Scattered Fields

We review the history of diffuse ultrasonic waves in solids with emphasis on recent developments in field-field correlations and their identification with Green’s function. The basic principles appear to be well understood now, and the identity between these two waveforms has been proven under a variety of assumed conditions that guarantee a diffuse field. Promise for practical passive imaging is good; nevertheless, measurements sometimes fail to fully agree with theory. We ascribe this in some cases to incomplete convergence — insufficient amounts of data have been processed. In other cases, it is probably because of a lack of perfect diffuseness; ambient nonmultiply scattered fields are often not equipartitioned and imperfectly diffuse.

Extraction of thermal Green's function using diffuse fields: a passive approach applied to thermography

2016 ◽  
Author(s):  
Margherita Capriotti ◽  
Simone Sternini ◽  
Francesco Lanza di Scalea ◽  
Stefano Mariani
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Diffuse Fields

scholarly journals Wavenumber Imaging of Near-Surface Defects in Rails using Green’s Function Reconstruction of Ultrasonic Diffuse Fields

Sensors ◽  
2019 ◽  
Vol 19 (17) ◽  
pp. 3744 ◽  
Author(s):  
Hui Zhang ◽  
Haiyan Zhang ◽  
Jiayan Zhang ◽  
Jianquan Liu ◽  
Wenfa Zhu ◽  
...  
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Phased Array ◽  
Surface Defects ◽  
Measured Signal ◽  
Near Surface ◽  
Diffuse Fields ◽  
Ultrasonic Phased Array ◽  
The Cross ◽  
Function Reconstruction

Wavenumber imaging with Green’s function reconstruction of ultrasonic diffuse fields is used to realize fast imaging of near-surface defects in rails. Ultrasonic phased array has been widely used in industries because of its high sensitivity and strong flexibility. However, the directly measured signal is always complicated by noise caused by physical limitations of the acquisition system. To overcome this problem, the cross-correlations of the diffuse field signals captured by the probe are performed to reconstruct the Green’s function. These reconstructed signals can restore the early time information from the noise. Experiments were conducted on rails with near-surface defects. The results confirm the effectiveness of the cross-correlation method to reconstruct the Green’s function for the detection of near-surface defects. Different kinds of ultrasonic phased array probes were applied to collect experimental data on the surface of the rails. The Green’s function recovery is related to the number of phased array elements and the excitation frequency. In addition, the duration and starting time of the time-windowed diffuse signals were explored in order to achieve high-quality defect images.

scholarly journals Phase Coherence Imaging for Near-Surface Defects in Rails Using Cross-Correlation of Ultrasonic Diffuse Fields

Metals ◽  
2019 ◽  
Vol 9 (8) ◽  
pp. 868
Author(s):  
Haiyan Zhang ◽  
Mintao Shao ◽  
Guopeng Fan ◽  
Hui Zhang ◽  
Wenfa Zhu ◽  
...  
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Cross Correlation ◽  
Phased Array ◽  
Surface Defects ◽  
Performance Indicator ◽  
Phase Coherence ◽  
Near Surface ◽  
Diffuse Fields ◽  
Coherence Imaging

In this paper, phase coherence imaging is proposed to improve spatial resolution and signal-to-noise ratio (SNR) of near-surface defects in rails using cross-correlation of ultrasonic diffuse fields. The direct signals acquired by the phased array are often obscured by nonlinear effects. Thus, the output image processed by conventional post-processing algorithms, like total focus method (TFM), has a blind zone close to the array. To overcome this problem, the diffuse fields, which contain spatial phase correlations, are applied to recover Green’s function. In addition, with the purpose of improving image quality, the Green’s function is further weighted by a special coherent factor, sign coherence factor (SCF), for grating and side lobes suppression. Experiments are conducted on two rails and data acquisition is completed by a commercial 32-element phased array. The quantitative performance comparison of TFM and SCF images is implemented in terms of the array performance indicator (API) and SNR. The results show that the API of SCF is significantly lower than that of TFM. As for SNR, SCF achieved a better SNR than that of TFM. The study in this paper provides an experimental reference for detecting near-surface defects in the rails.

GREEN'S FUNCTION MATCHING METHOD FOR REALISTIC CALCULATIONS OF INTERFACES

1985 ◽  
Vol 46 (C4) ◽  
pp. C4-321-C4-329 ◽  
Author(s):  
E. Molinari ◽  
G. B. Bachelet ◽  
M. Altarelli
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Matching Method
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