Correlation Based Ultrasound Imaging in NDT and SHM · 2020. 11. 27. · 1 CORRELATION-BASED...
Transcript of Correlation Based Ultrasound Imaging in NDT and SHM · 2020. 11. 27. · 1 CORRELATION-BASED...
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CORRELATION-BASED ULTRASOUND IMAGING IN NDT AND SHM
Patrice Masson1*, Nicolas Quaegebeur1
1GAUS, Dept Génie mécanique, Université de Sherbrooke, Sherbrooke (QC) J1K 2R1, Canada
*Corresponding Author: [email protected]
ABSTRACT
Ultrasound imaging is implemented either with bulk waves using phased-array (PA) transducers in classical Non-Destructive
Testing (NDT) techniques, or with guided waves using compact or distributed arrays of in situ bonded transducers in Structural
Health Monitoring (SHM). However, due to limitations of ultrasound-based NDT or SHM methods in terms of contrast, resolution
and penetration depth, the characterization of precursor damage in structures such as composites is still challenging. High-
performance damage imaging techniques based on the Full Matrix Capture (FMC) technique, such as Total Focusing Method
(TFM), can now be implemented in portable Graphics Processing Units (GPU). This work discusses advanced super-resolution
signal processing algorithms such as multi-modal plane wave imaging, phase coherence imaging, and correlation-based imaging.
The latter imaging approach implements the correlation of measured signals with theoretical propagated signals computed over a
given grid of points. The technique considers transducer directivity and dynamics, and also the refraction and transmission at the
interface between the wedge and the host structure, such that the number of required array elements for a given imaging
performance can be greatly reduced. Simulation and experimental results in NDT demonstrate that the correlation-based imaging
approach offers better separation between scatterers with the same number of transducers, or similar separation with a much lower
number of transducers Simulation and experimental results in SHM demonstrate that the correlation-based imaging approach
offers better resolution and increased robustness for damage localization in metallic and composite structures.
KEYWORDS: Ultrasound imaging, correlation-based imaging, phased-array transducers, damage localization.
1. INTRODUCTION
The Non-Destructive Testing (NDT) and Structural Health Monitoring (SHM) industries are still defining a framework for the use
of high-performance damage imaging based on Full Matrix Capture (FMC) technique, such as Total Focusing Method (TFM).
The advent of Graphics Processing Units (GPU) now enables the implementation of super-resolution signal processing algorithms
such as multi-modal plane wave imaging [1] or phase coherence imaging [2]. The correlation-based algorithm Excitelet allows
super-resolution imaging at much lower frequencies as compared to conventional NDT or SHM techniques. Excitelet inherently
considers a detailed model of the full wave propagation path, with the transducer dynamics and the coupling interfaces.
2. CORRELATION-BASED IMAGING
The Excitelet imaging approach has been first derived in the case of guided wave inspection in SHM using compact or sparse
transducer arrays [3], extended to bulk wave inspection of isotropic structures using contact probes [4] and to the case of inspection
using an angle wedge. The approach is based on the correlation between measured signals extracted from FMC and theoretical
predictions. For each point of a defined grid on the inspected structure, theoretical pitch-catch signals are pre-
computed using Spatial Impulse Response (SIR) method and stored in memory in order to be correlated with real-time acquired
signals . Eq. 1 presents the correlation coefficient calculated for each pixel and used to plot the image.
(1)
2.1 ASSESSMENT IN NDT
The Excitelet imaging approach has been validated experimentally for the imaging of holes with P waves, first in direct contact on
an aluminum block of dimensions 200 x 100 x 50 mm, and then through a wedge on a carbon steel block of dimensions 210 x 150
x 30 mm. Imaging results obtained using Excitelet and presented in Fig. 1 illustrate the effect of varying the number N of transducer
elements used, when compared with the standard TFM.
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Test case TFM Excitelet
N=
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N=
16
N=
32
N=
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Fig. 1 Test cases for hole imaging in NDT, with TFM and Excitelet, and with varying number N of transducer elements.
2.2 ASSESSMENT IN SHM
The Excitelet imaging approach has also been validated for the imaging of defects on plates with guided waves. Imaging results
presented in Fig. 2 illustrate the simulated defect localization performance of Excitelet on a unidirectional composite structure,
when compared with a standard delay-and-sum (DAS) approach.
Test case DAS Excitelet
Fig. 2 Test case for simulated defect (red circle) localization on unidirectional composite in SHM, with DAS and Excitelet.
3. CONCLUSIONS
The correlation-based imaging approach Excitelet inherently considers transducer directivity and dynamics, and also allows for
compensating diffraction, refraction and dispersion in the wave propagation path. The approach has been implemented in NDT
and SHM applications and benchmarked against standard approaches. NDT assessment has shown that Excitelet allows to greatly
reduce the number of required transducers without loss of imaging quality. SHM assessment has demonstrated that the approach
can be tailored to the application, for example by considering non-isotropic propagation in plate structures.
ACKNOWLEDGMENT
This work has been conducted with the financial support from the Natural Sciences and Engineering Research Council of Canada
(NSERC). The authors would like to thank all the students involved in the development and validation of Excitelet.
REFERENCES
[1] Le Jeune, L., Robert, S., Lopez Villaverde, E., Prada, C., “Plane wave imaging…”, Ultrasonics, 64, 128-138, 2016.
[2] Camacho, J., Parrilla, M., Fritsch, C., “Phase coherence imaging”, IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 56(5), 958-974, 2009.
[3] Quaegebeur, N., Masson, P., Langlois-Demers, D., Micheau, P., "Dispersion-based imaging…", Smart Mater. Struct., 20, 12p., 2011.
[4] Quaegebeur, N., Masson, P., "Correlation-based imaging technique using ultrasonic…", Ultrasonics, 58(2), 1056-1064, 2012.