2
1 CORRELATION-BASED ULTRASOUND IMAGING IN NDT AND SHM Patrice Masson 1* , Nicolas Quaegebeur 1 1 GAUS, 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. More info about this article: http://www.ndt.net/?id=25548 More info about this article: http://www.ndt.net/?id=25567
