Abstract
Multilayer composite structures have been widely used in industrial manufacturing, and nondestructive testing of these multilayer structures is to ensure their reliable quality and performance. Currently, ultrasonic total focusing method (TFM) imaging using full‐matrix capture (FMC) technology has been proven to sense small defects in a single homogeneous medium and improve the imaging signal‐to‐noise ratio. However, these algorithms cannot be accurately applied to imaging of multilayer composite structures, due to the acoustic impedance variation and because reflection and refraction occur at the interface between the layers, which makes it very difficult to calculate the ultrasonic propagation path and time. To solve this problem, a root‐mean‐square (RMS) velocity algorithm for total focusing imaging of multilayer structures is proposed in the article. Based on the theory of RMS velocity for processing of seismic data, the approximated delays can be easily and quickly calculated by a hyperbolic time‐distance relationship under circumstances of short lateral distance and horizontal layers. The performance of the proposed algorithm is evaluated by total focusing imaging of a two‐layer medium with drilled holes and conducted by the finite element simulation. To further improve imaging efficiency, the partial data in the full‐matrix data were used for imaging which is the simplified matrix focusing method (SFM). The results verify that the proposed methods are capable of total focusing imaging of two‐layered structures. However, the imaging performance and efficiency of these algorithms are different.
Highlights
Multilayer structures play an important role in the production and manufacturing of industrial products, such as solid rocket motors, aircraft skin, and generator blade, due to their advantages in corrosion protection, wear-resistant and strength enhancement, and so on [1, 2]
Different total focus algorithms have been proposed [14,15,16,17,18,19]. Note that both the algorithms based on total focus method (TFM) are only suitable for homogeneous and isotropic materials. erefore, these algorithms are invalid for multilayer structures as the acoustic impedance varies between different layers
We propose an ultrasonic TFM, based on root-mean-square (RMS) velocity, which is derived from processing of seismic data
Summary
Multilayer structures play an important role in the production and manufacturing of industrial products, such as solid rocket motors, aircraft skin, and generator blade, due to their advantages in corrosion protection, wear-resistant and strength enhancement, and so on [1, 2]. Different total focus algorithms have been proposed [14,15,16,17,18,19]. Note that both the algorithms based on TFM are only suitable for homogeneous and isotropic materials. The method in seismic imaging is used for reference, and the phase shift migration (PSM) algorithm [21] based on the plane wave theory was employed to synthetic aperture focusing technique (SAFT) of multilayer structures. We propose an ultrasonic TFM, based on root-mean-square (RMS) velocity, which is derived from processing of seismic data.
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