The paper provides a state of the art review of guided wave based structural health monitoring (SHM). First, the fundamental concepts of guided wave propagation and its implementation for SHM is explained. Following sections present the different modeling schemes adopted, developments in the area of transducers for generation, and sensing of wave, signal processing and imaging technique, statistical and machine learning schemes for feature extraction. Next, a section is presented on the recent advancements in nonlinear guided wave for SHM. This is followed by section on Rayleigh and SH waves. Next is a section on real-life implementation of guided wave for industrial problems. The paper, though briefly talks about the early development for completeness,. is primarily focussed on the recent progress made in the last decade. The paper ends by discussing and highlighting the future directions and open areas of research in guided wave based SHM.

https://iopscience.iop.org/article/10.1088/0964-1726/25/5/053001/pdf

Guided wave based structural health monitoring: A review

Review of magnetostrictive patch transducers and applications in ultrasonic nondestructive testing of waveguides.

A review of ultrasonic testing applications in additive manufacturing: Defect evaluation, material characterization, and process control.

Power ultrasound and its applications: A state-of-the-art review.

The metasurface concept has a significant potential due to its novel wavefront-shaping functionalities that can be critically useful for ultrasonic and solid wave-based applications. To achieve the desired functionalities, elastic metasurfaces should cover full 2π phase shift and also acquire full transmission within subwavelength scale. However, they have not been explored much with respect to the elastic regime, because the intrinsic proportionality of mass-stiffness within the continuum elastic media causes an inevitable trade-off between abrupt phase shift and sufficient transmission. Our goal is to engineer an elastic metasurface that can realize an inverse relation between (amplified) effective mass and (weakened) stiffness in order to satisfy full 2π phase shift as well as full transmission. To achieve this goal, we propose a continuum elastic metasurface unit cell that is decomposed into two substructures, namely a mass-tuning substructure with a local dipolar resonator and a stiffness-tuning substructure composed of non-resonant multiply-perforated slits. We demonstrate analytically, numerically, and experimentally that this unique substructured unit cell can satisfy the required phase shift with high transmission. The substructuring enables independent tuning of the elastic properties over a wide range of values. We use a mass-spring model of the proposed continuum unit cell to investigate the working mechanism of the proposed metasurface. With the designed metasurface consisting of substructured unit cells embedded in an aluminum plate, we demonstrate that our metasurface can successfully realize anomalous steering and focusing of in-plane longitudinal ultrasonic beams. The proposed substructuring concept is expected to provide a new principle for the design of general elastic metasurfaces that can be used to efficiently engineer arbitrary wave profiles.

Mass-stiffness substructuring of an elastic metasurface for full transmission beam steering

The lowest-branch torsional guided wave is very effective in pipe damage inspection because of its non-dispersive characteristics, but it cannot be used for the simultaneous identification of axial and circumferential locations of a defect in a pipe. Motivated by recent developments in magnetostrictive transducer technology, which is especially efficient in torsional and shear wave generation, the goal of this investigation is to extend this technology for simultaneous identification of the axial and circumferential locations of cracks by using shear horizontal (SH) waves. Unlike the conventional magnetostrictive patch method using a single complete patch wound around the pipe's circumference, the proposed method segments the patch into several pieces to generate SH waves propagating over the pipe surface. Accordingly, SH waves in a pipe are generated and sensed individually by a meander coil placed separately on each segment. By using two sets of segmented-patch arrays separated by some distance, the cylindrical surface of a pipe can be inspected both axially and circumferentially. After the underlying angular profile of the patch segment is investigated, experiments identifying the axial and circumferential locations of multiple cracks in a pipe are carried out to demonstrate the potential of the proposed methodology.

Shear-horizontal wave-based pipe damage inspection by arrays of segmented magnetostrictive patches

Small-sized defects in pipes can be better detected if the first nondispersive torsional mode is used in a higher frequency range. However, dispersive higher torsional modes accompany the first mode if the actuation frequency is above the first cutoff frequency, thereby making the detection difficult. This study proposes a new technique that is particularly useful for guided torsional waves in a pipe; it enhances the desired first nondispersive mode and suppresses the undesired second mode. The technique uses two transmitting transducers separated by an optimized distance and actuated with an optimized delay time. Unlike previous methods, such as a method tuning the delay time for desired mode enhancement and tuning the distance for undesired mode suppression, the proposed approach determines both the distance and delay time mainly to suppress the undesired second mode. With the selected values, the desired first mode is substantially enhanced. This phenomenon is unique in torsional waves, not longitudinal waves, for which delay time and distance controlling methods have been developed. After wave simulations were carried out to show why the proposed method is more effective for the case of torsional waves, several experiments using magnetostrictive transducers were performed to demonstrate the effectiveness of the proposed method.

Higher torsional mode suppression in a pipe for enhancing the first torsional mode by using magnetostrictive patch transducers

Waveguides may inevitably excite undesired modes and induce dispersion-related distortion. This is true when an elastic homogeneous waveguide is used to carry a pulse of which the center frequency is above its cutoff frequency. We show that these problems can be avoided if a properly-tailored phononic crystal (PC) structure is inserted inside a waveguide. In engineering the PC, we open the band gap of the undesired wave mode and make the dispersion curve of the desired wave branch straight in a target frequency band. Numerical simulations and ultrasonic experiments using the engineered PC confirmed the validity of the proposed approach.

Wave dispersion tailoring in an elastic waveguide by phononic crystals

A segmented magnetostrictive patch array transducer capable of generating a high frequency shear wave in a structure such as a rod or a pipe, a structural fault diagnosing apparatus including the segmented magnetostrictive patch array transducer, and a method of operating the segmented magnetostrictive patch array transducer are shown. The segmented magnetostrictive patch array transducer includes a plurality of magnetostrictive patches attached along a circumference of a rod member; a plurality of insulators that are disposed on the magnetostrictive patches; a plurality of meander coils, each of the meander coils comprising a plurality of coil lines extending along the circumference direction of the rod member on each of the insulators, wherein a current flows through adjacent coil lines in opposite directions to one another; and a plurality of magnets that respectively form a magnetic field along the circumferential direction of the rod member on the magnetostrictive patches.

Young Eui Kwon

Papers

Review of magnetostrictive patch transducers and applications in ultrasonic nondestructive testing of waveguides.

Shear-horizontal wave-based pipe damage inspection by arrays of segmented magnetostrictive patches

Higher torsional mode suppression in a pipe for enhancing the first torsional mode by using magnetostrictive patch transducers

Wave dispersion tailoring in an elastic waveguide by phononic crystals

Segmented magnetostrictive patch array transducer, apparatus for diagnosing structural fault by using the same, and method of operating the same