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.

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

Condition-based maintenance refers to the installation of permanent sensors on a structure/system. By means of early fault detection, severe damage can be avoided, allowing efficient timing of maintenance works and avoiding unnecessary inspections at the same time. These are the goals for structural health monitoring (SHM). The changes caused by incipient damage on raw data collected by sensors are quite small, and are usually contaminated by noise and varying environmental factors, so the algorithms used to extract information from sensor data need to focus on sensitive damage features. The developments of SHM techniques over the last 20 years have been more related to algorithm improvements than to sensor progress, which essentially have been maintained without major conceptual changes (with regards to accelerometers, piezoelectric wafers, and fiber optic sensors). The main different SHM systems (vibration methods, strain-based fiber optics methods, guided waves, acoustic emission, and nanoparticle-doped resins) are reviewed, and the main issues to be solved are identified. Reliability is the key question, and can only be demonstrated through a probability of detection (POD) analysis. Attention has only been paid to this issue over the last ten years, but now it is a growing trend. Simulation of the SHM system is needed in order to reduce the number of experiments.

/pdf/structural-health-monitoring-for-advanced-composite-53zg0e4jun.pdf

Structural Health Monitoring for Advanced Composite Structures: A Review

Phononic crystals and metamaterials can sculpt elastic waves, controlling their dispersion using different mechanisms. These mechanisms are mostly Bragg scattering, local resonances, and inertial amplification, derived from ad hoc, often problem-specific geometries of the materials' building blocks. Here, we present a platform that ultilizes a lattice of spiraling unit cells to create phononic materials encompassing Bragg scattering, local resonances, and inertial amplification. We present two examples of phononic materials that can control waves with wavelengths much larger than the lattice's periodicity. (1) A wave beaming plate, which can beam waves at arbitrary angles, independent of the lattice vectors. We show that the beaming trajectory can be continuously tuned, by varying the driving frequency or the spirals' orientation. (2) A topological insulator plate, which derives its properties from a resonance-based Dirac cone below the Bragg limit of the structured lattice of spirals.

/pdf/spiral-based-phononic-plates-from-wave-beaming-to-1k687u5c0f.pdf

Spiral-Based Phononic Plates: From Wave Beaming to Topological Insulators

A damage detection method based on an innovative 2D phased sensor array made of piezoelectric paint is proposed for in situ damage detection of a thin isotropic panel using guided Lamb waves A design analysis of candidate 2D arrays based on spiral, cruciform and circular element layouts is performed In this study, a 2D phased sensor array with a spiral configuration is fabricated using a piezoelectric composite (piezopaint) patch and used for detecting damages in an aluminum panel Steered array responses are generated from the raw sensor signals using a directional filtering algorithm based on phased array signal processing The fundamental flexural (or transverse), A0 mode, of the guided Lamb waves is used though the sensing and analysis technique is not limited to the mode used in this work To enhance the proposed analysis technique, empirical mode decomposition (EMD) and a Hilbert–Huang transform (HHT) are applied A new damage detection algorithm including threshold setting and damage index (DI) calculation is developed and implemented for detecting damages in the form of holes and a simulated crack The characteristic damage indices consistently increase as damage size grows

https://iopscience.iop.org/article/10.1088/0964-1726/19/7/075017/pdf

Piezoelectric-paint-based two-dimensional phased sensor arrays for structural health monitoring of thin panels

This paper presents the investigation of a directional magnetostrictive patch transducer (MPT) composed of a highly textured Galfenol (Fe?Ga alloy) patch in the use of ultrasonic guided Lamb wave (GLW) inspection techniques for isotropic planar structures. Recently, the actuation and sensing performance of an MPT using a disc patch made of polycrystalline nickel was reported, based on GLW testing in thin aluminum plates. The nickel-based MPT appeared to have omnidirectional GLW sensitivity in the metallic plate because of the isotropic magnetostrictive nature of polycrystalline nickel with random orientation. In this work, we investigated two viable methods to control and improve MPT?s directional sensitivity for detecting GLWs in metallic plate structures. First, we proposed a circular MPT (CMPT) using the highly textured Galfenol patch with a large magnetostriction of ?270 ppm along a preferred orientation parallel to the patch?s rolling direction. The CMPT exhibited outstanding sensitivity to incoming GLWs along the direction of the patch in a thin aluminum plate. This was mainly due to the unique anisotropic magnetostriction effect of the textured Galfenol patch. In addition to the use of the Galfenol material, we developed a novel cruciform MPT (XMPT) containing four solenoid sensing coils that possessed individual directional sensing preferences, corresponding to the orientations of the sensing coils. The directional sensing performance of the XMPT was initially validated by using the polycrystalline nickel patch with the isotropic magnetostrictive characteristic, exhibiting the remarkable directionality attributes of the individual sensing elements. Of particular interest was that the XMPT combined with the highly textured Galfenol patch demonstrated excellent directional sensitivity corresponding to the Galfenol?s preferred orientation. And the directional sensing feature was noticeably enhanced by incorporating the textured Galfenol patch into the proposed XMPT system.

https://iopscience.iop.org/article/10.1088/0964-1726/23/9/095035/pdf

Directional magnetostrictive patch transducer based on Galfenol’s anisotropic magnetostriction feature

In this paper, we investigate ultrasonic guided wave (GW) sensing performance of a magnetostrictive composite patch sensor (MCPS) in use of a structural health monitoring system for metallic plates. A composite patch of the MCPS consisted of magnetostrictive powders and binding epoxy, where flake- and granular-type Galfenol and spherical nickel particles were considered to examine the effect of particle shape on sensing capability and directional sensitivity. Galfenol flakes with the surface parallel to the (200) crystallographic planes exhibit good field sensitivities in magnetic and magnetostrictive properties when an in-plane field is applied. Shape anisotropy disappears for the conventional spherical shape, accompanying with a large magnetizing field and poor field sensitivity. The 60-kHz GW was successfully captured by the three different MCPSs. Among them, the MCPS with the Galfenol flakes showed the best sensing performance because of their excellent field sensitivities in magnetization and magnetostriction. In addition, the composite patch was configured into a uniaxial comb shape to promote the magnetic shape anisotropy feature of the magnetostrictive patch by itself. Due to the unique configuration of the Galfenol composite patch, the equivalent MCPS presented outstanding directional sensitivity for the GWs propagating along the orientation of the comb fingers.

Ultrasonic guided wave sensing performance of a magnetostrictive transducer using Galfenol flakes-polymer composite patches

Exploiting hand gestures for non-verbal communication has extraordinary potential in HCI. A data glove is an apparatus widely used to recognize hand gestures. To improve the functionality of the data glove, a highly stretchable and reliable signal-to-noise ratio sensor is indispensable. To do this, the study focused on the development of soft silicone microchannel sensors using a Eutectic Gallium-Indium (EGaIn) liquid metal alloy and a hand gesture recognition system via the proposed data glove using the soft sensor. The EGaIn-silicone sensor was uniquely designed to include two sensing channels to monitor the finger joint movements and to facilitate the EGaIn alloy injection into the meander-type microchannels. We recruited 15 participants to collect hand gesture dataset investigating 12 static hand gestures. The dataset was exploited to estimate the performance of the proposed data glove in hand gesture recognition. Additionally, six traditional classification algorithms were studied. From the results, a random forest shows the highest classification accuracy of 97.3% and a linear discriminant analysis shows the lowest accuracy of 87.4%. The non-linearity of the proposed sensor deteriorated the accuracy of LDA, however, the other classifiers adequately overcame it and performed high accuracies (>90%).

Hand Gesture Recognition Using EGaIn-Silicone Soft Sensors.

This paper presents the preliminary evaluation results of a magnetorheological elastomer (MRE) patch fabricated by oriented flake-type Fe–Ga alloy (Galfenol) particles bound in a silicone polymer matrix. A force/pressure sensor, based on the Galfenol MRE patch, was developed to verify the advantages of the MRE with magnetically aligned Galfenol flakes. Hall effect sensors embedded in the MRE sensing device detect the change of magnetic flux density due to the deformation of the Galfenol MRE patch induced by an externally applied force with non-magnetic disturbance. The Galfenol flakes are standing up and well aligned to the out-of-plane of the MRE patch using a 1 in diameter permanent magnet, parallel to the applied force direction to improve sensing performance and sensitivity. It was measured at $\sim 105$ mV in the sensor response to a thumb pressure, which exhibited five times higher sensing performance compared to that of another MRE patch with the Galfenol flakes aligned to the in-plane direction.

Byungseok Yoo

Papers

Piezoelectric-paint-based two-dimensional phased sensor arrays for structural health monitoring of thin panels

Directional magnetostrictive patch transducer based on Galfenol’s anisotropic magnetostriction feature

Ultrasonic guided wave sensing performance of a magnetostrictive transducer using Galfenol flakes-polymer composite patches

Hand Gesture Recognition Using EGaIn-Silicone Soft Sensors.

Evaluation of Magnetorheological Elastomers With Oriented Fe–Ga Alloy Flakes for Force Sensing Applications