Omnidirectional Shear-horizontal Wave Research Articles

Guided Wave Studies for Enhanced Acoustic Emission Inspection

ABSTRACT Fundamental elements of wave mechanics are covered with respect to Acoustic Emission (AE) analysis in Nondestructive Evaluation (NDE) and Structural Health monitoring (SHM). Emphasis is placed on aspects of Ultrasonic Guided Waves that travel in a structure due to elastic wave emissions from a defect source as defects continue to grow. The AE Method is based on the emission of elastic waves from a particular source as failure is initiated – crack, corrosion, leak, and so on. The AE signal received is a function of the source orientation and location as well as wave velocities in the structure, sensor types and positions, arrival time differences, thicknesses of the structure, and specimen structural variations. Such topics as Guided wave physical and theoretical considerations, Ultrasonic Guided wave accomplishments, and enhanced AE by considering various guided wave concepts are discussed. The topic of Acousto-Ultrasonics and its impact on guided wave understanding is also reviewed. This paper illustrates how principles in Ultrasonic Guided waves can be applied to Acoustic Emission. Besides basic issues, Acoustic Emission enhancement possibilities are based on recent studies of Ultrasonic Guided Waves and the use of Shear Horizontal guided waves in Acoustic Emission along with an omni-directional shear horizontal wave transducer. Editor’s note: Joseph L. Rose, PhD, is the recipient of the 2021 ASNT Research Recognition for Sustained Excellence. Rose presented on “Guided Wave Studies for Enhanced Acoustic Emission Inspection” during the ASNT Research Symposium which was held 27–29 April 2021.

Quantitative damage imaging in plates based on SH0 mode tomographic approach

Exploring the non-dispersive property, the fundamental shear horizontal mode SH0 can be widely used in the detection and monitoring of defects in plate like structures. To combine an approach of ultrasonic guided waves and a tomographic technique by adapting a reconstruction algorithm for the probabilistic inspection of damage (RAPID) using a double-turn coil omnidirectional shear horizontal wave magnetostrictive patch transducer (OSH-MPT) with three different kinds of array arrangements is presented here for imaging. SH0 can propagate in a composite material through bonding double-turn coil OSH-MPT. To get the information about a damage, RAPID can be utilized based on direct wave of SH0 mode. It is advantageous to monitor a composite plate where the velocity of direct waves depends on the propagation direction. In this paper, a surface simulated defect, using a small cylinder, in a composite material is quantitatively evaluated by using shear horizontal mode tomographic approach. Using modified approaches, to eliminate the effect of uneven probability distribution of the array (UPDA) in RAPID, imaging is obtained. Three different array setups of transducers have been tested to monitor the influence on the images. It has been presented that the image generated by the proposed approach with the optimal transducer array and shape factor, β, calculation method has great influence on the real defect shape. To improve the imaging results threshold is selected based on the receiver operating characteristic (ROC) curve. The proposed approach can be used to detect, locate and image the surface defects in plate like structures.

Bi-annular shear-horizontal wave MPT tailored to generate the SH1 mode in a plate

The use of a specific wave mode is critical in ultrasonic non-destructive evaluations but it is difficult to generate a specific mode, especially a higher mode at a frequency where there exist multiple wave modes. Here, we propose a compact omnidirectional shear-horizontal wave MPT (magnetostrictive patch transducer) having two annular magnetostrictive patches for the generation of a nearly pure SH1 (second shear-horizontal) mode in a plate for frequencies above the first cutoff frequency. While a common wavelength-matching approach would typically require the use of several patches and does not appear completely to eliminate unwanted omnidirectional wave modes, the proposed MPT, with only two annular patches, generates the desired SH1 mode predominantly with the unwanted SH0 (first shear-horizontal) mode nearly eliminated. For the design, the geometries of the annular patches are optimally configured to maximize the ratio of the SH1 mode to the SH0 mode. Numerical simulations and experiments confirm the effectiveness of the proposed bi-annular shear-horizontal wave MPT. Because the SH1 mode near the first cutoff frequency is highly dispersive, the developed transducer is expected to be critically useful in various applications, such as ultrasonic inspections of wall thinning.

Development of an Omnidirectional Shear Horizontal Wave Transducer for Structural Health Monitoring

In this work, an omnidirectional SH0 mode transducer is proposed. It is based on a simple arrangement of common piezoelectric ceramics attached to a steel disc, and is capable of generating a relatively high ratio of SH0 to other modes omnidirectionally. A parametric study of the components which constitute the transducer was performed through finite element modelling in order to characterize and investigate the influence of geometry on its frequency–response. The validation of modelling results was performed through the construction of two transducers with different configurations. Analysis of the relationship between the intensity of each mode generated in a plate and the characteristics of the SH0 mode emission allowed the identification of frequencies that presented the most desirable response.

Development of an Omni-Directional Shear Horizontal Wave Transducer Based on a Radially Poled Piezoelectric Ring

For the inspection of large plate-like structures, the omni-directional guided wave transducer-based system has been regarded as an effective tool since only a few transducers are required to cover the entire inspection area without blind zones. In comparison with Lamb waves, the shear horizontal (SH) wave is more promising because its fundamental mode is non-dispersive. In this work, we proposed an omni-directional SH wave piezoelectric transducer (OSH-PT) based on a radially poled $$\hbox {d}_{24}$$ -mode PZT ring. Both the finite element simulations and experiments were carried out to demonstrate its performance in generating and receiving $$\hbox {SH}_{0}$$ wave. Results showed that the radially poled OSH-PT could generate single-mode $$\hbox {SH}_{0}$$ wave and receive $$\hbox {SH}_{0}$$ wave only over a wide frequency range from 70 to 200 kHz. The obtained signal-to-noise ratio can reach up to 26 dB in generation and 24 dB in reception. The omni-directivity of this OSH-PT is also very good with the deviation only about 6% in both generation and reception of $$\hbox {SH}_{0}$$ wave. Considering its easy fabrication, low cost and superior performances, this proposed OSH-PT may promote the applications of $$\hbox {SH}_{0}$$ wave-based inspection in structural health monitoring and nondestructive testing.

Omnidirectional shear horizontal wave based tomography for damage detection in a metallic plate with the compensation for the transfer functions of transducer

Guided-wave based damage detection for plates has been widely studied for structural health monitoring. Because most of the earlier studies used dispersive Lamb waves, substantial efforts had to be made to alleviate the dispersive and multi-modal nature of Lamb waves and the effect of surface conditions. In contrast, shear-horizontal (SH) waves have better propagation characteristics suitable for the detection of damages in plates, but SH waves have not been widely used due to the lack of efficient methods to generate and sense omnidirectional SH waves. The objective of this study is to construct diagnostic images of damaged plates by using omnidirectional SH waves with a special emphasis on the compensation of the frequency-dependence of the SH wave transducers. The compensation is necessary to have reliable diagnostic images because its frequency-dependent characteristics considerably can affect imaging quality if they are not considered. Consequently, simplified, yet effective, models representing the transfer functions of the omnidirectional SH wave magnetostrictive patch transducer (OSH-MPT) are developed in this paper. To visualize the position and shape of the structural damages in a metallic plate, the virtual time-reversal imaging method is used and two alternative techniques are considered to compensate for the effects of the transfer functions of transducers in the imaging processes. The imaging results after the compensation appear quite promising, suggesting that the omnidirectional SH wave based enhanced time-reversal imaging can be an efficient inspection method for plate structures.

A variable-frequency structural health monitoring system based on omnidirectional shear horizontal wave piezoelectric transducers

Structural health monitoring (SHM) is of great importance for engineering structures as it may detect the early degradation and thus avoid life and financial loss. Guided wave based inspection is very useful in SHM due to its capability for long distance and wide range monitoring. The fundamental shear horizontal (SH0) wave based method should be most promising since SH0 is the unique non-dispersive wave mode in plate-like structures. In this work, a sparse array SHM system based on omnidirectional SH wave piezoelectric transducers (OSH-PT) was proposed and the multi data fusion method was used for defect inspection in a 2 mm thick aluminum plate. Firstly, the performances of three types OSH-PTs was comprehensively compared and the thickness-poled d15 mode OSH-PT used in this work was demonstrated obviously superior to the other two. Then, the signal processing method and imaging algorithm for this SHM system was presented. Finally, experiments were carried out to examine the performance of the proposed SHM system in defect localization and imaging. Results indicated that this SHM system can locate a through hole as small as 0.12λ (4 mm) in diameter (where λ is the wavelength corresponding to the central operation frequency) under frequencies from 90 to 150 kHz. It can also locate multiple defects accurately based on the baseline subtraction method. Obviously, this SHM system can detect larger areas with sparse sensors because of the adopted single mode, non-dispersive and low frequency SH0 wave which can propagate long distance with small attenuation. Considering its good performances, simple data processing and sparse array, this SH0 wave-based SHM system is expected to greatly promote the applications of guided wave inspection.

A uniform-sensitivity omnidirectional shear-horizontal (SH) wave transducer based on a thickness poled, thickness-shear (d15) piezoelectric ring

The fundamental shear horizontal (SH0) wave in plates is of great importance in the field of nondestructive testing (NDT) and structural health monitoring (SHM) since it is the unique non-dispersive guided wave mode. For practical applications, a phased array system based on omnidirectional SH0 wave transducers is most useful as it can cover a wide range of a plate. However, so far very few omnidirectional SH wave transducers have been developed. In this work, we proposed an omnidirectional SH wave piezoelectric transducer (OSH-PT) based on a thickness poled piezoelectric ring. The ring is equally divided into twelve sectors and the electric field is circumferentially applied, resulting in a new thickness-shear (d15) mode. Finite element analysis shows that the proposed OSH-PT can excite single-mode SH0 wave and receive the SH0 wave only. Experiments were then conducted to examine the performance of the proposed OSH-PT. Results indicated that it can generate and receive single-mode SH0 wave in a wide frequency range with nearly uniform sensitivities along all directions. Considering its quite simple configuration, compact size and low cost, the proposed OSH-PT is expected to greatly promote the applications of SH waves in the field of NDT and SHM.

Damage imaging in composite plate by using double-turn coil omnidirectional shear-horizontal wave magnetostrictive patch transducer array

A double-turn coil omnidirectional shear-horizontal wave magnetostrictive patch transducer (DC-OSH-MPT) array for imaging detection in composite plates is presented. DC-OSH-MPT parameters are optimized in order to make the transducer more suitable for imaging detection. Besides, the characteristic of the lowest shear-horizontal mode (SH0), which can be generated and received in all directions by DC-OSH-MPT bonded on composite plate, is beneficial for imaging. Due to anisotropic property of composite plates, the group velocity of SH0 mode in different azimuthal directions was experimentally measured in damage imaging. The location of simulated damage, which was a bonded circular cylinder, was estimated by DC-OSH-MPT array and ellipse imaging algorithm. The imaging results have demonstrated that DC-OSH-MPT can be considered as a spatially distributed array suitable for imaging detection in composite plates.

An omnidirectional shear-horizontal guided wave EMAT for a metallic plate

We propose a new electromagnetic acoustic transducer (EMAT) for generation and measurement of omnidirectional shear-horizontal (SH) guided waves in metallic plates. The proposed EMAT requires a magnetic circuit configuration that allows omnidirectional SH wave transduction. It consists of a pair of ring-type permanent magnets that supply static magnetic fluxes and a specially wound coil that induces eddy currents. The Lorentz force acting along the circumferential direction is induced by the vertical static magnetic flux and the radial eddy current in a plate, resulting in omnidirectional SH wave generation. To maximize the transducer output at given excitation frequencies, optimal EMAT configurations are determined by numerical simulations and validated by experiments. The omnidirectivity of the proposed EMAT is also confirmed by the simulations and experiments.

Generation of omni-directional shear-horizontal waves in a ferromagnetic plate by a magnetostrictive patch transducer

It this study, omni-directional shear-horizontal wave transduction is performed using a specially configured magnetostrictive patch transducer in a ferromagnetic plate. For a ferromagnetic plate, unavoidable magnetic flux leakage into the plate not only results in poor transduction efficiency but also generates unwanted waves within the plate. These problems must be overcome to inspect ferromagnetic plates using the transducer. Therefore, we investigate the reasons for the poor performance and propose a method to improve its performance. The effectiveness of the proposed method was validated through simulations and experiments.

Development of an omni-directional shear-horizontal wave magnetostrictive patch transducer for plates

As an effective tool to inspect large plates, omni-directional guided wave transducers have become more widely used to form phased-array inspection systems. While omni-directional Lamb wave transducers have been successfully utilized in the systems, omni-directional Shear-Horizontal (SH) wave transducers have not been investigated. In this paper, we propose an omni-directional SH magnetostrictive patch transducer that consists of an annular magnetostrictive patch, a toroidal coil and a permanent magnet. After presenting the unique configuration of the proposed transducer and its working principle, the omni-directivity of the developed transducer is verified through simulations and experiments conducted in an aluminum plate. The frequency characteristics of the proposed transducer depending on the patch size are also investigated as the underlying reference data for future construction of an SH phased-array system.