Peking University publishes research results to open a new era of structural health monitoring

[ Instrument Network Instrument Development ] Recently, the international ultrasound magazine Ultrasonics published a paper entitled "A practical omni-directional SH wave transducer for structural health monitoring based on two thickness-poled piezoelectric half-rings". (Q. Huan et. al, Ultrasonics, 2019 (94): 342-349). In this paper, a horizontal shear (SH) wave piezoelectric transducer with simple structure and widely used in structural health monitoring is proposed. It consists of two oppositely-polarized half-rings that apply a toroidal electric field to easily excite an isotropic non-dispersive horizontal shear wave SH0. The publication of this work marks the international leading position of Li Faxin's research group in the field of ultrasonic guided wave detection/monitoring, opening up a new era of basic research and applied research of SH wave. The first author of the thesis is a Ph.D. student at the Peking University Institute of Technology.

Figure 1. Double-half-ring SH-wave transducer and its excited omnidirectional SH wave

In recent years, structural health monitoring (SHM) has become more and more important in modern industry. For large structures such as pipelines and plates, ultrasonic guided wave monitoring methods have unique advantages such as long propagation distance and accurate positioning. However, the commonly used Lamb guided waves are subject to many limitations in practical applications due to problems such as multi-modality and dispersion. In contrast, the zero-order SH guided wave is completely non-dispersive and has only one displacement component, which has obvious advantages in computational analysis and signal processing, but the pure SH wave is difficult to be excited by the piezoelectric transducer.

As early as the end of the 1970s, American scholar Thompson et al. had used electromagnetic ultrasonic transducer (EMAT) to excite pureer SH waves in the plate. However, the biggest problem of EMAT is that the energy conversion efficiency is too low, so it is necessary to use a high-power transmitting device and a signal amplifying receiving circuit to work. The detection distance is limited, and it is difficult to exert the advantage of guiding wavelength distance detection, and the device is cumbersome and cannot be used for the structure. Health Monitoring (SHM). In the paper (IEEE. UFFC., 2011), the leading scholar in the field of non-destructive testing, the Royal College of British Academy of Sciences, and Professor Peter Cawley of Imperial College of Technology pointed out: "The SH guided wave in the plate and the torsional guided wave in the pipeline are difficult to use. Piezoelectric transducer excitation." In the paper (NDT&E International., 2012), a leading scholar in the field of electromagnetic ultrasound, Professor Ogi of Osaka University, Japan commented: "The current research on SH waves is mainly focused on theoretical and numerical simulations because SH waves are difficult to excite with piezoelectric transducers." . Therefore, the use of piezoelectric transducers to excite pure SH waves has been a difficult problem in this field, which seriously restricts the progress of basic research and applied research of SH waves.

Li Faxin's research group has been engaged in the research of SH wave excitation method since 2015. Due to years of accumulation in the field of iron voltage and electricity, it has made breakthroughs in more than three years, and has developed three generations of shear-type SH wave piezoelectric transformers. The energy device can be conveniently used for the basic research of SH wave and the application research of detection and monitoring.

The first generation: in-plane shear d36 type and synthetic d36 type SH wave piezoelectric transducer

Conventional PZT piezoelectric ceramics are transversely isotropic due to polarization, and there is no deformation mode of d36 in-plane shearing. The research group developed the deformation mode of d36 in-plane shear (APL2015) in PZT ceramics for the first time through stress-induced ferroelastic domain change (see Figure 2, left). The use of d36 PZT ceramics can excite SH waves (Miaoet al, JAP 2016), but also stimulates Lamb waves. They improved the design and prepared a synthetic d36 piezoelectric ceramic (Li, Miao, JAP 2016) by two-dimensional reverse polarization (see Figure 2). It excites the SH0 wave better, but still can't motivate it. Pure SH0 wave. They proposed a method of applying a two-dimensional reverse electric field to a thickness-polarized PZT ceramic segmented electrode (see Figure 2, right) to excite and receive SH0 waves (Huanet al, Ultrasonics 2018) in a narrower frequency band. This method can be applied to commercial thickness-polarized PZT, which is suitable for laboratory research, but it is not suitable for a wide range of applications due to troublesome leads.

Second generation: in-plane shear d24 type SH wave piezoelectric transducer

In 2016, they proposed a new in-plane shear d24 mode (Fig. 3 left), which uses a d24 piezoelectric transducer to excite a single-mode non-dispersive zero-order SH wave in a flat plate, while also selectively Receive SH waves (filter out Lamb waves). This work was published in November 2016 in the field of authoritative journal Smart Mater Struct, which was selected as one of the 23 highlights of the journal in 2016 (more than 540 papers throughout the year). They then used a pair of dimensionally optimized d24 piezoelectric sheets (in Figure 3) to produce a bidirectional focused SH wave transducer suitable for studying the fundamental properties of SH waves and exciting circumferential SH guided waves in large diameter pipes. (Miao et al, Ultrasonics 2018). At the same time, they use a ring-shaped d24 piezoelectric array (Fig. 3 right) to excite a non-dispersive zero-order torsional wave T(0,1) in a circular tube (Miao et al, Smart Mater Struct2017) for long distances. The structural health monitoring of the pipeline provides a convenient and feasible technical solution.

Third generation: omnidirectional SH wave piezoelectric transducer

The first and second generation SH wave transducers in the front are either directional (cross pointing) or bidirectional (focusing) types, and omnidirectional SH wave transducers are required for board structure detection/monitoring. The research group used the d24 piezoelectric flat ring array to synthesize the circumferential polarization (Fig. 4 left), and realized the omnidirectional SH wave transducer (Miao et al, Ultrasonics 2017). The sensitivity deviation in each direction is about 15%, acceptable but not ideal. They further proposed an omnidirectional SH-wave transducer (Huanet al, Smart Mater Struct 2017) based on a new thickness-polarized, thickness-sheared d15 mode piezoelectric ring 12 (see Figure 4), the transducer High signal-to-noise ratio (~24dB) and good uniformity (sensitivity deviation 6-7%) have been verified in the health monitoring of sparse array structure. Compared with the structural health monitoring system based on Lamb wave, the non-dispersive SH0 guided wave monitoring system has obvious advantages such as high defect positioning accuracy, simple signal processing and variable frequency operation. However, such a 12-element transducer is still inconvenient in large-scale applications.

The newly released omnidirectional SH-wave transducer based on the bimorph half-ring has simple structure and excellent performance, which completely solves the technical problem of SH wave used in structural health monitoring. It is foreseeable that in the near future, the research pattern in the field of guided wave detection/monitoring will change, and the detection/monitoring method based on SH wave will be rapidly developed. Moreover, the double-half-ring SH-wave piezoelectric transducer is also a perfect torsional vibration transducer that can excite non-dispersive torsional guided waves T(0,1) in a round bar or a circular tube. Therefore, it will be widely used in the fields related to ultrasound and vibration.

The above-mentioned development of the SH wave transducer has been applied in Tufts University, Beijing Institute of Technology, Xiamen University, Dalian University of Technology and other units, and the response is very good. Relevant work has won the first prizes of the first and second international seminars on rail transit structural health monitoring (2016, 2018, all only), and the first prize of the best paper of China Railway Society (2019).

(Original title: Li Faxin, a research group, published the research results of SH ultrasonic guided wave transducer to open a new era of structural health monitoring)