Excitation of unidirectional SH wave within a frequency range of 50 kHz by piezoelectric transducers without frequency-dependent time delay

Highlights

• A method for unidirectional generation of SH wave without frequency-dependent time delay was proposed.

• Two types of unidirectional SH wave transducer configurations were designed.

• Simulations and experiments validated the proposed unidirectional transducers.

Abstract

Unidirectional generation of a pure guided wave mode is of practical importance in structural health monitoring (SHM). Currently, unidirectional propagation guided waves can only be generated by using transducer array in a phased array form, which is limited within a very narrow frequency range. If a variable-wavelength unidirectional wave source is required, both the transducer spacing and time delay usually need to be changed with frequency, which is against the permanent arrangement demand of transducers in SHM. In this work, inspired by the unique features of bidirectional SH₀ wave (the fundamental shear horizontal mode) piezoelectric face-shear and thickness-shear transducers, a new method was proposed for generating unidirectional SH₀ wave. Two types of transducer configurations (double-side and single-side) were designed to unidirectionally generate SH₀ wave without frequency-dependent time delay. Both finite element simulations and experiments were conducted to validate the unidirectional features of the proposed double-side and single-side transducer configurations. Results shows that the double-side transducer configuration is capable of generating unidirectional propagation SH₀ wave at the frequency range from 100 kHz to 150 kHz, while the corresponding working frequency range for the single-side one is from 100 kHz to 140 kHz. The proposed method provides a cheap way for generation of the unidirectional SH₀ wave within a certain frequency range without adjusting the relative transducer spacing and the time delay with frequency, so it will have great potential applications in SHM.

Introduction

Due to its unique nondispersive characteristic, the fundamental shear horizontal wave (SH₀ wave) is believed to be an ideal wave mode for structural health monitoring (SHM) and non-destructive evaluation (NDE) of plate-like structures. However, SH₀ wave is less-used than Lamb waves in the past decades possibly due to the fact that controllable generation of a pure SH₀ wave is difficult. Therefore, it is essential to develop ultrasonic transducers for exciting pure SH₀ wave with controllable radiation patterns.

The current SH₀ wave transducers can be classified into omnidirectional and directional transducers according to their radiation patterns [1]. Omnidirectional transducers are preferred to constitute phased array inspection systems, since omnidirectional B-scan can be realized to inspect the whole plate over a full 360° [2]. Lorentz-force-based [3], [4], magnetostrictive [5], piezoelectric face-shear [6] and thickness-shear [7], [8], [9] transducers are currently available four kinds of omnidirectional SH₀ wave transducers. All of them generate omnidirectional SH₀ wave by inducing axisymmetric tangential tractions in the waveguide (Note that inducing face-shear deformation is physically equivalent to applying tangential tractions [1], [10]), although the effective tractions are produced by different mechanisms for different kinds of transducers [1].

Directional SH₀ wave transducers are also of great importance in many applications, since wave energy can be directly focused along given directions by a single directional transducer without complex electronics [11], [12]. Directional transducers can be either bidirectional or unidirectional. Bidirectional propagation SH₀ wave is relatively easier to obtain than the unidirectional one. The first Lorentz-force-based SH₀ wave transducer is exactly a bidirectional device [13], which is known as the periodic permanent magnet electromagnetic acoustic transducer (PPM-EMAT). Alternating in-plane body forces can be generated in the waveguide by the PPM-EMAT [14], [15], so Lamb waves in the direction parallel to the body forces are suppressed due to the destructive interference while bidirectional SH₀ wave is generated in the perpendicular direction. Based on the same excitation mechanism, thickness-shear piezoelectric transducers for exciting bidirectional SH₀ wave were also proposed [16], [17]. Unlike the PPM-EMATs and thickness-shear piezoelectric transducers, magnetostrictive EMATs [18], [19], [20] and face-shear piezoelectric transducers [12] need to suppress the SH₀ wave in the unwanted directions to obtain bidirectional SH₀ wave, since a pure in-plane shear deformation will intrinsically generate SH₀ wave in four orthogonal directions (0°, 90°, 180° and 270°) [1]. Inducing symmetric face-shear deformations is an effective method to eliminate the SH₀ wave in the unwanted directions [12]. Based on this principle, several bidirectional SH₀ wave piezoelectric transducers have been developed [12], [21], [22].

Efficient unidirectional generation of an ultrasonic wave can be realized by using two bidirectional transducers with a predefined space interval and a proper excitation time delay. This phased array setup results in destructive interference in the weakened side, whilst constructive interference occurs in the opposite direction. This principle has been exploited to unidirectionally generate Lamb waves in plates [23], [24], [25], longitudinal [26] and torsional [27] guided waves in pipes. Recently, based on the same principle, unidirectional generation of the SH₀ wave is also realized by using two pairs of thickness-shear piezoelectric strips [28] and new-designed PPM-EMATs [29], [30]. However, this unidirectional generation principle requires that the wave source spacing is precisely matched with the wavelength, so the unidirectional transducers (or transducer array) mentioned above are only valid within a very narrow frequency range. In other words, if a variable-wavelength unidirectional wave source is required, both the transducer spacing and time delay usually need to be changed with frequency, which is against the permanent arrangement demand of transducers in SHM. Therefore, it is essential to develop a new method to unidirectionally generate ultrasonic waves over a wide frequency range.

In this work, based on the unique features of bidirectional SH₀ wave piezoelectric face-shear and thickness-shear transducers, a new method was proposed to unidirectionally excite the SH₀ wave within a certain frequency range without frequency-dependent time delay. Firstly, the proposed principle of unidirectional wave generation was explained and two types of transducer configurations based on this principle were designed. The transducer configurations’ performances on unidirectional generation of the SH₀ wave were then validated by finite element simulations. Finally, experiments were conducted to investigate the unidirectivity of the SH₀ wave generated by the proposed transducer configurations. Results validate these designs very well. The proposed methodology may be useful in many applications, since it provides a cost-effective method to generate unidirectional SH₀ wave.