Piezoelectric Transducers For Excitation Of Unidirectional Propagation SH Guided Wave

The unidirectional generation of guided wave is of great importance in structural health monitoring（SHM）and non-destructive testing（NDE）.Focusing the incident wave beam along a given direction will not only save input power but also simplify the interpretation of signals.However,the guided waves induced by a disturbance in an isotropic plate-like structure are usually multi-mode and propagate along all 360-degree directions,which makes the realization of the controllable excitation of a single wave mode propagation along a given direction remaining as a challenging issue.In this dissertation,methods for excitation of unidirectional propagation SH₀ wave（the fundamental shear horizontal wave）were proposed and several unidirectional SH₀ wave piezoelectric transducers were developed.The main contributions are listed as follow:（1）Base on the unique feature of bidirectional SH₀ wave piezoelectric face-shear and thickness-shear piezoelectric transducers,a new method was proposed for generating the unidirectional SH₀ wave.Two types of transducer configurations（double-side and single-side）were designed to unidirectionally generate SH₀ waves.The double-side transducer configuration is a good tool for unidirectional generation of the SH₀ waves in laboratory studies,while the single-side transducer configuration is more preferred in realistic applications due to its easy installation feature.The performances of the proposed transducer configurations were exanimated by both finite element simulations and experimental measurements.The results indicated that both two configurations could unidirectionally generate pure SH₀ wave within a frequency range of about 50 k Hz without frequency-dependent time delay.（2）The thickness-shear vibration characteristics of a piezoelectric wafer bonded on a waveguide were numerically investigated,it was found that the properties of the bonding layer will induce an equivalent phase delay between the excitation signal and vibration response of the lower surface of the wafer.Moreover,such phase delay can be changed by adjusting the size of the wafer.Based on these findings,a methodology for unidirectional generation of SH₀wave without time delay was proposed.（3）Based on the proposed unidirectional generation method,two unidirectional SH₀wave piezoelectric transducers with different topological structures were then developed.One transducer is made of rectangle piezoelectric wafers so it was named as the rectangle unidirectional transducer,while the other one consists of several rhombus piezoelectric wafers so it was abbreviated as the rhombus unidirectional transducer.Finite element simulation and experimental measurements show that the both transducers were able to generate unidirectional SH₀ wave without excitation time delay.The rectangle unidirectional transducer was capable of directionally generate the SH₀ wave in the range about 150 k Hz to 190 k Hz,restricting the SH₀ wave beam in the range about 60°with merely negligible Lamb waves.While the rhombus unidirectional transducer was able to effectively excite the unidirectional propagation SH₀wave from 160 k Hz to 200 k Hz,with a small beam divergence angle less than 60°.