| Ultrasonic guided waves in solids are widely used in the fields of non-destructive testing（NDT）and structural health monitoring（SHM）due to their properties of long propagation distance,wide detection range and sensitivity to structural damages.At present,the existing NDT/SHM sensors can only pick up the 1D vibration displacement/velocity of ultrasonic guided waves,and there is still a lack of the way for real-time sensing the 3D vibration displacement/velocity of guided waves at one point,which restricts the development of damage detection based on multi-mode guided waves.In the past two decades,capacitive micro-machined ultrasonic transducers（CMUTs）have achieved great developments and have shown inestimable application prospects in the ultrasonic medicine.Based on the advantages of micro-electro-mechanical system（MEMS）,the size of one CMUT cell can be processed to the order of micrometers or even nanometers,which is far smaller than the wavelength of ultrasonic guided waves.Therefore,it is expected to achieve real-time sensing of 3D guided waves at one point using CMUTs.In this work,we propose a novel 3D-CMUT for sensing 3D guided waves,develop the relevant theories for designing 3D-CMUT,and explore the potential applications of3D-CMUT in NDT/SHM.The main contents are as follows:（1）Based on the control equations of ultrasonic guided waves in plates,the wave structures of low-order A0,S0,and SH0 modes are analyzed.For sensing the out-of-plane and the in-plane displacement/velocity of different guided modes,an out-of-plane-sensing CMUT cell and an in-plane-sensing CMUT cell are designed,respectively.On this basis,an analytical small-signal model based on the“spring-mass-damper”model and the parallel-plate capacitor model,are developed for theses two types of CMUT cell,through which the vibration velocity,output current,and sensitivity of an out-of-plane-sensing CMUT cell/array and an in-plane-sensing CMUT cell/array can be directly calculated by the structural and material parameters,thus,providing a theoretical way to design CMUT for sensing guided waves.（2）The finite element method（FEM）simulation models of an out-of-plane-sensing CMUT cell/array and an in-plane-sensing CMUT cell/array are respectively established to verify the accuracy of the analytical small-signal model.By selecting different structural parameters and DC bias voltages,the CMUT cells resonating at different frequencies are designed,of which the vibration modes and eigenfrequencies,as well as the working behaviors in steady-state,frequency-domain and time-domain,are analyzed.It is found that the results of analytical small-signal model are almost consistent with those of FEM simulation,if the used 1stnatural frequencies of these two method are identical.On this basis,the effects of air damping,the cell number in array,the cell spacing in array,the input displacement amplitude,and the DC bias voltage amplitude on the sensing behaviors of these CMUT cells are discussed.Relevant conclusions may be significant for guiding the design of sensors.（3）For the aim of meeting requirements of broadband sensing,the broadband out-of-plane-sensing CMUT and the broadband in-plane-sensing CMUT are developed.By combining CMUT cells resonating at different frequencies and changing the applied DC bias voltage differences,we propose a novel broadband CMUT,which are based on the mechanism of phase reversal.On this basis,the working behaviors of the phase-reversal broadband CMUT in coarse vacuum and air damping environments are studied both in theory and FEM simulation.It is found that the pass band and stop band performances of phase-reversal broadband CMUT are significantly better than those of traditional broadband CMUT.In order to adapting broadband CMUTs for sensing guided waves in different frequency-bands,an optimized method combined with the genetic algorithm,the analytical small-signal model,and the FEM simulation is proposed,and the influences of frequency-band width,frequency-band position,and frequency offset on the design of broadband CMUTs are discussed.Based on this method,the design of a broadband CMUT with any flat pass band can be realized in theory.（4）3D-CMUT is developed to meet the requirements of sensing 3D guided waves at one point.Through the combination of out-of-plane-sensing CMUT cells and in-plane-sensing CMUT cells,CAD layouts of 3D-CMUT for sensing narrow-band 3D guided waves and for sensing broadband 3D guided waves,as well as the way to unify sensitivities,are proposed respectively.On this basis,the simplified combination of an out-of-plane-sensing CMUT cell and two orthogonally placed in-plane-sensing CMUT cells are taken as an example,to study the working behaviors of 3D-CMUT,and to verify the feasibility of sensing A0,S0 and SH0guided waves using 3D-CMUT.Finally,the potential applications of 3D-CMUT in the field of NDT/SHM is explored,including sensing the 3D acoustic emission signals from the falling ball impacts on a non-destructive thin plate,and imaging damages in the damaged thin plate by combining the received 3D scattering signals,the DORT-MUSIC algorithm and the image fusion together.This work has developed a micro sensor for real-timely detecting the 3D vibrations of guided waves at one point.Relevant results may provide a device support for the damage detection using multi-mode guided waves,and may promote the development of the damage detection technology using multi-mode guided waves. |
