Selective Actuation and Sensing of Antisymmetric Waves Using Shear-mode Piezoelectric Transducers

摘要

One of the most advanced techniques for structural health monitoring is based on analysis of ultrasonic guided waves propagated through plate-like structures. Analysis of changes in propagated guided wave signals enables detection, location, characterization and quantification of damage. Characterization is possible because different guided wave propagation modes interact with various damage forms in different ways including reflection, attenuation, distortion, and mode conversion. An obstacle to guided wave analysis is the complexity of the received signals which can contain multiple guided wave modes that propagate with different velocities. The result is complex and superposed wave signals that are difficult to interpret. A common technique to deal with the multiple propagation modes of guided waves is to focus on the first wave packet arrival which has to be the fastest traveling propagation mode traveling the shortest path. At relatively low frequencies, the fastest mode of propagation is the fundamental symmetric Lamb wave propagation mode. Unfortunately, symmetric strain wave propagation is not sensitive to certain forms of damage. Antisymmetric wave propagation modes have been found to be sensitive to some different forms of damage, but are challenging to work with. At relatively low frequencies the fundamental antisymmetric mode propagates slower than the fundamental symmetric mode. Reflections, dispersion, and mode conversions create the potential for constructive and destructive interference in a signal that contains both symmetric and antisymmetric propagation modes. Some techniques have been developed to selectively excite and detect specific propagation modes to support signal analysis but many of these are inherently tuned to specific frequencies, require precise geometries, are inefficient, or employ multiple actuators or sensors. This paper presents recent research into selectively actuating and sensing antisymmetric strain waves using shear-mode piezoelectric transducers internally located at the neutral axis of a structure for damage detection. This approach employs the inherent coupling between transverse shear and bending to actuate and sense antisymmetric waves using shear mode transducers. Finite element (FE) models were created to simulate wave propagation in structures containing shear mode transducers at the neutral axis. FE model results were compared against experimental results and classical theory for validation and analysis. The FE models were then used to further analyze the wave propagation and output signals with geometry to cause mode conversion and additional, more traditional surface mounted d_3i transducers. Results indicate that actuating shear mode transducers at the neutral axis only produced AO propagation modes in the polarization direction. Other wave modes could develop due to mode conversion from interaction between the antisymmetric wave and structural discrepancies, defects or damage. An inverse property was also found when using shear mode transducers located at the neutral axis as sensors; they did not detect symmetric waves and functioned as a hardware based filter producing signals that exclusively represent antisymmetric deformation. The combination of selective actuation and selective sensitivity provides a powerful tool for ultrasonic signal analysis in SHM.