Piezoelectric materials have been widely used for structural sensing due to the linear electromechanical coupling effect. Flexoelectricity, generally existing in all dielectrics, describes the linear inhomogeneous electromechanical coupling phenomena. Previous studies have shown that the direct flexoelectric effect is sensitive to the bending deformation. This study focuses on comparison and differences between two sensing mechanisms, that is, the flexoelectric and the piezoelectric effects, and explores their distinctive characteristics and potential applications. Based on the direct flexoelectric/piezoelectric effects, flexoelectric/piezoelectric materials treated as flexoelectric/piezoelectric sensors are applied to two ring models with identical dimensions. The mathematical models of thin elastic rings laminated with flexoelectric/piezoelectric sensors are established. Open-circuit sensing signals are derived for further evaluation and comparison. Sensing capabilities of these two materials and mechanisms are compared with respect to ring thickness, sensor thickness, and ring radius in case studies. Results show that the piezoelectric sensing signal appears to be sensitive for both bending-dominant and membrane-dominant vibrations, while the flexoelectric signal is much more prominent than the piezoelectric one for sensing of bending-dominant vibrations. For a flexible structure performing transverse vibration, the bending behavior is usually dominant. Thus, the flexoelectric effect could provide an effective alternative for structural sensing.
Flexoelectricity is known as the electromechanical coupling effect between the strain gradient and the polarization. In this study, a flexoelectric sensor is laminated on conical shells to monitor the natural modal signal distributions. The sensing mechanism of a generic flexoelectric sensor patch is presented first. Then, the spatially distributed microscopic sensing signal with respect to coordinates is evaluated in detail to reveal the modal signal distributions. Due to the gradient effect, the bending strain component is the only contribution to the total sensing signal. The total signal consists of two components resulting from the circumferential and longitudinal bending strain components. Analytical results show that the flexoelectric sensing signal induced by the longitudinal bending strain is the dominant contribution to the total signal for lower order modes; the contribution of the circumferential bending strain components increases while increasing the circumferential mode number. In lower modes, the optimal location of flexoelectric sensor is at the minor end and shifts to the middle for shallow shells. In higher modes, the optimal location is at the middle of the shell, but first shifts to the major end and then shifts to the minor end while increasing the semi-apex angle.
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