Towards a better understanding of the CMUTs potential for SHM applications

The ability of capacitive micromachined ultrasonic transducer (CMUTs) to design broadband sensors for structural health monitoring (SHM) is studied through both multi-frequency and bandwidth aspects. Elementary cells are composed of circular membranes fabricated using the standard MUMPS Process. The multi-frequency aspect, which involves different individual membranes from 50 to 250 mu m radius, is theoretically addressed through a numerical modeling. The targeted frequency range, consistent with the SHM application, is then between 80 kHz and 2 MHz. Geometrical features induced by the manufacturing process greatly affect the dynamic properties of the membranes and this is experimentally validated. The bandwidth aspect is also addressed on an array of identical 100 mu m radius membranes thus involving their intrinsic capabilities. Harmonic excitation with targeted frequencies 300, 530 and 800 kHz, below and beyond the resonance frequency of the membranes, are performed. The influence of the bias voltage VDC on the signal-to-noise ratio is studied according to the excitation frequency. As a result, a signalto-noise of 20 dB is achieved around the resonance frequency. Finally, the circular membranes array is tested for acoustic emission sensing through a pencil lead break test. In spite of a low signal-to-noise ratio, acoustic events are clearly detected. The multi-frequency aspect and the large bandwidth capability of the CMUTs are hence demonstrated and highlight the adaptability of the sensor to its environment. (C) 2020 Elsevier B.V. All rights reserved.