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 µm to 250 µm 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 µm radius membranes thus involving their intrinsic capabilities. Harmonic excitation with targeted frequencies 300 kHz, 530 kHz and 800 kHz, below and beyond the resonance frequency of the membranes, are performed. The influence of the bias voltage V DC on the signal-to-noise ratio is studied according to the excitation frequency. As a result, a signal-to-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-tonoise 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.
In this work, a smart composite structure with damping and stiffness control ability is proposed. The multilayered arrangement has a shape memory core, whose damping and stiffness are tuned by temperature control. The structure is divided in several zones, each of them can be heated in real time using temperature regulation to reach the expected mechanical properties provided by the strong temperature and frequency dependency of the stiffness and loss factor of the viscoelastic core. The heat flux, which is used to tune the mechanical properties, is provided by copper tracks printed on an electronic board used as skin of the sandwich. The paper presents a model-based design process, including thermal and mechanical simulations, providing the gradient temperature fields which are required to obtain a specific damping and stiffness. Experimental tests are finally presented, considering three configurations with various temperature sets corresponding to three different compromises between static stiffness and dynamic damping.
A MEMS sensor dedicated to SHM applications is presented. The MEMS is made of a Capacitive Micromachined Ultrasonic Transducer (CMUT) chip composed of circular membranes array. The radius of the membranes vary between 50 µm and 250 µm and hence the associated resonance frequencies between 80 kHz and 2 MHz. A wide frequency bandwidth is then available for acoustic measurements. A testing campaign is conducted in order to characterize the MEMS sensor's behavior when subjected to single-frequency and broadband excitation stimuli. The single-frequency excitations are produced with specific piezoelectric transducers from 300 kHz to 800 kHz. The Fast Fourier Transform (FFT) of the measured signal from the CMUT is centered as expected on the excitation frequency. The broadband excitation is obtained with a pencil lead break. In this case, the FFT of the measured signal is centered on the resonance frequency of the membrane. These characterizations point out the DC bias voltage applied to the CMUT as a major parameter for controlling the sensitivity of the sensor. The CMUT sensor proves to be sufficiently sensitive to monitor these sources. This work highlights the relevant prospective capacities of the CMUT sensor to collect data in structural health monitoring applications. This sensor technology could be externally deployed, or even integrated into a composite structure, in order to monitor the structure by the CMUT detection, either by active ultrasound tests or by passive acoustic emission.
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