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 varies 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.
Viscoelastic materials are widely used to control vibrations. However, their mechanical properties are known to be frequency and temperature-dependent. Thus, in a narrow frequency bandwidth, there is an optimal temperature that corresponds to a maximum loss factor and it is tricky to get a high damping level over a wide frequency range. Furthermore, an optimal temperature for a maximum structural damping leads to a low static stiffness because the peak of the loss factor is obtained during the glass transition when the storage modulus is decreasing. In order to obtain a compromise between stiffness and damping it is suggested to use a viscoelastic material which properties are functionally graded thanks to a non-uniform temperature field over the structure. In this work, a composite structure has been designed integrating a viscoelastic core and a heat control device. The optimal temperature field has been obtained through the minimization of a cost function that reflects the compromise between structural damping over a wide frequency band and high static rigidity. The experimental validation has been performed on a reduced scale airplane model: the composite wings are sandwich structures made of aluminum skins and a viscoelastic core in tBA/PEGDMA with a non-uniform temperature field and skins are in an aluminum and FR-4. A broadband excitation is produced with a shaker and the measurements are performed with a set of accelerometers. Several temperature fields are tested. The frequency response functions show the compromise obtained between static and dynamic behaviors when using the optimal temperature field determined by numerical simulation.
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