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This paper deals with a multiphysics numerical modelling via finite element method (FEM) of an air-coupled array piezoelectric micromachined ultrasonic transducers (PMUTs). The proposed numerical model is fully 3D with the following features: the presence of the fabrication induced residual stresses, which determine a geometrically non-linear initial deformed configuration of the diaphragms and a remarkable shift of the fundamental frequency; the multiple coupling between different physics, namely electro-mechanical-coupling for the piezo-electric model, acoustic-structure interaction at the acoustic-structure interface and pressure acoustics in the surrounding air. The model takes into account the complete set of PMUTs belonging to the silicon die in a 4 × 4 array configuration and the protective package, as well. The results have been validated by experimental data, in terms of initial static pre-deflected configuration of the diaphragms and frequency response function of the PMUT. The numerical procedure was applied, to analyze different package configurations of the device, to study the influence of the holes on the acoustic transmission in terms of SPL and propagation pattern and consequently extract a set of design guidelines. [ABSTRACT FROM AUTHOR]

The design of innovative metamaterials with robust and reliable performances is attracting increasing interest in the scientific community because of their unique properties and for their unexplored potential. In particular, dynamical properties of periodic structures are widely studied specifically for their bandgap opening characteristic, which enables the design of structures with unprecedented dynamical behaviour. In the present work an ultra-wide three-dimensional bandgap is presented, with extremely low frequency range of operation. Numerical simulations and analytical models are proposed to prove the claimed properties, together with experiments carried out on a prototype built by means of additive manufacturing. [ABSTRACT FROM AUTHOR]

The energy harvesting capability of a graded metamaterial is maximised via reinforcement learning (RL) under realistic excitations at the microscale. The metamaterial consists of a waveguide with a set of beam-like resonators of variable length, with piezoelectric patches, attached to it. The piezo-mechanical system is modelled through equivalent lumped parameters determined via a general impedance analysis. Realistic conditions are mimicked by considering either magnetic loading or random excitations, the latter scenario requiring the enhancement of the harvesting capability for a class of forcing terms with similar but different frequency content. The RL-based optimisation is empowered by using the physical understanding of wave propagation in a such local resonance system to constrain the state representation and the action space. The procedure outcomes are compared against grading rules optimised through genetic algorithms. While genetic algorithms are more effective in the deterministic setting featuring the application of magnetic loading, the proposed RL-based proves superior in the inherently stochastic setting of the random excitation scenario. [ABSTRACT FROM AUTHOR]

This work presents an investigation on the dynamics of a magnetically frequency up-converted piezoelectric energy harvester. The magnetic interaction arises between a tip magnet on a piezoelectric bimorph and a moving magnet. The latter is controlled through a low-frequency shaker at a fixed frequency of 3 Hz. We investigate in detail the effect of increasing velocity interaction between the magnets at the same input frequency, in terms of frequency up-conversion (i.e. migration from slow to fast dynamics). We show that, for increasing velocity of interaction, the frequency of the first bending mode of the piezoelectric bimorph gradually appears. Also material nonlinearities are observed as a frequency shift of the bending mode. The work shows also that the magnetic plucking phenomenon can be reliably simulated by means of a reduced order model that takes into account material nonlinearities. [Display omitted] • Efficient frequency up-conversion depends on the velocity of interaction. • Material nonlinearities can arise in the magnetic plucking. • Reduced order model can be used for reliable simulation of the nonlinear harvester. [ABSTRACT FROM AUTHOR]

Aim of this work is the synthesis of auxetic structures using a topology optimization approach for micropolar (or Cosserat) materials. A distributed compliant mechanism design problem is formulated, adopting a SIMP-like model to approximate the constitutive parameters of 2D micropolar bodies. The robustness of the proposed approach is assessed through numerical examples concerning the optimal design of structures that can expand perpendicularly to an applied tensile stress. The influence of the material characteristic length on the optimal layouts is investigated. Depending on the inherent flexural stiffness of micropolar solids, truss-like solutions typical of Cauchy solids are replaced by curved beam-like material distributions. No homogenization technique is implemented, since the proposed design approach applies to elements made of microstructured material with prescribed properties and not to the material itself. [ABSTRACT FROM AUTHOR]

This work presents a novel development of the impact-based mechanism for piezoelectric vibration energy harvesters. More precisely, the effect of an impacting mass on a cantilever piezoelectric transducer is studied both in terms of the tip mass value attached to the cantilever and impact position to find an optimal condition for power extraction. At first, the study is carried out by means of parametric analyses at varying tip mass and impact position on a unimorph MEMS cantilever, and a suitable physical interpretation of the associated electromechanical response is given. The effect of multiple impacts is also considered. From the analysis, it emerges that the most effective configuration, in terms of power output, is an impact at the cantilever tip without a tip mass. By changing the value of the tip mass, a sub-optimal impact position along the beam axis can also be identified. Moreover, the effect of a tip mass is deleterious on the power performance, contrary to the well-known case of a resonant energy harvester. A mesoscale prototype with a bimorph transducer is fabricated and tested to validate the computational models. The comparison shows a good agreement between numerical models and the experiments. The proposed approach is promising in the field of consumer electronics, such as wearable devices, in which the impact-based device moves at the frequencies of human movement and is much lower than those of microsystems. [ABSTRACT FROM AUTHOR]

The application of piezoelectric composites in energy harvesters is continuously increasing even at the microscale, with the immediate corollary of a fundamental need for improved computational tools for optimization of performances at the design level. In this paper, a refined, yet simple model is proposed with the aim of providing fast and insightful solutions to the multi-physics problem of energy harvesting via piezoelectric layered structures. The main objective is to retain a simple structural model (Euler–Bernoulli beam), with the inclusion of effects connected to the actual three-dimensional shape of the device. A thorough presentation of the analytical model is presented, along with its validation by comparison with the results of fully 3D computations. [ABSTRACT FROM AUTHOR]

In this work we present a numerical, multi-scale approach to estimate the strength of a wafer-to-wafer metallic thermo-compression bonding. Following a top-down approach, the mechanical problem is handled at three different length scales. Taking into account control variables such as temperature, overall applied force over the wafer and contact surface roughness, it is shown that the proposed approach is able to provide an estimate of the sealing properties, especially in terms of bonding strength. [ABSTRACT FROM AUTHOR]

This work proposes a mono-axial piezoelectric energy harvester based on the innovative combination of magnetic plucking and indirect impacts, e.g., impacts happening on the package of the harvester. The harvester exploits a permanent magnet placed on a non-magnetic mass, free to move within a predefined bounded region located in front of a piezoelectric bimorph cantilever equipped with a magnet as the tip mass. When the harvester is subjected to a low-frequency external acceleration, the moving mass induces an abrupt deflection and release of the cantilever by means of magnetic coupling, followed by impacts of the same mass against the harvester package. The combined effect of magnetic plucking and indirect impacts induces a frequency up-conversion. A prototype has been designed, fabricated, fastened to the wrist of a person by means of a wristband, and experimentally tested for different motion levels. By setting the magnets in a repulsive configuration, after 50 s of consecutive impacts induced by shaking, an energy of 253.41 μJ has been stored: this value is seven times higher compared to the case of harvester subjected to indirect impacts only, i.e., without magnetic coupling. This confirms that the combination of magnetic plucking and indirect impacts triggers the effective scavenging of electrical energy even from low-frequency non-periodical mechanical movements, such as human motion, while preserving the reliability of piezoelectric components. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the numerical simulation of piezolaminated microplates adopted as actuators in micropumps. The performances of piezoelectric actuation are critically assessed by means of comparisons with devices based on the electrostatic force. In order to perform accurate simulations of the micropump behavior, the theory of laminates is adopted, account taken of the piezoelectric coupling. Both static and dynamic analyses have been performed, in order to obtain information on the optimal configuration for the micropump. [Copyright &y& Elsevier]

Abstract: We present a numerical study aimed to define the overall elastic domain of thin polysilicon films subjected to in-plane loadings. Homogenized properties are obtained for digital polycrystalline microstructures, generated in a representative volume element (RVE) through Voronoi tessellations. To locate the elastic limit, three micromechanical sources of dissipation are allowed for: (i) trans-granular cracking, as due to tensile stresses attaining the local strength inside silicon grains; (ii) inter-granular failure, as due to coupled normal-shear tractions attaining a local effective strength along grain boundaries; (iii) trans-granular phase transformation, as due to compressive stresses attaining a critical threshold inside silicon grains. Results of the homogenization procedure show that Rankine-type overall domains are too crude approximations to the polysilicon elastic envelope, since corners arise because of switching among the three dissipative modes allowed for. Outcomes of the analysis allow also to estimate the size of the RVE required to get objective overall polysilicon properties: according to data already available in the literature, it is shown that the RVE has to gather at least a few hundreds of grains. [Copyright &y& Elsevier]

We present a new multiphysics Reduced Order Model (ROM) for the electro-mechanical-acoustic analysis of large arrays of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs). The solution strategy consists of two main stages. In the former one, the eigenvalue piezoelectric problem is solved on a representative PMUT and a few representative modes are identified. In the latter stage, the piezoelectric-acoustic problem is addressed by representing the mechanical response of each PMUT as a linear combination of the selected modes, each mode corresponding to a single Degree Of Freedom (DOF). The governing equations of the coupled problem are integrated by means of a time-marching staggered algorithm. The proposed formulation is benchmarked with the commercial software COMSOL Multiphysics 5.2, in the case of a single transducer in transmitting (TX) and receiving (RX) phases. The Finite Element Reduced Order Model (FE-ROM) provides a very good accuracy and largely reduced CPU time with respect to the standard coupled piezoelectric-acoustic full-order approach. Since the novel approach has been conceived to address beamforming, focusing and RX problems, the model is next applied to the simulation of pressure waves propagation, involving an array of several vibrating PMUTs. Two case studies are reported. In the former, acoustic energy has been focused at a preselected point along the transversal acoustic axis of the device by means of an array of 11 × 11 PMUTs; in the latter, a RX phase has been simulated with a 7 × 7 cluster of PMUTs, considering an incoming plane pressure wave. [ABSTRACT FROM AUTHOR]

Microsystems (or Micro Electro Mechanical Systems, MEMS) are important components of many popular products in the consumer market and are one of the enabling ingredients of incoming industrial revolutions like Industry 4.0 and Internet of Things (IoT). Behind many Microsystems there are important mechanical principles and coupling effects that must be completely mastered starting from the design phase. More sophisticated and smaller devices also imply to consider many non-linear effects that can be strictly related to the mechanical response or to coupled electro-thermo-mechanical phenomena. This paper contains an overview of key mechanical aspects in design and reliability of Microsystems with a particular focus on non-linear dynamics of oscillators in inertial sensors. • State of the art on non-linear mechanical phenomena in microsystems. • Focus of the paper on resonant inertial micro sensors. • Various examples on non-linearities in real cases. • Discussion on future research directions. [ABSTRACT FROM AUTHOR]

The mechanical characterization of a 4 × 4 air-coupled array of Piezoelectric Micromachined Ultrasonic Transducers (PMUTs) is presented. The experimental campaign consists of three set of experimental tests, namely: topography measurements, small signal dynamic measurements, and vibrometry in the non-linear dynamic regime. The behavior of three different kinds of PMUT are reported. They differ according to the thermo-electrical treatment that has been applied to the piezoelectric material. The presence of the fabrication induced residual stresses is investigated and the treatment effect is evaluated in terms of the initial deflected configuration. The results reported in this paper represent an experimental mechanical investigation useful for the design of PMUT structures with advanced functionalities in the linear and non-linear regime. [ABSTRACT FROM AUTHOR]

Phononic crystals/metamaterials are attracting increasing interest because of their large variety of applications at both the macro and micro scales. In this work, a new metamaterial plate (metaplate) composed of innovative phononic crystal unit-cells is presented, numerically studied, fabricated at the microscale and experimentally tested. Numerical simulations and experimental tests demonstrate a complete 3D phononic bandgap that guarantees a complete vibration isolation in a certain range of frequency. Moreover, its compatibility with Micro Electro Mechanical Systems (MEMS) fabrication processes suggests applications for vibration isolation of MEMS resonant devices. The measured transmission diagram shows a -30 dB attenuation level, which is in good agreement with numerical predictions. The proposed design opens up new perspectives for the development of vibration isolation applications for MEMS resonators. [2020-0069] [ABSTRACT FROM AUTHOR]

Micro-Electro-Mechanical Systems (MEMS) gyroscopes are inertial sensors for the measurement of angular rates. They have a variety of applications from consumer electronics to drones and the need of stability against environmental fluctuations, such as temperature, is a key factor in order to avoid expensive calibration procedures. Frequency Modulation (FM) has been recently proposed as an innovative working principle for MEMS gyroscopes and as the desired solution in terms of stability against environmental fluctuations. In this paper, the FM working principle is formalized for the three-axial case for the first time and the governing equations are derived both in the idealized case of a point-mass gyroscope and in the real case of a distributed-mass gyroscope. Moreover, the mechanical structure of the first three-axial MEMS FM gyroscope is proposed and studied. Preliminary experimental measurements prove the validity of both the model and the simulations results employed during the design process. The proposed structure overcomes lots of the constraints of the surface micromachining fabrication processes and represents an important step towards the development of a new class of MEMS gyroscopes. [ABSTRACT FROM AUTHOR]

In this paper, the transient dynamic response of shear type multi-storey buildings subjected to earthquake ground motion is generated via adversarial learning technique under different damage conditions, starting from the relevant undamaged responses. A Representation Generative Adversarial Network (RepGAN) is trained on a database of synthetic accelerograms to obtain the responses of the buildings in their undamaged state and in case of plausible damage patterns. Each structural response, represented by a set of time histories to catch the lateral storey displacements/accelerations, is encoded to learn its hidden features and infer the associated damage class. By re-sampling the encoded latent space, it is shown how to switch from the undamaged to the damaged class and to decode the damaged response. The proposed methodology enables damage classification in shear-type multi-storey buildings proving that it can successfully detect damage and assess two different damage severity levels whenever the time-history of a sufficient number of floors is monitored. To outline the generalization capability of the proposed approach, the signal reconstruction is quantitatively assessed for all damage conditions and even in case of a damage condition different from the one corresponding to the encoded signal. • Standard generative adversarial networks (GAN) lack a structured latent manifold. • Adversarial autoencoders (AAE) show excellent reconstruction but poor inference. • InfoGAN forces the unsupervised learning of a disentangled latent space. • RepGAN combines InfoGAN and AAE to reach bijection between data and latent code. • RepGAN generates realistic transient response under different damage conditions. [ABSTRACT FROM AUTHOR]

A reinforcement learning approach to design optimised graded metamaterials for mechanical energy confinement and amplification is described. Through the proximal policy optimisation algorithm, the reinforcement agent is trained to optimally set the lengths and the spacing of an array of resonators. The design optimisation problem is formalised in a Markov decision problem by splitting the optimisation procedure into a discrete number of decisions. Being the physics of graded metamaterials governed by the spatial distribution of local resonances, the space of possible configurations is constrained by using a continuous function for the resonators arrangement. A preliminary analytical investigation has been performed to characterise the dispersive properties of the analysed system by treating it as a locally resonant system. The outcomes of the optimisation procedure confirms the results of previous investigations, highlighting both the validity of the proposed approach and the robustness of the systems of graded resonators when employed for mechanical energy confinement and amplification. The role of the resonator spacing is shown to be secondary with respect to the resonator lengths or, in other words, with respect to the oscillation frequencies of the resonators. However, it is also demonstrated that reducing the number of resonators can be advantageous. The outcomes related to the joint optimisation of the resonator lengths and spacing, thanks also to the adaptive control of the analysis duration, overcome significantly the performance of previously known systems by working almost uniquely on enlarging the time in which the harvester oscillations take place without amplifying these oscillations. The proposed procedure is suitable to be applied to a wide range of design optimisation problems in which the effect of the design choices can be assessed through numerical simulations. • Resonator lengths and spacing of a rainbow based metamaterial as design parameters. • Constraining of the design space based on physical considerations. • Formalisation of the design optimisation problem as a Markov decision process. • Actor–critic reinforcement learning for solving the Markov decision process. • Best grading rule increases the extension of the target oscillation time. [ABSTRACT FROM AUTHOR]

The correct evaluation of adhesion energy is a challenging task and even more difficulties are found when dealing with real-life surfaces in Micro-Electro-Mechanical Systems (MEMS). In this paper, we devise an effective tool for such a computation, on the basis of an innovative, yet simple, geometrical model for the description of rough surfaces by means of spherical caps. A mechanical model is devised for the adhesive contact of spherical surfaces: the model accounts for the presence of elastic and plastic deformation and introduces the effects of both van der Waals forces and capillary attraction. The correctness of the geometrical description has been checked with reference to several measurements on realistic surfaces. The obtained results, in terms of adhesion energy, have been compared with experimental data, which are available in the literature. The agreement between measurements and numerical outcomes is satisfactory and by far better than for previously proposed models. [ABSTRACT FROM AUTHOR]

Phononic Crystal (PnC) Gradient Index (GRIN) lenses have been intensively investigated in recent years for their promising applications in energy harvesting. Here we propose and verify, both numerically and experimentally, three designs of PnC GRIN lenses with amplification factors at the focal points of 4.33x, 7.35x and 7.49x. A design procedure based on the combination of two mechanisms of refraction index gradient formation is employed to boost the efficiency of the lens. Moreover, thanks to the planarity of the design and to the single-phase constitutive material, the proposed lenses are fully compatible with microfabrication processes and can be therefore employed as micro energy harvesters after proper miniaturization. • Three designs of Phononic Crystals (PnC) Gradient-Index (GRIN) lenses are proposed. • Two refractive index gradient formation mechanisms are combined. • Amplification factors at the focal points of 4.33x, 7.35x and 7.49x are achieved. • Numerical simulations and experiments are in good agreement. • Compatibility with standard micro-machining fabrication process is guaranteed. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a domain decomposition technique is proposed to reduce the computational burden of three-dimensional numerical analyses of fracture processes in polycrystalline solids under dynamic loads. The material considered is polysilicon, which is the most widely employed structural material for Micro-Electro-Mechanical Systems (MEMS). The algorithm extends the proposal of Gravouil and Combescure (2001) to fracture processes involving the presence of discrete cracks propagating both inside the sub-domains and across the interfaces between neighbouring sub-domains. The case of brittle or quasi-brittle fracture is addressed by means of a cohesive approach. Alternative choices for the spatial subdivision strategy are compared. Numerical examples concerning known fracture tests are used for validation. Two applications to real MEMS are presented. [Copyright &y& Elsevier]

Numerical simulations of the transient, dynamical behaviour of MEMS microphones enlighten the mechanisms leading to possible failure during guided free fall tests. In particular, the leading role of the imposed motion at the anchors for the thin silicon membrane and the holed backplate nearby are evidenced in comparison with the air over-pressure arising in the fluid region squeezed underneath a falling jig. At the end of a numerical procedure, moving from the macro-scale to the micro-scale, elastic stress waves travelling from the impact zone reach the microphone system at its anchors, where this load combines with the air-overpressure arriving there through the narrow air ducts connected with the exterior environment. It is found that several failure mechanisms can alternatively occur, involving the membrane or, more likely, the backplate, depending on the phase balance between the two loading signals. This contribution helps to move ahead with respect to a classical pass/not pass approach for guided free fall tests, and to possibly envisage new design choices. • Guided free fall tests on MEMS microphones registered by high speed camera • Jig macro-scale FSI simulations get the pressure histories in the MEMS chamber • Displacement histories at MEMS anchors by macro-scale mechanical simulations • Combination of imposed motion and air over-pressure can break the backplate • Variant geometries slightly improve the mechanical behaviour [ABSTRACT FROM AUTHOR]

• Within a structural health monitoring framework, we propose a simulation-based strategy that exploits vibration and thermal data to manage the confounding influence of temperature fluctuations on the damage identification process. • The proposed strategy combines model order reduction techniques and deep learning algorithms. • The tasks of damage detection and damage localization are handled as a supervised classification exploiting a fully convolutional neural network architecture. • In order to speed up the offline assembling of the training dataset of vibration and temperature recordings, a parametric order reduction of the thermo-mechanical model of the monitored structure is exploited. • By correlating the effects of temperature fluctuations on the vibration recordings to the thermo-mechanical coupling, environment-insensitive and damage-sensitive features are automatically selected and extracted by the neural network. • Two case studies has been adopted to validate the proposed procedure, respectively involving a cantilever beam and a portal frame. Data-driven approaches to structural health monitoring (SHM) have been recently shown to be a powerful paradigm, helping to lead to an evolution of traditional scheduled-based maintenance methodologies towards condition-based ones. Nevertheless, only few of them provide monitoring scenarios accounting for the varying loading and environmental conditions, which can potentially lead to misinterpretations of the structural state. In this paper, we propose a damage localization strategy that efficiently exploits vibration and temperature data to account for the effects of temperature fluctuations on the structural response. By allowing for a finite number of a priori defined damage scenarios, deep learning techniques are used to handle the damage localization task as a supervised classification, conditioned on temperature data. The training dataset is generated through a parametrized thermo-mechanical model of the structure, under a prescribed variability of loading and thermal conditions. To relieve the computational burden associated to the data generation process, a parametric order reduction strategy is also exploited. Results relevant to two case studies, a cantilever beam and a portal frame, are adopted to testify the capability of the proposed procedure to locate the damage, also when characterized by a rather small reduction of the local stiffness properties. [ABSTRACT FROM AUTHOR]

Abstract: To overcome the computational burden associated to the three-dimensional finite element simulation of fracture phenomena in polysilicon MEMS during dynamic loading, like e.g. impacts, a domain decomposition technique is used. The approach extends a method developed for linear elastic materials, by including cohesive crack propagation and it allows for the simulation of inter and trans-granular fracture initiation and propagation in polycrystals and it is a step forward in the construction of a complete simulation tool for the description of fracture phenomena in microsystems. Applications to critical MEMS details show encouraging results in reproducing local failure mechanisms. [Copyright &y& Elsevier]

In this work we exploit a multi-scale framework to model the shock-induced failure of polysilicon micro electro-mechanical systems (MEMS), and study the impact of uncertainties at polycrystal length-scale on the results. Because of polysilicon brittleness, MEMS sensors almost instantaneously fail by micro-cracking when subjected to shocks. Since the length of the zone where such micro-cracking is spreading can amount to 5-10% of the characteristic grain size, the morphology of polysilicon films constituting the movable parts of the MEMS is explicitly modeled at the micro-scale within a cohesive approach. Focusing on shocks induced by accidental drops, forecasts of MEMS failure are obtained through a Monte Carlo methodology, wherein statistics of the polycrystalline morphology are accounted for. Outcomes, in terms of failure mode and drop height leading to failure, are shown to correctly represent available experimental evidences relevant to a commercial micro-device. [ABSTRACT FROM AUTHOR]

Within a structural health monitoring (SHM) framework, we propose a simulation-based classification strategy to move towards online damage localization. The procedure combines parametric Model Order Reduction (MOR) techniques and Fully Convolutional Networks (FCNs) to analyze raw vibration measurements recorded on the monitored structure. First, a dataset of possible structural responses under varying operational conditions is built through a physics-based model, allowing for a finite set of predefined damage scenarios. Then, the dataset is used for the offline training of the FCN. Because of the extremely large number of model evaluations required by the dataset construction, MOR techniques are employed to reduce the computational burden. The trained classifier is shown to be able to map unseen vibrational recordings, e.g. collected on-the-fly from sensors placed on the structure, to the actual damage state, thus providing information concerning the presence and also the location of damage. The proposed strategy has been validated by means of two case studies, concerning a 2D portal frame and a 3D portal frame railway bridge; MOR techniques have allowed us to respectively speed up the analyses about 30 and 420 times. For both the case studies, after training the classifier has attained an accuracy larger than 85 %. [ABSTRACT FROM AUTHOR]

Failures on MEMS microphones placed on dummy smartphone are observed during a guided free fall test. The instants before and after the first impact are registered with a high-speed camera and the displacement/velocity signals are used as input for three-dimensional finite element analyses at different scales to obtain insights at local level to judge whether possible ruptures arise. The numerical procedure follows a multi-scale, top-down approach and it can adopt both fluid-structure interaction and purely mechanical analyses. Finally, two loadings are transferred at the micro-scale to the thin silicon membrane representing the microphone: it is found that, in some drop cases, failures depend from the combination of the fluid-induced (air over-pressure) and of the solid-induced input (relative displacements at the anchors), and not from a single domain loading condition. • Guided free fall tests on MEMS microphones registered by high speed camera • Jig macro-scale FSI simulations get the pressure histories in the MEMS chamber. • Displacement histories at MEMS anchors by macro-scale mechanical simulations • Both pressure and displacement histories are applied to a MEMS micro-scale model. • It is the combination of the signals to induce failures in the thin membrane. [ABSTRACT FROM AUTHOR]

This investigation aims to study the flexural behaviour of reinforced concrete (RC) beams strengthened with near-surface mounted (NSM) carbon fibre reinforced polymer prestressed concrete prisms (CFRP-PCPs). Eight RC beams were tested under monotonic loading until the failure load was reached. One beam was un-strengthened to act as a control beam. The other seven beams were strengthened with non-prestressed or prestressed NSM CFRP-PCPs. The effects of bond length, prestress level, and concrete type of the CFRP-PCPs on the flexural capacity, flexural crack and deflection are discussed in this paper. The results indicate that the flexural capacity of RC beams strengthened with NSM CFRP-PCPs was greater than the control beam. An obvious improvement was discovered in the crack resistance when the RC beams were strengthened with prestressed NSM CFRP-PCPs. The strengthened beams showed a higher first-cracking, yielding, and ultimate load as the bond length and prestress level of CFRP-PCPs increased up to a critical level. The beams strengthened with CFRP-PCPs, which were cast with ultra-high performance concrete (UHPC), exhibited greater load capacity than the corresponding beams with epoxy resin mortar. The analytical model of flexural response for the NSM CFRP-PCPs strengthening beams is presented. The analytical results are in good agreement with the experimental results, which revealed the NSM CFRP-PCPs is an effective technique for flexural strengthening of the RC beams. [ABSTRACT FROM AUTHOR]

This paper presents a new metamaterial plate (metaplate) made by a periodic repetition of 'frame-springs-mass' systems for vibration isolation. Numerical studies and experimental measurements demonstrate that the proposed structure show a complete 3D phononic bandgap in the low-frequency range. In the numerical computation of the transmission diagram, the self-weight gravity, the material damping, the geometry of the metaplate and different boundary conditions are considered. A prototype of the proposed metaplate, made of Polyamide, is fabricated by means of additive manufacturing and experimentally tested. The measured transmission diagram is in good agreement with numerical predictions, thus proving the proposed design and promoting potential applications such as low-frequency vibration isolation and stress wave mitigation. [ABSTRACT FROM AUTHOR]

• Modes separation provides strong vibration attenuation even in imperfect structures. • Few lattice elements are enough for vibration attenuation based on modes separation. • Imperfections and misalignments could alter the pattern of wave transmission. • Increasing the number of cells, the influence of imperfections decreases. In this manuscript we experimentally study a modular metamaterial that is able to provide mechanical insulation for elastic wave propagation. Specifically, the structural design is based on the modes separation principle, that entails the presence of ultra-wide bandgap at low frequencies, also in presence of few lattice elements. In this paper, we present a simple manufacturing process for the unit cell realization. This method, however, introduces imperfections given by folding and bonding operations: we demonstrate that upon a careful unit cell design and thanks to modes separation, the presence of defects is not relevant from the dynamical point of view and does not alter the filtering behavior. Moreover, we consider a simple assembly with repetition along a single direction, in order to investigate the metamaterial robustness with respect to the lack of 3D periodicity. Experimental data are compared to numerical simulations demonstrating the advantages of such design for real-life vibration isolation problems. [ABSTRACT FROM AUTHOR]

Micro-electro-mechanical systems (MEMS) have enabled new techniques for the miniaturization of sensors suitable for Structural Health Monitoring (SHM) applications. In this study, MEMS-based sensors, specifically Piezoelectric Micromachined Ultrasonic Transducers (PMUT), are used to evaluate and monitor the pre-tensioning of a bolted joint structural system. For bolted joints to function properly, it is essential to maintain a suitable level of pre-tensioning. In this work, an array of PMUTs attached to the head and to the end of a bolt, serve as transmitter and receiver, respectively, in a pitch-catch Ultrasonic Testing (UT) scenario. The primary objective is to detect the Change in Time of Flight (CTOF) of the acoustic wave generated by the PMUT array and propagating along the bolt's axis between a non-loaded bolt and a bolt in service. To model the pre-tensioning of bolted joints and the transmission of the acoustic wave to and from a group of PMUTs through the bolt, a set of numerical models is created. The CTOF is found to be linearly related to the amount of pre-tensioning. The numerical model is validated through comparisons with the results of a preliminary experimental campaign.

A thermo-mechanical wafer-to-wafer bonding process is studied through experiments on the glass frit material and thermo-mechanical numerical simulations to evaluate the effect of the residual stresses on the wafer warpage. To experimentally characterize the material, confocal laser profilometry and scanning electron microscopy for surface observation, energy dispersive X-ray spectroscopy for microstructural investigation, and nanoindentation and die shear tests for the evaluation of mechanical properties are used. An average effective Young's modulus of 86.5 ± 9.5 GPa, a Poisson's ratio of 0.19 ± 0.02, and a hardness of 5.26 ± 0.8 GPa were measured through nanoindentation for the glass frit material. The lowest nominal shear strength ranged 1.13 ÷ 1.58 MPa in the strain rate interval to 0.33 ÷ 4.99 × 10-3 s-1. To validate the thermo-mechanical model, numerical results are compared with experimental measurements of the out-of-plane displacements at the wafer surface (i.e., warpage), showing acceptable agreement.

This paper presents a numerical reduced-order modeling (ROM) approach for complex multi-layered arrays of piezoelectric micromachined ultrasonic transducers (PMUTs). The numerical modeling technique adopted to generate an array of PMUTs consisting of a considerable number of transducers allows for a large reduction in computational cost without reducing accuracy. The modeling idea is based on coupling shell elements applied to the PMUT structural layers with 3D-solid elements applied to the piezoelectric layer. A set of eigenfrequency and frequency domain analyses are presented considering a single ROM of a PMUT performing in different ambients and the performing central frequencies are obtained for every considered scenario. A unique arrangement of 228 PMUTs is presented and tested for its ability to transmit and receive acoustic waves. The operating frequency band of the array and the level of interference and cross-talk among different PMUTs in the near field are estimated. Finally, the results from a preliminary experimental test performed to analyze the acoustic abilities of an 8 × 8 array of PMUTs are presented. A corresponding numerical model is created and the obtained results matched the experimental data, leading to a validation of the modeling technique proposed in this work.

The design and the combination of innovative metamaterials are attracting increasing interest in the scientific community because of their unique properties that go beyond the ones of natural materials. In particular, auxetic materials and phononic crystals are widely studied for their negative Poisson's ratio and their bandgap opening properties, respectively. In this work, auxeticity and phononic crystals bandgap properties are properly combined to obtain a single phase periodic structure with a tridimensional wide tunable bandgap. When an external tensile load is applied to the structure, the auxetic unit cells change their configurations by exploiting the negative Poisson's ratio and this results in the tuning, either hardening or softening, of the frequencies of the modes limiting the 3D bandgap. Moreover, the expansion of the unit cell in all the directions, due to the auxeticity property, guarantees a fully 3D bandgap tunability of the proposed structure. Numerical simulations and analytical models are proposed to prove the claimed properties. The first experimental evidence of the tunability of a wide 3D bandgap is then shown thanks to the fabrication of a prototype by means of additive manufacturing.