Electromechanical contact elements for modelling adhesion and interfacial interactions in electrospun nanofibers systems

Abstract The analysis of deformation and interactions during the electromechanical contact between surfaces with non-matching meshes is important for advanced applications such as mechanical energy harvesting and pressure/force sensors using flexible piezoelectric devices made of polymeric nanowires. The node-to-segment (NTs) and the node-to-surface (NTS) algorithms are widely employed discretization techniques despite well known limitations in problems where the identification of the master segment/surface related to a slave-node is ambiguous or impossible. The objectives of this work is to extend the classical formulation to electromechanical interfaces using automatic differentiation technologies to derive and implement the resulting numerical equations. In particular, the contact contributions to the stiffness matrix and to the residual vector are derived and an adhesion behaviour is also added into the constitutive law. Then, some applications to selected practical problems are presented

Parameter identification strategy for online detection of faults in smart structures for energy harvesting and sensing

Abstract In this work, we propose a simple computational method to detect faults in smart piezoelectric structures based on a synchronization strategy. The flexible smart structures are in general described as distributed systems governed by partial differential equations. Numerical discetization is employed to derive a reduced order model such as his dynamic response is simulated solving only ordinary differential equations. Then, the parameter identification strategy is formalized as a dynamic optimization and evolution problem through a further proper set of ordinary differential equations. Lyapunov' theorems are employed to derive an integral type identification algorithm and to ensure the convergence of the procedure. The method is suitable to assess and model nonlinearities in the response of a flexible piezoelectric smart device due to material degradation or local failure. These features are very important to detect faults in the structure and to assess the system reconfiguration properties in real time

A multiscale-multiphysics strategy for numerical modeling of thin piezoelectric sheets

Flexible piezoelectric devices made of polymeric materials are widely used for micro- and nano-electro-mechanical systems. In particular, numerous recent applications concern energy harvesting. Due to the importance of computational modeling to understand the influence that microscale geometry and constitutive variables exert on the macroscopic behavior, a numerical approach is developed here for multiscale and multiphysics modeling of piezoelectric materials made of aligned arrays of polymeric nanofibers. At the microscale, the representative volume element consists in piezoelectric polymeric nanofibers, assumed to feature a linear piezoelastic constitutive behavior and subjected to electromechanical contact constraints using the penalty method. To avoid the drawbacks associated with the non-smooth discretization of the master surface, a contact smoothing approach based on B\'ezier patches is extended to the multiphysics framework providing an improved continuity of the parameterization. The contact element contributions to the virtual work equations are included through suitable electric, mechanical and coupling potentials. From the solution of the micro-scale boundary value problem, a suitable scale transition procedure leads to the formulation of a macroscopic thin piezoelectric shell element.Comment: 11 pages, 6 pages, 21 reference

Computational homogenization of fibrous piezoelectric materials

Flexible piezoelectric devices made of polymeric materials are widely used for micro- and nano-electro-mechanical systems. In particular, numerous recent applications concern energy harvesting. Due to the importance of computational modeling to understand the influence that microscale geometry and constitutive variables exert on the macroscopic behavior, a numerical approach is developed here for multiscale and multiphysics modeling of thin piezoelectric sheets made of aligned arrays of polymeric nanofibers, manufactured by electrospinning. At the microscale, the representative volume element consists in piezoelectric polymeric nanofibers, assumed to feature a piezoelastic behavior and subjected to electromechanical contact constraints. The latter are incorporated into the virtual work equations by formulating suitable electric, mechanical and coupling potentials and the constraints are enforced by using the penalty method. From the solution of the micro-scale boundary value problem, a suitable scale transition procedure leads to identifying the performance of a macroscopic thin piezoelectric shell element.Comment: 22 pages, 13 figure

A two-step hybrid approach for modeling the nonlinear dynamic response of piezoelectric energy harvesters

An effective hybrid computational framework is described here in order to assess the nonlinear dynamic response of piezoelectric energy harvesting devices. The proposed strategy basically consists of two steps. First, fully coupled multiphysics finite element (FE) analyses are performed to evaluate the nonlinear static response of the device. An enhanced reduced-order model is then derived, where the global dynamic response is formulated in the state-space using lumped coefficients enriched with the information derived from the FE simulations. The electromechanical response of piezoelectric beams under forced vibrations is studied by means of the proposed approach, which is also validated by comparing numerical predictions with some experimental results. Such numerical and experimental investigations have been carried out with the main aim of studying the influence of material and geometrical parameters on the global nonlinear response. The advantage of the presented approach is that the overall computational and experimental efforts are significantly reduced while preserving a satisfactory accuracy in the assessment of the global behavior

Numerical analysis of FRP strengthened masonry structures

Tese de Doutoramento em Engenharia CivilMasonry structures have always been used since the dawn of construction, and nowadays, due to aging, material degradation, settlements, and structural alterations, members often need strengthening to re-establish their performance. In this frame, fiber-reinforced polymer (FRP) composites in the form of bonded laminates applied to the external surface can be a viable strengthening solution provided that they comply with the cultural value of the building. Despite research efforts in the last years, for the seismic analysis of strengthened masonry system, reliable numerical models, endowed with accuracy, high efficiency and good convergence properties, are still lacking. In this thesis, numerical approaches to model FRP strengthened masonry structures are discussed and in the first part, a material model suitable for micro-modeling of the FRP-masonry interfacial behavior implemented in the Diana FEM program with a user-subroutine is presented. This micro-modeling approach based on interface elements within the framework provided from the theory of multi-surface plasticity is then used to assess the global behavior of a different type of finite element that was implemented in the OpenSees framework. This new element is extremely effective for the seismic analysis of masonry buildings because it drastically reduces the number of degrees of freedom (DOF) of the FEM model. Each panel in the structure can be modeled by using a single “MultiFan” element based on a simplification of the material behavior and the stress field within the panel. The approach proposed is validated through comparison with the results obtained according the simplified model proposed in the recent Italian Code DM2008 modified and extended to include the effect of FRP pier retrofits. Numerical results are validated by comparison with experimental results from tests performed at the University of Pavia, Italy, and the Georgia Institute of Technology, USA and the usefulness of the proposed approaches for solving engineering problems is demonstrated. In particular, macro-modeling shows a satisfactory degree of accuracy at the global level, and, at the same time, is efficient enough, from the computational point of view, to analyze complex assemblages of masonry buildings, including cyclic loads effects and FRP strengthening.As estruturas de alvenaria têm sido usadas desde sempre na construção, mas o seu envelhecimento, a degradação material, os assentamentos e as alterações estruturais têm levado à necessidade do seu reforço para garantir um desempenho adequado. Neste contexto, o uso de materiais compósitos com matriz polimérica (FRP) aplicados externamente no reforço de estruturas pode ser uma solução viável, desde que respeite o valor cultural da construção. Apesar dos esforços de pesquisa dos últimos anos, a análise sísmica de estruturas de alvenaria reforçadas com FRP ainda carece de modelos numéricos precisos e mais eficientes. Nesta tese são estudadas ferramentas numéricas para representar o reforço com FRP em estruturas de alvenaria. Na primeira parte apresenta-se um modelo material adequado à micromodelação do comportamento da interface FRP-alvenaria, desenvolvido e implementado no programa de elementos finitos Diana. A micro-modelação, baseada em elementos de interface onde a teoria da plasticidade é aplicada a multi-superfícies de cedência, foi posteriormente usada para avaliar o comportamento global de um outro tipo de elemento, implementado no programa OpenSees. Este novo elemento é adequado à análise sísmica de edifícios em alvenaria pois reduz o número de graus de liberdade do modelo estrutural. A abordagem proposta nesta tese é validada através da comparação com os resultados obtidos de acordo com os modelos propostos no recente código italiano DM2008, modificado e ampliado para incluir o efeito do reforço com FRP. Os resultados numéricos são validados por comparação com os resultados experimentais realizados na Universidade de Pavia (Itália) e no Instituto de Tecnologia da Geórgia (EUA). De uma forma geral, obteve-se uma boa comparação entre os resultados experimentais e numéricos a nível global e, ao mesmo tempo, eficiência do ponto de vista computacional, para analisar a complexidade do conjunto em alvenaria, incluindo os efeitos cíclicos de cargas e reforço com FRP.In questi ultimi anni, la necessità di sviluppare ed implementare in codici di calcolo modelli numerici affidabili per l’analisi del comportamento di strutture in muratura sta assumendo sempre più rilevanza scientifica in particolare alla luce di eventi tragici come il recente terremoto dell’Aquila. Contemporaneamente, tra le varie tipologie di rinforzo strutturale, i materiali compositi fibro-rinforzati (FRP) hanno mostrato di essere una soluzione valida per il ripristino di edifici in muratura esistenti. In questa tesi, partendo da una attenta analisi dello stato dell’arte, differenti approcci numerici per la modellazione di strutture in muratura rinforzate con i materiali compositi sono impiegati allo scopo di individuare e poi implementare in codici di calcolo agli elementi finiti (Diana ed OpenSees) dei modelli costitutivi adatti per la valutazione della sicurezza strutturale di singoli elementi (muri e archi) o edifici in presenza di FRP. Sulla base dei risultati prodotti da una recente campagna sperimentale, si è sviluppato un modello costitutivo in grado di descrivere il comportamento meccanico dell’interfaccia muratura-FRP. La formulazione matematica si fonda sulla teoria incrementale della plasticità dove la relazione tensioni-deformazioni è definita attraverso una matrice di rigidezza tangente del materiale che a sua volta è funzione della forma delle superfici di snervamento e delle leggi di incrudimento adottate. L’introduzione in un modello costitutivo esistente di una legge di hardening/softening multi-lineare si è rivelata efficace nel cogliere la natura complessa del comportamento dell’interfaccia FRP-muratura come evidenziato dalle simulazioni di test di aderenza effettuati su substrati sia piani che curvi. La corretta calibrazione del modello ha poi consentito di riprodurre con buona approssimazione il comportamento di archi in muratura rinforzati con FRP all’intradosso ed estradosso. L’approccio basato sulla micromodellazione è poi impiegato per la validazione di un nuovo elemento finito in grado di descrivere il comportamento degli edifici in muratura con tecniche di macromodellazione. Il nuovo macroelemento è basato su una schematizzazione a ventaglio dello stato tensionale al suo interno e sull’introduzione di cerniere plastiche sulle facce estreme superiore ed inferiore in grado attraverso tecniche di condensazione di introdurre differenti criteri di collasso sia a taglio che a flessione anche in presenza di eventuali rinforzi in materiale composito. Le cerniere plastiche introdotte consentono di identificare il comportamento strutturale di pannelli murari sia in presenza di carichi monotonici che ciclici. In fine, vengono presentati alcuni confronti tra i risultati numerici ottenuti con la discretizzazione a macroelementi ed i risultati sperimentali su edifici in muratura ottenuti presso i laboratori dell’Università di Pavia (Italia) e del Georgia Institute of Technology (Stati Uniti). I due prototipi analizzati sono considerati due validi benchmark per quanto riguarda edifici soggetti a carichi ciclici, il primo in assenza di materiali di rinforzo, il secondo in presenza di FRP per prevenire meccanismi di crisi a flessione e taglio

Cooperativity in the enhanced piezoelectric response of polymer nanowires

We provide a detailed insight into piezoelectric energy generation from arrays of polymer nanofibers. For sake of comparison, we firstly measure individual poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFe)) fibers at well-defined levels of compressive stress. Under an applied load of 2 mN, single nanostructures generate a voltage of 0.45 mV. We show that under the same load conditions, fibers in dense arrays exhibit a voltage output higher by about two orders of magnitude. Numerical modelling studies demonstrate that the enhancement of the piezoelectric response is a general phenomenon associated to the electromechanical interaction among adjacent fibers, namely a cooperative effect depending on specific geometrical parameters. This establishes new design rules for next piezoelectric nano-generators and sensors.Comment: 31 pages, 11 figures, 1 tabl

Optimization of SAW Sensors for Nanoplastics and Grapevine Virus Detection

In this work, we report the parametric optimization of surface acoustic wave (SAW) delay lines on Lithium niobate for environmental monitoring applications. First, we show that the device performance can be improved by acting opportunely on geometrical design parameters of the interdigital transducers such as the number of finger pairs, the finger overlap length and the distance between the emitter and the receiver. Then, the best-performing configuration is employed to realize SAW sensors. As aerosol particulate matter (PM) is a major threat, we first demonstrate a capability for the detection of polystyrene particles simulating nanoparticulates/nanoplastics, and achieve a limit of detection (LOD) of 0.3 ng, beyond the present state-of-the-art. Next, the SAW sensors were used for the first time to implement diagnostic tools able to detect Grapevine leafroll-associated virus 3 (GLRaV-3), one of the most widespread viruses in wine-growing areas, outperforming electrochemical impedance sensors thanks to a five-times better LOD. These two proofs of concept demonstrate the ability of miniaturized SAW sensors for carrying out on-field monitoring campaigns and their potential to replace the presently used heavy and expensive laboratory instrumentation

Multiscale modeling of piezoelectric materials

This paper focuses on numerical strategies to predict the behavior of piezoelectric materials and devices characterized by heterogenous microstructural features. Several of these materials are attractive for technological applications including mechanical energy harvesting and pressure/force sensors. After a general introduction on the linear piezoelastic problem, two multiscale strategies are presented and applied to the solution of simple but significant problems frequently encountered in nanotechnology test setups. The first strategy consists in classical homogenization based on the choice of a representative volume element and on the classical micro-macro work equality known as Hill’s lemma. The second strategy is based on the so called FE2 method, where the microscale average response resulting from an homogenization procedure is directly used as a constitutive model at each quadrature point at the macroscale. Both strategies have been implemented within an advanced numeric framework based on the authomatic differentiation technique

Modelling and parameter identification of electromechanical systems for energy harvesting and sensing

Advanced modelling of electro-mechanical systems for energy harvesting (EH) and sensing is important to develop reliable self-powered autonomous electronic devices and for structural health monitoring (SHM). In this perspective, a novel computational approach is here proposed for both real-time and off-line parameter identification (PI). The system response is governed by a set of four partial differential equations (PDE) where the three displacement components and the electrical potential are the unknowns. Firstly, the finite element (FE) method is used to reduce the PDE problem into a set of ordinary differential equations (ODE). Then, a state- space model is derived with the aim to limit the PI problem to a subset of unknowns. After that, an identification error is introduced and the Lyapunov theory is used to derive the PI algorithm. The numerical implementation is based on a sensitivity analysis feedback block. The overall proposed computational strategy is robust and results in an exponential asymptotic convergence. The accuracy of the PI method is demonstrated by analysing the time–domain response of an array of piezoelectric bimorphs subjected to low–frequency structural random vibrations. The selected case–study is an existing cable–stayed bridge, for which an extensive dynamic monitoring campaign has provided the experimental data. Once time histories of the device response are obtained through time–dependent dynamic FE simulations, the PI algorithm is used to determine the unknown lumped coefficients of the state-space model. The comparison between FE method and lumped parameters model in terms of tip displacement and output voltage demonstrates the superior predictive capability of the new PI algorithm. As a result of the sensitivity analysis, guidelines to assess the optimal array configuration are also provided