Analisi FEM della micromeccanica elastoplastica di materiali compositi unidirezionali con matrice epossidica

Lo scopo del presente lavoro è quello di studiare il comportamento micromeccanico dei materiali compositi unidirezionali in resina epossidica, mediante un'analisi agli elementi finiti con il software commerciale ABAQUS. L'obiettivo principale è quello di indagare sull'interazione tra fibra e matrice in un materiale composito, e sull'influenza della loro risposta a sollecitazioni esterne sulle proprietà meccaniche dello stesso. Nello specifico, è stato analizzato quanto accade a livello microscopico in un materiale composito unidirezionale, focalizzando l'attenzione sulla ripartizione del carico, sul calcolo della matrice di rigidezza omogeneizzata e sul comportamento elastoplastico della cella elementare del materiale stesso. In primis è stato realizzato un modello agli elementi finiti per l’estrapolazione della matrice di rigidezza del composito, con l’ipotesi che fibra e matrice fossero materiali omogenei, isotropi ed elastici. Una volta validato tale modello con le più avanzate teorie di calcolo, siamo passati all’analisi della risposta del composito in esame in termini di curve sforzo-deformazione. Il comportamento non lineare dell’intero composito, specialmente nel caso di sollecitazioni dominate dalla matrice, ci ha indotto ad ottimizzare la risposta della matrice stessa attribuendole un comportamento elastoplastico, poi implementato nel modello agli elementi finiti. Il nuovo modello ottenuto per la matrice è stato dunque settato sulle curve sperimentali della stessa e, una volta verificato, è stato introdotto nel modello del composito. Da quest’ultimo sono state estrapolate le curve sforzo-deformazione del composito, che sono state subito dopo confrontate con dei dati trovati in letteratura

A numerical micro-mechanical study of the influence of fiber–matrix interphase failure on carbon/epoxy material properties

A finite element micromechanical study of unidirectional carbon–epoxy system is performed in order to investigate the role of fiber–matrix debonding in the degradation of mechanical properties and in the onset of failure for this class of composite materials. The presence of interphase flaws, that can be induced during the manufacturing processes, into micro-scale FE models is obtained by means of an original damage injection technique developed by the authors. The fibers are considered as transversally isotropic solids and the matrix is modeled as an isotropic, elasto-plastic, material with damage. The effect of fiber–matrix debonding is analyzed by means of a quasi 3-D unitary cell with a single fiber, with periodic boundary conditions, for different loading cases. Subsequently, multi-fiber representative volume elements are investigated with the same boundary and loading conditions. Finally, the effect of a 3-D debonding propagation is studied via single fiber model with an increased fiber-wise depth

An All-Solid-State Coaxial Structural Battery Using Sodium-Based Electrolyte

The transition to a sustainable society is paramount and requires the electrification of vehicles, the grid, industry, data banks, wearables, and IoT. Here, we show an all-solid-state structural battery where a Na+-based ferroelectric glass electrolyte is combined with metallic electrodes/current collectors (no traditional cathode present at fabrication) and thin-ply carbon-fiber laminates to obtain a coaxial multifunctional beam. This new concept aims to optimize the volume of any hollow beam-like structure by integrating an electrochemical system capable of both harvesting thermal and storing electrical energy while improving its mechanical performance. The coaxial cell is a coaxial cable where the dielectric is ferroelectric. The electrochemical results demonstrated the capability of performing three-minute charges to one-day discharges (70 cycles) and long-lasting discharges (>40 days at 1 mA) showing an energy density of 56.2 Wh center dot L-1 and specific energy of 38.0 Wh center dot kg(-1), including the whole volume and weight of the structural cell. This is the highest specific energy among safe structural cells, while no Na+-based structural cells were found in the literature. The mechanical tests, instead, highlighted the coaxial cell capabilities to withstand severe inelastic deformation without compromising its functionalities, while increasing the flexural strength of the hosting structure. Moreover, the absence of alkali metals and liquid electrolytes together with its enhanced thermal properties makes this coaxial structural battery a valid and safe alternative as an energy reservoir for all the applications where traditional lithium-ion batteries are not suitable

Structural Batteries: A Review

Structural power composites stand out as a possible solution to the demands of the modern transportation system of more efficient and eco-friendly vehicles. Recent studies demonstrated the possibility to realize these components endowing high-performance composites with electrochemical properties. The aim of this paper is to present a systematic review of the recent developments on this more and more sensitive topic. Two main technologies will be covered here: (1) the integration of commercially available lithium-ion batteries in composite structures, and (2) the fabrication of carbon fiber-based multifunctional materials. The latter will be deeply analyzed, describing how the fibers and the polymeric matrices can be synergistically combined with ionic salts and cathodic materials to manufacture monolithic structural batteries. The main challenges faced by these emerging research fields are also addressed. Among them, the maximum allowable curing cycle for the embedded configuration and the realization that highly conductive structural electrolytes for the monolithic solution are noteworthy. This work also shows an overview of the multiphysics material models developed for these studies and provides a clue for a possible alternative configuration based on solid-state electrolytes

The Latest Trends in Electric Vehicles Batteries

Global energy demand is rapidly increasing due to population and economic growth, especially in large emerging countries, which will account for 90% of energy demand growth to 2035. Electric vehicles (EVs) play a paramount role in the electrification revolution towards the reduction of the carbon footprint. Here, we review all the major trends in Li-ion batteries technologies used in EVs. We conclude that only five types of cathodes are used and that most of the EV companies use Nickel Manganese Cobalt oxide (NMC). Most of the Li-ion batteries anodes are graphite-based. Positive and negative electrodes are reviewed in detail as well as future trends such as the effort to reduce the Cobalt content. The electrolyte is a liquid/gel flammable solvent usually containing a LiFeP6 salt. The electrolyte makes the battery and battery pack unsafe, which drives the research and development to replace the flammable liquid by a solid electrolyte

Potenziamento di un impianto di depurazione con un processo a letto mobile, sperimentazione in un impianto pilota

Sono stati studiati i dati dei campionamenti dell'impianto di depurazione di Sona-Sommacampagna (Vr) forniti dall'azienda Acque Veronesi s.c.a.r.l.; è stato realizzato un impianto pilota a corpi di riempimento, prima in configurazione a biomassa adesa, poi a biomassa sospesa, e sono stati confrontati i rendimenti depurativi con l'impianto principal

Structural Batteries: A Review

Structural power composites stand out as a possible solution to the demands of the modern transportation system of more efficient and eco-friendly vehicles. Recent studies demonstrated the possibility to realize these components endowing high-performance composites with electrochemical properties. The aim of this paper is to present a systematic review of the recent developments on this more and more sensitive topic. Two main technologies will be covered here: (1) the integration of commercially available lithium-ion batteries in composite structures, and (2) the fabrication of carbon fiber-based multifunctional materials. The latter will be deeply analyzed, describing how the fibers and the polymeric matrices can be synergistically combined with ionic salts and cathodic materials to manufacture monolithic structural batteries. The main challenges faced by these emerging research fields are also addressed. Among them, the maximum allowable curing cycle for the embedded configuration and the realization that highly conductive structural electrolytes for the monolithic solution are noteworthy. This work also shows an overview of the multiphysics material models developed for these studies and provides a clue for a possible alternative configuration based on solid-state electrolytes

Studio del funzionamento dell'impianto di depurazione di San Bonifacio

Studio della capacità di rimozione dei principali inquinanti (SST, BOD, COD, sostanze azotate, fosforo) dell'impianto a fanghi attivi di San Bonifacio (Vr): rilevamento delle portate medie in ingresso ed uscita, calcolo dei carichi inquinanti, calcolo delle rese di rimozione, valutazione delle massime capacità di trattamento e delle eventuali capacità residue

Composite failure properties and non-linearity evaluation via statistical micromechanical analysis

Composite materials are a consolidated reality in the worldwide technological scenario. Their specific stiffness and strength, merged to high flexibility in manufacturing, render composites ideal materials for aerospace structural applications. While ensuring many advantages, only a little part of their true potential can be exploited since some of their degradation and failure mechanisms have not been completely understood yet. One of the more concerning aspect is the intrinsic brittleness of traditional aerospace graded composites that causes undesirably abrupt structural failures. A possible way to mitigate such behaviour is to develop composites with hybrid fiber systems. This approach can be effectively pursued once failure mechanisms at the constituents scale (microscale) have been clearly understood and tools developed to analyze composite at such scale. The aim of this thesis is twofold: firstly to develop high-fidelity micro-mechanical models capable to give an insight of the damage onset at the constituents level and, secondly, to move first steps into the field of composite hybridization as a toughening mechanism. The proposed micro-mechanical modeling approach ensures an accurate reproduction of unidirectional composites together with a detailed characterization of their constituents: fibers, matrix and inter-phase. In order to quantify the fibers dispersion, an analysis tool for unidirectional composite micrographs has been developed and endowed with an algorithm for statistical spatial analyses. Moreover, information on the fibers pattern have been collected to feed a code that automatically generate of representative volume elements with statistically equivalent fibers distributions. Once the material geometries have been validated, the other crucial point in the definition of high-fidelity models is the characterization of the constituents mechanical behaviour. The matrix is modeled as an isotropic elasto-palstic damageable solid. A pressure dependent yield surface has been implemented and hardening functions calibrated with experimental data reported in recent literature. The fibers have been characterized as brittle damageable transversely isotropic solids. Crack band theory has been implemented for both fiber and matrix damage modes in order to avoid mesh sensitivity issues. A bilinear traction separation cohesive zone model approach has been introduced for the simulation of the interphase. All these constitutive behaviours have been defined into the commercial software Abaqus by means of a user-defined material subroutine (UMAT). Linear elastic simulations have been performed to check the modeling procedure and results have been compared to experimental values of material moduli. Non-linear simulations have been performed in order to investigate the effect of the fiber-matrix interphase and the composites failure modes due to microdebonding and to the curing process. Micro-debonding between the matrix and the fibers has been introduced via a state variable initialization procedure through a dedicated Fortran subroutine (SDVINI). A numerical reproduction of single fiber fragmentation test has been performed and fed with experimental data. The effects on material stiffness and damage onset and propagation are investigated under different loading conditions. By tailoring the matrix constitutive behaviour with a cure kinetic model, the effect of resin stiffening and shrinkage during the curing has been introduced and results have been produced for the transverse tensile loading condition. Nowadays the possibility of manufacturing very thin plies allows the use of hybridization techniques as a toughening strategy. In the second part of the thesis the effect of the hybridization is investigated for carbon composites of aeronautical interest built by using micro-ply CFRP prepregs. An experimental campaign has been carried out to investigate the mechanical behaviour of different fiber systems both in thin-ply hybrid and non-hybrid configuration. An analytical tool for the prediction of hybrid composite behaviour has been developed and used to asses the pseudo ductility of a number of hybrid solutions. Specimens have been manufactured for the testing of the selected fiber systems: T800 and HR40. Single-fiber tests have been performed on fibers and data analyzed via a statistical approach. Material configurations were compared in terms of longitudinal tensile response and fiber fracture toughness through the double edge notched test. Transverse tensile tests and in-plane shear tests have been carried out for a full definition of the reference hybrid system as well as for the thin-ply composite used in the hybridization process

Simulation of delaminations induced by low velocity impacts in composites: a sensitivity study to guide an effective use of cohesive elements

Low-velocity impacts (LVIs) can seriously harm composite laminates by causing both intralaminar (fiber failure and matrix cracking) and interlaminar damage (delaminations). Extensive delaminations, although hardly detectable by visual inspections, may reduce the strength of the structure and can lead to its premature failure under compressive loads. In aeronautics, extensive experimental campaigns are used to investigate the effects of low-velocity impacts on composite structures; such experimental effort might be significantly reduced if numerical simulations were used smartly. Even though numerical simulations of LVI events require both intralaminar and interlaminar damage models, most of the impact energy is spent to produce delaminations, hence particular care must be exerted to define and tune the interlaminar damage model. The cohesive zone model (CZM) is the most used approach to simulate both onset and propagation of delaminations. Nonetheless, recent studies simulating standard interlaminar fracture toughness tests showed that the simulation results are affected by both the constitutive parameters and the size of the cohesive elements. The authors propose to investigate the influence of the constitutive law parameters and of the dimensions of cohesive elements on LVI simulations results; the study focuses on the variation of delamination shape, depth and extension and compares the related computational costs. A first reference analysis is performed in order to verify the implemented intralaminar and interlaminar damage models. Experimental results of impactor force and displacement time histories, extension and depth of delaminations (obtained by means of ultrasonic inspections) are used as a term of reference for the sensitivity study. In order to perform LVI simulations in ABAQUS two distinct user-defined material routines (UMAT) are developed; UMATs model the intralaminar behavior in solid elements (C3D8) and the constitutive behavior of the cohesive elements (COH3D8). The intralaminar routine implements a continuum-damagemechanics- based model with a smeared crack formulation for both fiber failure and matrix cracking; given the aim of the present study, the intralaminar constitutive parameters are kept constant throughout the sensitivity analyses. The interlaminar damage model uses a traction-separation bi-linear constitutive law to describe the initial elastic behavior and the subsequent progressive softening. The proposed sensitivity analysis wants to point out the relative importance of the intralaminar phenomena compared to the interlaminar ones in lowvelocity impact simulations on a carbon/epoxy material system. At the same time the authors aim at defining guidelines for the effective use of cohesive elements for such analyses through a systematic comparison between the numerical results and experimental ones. The study also evaluates the effects of different cohesive parameters on the computational costs of the simulations