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LUKE MIZZI

Ricercatore t.d. art. 24 c. 3 lett. B
Dipartimento di Scienze e Metodi dell'Ingegneria

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Pubblicazioni

2024 - Mechanical properties and failure modes of additively-manufactured chiral metamaterials based on Euclidean tessellations: an experimental and finite element study [Articolo su rivista]
Mizzi, Luke; Simonetti, Arrigo; Spaggiari, Andrea
abstract

Purpose The “chiralisation” of Euclidean polygonal tessellations is a novel, recent method which has been used to design new auxetic metamaterials with complex topologies and improved geometric versatility over traditional chiral honeycombs. This paper aims to design and manufacture chiral honeycombs representative of four distinct classes of 2D Euclidean tessellations with hexagonal rotational symmetry using fused-deposition additive manufacturing and experimentally analysed the mechanical properties and failure modes of these metamaterials. Design/methodology/approach Finite Element simulations were also used to study the high-strain compressive performance of these systems under both periodic boundary conditions and realistic, finite conditions. Experimental uniaxial compressive loading tests were applied to additively manufactured prototypes and digital image correlation was used to measure the Poisson’s ratio and analyse the deformation behaviour of these systems. Findings The results obtained demonstrate that these systems have the ability to exhibit a wide range of Poisson’s ratios (positive, quasi-zero and negative values) and stiffnesses as well as unusual failure modes characterised by a sequential layer-by-layer collapse of specific, non-adjacent ligaments. These findings provide useful insights on the mechanical properties and deformation behaviours of this new class of metamaterials and indicate that these chiral honeycombs could potentially possess anomalous characteristics which are not commonly found in traditional chiral metamaterials based on regular monohedral tilings. Originality/value To the best of the authors’ knowledge, the authors have analysed for the first time the high strain behaviour and failure modes of chiral metamaterials based on Euclidean multi-polygonal tessellations.

2023 - A rosette approach for the determination of the compliance matrix [Articolo su rivista]
Farrugia, Ps; Mizzi, L; Gatt, R; Grima, Jn
abstract

The computation of the Young's moduli and Poisson's ratio using finite element analysis is comparatively straight forward in the case of systems with non-orthogonal lattice vectors. However, the corresponding calculation of the shear coupling coefficients and the shear modulus is more demanding and less readily attainable. Yet these are needed in order to determine the compliance matrix, which is essential in order to obtain a complete description of the mechanical properties of a structure in any direction. Based on these considerations, this work aims at providing a general methodology for the computation of the complete compliance matrix for systems with non-orthogonal lattice vectors from measurements of the Young's modulus and Poisson's ratio obtained using a suitable rosette of unit cells. The theoretical framework will be outlined and then applied on an accordion-like honeycomb that does not possess a rectangular unit cell and has potential applications in cellular scaffolding, particularly of heart muscle tissue. For the computation, the values of the Young's moduli and Poisson's ratio from three differently-orientated non-rectangular unit cells were obtained using finite element simulations allowing the determination of the complete compliance matrix. The results were validated by comparison with numerical simulations carried out on another unit cell having a different orientation from the other three.

2023 - Auxetic behavior obtained through the large deformations of variants of the rectangular grid [Articolo su rivista]
Farrugia, P. -S.; Gatt, R.; Mizzi, L.; Grima, J. N.
abstract

The deformation behavior of intersecting ligaments forming variants of the square and rectangular grids under mechanical compression was investigated. It was shown that such systems are able to exhibit a negative incremental Poisson’s ratio at relatively large axial compressive strains. Numerical simulations and experimental studies indicated that the extent of auxeticity depends on the relative offset of successive ligaments, the relative lengths of the ligaments as well as on their thickness. It was also shown that there are two distinct modes of deformation, one resembling that of the reentrant hexagonal honeycomb and the other that of the meta-tetrachiral system.

2023 - Design of isotropic 2D chiral metamaterials based on monohedral pentagonal tessellations [Articolo su rivista]
Mizzi, Luke; Grasselli, Luigi; Spaggiari, Andrea; Gatt, Ruben; Farrugia, Pierre-Sandre; N Grima, Joseph
abstract

A novel class of transversely-isotropic metamaterials with the potential to exhibit a wide range of Poisson's ratios and stiffnesses is proposed in this work. These metamaterials, which also have the potential to exhibit auxetic behaviour, are formed through the chiralisation of 2D monohedral Euclidean pentagonal tessellations. Through Finite Element simulations and experimental testing on additively manufactured prototypes, we show that these systems can exhibit Poisson's ratios which encompass the entire range of transverse isotropicity, i.e. +1 to -1, and that the mechanical properties of these structures can be tailored considerably through variation of the geometric parameters without a loss of global symmetry and isotropy. The level of versatility observed in these new metamaterials exceeds by far that which is commonly found in traditional and well-known isotropic auxetic systems such as hexachiral honeycombs. In addition, analytical expressions pertaining to the geometric limits which define the realisability of this new class of auxetic metamaterials have also been derived and presented. The findings of this work demonstrate that pentagonal tessellations have considerable potential for the development of novel metamaterials and that the geometric constraints associated with transverse isotropy need not necessarily be an insurmountable barrier for the design of metamaterials with tailorable and versatile mechanical properties.

2023 - Development and prototyping of SMA-metamaterial biaxial composite actuators [Articolo su rivista]
Mizzi, L; Hoseini, Sf; Formighieri, M; Spaggiari, A
abstract

Shape memory alloys (SMA) are excellent candidates for implementation in actuator systems due to their ability to recover their original shape after high-strain loading through a thermally-induced phase transition. In this work, we propose and develop a novel SMA-metamaterial actuator which is capable of exhibiting a reversible, global elongation in multiple directions induced by the unidirectional contraction upon heating of a single SMA component. This actuator consists of (a) an SMA component, (b) a bias component and (c) the metamaterial geometry, with each component having a distinct function: (a) actuation activation, (b) reversibility of actuation upon deactivation and (c) amplifying and re-directing the uni-directional SMA actuation globally throughout the actuator, respectively. A prototype actuator was designed and tested in various configurations over multiple activation/deactivation cycles in order to demonstrate the functionality and reusability of this system. Furthermore, a theoretical model which predicts the actuation stroke of the system on the basis of the material properties of the SMA and bias components as well as the geometry of the metamaterial system was developed and validated. The findings of this work demonstrate the considerable potential of SMA-metamaterial actuators for implementation in systems requiring a multi-axial actuation output.

2023 - Micro-scale graded mechanical metamaterials exhibiting versatile Poisson’s ratio [Articolo su rivista]
Dudek, K. K.; Mizzi, L.; Iglesias Martínez, J. A.; Spaggiari, A.; Ulliac, G.; Gatt, R.; Grima, J. N.; Laude, V.; Kadic, M.
abstract

2022 - Bio-inspired auxetic mechanical metamaterials evolved from rotating squares unit [Articolo su rivista]
Sorrentino, Andrea; Castagnetti, Davide; Mizzi, Luke; Spaggiari, Andrea
abstract

This work investigates the mechanical response of bio-inspired titanium mechanical metamaterials with negative Poisson’s ratio evolved from rotating squares unit. The systems were designed and optimized using finite element analysis, with peculiar focus on the shape profile at the interconnection regions of the rotating units. The proposed solution consists of a combined auxetic rotating/chiral architecture with enhanced mechanical and topological properties which exhibits a 3% of global elastic strain of the structure and a Poisson’s ratio equal to -0.94. Numerical results are in good agreement with those obtained from experimental tests on a 3D printed Onyx prototype. We also examined the effect on the structural response of the metamaterial subject to off-axis mechanical conditions revealing the strong correlation to the geometrically-related anti-tetrachiral honeycombs. The work confirms the great potential of biologically inspired auxetic metamaterials, which can be designed to obtain tailored mechanical properties while improving the elastic strains capabilities of the system.

2022 - Novel chiral honeycombs based on octahedral and dodecahedral Euclidean polygonal tessellations [Articolo su rivista]
Mizzi, L.; Spaggiari, A.
abstract

In this work, we explored the effect of ‘chiralisation’, i.e. the introduction of geometric chiral characteristics, on the mechanical properties of Euclidean polygonal tessellations containing octahedral or dodecahedral elements. This geometric transformation resulted in the design of three novel auxetic metamaterials which have the potential to exhibit large negative Poisson's ratios (ca. −1) coupled with high levels of in-plane isotropy. We have also examined the influence of the introduction of chiral nodes on the type of geometric arrangement (i.e. whether original or dual) of the base tessellation and also show how the extent of auxeticity may be controlled by tuning the geometric parameters of these systems. This work confirms the potential of Euclidean polygonal tessellations for the design of novel auxetic metamaterials and provides new insights into the deformation mechanisms and geometric conditions which impart this anomalous property.

2021 - A Comparison between Rotating Squares and Anti-Tetrachiral Systems: Influence of Ligaments on the Multi-Axial Mechanical Response [Articolo su rivista]
Mizzi, L; Sorrentino, A; Spaggiari, A; Castagnetti, D
abstract

Rotating unit systems are one of the most important and well-known classes of auxetic mechanical metamaterials. As their name implies, when loaded, these systems deform primarily via rotation of blocks of material, which may be connected together either directly through joints (or ‘joint-like’ connections made by overlapping vertices of the rotating units) as in the case of rotating rigid polygonal-unit systems or by ligaments/ribs as in the case of chiral honeycombs. In this work, we used Finite Element Analysis to investigate the effect which the presence/absence of ligaments has on the on-axis and off-axis mechanical properties of these systems by analysing two of the most well-known structures which characterise these two cases: the rotating square system and the anti-tetrachiral honeycomb. It was found that while the presence of ligaments has a negligible effect on the on-axis Poisson’s ratio of these systems, it has a profound influence on nearly all other mechanical properties as well as on the off-axis loading behaviour. Systems with ligaments were found to exhibit a high level of anisotropy and also a severely reduced level of stiffness in comparison to their non-ligamented counterparts. On the other hand, the rotating square system suffers from high localized stress-intensities and has a very low strain-tolerance threshold. In addition, an optimized ‘hybrid’ geometry which is specifically designed to capture the best features of both the anti-tetrachiral and rotating square system, was also analysed. This work shows the main differences between ligament-based and non-ligament-based auxetic structures and also highlights the importance of considering the off-axis mechanical response in addition to the on-axis properties when investigating such systems.

2021 - Auxetic mechanical metamaterials with diamond and elliptically shaped perforations [Articolo su rivista]
Mizzi, L.; Attard, D.; Evans, K. E.; Gatt, R.; Grima, J. N.
abstract

Mechanical metamaterials are systems which derive their mechanical properties from their structure rather than their intrinsic material composition. In this work, we investigate a class of highly anisotropic mechanical metamaterials designed by the introduction of diamond and elliptically shaped perforations which possess the ability to show auxetic behaviour. By the use of finite element simulations, we show how these highly tuneable systems have the potential to exhibit a large range of Poisson’s ratios, ranging from highly positive to giant negative values, simply by altering the geometric parameters and orientation of the perforations. The anomalous properties of these systems have also been shown to be retained over significant tensile strain ranges, highlighting the vast potential applicability and functionality of these mechanical metamaterials.

2021 - Chiralisation of Euclidean polygonal tessellations for the design of new auxetic metamaterials [Articolo su rivista]
Mizzi, L.; Spaggiari, A.
abstract

Chiral honeycombs are one of the main classes of mechanical metamaterials with the potential to exhibit auxetic behaviour. In this work, we propose a new class of chiral metamaterials based on uniform Euclidean tessellations and their dual counterparts. In total, ten new structures were designed and analysed using Finite Element analysis under periodic boundary conditions, with eight of these systems showing the capability of possessing a negative Poisson's ratio. The relationship between the various geometric parameters defining the systems and the resultant mechanical properties was also studied. We show that ‘chiralisation’, i.e. introduction of chirality and rotational elements within the system, has the ability to transform even complex geometries, which in their original state possess a high positive Poisson's ratio, into auxetic metamaterials and hope that this work can act as a blueprint for the design of auxetic structures with novel topologies.

2021 - Function from configurational degeneracy in disordered framework materials [Articolo su rivista]
Reynolds, E. M.; Wolpert, E. H.; Overy, A. R.; Mizzi, L.; Simonov, A.; Grima, J. N.; Kaskel, S.; Goodwin, A. L.
abstract

The existence of correlated disorder in molecular frameworks is an obvious mechanism by which unusual cooperative phenomena might be realised. We show that the use of local-symmetry lowering approaches can allow ostensibly high-symmetry framework structures to harbour exotic disordered states often studied in the context of spin lattice models. These states exhibit strongly cooperative behaviour that might be exploited in anomalous mechanical, host/guest, and information storage behaviour. Our contribution focuses in particular on the concepts of (i) combinatorial mechanics, (ii) adaptive flexibility, and (iii) error-correcting data storage in framework materials.

2021 - Implementation of periodic boundary conditions for loading of mechanical metamaterials and other complex geometric microstructures using finite element analysis [Articolo su rivista]
Mizzi, L.; Attard, D.; Gatt, R.; Dudek, K. K.; Ellul, B.; Grima, J. N.
abstract

The implementation of periodic boundary conditions (PBCs) is one of the most important and difficult steps in the computational analysis of structures and materials. This is especially true in cases such as mechanical metamaterials which typically possess intricate geometries and designs which makes finding and implementing the correct PBCs a difficult challenge. In this work, we analyze one of the most common PBCs implementation technique, as well as implement and validate an alternative generic method which is suitable to simulate any possible 2D microstructural geometry with a quadrilateral unit cell regardless of symmetry and mode of deformation. A detailed schematic of how both these methods can be employed to study 3D systems is also presented.

2021 - Rotating squares auxetic metamaterials with improved strain tolerance [Articolo su rivista]
Sorrentino, Andrea; Castagnetti, Davide; Mizzi, Luke; Spaggiari, Andrea
abstract

Rotating squares auxetic metamaterials have the peculiar feature of a negative Poisson’s ratio. This work proposes and examines how an innovative variable arcs fillet solution, at the interconnection regions between the rotating units, improves the structural response of a titanium alloy-based rotating squares metamaterial. Through a 2D finite element (FE) model of the auxetic structure, we investigated and optimized two fillet configurations: first, a double circular arcs profile; second, a combined elliptical and circular arc fillet. According to the FE results, the optimal configuration of the combined elliptical and circular arc fillet allows an overall 3% elastic strain of the metamaterial, with a Poisson’s ratio (PR) equal to ca. −1. In order to assess the deformation behavior of the proposed metamaterial, we performed a tensile test on a prototype of the optimal solution, 3D printed in Onyx material. The experimental displacement field of the specimen, measured through digital image correlation, exhibited excellent agreement with the FE predictions, with a PR equal to ca. −1 up to a 3% overall strain.

2021 - Stress concentrations in skew pressurized holes: A numerical analysis [Articolo su rivista]
Mizzi, Luke; Spaggiari, Andrea
abstract

This work provides a numerical analysis of the stress concentration factor in an elastic solid containing non-aligned and non-concurrent circular holes subjected to an internal pressure. Using Finite Element Analysis, a variety of systems with a range of geometric configurations and loading conditions were simulated and the trends observed were analysed using a qualitative and statistical analysis in order to determine the correlation between these factors and the maximum stress concentration in these systems. Furthermore, simple empirical models were calibrated on the simulation results and used to plot reference graphs which may be employed to predict the stress concentration factor of these systems according to several geometric parameters and loading conditions. The numerical results and the empirical models presented here also show good agreement with previously derived analytical results based on 2D models and are expected to provide a useful tool for the designer of such systems especially for fatigue problems in fluid-power systems, where a straightforward evaluation of the stress concentration factor in pressurized hole systems is needed.

2020 - 2D auxetic metamaterials with tuneable micro-/nanoscale apertures [Articolo su rivista]
Mizzi, L.; Salvati, E.; Spaggiari, A.; Tan, J. -C.; Korsunsky, A. M.
abstract

Modern advanced manufacturing technologies have made possible the tailored design and fabrication of complex nanoscale architectures with anomalous and enhanced properties, including mechanical and optical metamaterials; structured materials which are able to exhibit unusual mechanical and optical properties that are derived from their geometry rather than their intrinsic material properties. In this work, we fabricated for the first time an ultrathin 2D auxetic metamaterial with nanoscale geometric features specifically designed to deform in-plane by using focused-ion-beam milling to introduce patterned nano-slits within a thin membrane. The system was mechanically loaded in-situ and exhibited in-plane dominated deformation up to 5% tensile strain and a Poisson's ratio of −0.78. Furthermore, the porosity and aperture shape of the metamaterial have been shown to change considerably upon the application of strain, with pore dimensions showing a fourfold increase at 5% strain. This mechanically-controlled tuneability makes this metamaterial system an ideal candidate for use as a reconfigurable nano-filter or a nano light-modulator.

2020 - Assessing the individual microbial inhibitory capacity of different sugars against pathogens commonly found in food systems [Articolo su rivista]
Mizzi, L.; Maniscalco, D.; Gaspari, S.; Chatzitzika, C.; Gatt, R.; Valdramidis, V. P.
abstract

Highly concentrated sugar solutions are known to be effective antimicrobial agents. However, it is unknown whether this effect is solely the result of the collective osmotic effect imparted by a mixture of sugars or whether the type of carbohydrate used also has an individual chemical effect on bacterial responses, that is, inhibition/growth. In view of this, in this work, the antimicrobial properties of four sugars, namely, glucose, fructose, sucrose and maltose against three common food pathogens; Staphylococcus aureus, Escherichia coli and Salmonella enterica, were investigated using a turbidimetric approach. The results obtained indicate that the type of sugar used has a significant effect on the extent of bacterial inhibition which is not solely dependent on the water activity of the individual sugar solution. In addition, while it was shown that high sugar concentrations inhibit bacterial growth, very low concentrations show the opposite effect, that is, they stimulate bacterial growth, indicating that there is a threshold concentration upon which sugars cease to act as antimicrobial agents and become media instead. Significance and Impact of the Study: In this work, an analysis on the antimicrobial properties of glucose, fructose, sucrose and maltose in solution was conducted using a turbidimetric approach. Our findings indicate that while, as expected, all of these sugars exhibit significant antimicrobial effects at high concentrations, at low concentrations they appear to act as substrates for the bacteria which results in enhanced microbial growth instead of inhibition. In addition, the results obtained also suggest that the resultant osmotic stress imparted by the sugar solutions is not the only factor which determines their antimicrobial activity and that other chemical factors may be playing a significant role.

2020 - Design of shape memory alloy sandwich actuators: an analytical and numerical modelling approach [Articolo su rivista]
Mizzi, Luke; Spaggiari, Andrea; Dragoni, Eugenio
abstract

Shape memory alloy (SMA)-based actuator composites are characterised by a high force output which is activated by a temperature increase. In this work we exploit this property to design sandwich structures with SMA-matrix composite actuator skins capable of exhibiting a reversible, tailored flexural response. A theoretical model which predicts the resultant deflection and flexural moment produced as a result of selectively actuating one of the system skins was developed and confirmed using a multi-step Finite Element (FE) analysis which takes into account the fabrication pathway through which these systems may be manufactured. The model correlates the geometric parameters and material properties of the various components making up the system and provides a quantitative description of the role which each variable plays in determining the overall sandwich actuator performance. This is necessary for the future production and implementation of such systems in real-life applications.

2020 - HPLC analysis of phenolic compounds and flavonoids with overlapping peaks [Articolo su rivista]
Mizzi, L.; Chatzitzika, C.; Gatt, R.; Valdramidis, V.
abstract

The identification and quantification of phenolic compounds and flavonoids in various natural food products is typically conducted using HPLC analysis. Their analysis is particularly complex since most natural food products contain a large number of different phenolic compounds, many of which have similar chemical characteristics such as polarity, which makes complete separation of all eluents extremely difficult. In this work we present and validate a method for the quantitative determination of the concentration of two compounds with similar retention times, i.e. they show overlapping peaks in a mixed solution. Two pairs of phenolic compounds were investigated: caffeic and vanillic acids and ferulic and p-coumaric acids. This technique takes advantage of the different absorbances of the two phenolic compounds in the eluent at various wavelengths and can be used for the quantitative determination of the concentration of these compounds even if they are not separated in the HPLC column. The presented method could be used to interpret the results of HPLC analysis of food products which possess a vast spectrum of phenolic compounds and flavonoids.

2020 - Highly stretchable two-dimensional auxetic metamaterial sheets fabricated via direct-laser cutting [Articolo su rivista]
Mizzi, L.; Salvati, E.; Spaggiari, A.; Tan, J. -C.; Korsunsky, A. M.
abstract

The design and production of multifunctional materials possessing tailored mechanical properties and specialized characteristics is a major theme in modern materials science, particularly for implementation in high-end applications in the biomedical and electronics industry. In this work, a number of metamaterials with perforated architectures possessing the ability to exhibit a plethora of 2D auxetic responses with negative Poisson's ratios ranging from quasi-zero to large negative values (lower than −3.5), stiffnesses, stretchability and surface coverage properties were manufactured. These systems were produced through the introduction of microstructural cuts in a rubber sheet using direct laser cutting, and analysed using a dual approach involving experimental tests and Finite Element Analysis. In addition to examining the mechanical properties of the perforated metamaterials, the influence of edge effects and material thickness on the deformation behaviour of these systems were investigated, with re-entrant systems shown to possess anomalous deformation profiles which are heavily dominated by the boundary regions. These findings highlight the effectiveness of this method for the fabrication of auxetic metamaterial sheets as well as the large variety of mechanical properties, deformation mechanisms and load responses which may be obtained through what may be effectively described as simply the introduction of patterned cuts in a thin sheet.

2020 - Lightweight mechanical metamaterials designed using hierarchical truss elements [Articolo su rivista]
Mizzi, Luke; Spaggiari, Andrea
abstract

Rotating unit systems constitute one of the main classes of auxetic metamaterials. In this work, a new design procedure for lightweight auxetic systems based on this deformation mechanism is proposed through the implementation of a hierarchical triangular truss network in place of a full block of material for the rotating component of the system. Using numerical simulations in conjunction with experimental tests on 3D printed prototypes, the mechanical properties of six types of auxetic structures, which include a range of rotating polygons and chiral honeycombs, were analysed under the application of small tensile loads. The results obtained show that there is almost no difference in the Poisson's ratios obtained from the regular, full structures and the ones made from triangular truss systems despite the latter, in some cases, being 80% lighter than the former. This indicates that these systems could be ideal candidates for implementation in applications requiring lightweight auxetic metamaterial systems such as in the aerospace industry.

2019 - Analysis of the Deformation Behavior and Mechanical Properties of Slit-Perforated Auxetic Metamaterials [Articolo su rivista]
Mizzi, L.; Grima, J. N.; Gatt, R.; Attard, D.
abstract

Perforated systems constitute one of the most important classes of mechanical metamaterials. In this work, two types of “I”-shaped slit perforation patterns are proposed which may be used to design perforated systems that mimic the deformation mechanisms of a variety of re-entrant and anti-tetrachiral honeycomb systems. Using finite element analysis, it is shown how these systems have the potential to exhibit a large spectrum of negative Poisson's ratios, ranging from extremely negative to zero, which are retained over a wide tensile strain range. A detailed analysis of the deformation behavior of these systems is also presented along with a comparison of the changes in overall expansion and Poisson's ratios of both systems observed upon loading with those predicted by previously formulated theoretical models of re-entrant and anti-tetrachiral systems. It is hoped that this work will be of considerable aid in the efforts of scientists to understand the underlying principles governing the production of auxetic mechanical metamaterials through the use of perforations and also stimulate further research on how these and similar mechanisms may be implemented in perforated systems to design other metamaterials with anomalous mechanical properties.

2019 - CONSULENZA TECNICA SU ANALISI STRUTTURALE E PROGETTAZIONE RIDUTTORE DI VELOCITÀ MAZZONI [Altro]
Spaggiari, Andrea; Castagnetti, Davide; Dragoni, Eugenio; Mizzi, Luke
abstract

Fornire consulenza tecnica per l’analisi strutturale di un riduttore di velocità per accoppiamento di motori endotermici a benzina e pompe acqua a pistoni assiali. Lo studio è finalizzato all’analisi di un riduttore commerciale di riferimento, al fine di valutarne le prestazioni, e allo studio della gamma di riduttori di comune uso presso Mazzoni. Sulla base dei primi due passi si andrà a studiare la fattibilità tecnica per la produzione, da parte del Committente, di una piattaforma di riduttori modulari, adatti alla gamma di prodotti d’interesse.

2019 - CONSULENZA TECNICA SU SISTEMA HEART DAMPER [Altro]
Spaggiari, Andrea; Castagnetti, Davide; Dragoni, Eugenio; Mizzi, Luke
abstract

La collaborazione riguarda la consulenza tecnica per l’analisi della deformazione e la valutazione della resistenza di un heart damper in Nitinol per la terapia dell’insufficienza cardiaca tramite metodi numerici FEM

2019 - Design-oriented modelling of composite actuators with embedded shape memory alloy [Articolo su rivista]
Mizzi, Luke; Spaggiari, Andrea; Dragoni, Eugenio
abstract

Shape memory alloy (SMA) actuators have generated a great deal of interest in recent years due to their reusability and ability to exhibit a wide spectrum of actuation properties. In this work we present an analytical approach through which one may predict the actuation stroke as well as recovery potential of a two-component SMA-based composite actuator. The predictions of the analytical model were validated using Finite Element (FE) simulations on a composite SMA actuator designed in the form of an SMA strip embedded within an elastic matrix, where the shape memory effect of the SMA component was modelled using the numerical Souza-Auricchio model. The results obtained from the two approaches show extremely good agreement. The trends found upon altering various geometric and material parameters within the system provide a thorough understanding of how one can vary these parameters in order to obtain a tailored actuation and recovery response from the SMA-based actuator.

2019 - Quantitative assessment of tolerance response to stress after exposure to oregano and rosemary essential oils, carvacrol and 1,8-cineole in Salmonella Enteritidis 86 and its isogenic deletion mutants ∆dps, ∆rpoS and ∆ompR [Articolo su rivista]
Cariri, M. L.; de Melo, A. N. F.; Mizzi, L.; Ritter, A. C.; Tondo, E.; de Souza, E. L.; Valdramidis, V.; Magnani, M.
abstract

This study assessed the influence of rpoS, dps and ompR genes on the tolerance response of Salmonella Enteritidis 86 (SE86) to homologous and heterologous stressing agents after exposure to essential oils (EOs) from Origanum vulgare L. (oregano; OVEO) and Rosmarinus officinalis L. (rosemary; ROEO) and their major constituents (ICs), carvacrol (CAR) and 1,8-cineole (CIN), respectively, by modelling the log reduction over time. Minimum inhibitory concentration values of OVEO (1.25 μL/mL), CAR (0.62 μL/mL), ROEO (20 μL/mL) and CIN (10 μL/mL) against SE86 were always one-fold higher than those against ∆dps, ∆rpoS and ∆ompR mutants. Exposure to the same concentration of OVEO, CAR, ROEO or CIN caused higher reductions (up to 2.5 log CFU/mL) in ∆dps, ∆rpoS and ∆ompR mutants than in SE86 in chicken broth. In assays with homologous stressing agents, ompR, dps and rpoS influenced the tolerance to OEs or ICs. After adaptation to OVEO, CAR, ROEO and CIN, osmotolerance and acid tolerance of SE86 were influenced by rpoS gene, while thermotolerance of SE86 was influenced by ompR. Tolerance of SE86 to sodium hypochlorite after adaptation to OEs or ICs was influenced by rpoS and dps. These findings quantitatively describe for the first time the influence of rpoS, dps and ompR genes on the tolerance of Salmonella Enteritidis to OVEO, CAR, ROEO and CIN.

2019 - Tailoring the flexural response of sandwich structures with faces made from shape memory alloy composite actuators [Relazione in Atti di Convegno]
Mizzi, L.; Spaggiari, A.; Dragoni, E.
abstract

2018 - An analytical and finite element study on the mechanical properties of irregular hexachiral honeycombs [Articolo su rivista]
Mizzi, L.; Attard, D.; Gatt, R.; Farrugia, P. -S.; Grima, J. N.
abstract

The highly symmetric hexachiral honeycomb with an isotropic Poisson's ratio of -1 is one of the earliest and most well-known examples of auxetic mechanical metamaterials and has been proposed for numerous applications ranging from satellite antennas to stents. In this work, we propose a novel, less symmetric generic form of the hexachiral honeycomb that is made up of three sets of unequally sized ligaments rather than six identical ones and has the potential to exhibit a wide range of negative Poisson's ratios. Using finite element analysis, we investigated the deformation behaviour of these generic hexachiral systems and derived an analytical model which may be used to predict the Poisson's ratio of these systems. The results obtained indicate that the hexachiral topology is extremely versatile with respect to its potential to exhibit diverse mechanical properties and it is hoped that this added tunability will enhance the functionality and applicability of this class of auxetic mechanical metamaterials.

2018 - Mechanical metamaterials with star-shaped pores exhibiting negative and zero Poisson's ratio [Articolo su rivista]
Mizzi, L.; Mahdi, E. M.; Titov, K.; Gatt, R.; Attard, D.; Evans, K. E.; Grima, J. N.; Tan, J. -C.
abstract

The term “mechanical metamaterials” encompasses a wide range of systems whose anomalous mechanical properties arise primarily from their structure rather than composition. This unique characteristic gives them an edge over many conventional natural or readily available materials and makes them well-suited for a variety of applications where tailor-made mechanical properties are required. In this study, we present a new class of mechanical metamaterials featuring various star-shaped perforations, which have the potential to exhibit auxetic or zero Poisson's ratio (ν) properties. Using finite element modelling in conjunction with experimental measurements on 3D printed prototypes, we demonstrate that these star-shaped porous systems possess the potential to retain their unusual mechanical properties up to tensile strains exceeding 15%. By virtue of these exceptional properties, the proposed concept could be applied for engineering numerous potential applications in a wide range of fields.

2018 - Potential of mechanical metamaterials to induce their own global rotational motion [Articolo su rivista]
Dudek, K. K.; Wojciechowski, K. W.; Dudek, M. R.; Gatt, R.; Mizzi, L.; Grima, J. N.
abstract

The potential of several classes of mechanical metamaterials to induce their own overall rotational motion through the individual rotation of their subunits is examined. Using a theoretical approach, we confirm that for various rotating rigid unit systems, if by design the sum of angular momentum of subunits rotating in different directions is made to be unequal, then the system will experience an overall rotation, the extent of which may be controlled through careful choice of the geometric parameters defining these systems. This phenomenon of self-induced rotation is also confirmed experimentally. Furthermore, we discuss how these systems can be designed in a special way so as to permit extended rotations which allows them to overcome geometric lockage and the relevance of this concept in applications ranging from satellites to spacecraft and telescopes employed in space.

2018 - consulenza tecnica per l’analisi della deformazione e la valutazione della resistenza di un heart damper in Nitinol per la terapia dell’insufficienza cardiaca [Altro]
Spaggiari, Andrea; Dragoni, Eugenio; Castagnetti, Davide; Mizzi, Luke
abstract

– Analisi del materiale, del sistema di carichi e vincoli agenti sul sistema e della geometria attuale dell'heart damper – Analisi agli elementi finiti del sistema e previsione di tensioni e deformazioni – Analisi critica dei risultati e suggerimento delle possibili migliorie strutturali

2017 - Global rotation of mechanical metamaterials induced by their internal deformation [Articolo su rivista]
Dudek, K. K.; Gatt, R.; Mizzi, L.; Dudek, M. R.; Attard, D.; Grima, J. N.
abstract

In this work, we propose the concept that a device based on mechanical metamaterials can be used to induce and control its own rotational motion as a result of internal deformations due to the conversion of translational degrees of freedom into rotational ones. The application of a linear force on the structural units of the system may be fine-tuned in order to obtain a desired type of rotation. In particular, we show, how it is possible to maximise the extent of rotation of the system through the alteration of the geometry of the system. We also show how a device based on this concept can be connected to an external body in order to rotate it which result may potentially prove to be very important in the case of applications such as telescopes employed in space.

2017 - On the dynamics and control of mechanical properties of hierarchical rotating rigid unit auxetics [Articolo su rivista]
Dudek, K. K.; Gatt, R.; Mizzi, L.; Dudek, M. R.; Attard, D.; Evans, K. E.; Grima, J. N.
abstract

In this work, we investigate the deformation mechanism of auxetic hierarchical rotating square systems through a dynamics approach. We show how their deformation behaviour, hence their mechanical properties and final configuration from a given applied load, can be manipulated solely by altering the resistance to rotational motion of the hinges within the system. This provides enhanced tunability without necessarily changing the geometry of the system, a phenomenon which is not typically observed in other non-hierarchical unimode auxetic systems. This gives this hierarchical system increased versatility and tunability thus making it more amenable to be employed in practical application which may range from smart filtration to smart dressings.

2016 - Auxetic Perforated Mechanical Metamaterials with Randomly Oriented Cuts [Articolo su rivista]
Grima, J. N.; Mizzi, L.; Azzopardi, K. M.; Gatt, R.
abstract

Perforated systems with quasi-disordered arrays of slits are found to exhibit auxetic characteristics almost as much as their traditional ordered "rotating-squares" counterparts. This provides a highly robust methodology for constructing auxetics that may be used for various practical applications such as skin grafting, where a high degree of precision may not always be achievable.

2016 - Planar auxeticity from elliptic inclusions [Articolo su rivista]
Pozniak, A. A.; Wojciechowski, K. W.; Grima, J. N.; Mizzi, L.
abstract

Composites with elliptic inclusions of long semi-axis a and short semi-axis b are studied by the Finite Element method. The centres of ellipses form a square lattice of the unit lattice constant. The neighbouring ellipses are perpendicular to each other and their axes are parallel to the lattice axes. The influence of geometry and material characteristics on the effective mechanical properties of these anisotropic composites is investigated for deformations applied along lattice axes. It is found that for anisotropic inclusions of low Young's modulus, when a + b → 1 the effective Poisson's ratio tends to -1, while the effective Young's modulus takes very low values. In this case the structure performs the rotating rigid body mechanism. In the limit of large values of Young's modulus of inclusions, both effective Poisson's ratio and effective Young's modulus saturate to values which do not depend on Poisson's ratio of inclusions but depend on geometry of the composite and the matrix Poisson's ratio. For highly anisotropic inclusions of very large Young's modulus, the effective Poisson's ratio of the composite can be negative for nonauxetic both matrix and inclusions. This is a very simple example of an auxetic structure being not only entirely continuous, but with very high Young's modulus. A severe qualitative change in the composite behaviour is observed as a/b reaches the limit of 1, i.e. inclusions are isotropic. The observed changes in both Poisson's ratio and Young's modulus are complex functions of parameters defining the composite. The latter allows one to tailor a material of practically arbitrary elastic parameters.

2016 - Progettazione e validazione di materiali compositi attivi rinforzati con fibra SMA per strutture adattative, nell’ambito del progetto Prin 2015 n. 2015RT8Y45-PE8 dal titolo Smart Composite Laminates [Altro]
Spaggiari, A.; Castagnetti, Davide; Dragoni, E.; Mizzi, Luke
abstract

Smart materials such as piezoelectrics and shape memory alloys (SMA) are receiving increasing attention due to their possible application in actuators technology, shape morphing structures, energy harvesters, and vibration control. However, their practical diffusion is limited due to restrictions associated with scarce mechanical properties, low electro-mechanical conversion rates, or difficulties in the modulation of their morphed shape while actuated. Overarching objective of this project is developing and characterizing innovative smart structures which can either serve as conductors, energy harvesters, or selectively modulate their shape (shape morphing) by combining innovative piezoelectric materials with SMAs to form a new class of smart structural composites. Final effort of this project is not only the development of innovative smart composite materials, but also the development of prototypal energy harvester and shape morphing structures to assess their effective smart capabilities. The proper development of such a technology involves a broad range of expertises. First, the development, optimization, and characterisation of such smart composite materials. Second, the formulation of tools capable of predicting the complex thermo-electro-mechanical behaviour of the envisioned structures to aid the optimization of their design. Third, the development of mechatronic techniques for the autonomous implementation of the morphing process, which passes through the creation of a robust control policy capable of selectively actuate the morphing structure as a function of its application. To tackle such a challenging process, we here envisage developing smart structures by utilizing both SMAs and innovative piezoelectric nanofibers. In particular, the piezoelectric polymeric nanofibers production technology has been recently developed by members of the proposed research team. These offer the twofold advantage of significantly increase the electromechanical conversion rate with respect to traditional piezoelectric materials, whereby their morphology allows their introduction into composite laminates at the production stage, resulting into a piezoelectric structural material. Similarly, SMA fibers will be utilized as reinforce for the composite. These allow for higher actuation loads and larger deformations, extending the application ranges. Analytical and numerical models of the thermo-electro-mechanical response will be developed and utilised for the optimisation of the active structures. Results from the proposed research will be finally applied to specific case studies, e.g. a micro-actuator, a energy harvester from a broadband excitation, and plates with shape morphing capabilities under selective control. The potential impact and importance of these goals on materials science, and for a wide spectrum of industrial applications, high-tech industry, and finally in actuating and sensing technology is indeed of extreme interest.

2015 - A force-field based analysis of the deformation mechanism in α-cristobalite [Articolo su rivista]
Gatt, R.; Mizzi, L.; Azzopardi, K. M.; Grima, J. N.
abstract

α-cristobalite is a metastable polymorph of silica which has the remarkable property of exhibiting a negative Poisson's ratio (auxetic). In this study, the mechanical properties and deformation mechanisms of this system were investigated through a force-field based approach. Besides simulations on the actual structure of α-cristobalite, a number of simulations where the rigidity of SiO2 tetrahedra was increased, were also conducted. This was done in order to obtain a clearer picture of the role which these tetrahedra play in determining the mechanical properties of this system. These systems were loaded in an off-axis direction in a number of planes about the [001] direction and the deformations observed were compared with those of type II rotating rectangle systems. It was discovered that the mechanical properties of α-cristobalite can be accurately described through the semi-rigid version of this model, even for cases where the tetrahedra were fully rigid.

2015 - Auxetic metamaterials exhibiting giant negative Poisson's ratios [Articolo su rivista]
Mizzi, L.; Azzopardi, K. M.; Attard, D.; Grima, J. N.; Gatt, R.
abstract

Auxetic metamaterials have generated a great deal of interest in recent years. In this letter, a novel approach which can be used to produce such auxetic metamaterials is presented. This method involves the introduction of patterned slit perforations within a sheet/block of material in order to create systems which resemble a large variety of auxetic systems ranging from rotating units to chiral honeycombs. These perforated systems have also been shown to have the potential to exhibit giant negative Poisson's ratios.

2015 - Hierarchical Auxetic Mechanical Metamaterials [Articolo su rivista]
Gatt, R.; Mizzi, L.; Azzopardi, J. I.; Azzopardi, K. M.; Attard, D.; Casha, A.; Briffa, J.; Grima, J. N.
abstract

Auxetic mechanical metamaterials are engineered systems that exhibit the unusual macroscopic property of a negative Poisson's ratio due to sub-unit structure rather than chemical composition. Although their unique behaviour makes them superior to conventional materials in many practical applications, they are limited in availability. Here, we propose a new class of hierarchical auxetics based on the rotating rigid units mechanism. These systems retain the enhanced properties from having a negative Poisson's ratio with the added benefits of being a hierarchical system. Using simulations on typical hierarchical multi-level rotating squares, we show that, through design, one can control the extent of auxeticity, degree of aperture and size of the different pores in the system. This makes the system more versatile than similar non-hierarchical ones, making them promising candidates for industrial and biomedical applications, such as stents and skin grafts.

2015 - Influence of translational disorder on the mechanical properties of hexachiral honeycomb systems [Articolo su rivista]
Mizzi, L.; Attard, D.; Gatt, R.; Pozniak, A. A.; Wojciechowski, K. W.; Grima, J. N.
abstract

Chiral honeycombs are one of the most important and oft studied classes of auxetic systems due to their vast number of potential applications which range from stent geometries to composites, sensors and satellite components. Despite numerous works on these systems, however, relatively few studies have investigated the effect of structural disorder on these structures. In view of this, in this study, the effect of translational disorder on hexachiral honeycombs were investigated through a Finite Element approach. It was found that this type of disorder has minimal effect on the Poisson's ratios of these systems provided that the ligament length to thickness ratio remains sufficiently large and the overall length to width ratio of the disordered system does not differ considerably from that of its ordered counterpart. This makes it ideal for use in various applications and products such as sandwich composites with an auxetic core.

2015 - Non-porous grooved single-material auxetics [Articolo su rivista]
Mizzi, L.; Gatt, R.; Grima, J. N.
abstract

Over the years, great advances have been made in the field of auxetic metamaterials where one of the main focuses was the production of systems which can be produced through simple and relatively inexpensive means. In this work, auxetic systems created through the introduction of elliptical grooves, meant to mimic the rotating units mechanism, are proposed and analyzed. These systems were found to have the potential to exhibit Poisson's ratios ranging from ca.-1 to the Poisson's ratio of the material of the system itself, with some systems also possessing the remarkable property of a zero Poisson's ratio. The final product is a non-porous metamaterial with potentially tailor-made Poisson's ratio properties.

2015 - Tailoring graphene to achieve negative poisson's ratio properties [Articolo su rivista]
Grima, J. N.; Winczewski, S.; Mizzi, L.; Grech, M. C.; Cauchi, R.; Gatt, R.; Attard, D.; Wojciechowski, K. W.; Rybicki, J.
abstract

Graphene can be made auxetic, through the introduction of vacancy defects. This results in the thinnest negative Poisson's ratio material at ambient conditions known so far, an effect achieved via a nanoscale de-wrinkling mechanism that mimics the behavior at the macroscale exhibited by a crumpled sheet of paper when stretched.

2014 - On the properties of real finite-sized planar and tubular stent-like auxetic structures [Articolo su rivista]
Gatt, R.; Caruana-Gauci, R.; Attard, D.; Casha, A. R.; Wolak, W.; Dudek, K.; Mizzi, L.; Grima, J. N.
abstract

Auxetics, i.e. systems with a negative Poisson's ratio, exhibit the unexpected property of becoming wider when stretched and narrower when compressed. This property arises from the manner in which the internal geometric units within the system deform when the system is submitted to a stress and may be explained in terms of 'geometry-deformation mechanism' based models. This work considers realistic finite implementations of the well known rotating squares system in the form of (i) a finite planar structure and (ii) a tubular conformation, as one typically finds in stents. It shows that although the existing models of the Poisson's ratios and moduli based on periodic systems may be appropriate to model systems where the geometry/deformation mechanism operate at the micro- or nano- (molecular) level where a system may be considered as a quasi infinite system, corrections to the model may need to be made when one considers finite structures with a small number of repeat units and suggests that for finite systems, especially for the 2D systems, the moduli as predicted by the periodic model may be significantly overestimating the moduli of the real system, even sometimes by as much as 200%. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2014 - On the suitability of hexagonal honeycombs as stent geometries [Articolo su rivista]
Mizzi, L.; Attard, D.; Casha, A.; Grima, J. N.; Gatt, R.
abstract

It is a well-known fact that the mechanical properties of coronary stents are defined mainly by two components, the constituting material and the design pattern of the stent itself. The latter especially has attracted the interest of entrepreneurs and scientists alike with a plethora of patents being issued for numerous stent designs. Despite this widespread interest, the suitability of said designs are seldom studied. Accordingly, in this work we have investigated the properties of stent designs based on the hexagonal honeycomb geometry. Stent patterns based upon re-entrant, non re-entrant, and hybrid honeycomb geometries were studied with respect to their behaviour at extremely high strains using Finite Element Analysis. The data collected indicates that although the non re-entrant and hybrid geometries may be more suited to stent designs than the re-entrant geometry in terms of tolerance to high strains, none of these systems convey all the ideal properties desired in a stent, even if the former two have the potential of exhibiting some of them. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2013 - A realistic generic model for anti-tetrachiral systems [Articolo su rivista]
Gatt, R.; Attard, D.; Farrugia, P. -S.; Azzopardi, K. M.; Mizzi, L.; Brincat, J. -P.; Grima, J. N.
abstract

Chiral systems are a class of structures, which may exhibit the anomalous property of a negative Poisson's ratio. Proposed by Wojciechowski and implemented later by Lakes, these structures have aroused interest due to their remarkable mechanical properties and numerous potential applications. In view of this, this paper investigates the on-axis mechanical properties of the general forms of the flexing anti-tetrachiral system through analytical and finite element models. The results suggest that these are highly dependent on the geometry (the ratio of ligament lengths, thicknesses, and radius of nodes) and material properties of the constituent materials. We also show that the rigidity of an anti-tetrachiral system can be changed without altering the Poisson's ratio. The anti-tetrachiral system, with the unit cell shown in red. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2013 - On the effect of the mode of connection between the node and the ligaments in anti-tetrachiral systems [Articolo su rivista]
Gatt, R.; Brincat, J. P.; Azzopardi, K. M.; Mizzi, L.; Grima, J. N.
abstract

Chiral systems may exhibit auxetic behavior, i.e. they may have a negative Poisson's ratio. This particular property has led to their being studied extensively by several authors. A Finite Elements Study is presented here, investigating the mode of connection between the nodes and ligaments in the anti-tetrachiral structure. The results show that the amount of gluing material used to attach the ligaments to the node will not affect the Poisson's ratio, but may have a large influence on the observed stiffness of the structure (Young's modulus). It is also shown that the stiffness of the glue will have a large effect on the mode of deformation of the chiral system. This change in mechanism was found to effect the stiffness of the structure but not its Poisson's ratios.