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Sandwich panels made with a bamboo core of different dimensions, packing geometries and facing materials are subjected to three-point bending tests and assessed through statistical and failure analysis. In addition to promoting a circular economy, this architecture holds great promise for replacing secondary structural components in sustainable construction and transportation facilities. The statistical analysis responses are associated with the equivalent density of the panels, flexural strength and modulus of the panels, skin stress and core shear strength and modulus. Individual bamboo rings are also characterised using physic mechanical and interfacial bonding tests. Treated aluminium face sheets provide the best mechanical performance compared to glass fibre-reinforced composite (GFRP) ones by increasing the overall properties of the sandwich panels. The specific face sheet material and void percentage affect the equivalent density, with lower values (~ 0.48 g/cm3) when using GFRP skins, larger bamboo rings and cubic packing. Sandwich panels with 30 mm bamboo rings and hexagonal packing provide higher flexural properties, i.e. ~ 43 MPa strength and ~ 7.6 GPa modulus, and skin stress (~ 288 MPa), while those with 20 mm bamboo rings have higher shear stiffness (~ 132 MPa) and resistance (~ 3.33 MPa). Sandwich panels made with aluminium skins show evident skin-polymer debonding, while those with GFRP have premature skin failure and lower structural performance. The proposed sandwich panels present remarkable and competitive mechanical performance concerning commercial analogous structures, generally used in the aeronautical and automotive industries.

This work evaluates a new eco-friendly surface treatment method for coir fibres based on sodium bicarbonate (NaHCO3). The fibres are treated with 10 wt% NaHCO3 solution for different periods of time (24, 96 and 168 h). A 2141 Full Factorial Design is established to investigate the effect of the type of matrix (epoxy and polyester) and treatment time (24, 96 and 168 h) on the mechanical properties of coir fibre composites. X-Ray diffraction (XRD) analysis reveals that the treatment does not promote a complete change of cellulose-I into cellulose-II. However, the degradation of hemicellulose and pectin of the fibre surface is evidenced by the thermogravimetric (TGA) curves. The chemical treatment contributes to increase the fibre density and to reduce its thermal stability. Single fibre tensile tests reveal a gradual increase in the elastic modulus attributed to the treatment time factor. Epoxy composites presented higher tensile and flexural strength in comparison to polyester composites, which exhibit higher tensile and flexural modulus. The alkaline treatment does not affect the mechanical strength of the composites. However, higher tensile and flexural moduli are obtained when the treatment time increases to 96 or 168 h. The proposed treatment proves to be feasible and efficient to increase the stiffness of coir fibre composites, besides being less damaging to the environment after disposal when compared to strongly alkaline solutions such as NaOH.

This work investigates the mechanical behaviour of a sustainable sandwich panel, consisting of bamboo rings core, treated aluminium skins and epoxy adhesive. A Design of Experiment (DoE) is used to identify the effects of bamboo diameters (30 and 45 mm) and aluminium skin treatments (alkaline degreasing and application of primer) on the mechanical and physical properties of sandwich panels. The aluminium skins treated with the wash primer significantly increase adhesion to the polymer, resulting in greater maximum load, flexural strength, maximum skin stress and maximum core shear stress; while the skins treated with NaOH resulted in a greater flexural and core shear modulus. Relatively more rigid and resistant structures are obtained with Ø30 mm rings, due to the increased surface contact area and the number of constraints on the core. The samples fail due to the skin fracture, implying an efficient face-core bond that is attributed to the proper absorption of the polymer by bamboo and the treatment of the aluminium surface. The proposed panels present good mechanical performance, proving to be a feasible and promising alternative for secondary structural applications.

Hybrid composites present unique features that meet various design requirements more efficiently and more economically than conventional composites, offering advantages such as high strength and stiffness, increased fatigue and impact resistance, among others. Ever-increasing demands for sustainability and ecologically friendly products have considerably amplified the academic and industrial interest in natural fibers in the last decades. Natural fibers, owing to their intrinsic low density, present excellent physical and mechanical specific properties. Natural fiber-based composites also reduce the environmental impact and carbon footprint, as well as energy consumption and fabrication expenses. However, they also present lower durability, high moisture absorption, and lower strength as compared to synthetic fiber composites. The hybridization of different natural fibers or natural fibers with synthetic fibers (or particles) can mitigate these problems, as the advantages of one type of fiber can compensate for the disadvantages of the other. This chapter presents the recent research and developments on hybrid natural fiber composites, regarding different types of hybridization and their particularities. The hybridization of two or more natural fibers, natural and synthetic fibers, as well as fibers (natural or synthetic) and particles, are discussed, focusing on the advantages and drawbacks of theses strategies on the mechanical properties of composite materials.

A full factorial design (2 231 ) has been used to investigate the effect of the use of sodium hydroxide fibre treatment, Portland cement and uniaxial pressure on the physical and mechanical properties of hybrid short coir fibre reinforced composites (HSCoirFRCs). The response variables considered in this work were the apparent density, porosity, tensile and flexural strength , the modulus of elasticity and the Charpy impact resistance. The alkali treatment contributed not only to reduce the apparent porosity , but also to increase the mechanical properties of the HSCoirFRCs. A reduction of the impact resistance and an increase of the apparent density was also identified after treatment. Cold pressing significantly affected the physical and mechanical properties of the HSCoirFRCs. Higher pressure levels enhanced the wettability of the fibres and, consequently, the mechanical performance of the composites. The incorporation of cement microparticles as a second reinforcement phase was however not effective, leading to decreased strength and an increased apparent density of the materials. The HSCoirFRC structure can be considered an economical and sustainable alternative for future secondary structural parts in lightweight transport applications.

Over the last few decades, a significant amount of research has been focused on the use of natural fibres as reinforcement in polymers, due to their intrinsic properties such as sustainability, easy availability and processing, biodegradability and moderate mechanical strength. Among natural fibres, coir is a low-cost fibre extracted from coconut palm which is extensively produced in Brazil. A full factorial design was carried out to investigate the effects of the manufacturing and composition parameters on the mechanical and physical properties of short coir fibre–reinforced composites (SCoirFRCs). The random short fibres were mixed with epoxy polymer and compacted by uniaxial pressure. The physical and mechanical responses, namely, apparent density, impact resistance, flexural strength and modulus, were investigated under a design of experiment approach. SCoirFRCs fabricated with 35% of fibre volume fraction, 375 g/m2 of fibre grammage and HY956 epoxy hardener type achieved higher flexural modulus and impact resistance, while those consisting of 30 vol% of coir fibres, HY956 type and 300 g/m2 of grammage revealed higher flexural strength. The findings revealed that the mechanical properties of SCoirFRCs are substantially dominated by the properties of the matrix phase and fibre wettability.

The hybrid configuration of bio-reinforced composites has established a new extended boundary for the development of pro-ecological technologies due to light weight, moderate specific strength, low cost, environmental benefits, and potential applications of natural components. This work investigates the physical and mechanical properties of hybrid composites made of sisal/glass fibres and Portland cement inclusions. A full factorial design was generated to identify the effects of the stacking sequence and cement particles on the flexural strength, flexural stiffness, apparent density, apparent porosity and water absorption of the composites. The significant contributions of these main factors and their interactions were determined via Design of Experiments (DoE) and Analysis of Variance (ANOVA). The fracture features and damage mechanisms of hybrid composite were also reported. The inclusion of cement microparticles led to an increased apparent porosity, as well as enhanced water absorption, flexural stiffness and flexural strength of the hybrid composites. The mechanical properties were strongly dependent on the fibre stacking sequence, which accounts for approximately 98% of the effects observed. Moreover, the stacking sequence affected the damage mechanism of the bio-composites. Finally, the replacement of glass fibres by unidirectional sisal reinforcements may potentially improve the specific properties in structural applications with an environmental sustainable footprint.

This work investigates the effect of silica nanoparticles functionalized with poly-diallyldimethylammonium chloride (PDDA) and silica microparticle inclusions (1.0 wt% and 3.5 wt%) on the impact resistance of hybrid carbon fibre reinforced composite laminates (HCFRCs) and tensile modulus of particle reinforced polymers (PRPs) via Full-Factorial Design of Experiments. The data were analysed with Analysis of Variance (ANOVA). The inclusion of particles led to reduced impact absorption of HCFRCs, except for composites with 1.0 wt% of silica in microscale, which provides an increase of 11.75% in the impact resistance. Microstructural analysis of fractured impact samples revealed pull-out as the predominant fracture mode in 1.0 wt% silica microparticle composites. Such mechanism leads to impact energy dissipation which may explain the increased impact resistance of these samples.

Surface modification induced by sodium carbonate on coir fibres was studied in view of its application as a reinforcing agent in polyester and epoxy polymer fibre-reinforced composites. Coir fibres were treated with a 10 wt% sodium carbonate solution for different periods of time (24, 96 and 168 h) at room temperature. The surface treatment was evaluated by scanning electron microscopy, X-ray diffraction and thermogravimetric analysis. Tensile, flexural and impact properties of treated and untreated coir fibre-reinforced composites were compared. Coir-reinforced epoxy and polyester composites were manufactured by the uniform dispersion of randomly oriented coir fibres within the polymeric matrix. Tensile and flexural modulus of coir-reinforced polyester composites increased nearly 28% and 25%, respectively after 96 h of coir treatment. An analogous increase of 31% and 17% was obtained for coir-reinforced epoxy composites. In addition, coir-polyester composites achieved superior tensile (∼17%) and flexural (∼5%) modulus and impact strength (∼193%) compared with coir-epoxy-derived composites. In contrast, coir-epoxy composites led to superior tensile and flexural strength. The experimental results revealed that sodium carbonate solution effectively removes hemicellulosic compounds, promotes swelling and increases superficial roughness of the fibres, improving the mechanical properties (modulus and strength) of the coir-reinforced composites studied.

This work describes the experimental and numerical behaviour of sandwich panels made of aluminium skins and bamboo core under low-velocity impact test. A statistical design is carried out to evaluate the effect of the bamboo diameter (O20 and O30 mm) and the adhesive type (epoxy and biopolymer) on the maximum load, energy to maximum load, total deflection and total energy of the panels, which are assessed through graphical and failure analysis. A non-linear finite element (FE) analysis is developed to simulate the low-velocity impact test and to predict the failure mechanisms of the skins, bamboo core and adhesive. The experimental results show that, unlike the adhesive type, the bamboo diameter variation does not significantly affect the impact properties. Sandwich panels made of epoxy adhesive exhibit greater rigidity and lower maximum load than those with biopolymer, resulting in premature core-face debonding. On the other hand, sandwich panels made with biopolymer have a greater capacity for absorbing energy and maintaining structural integrity. The numerical simulation indicates a good correlation with the experimental data for load–displacement impact curves, kinematic energy-time curves, perforation process and failure modes.

The incorporation of nano or micro ceramic particles into fibre reinforced composites (FRC) to enhance their stiffness and durability has been widely investigated. This mechanism has been attributed to the increase in stiffness of the polymeric matrix phase and shear strength of FRCs due to the presence of particles at the interlaminar regions. In order to elucidate such effect, hybrid single-lap joints consisted of ceramic particles and glass fibre reinforced composites were evaluated to better assess the mechanical interlocking effect provided by silica and cement inclusions. A full factorial design (2 3 ) was performed to identify the effect of the type of particle (silica and cement), particle weight fraction (2.5 and 5 wt%) and glass fibre grammage (200 and 600 g/m 2 ) on the apparent shear strength and adherent strength of single-lap joints under tensile loading. The ceramic particle inclusions led to increased apparent shear strength and adherent strength. The inclusion of 5 wt% ceramic particles into 600 g/m 2 cross-ply glass fibre composites enhanced both adherent and apparent shear strengths.

A significant amount of research has been focused on the use of ceramic nano/micro particles to enhance the strength and stiffness of polymeric matrices. This work evaluates the effect of Portland cement or crystalline silica (quartz) particle inclusions into epoxy polymer. Two experiments were conducted based on a full factorial design analysis. Experiment I investigated the effect of Portland cement amount (ASTM III), two types of hardeners (HY 951 and 956) and two curing times (7 and 28 days) on the compressive behaviour and density of particulate composites. Experiment II evaluated the incorporation of quartz or cement particles by mixing different mass fraction levels, considering 28 days of curing time and HY 951 hardener. The samples were prepared in a randomized manufacturing process and tested in compression. The mechanical properties were significantly affected by the type of hardener used. Both particles, considered in appropriate level set, can enhance the compressive strength and stiffness of the composites.

This work investigates epoxy composites reinforced by randomly oriented, short glass fibres and silica microparticles. A full-factorial experiment evaluates the effects of glass fibre mass fraction (15 and 20 wt%) and length (5 and 10 mm), and the mass fraction of silica microparticles (5 and 10 wt%) on the apparent density and porosity, as well as the compressive and tensile strength and modulus of the hybrid composites. Hybrid epoxy composites present significantly higher tensile strength (9%) and modulus (57%), as well as compressive strength (up to 15%) relative to pure epoxy.

This work further investigates the manufacture and characterisation of a sustainable sandwich panel made from aluminium skins and a recycled thermoplastic bottle cap core, an innovative concept proposed in a previous paper. A full factorial design based on Design of Experiments (DoE) and Analysis of Variance (ANOVA) techniques has highlighted the complex influence of three manufacturing parameters (type of polymeric adhesive, adhesive layer thickness layer and core packing topology) on the absolute and specific physical and flexural properties of the panels. The ANOVA revealed that the use of higher amount of epoxy polymer led to enhanced panel strength and stiffness. The cell packing topology, however, did not provide a significant effect on most panel properties. Discarded bottle caps have proven to be a promising lightweight and inexpensive honeycomb component for structural applications.

The apparent density, flexural modulus and strength of hybrid laminated composites were investigated through a full-factorial Design of Experiment (DoE) approach. Laminates were manufactured by hand lay-up using nine layers of glass fibre cross-ply fabric with an epoxy matrix phase reinforced with Portland cement microparticles. A first experiment investigated the effect of the inclusion site (particles in upper four layers, lower four layers, all layers or none), curing time (7 and 28 days) and compaction method (vacuum or uniaxial pressure). The fibre–matrix volume fraction and the particle mass fraction were fixed at 48.6/51.4% and 10% respectively. A second experiment investigated two distinct fibre–matrix volume fractions (48.6/51.4 and 29.6/70.4%) and five particle mass fractions (0, 2.5, 5.0, 7.5 and 10 wt%). Particle inclusions were restricted to the upper four layers, with 28 days of curing time and uniaxial compaction. The results were analysed via Analysis of Variance (ANOVA). A significant increase in flexural modulus and strength was observed at 28 days of curing time. Enhanced mechanical properties were obtained for laminates with particle inclusions only in the upper half of the structure, manufactured with 48.6/51.4% fibre-matrix volume fraction and uniaxial pressure. Higher flexural strength was achieved for composites manufactured with 51/49% fibre–matrix volume fraction and 2.5% of particle mass fraction. These fibrous-particulate hybrid composite laminates can be considered for future secondary structural parts in lightweight engineering applications.

The physical and mechanical properties of brown-pigmented particulate composites based on epoxy polymer reinforced with quartz particles were assessed experimentally. The best setup condition was chosen to repair Macaranduba and Cedrinho timber species. A full factorial design was conducted to evaluate the effect of the quartz particle (0, 40, 50, 60 wt-%) and pigment (0, 0.02 wt-%) inclusions on the density, apparent porosity, compressive modulus and strength. The composite made with 50 wt-% of quartz particles and 0.02 wt-% of pigment revealed promising characteristics for restoration works, being used to repair the upper (under compression) side of timber beams evaluated under four-point bending tests. The timber beams were evaluated in pristine, with defect and in repaired conditions. Repaired Cedrinho timber achieved similar mechanical behaviour when compared with pristine conditions. In contrast, the repair procedure was not successful in restoring the load capacity of damaged Macaranduba ti...

The bicritical properties of the three-dimensional classical anisotropic Heisenberg model in a crystal field are investigated through extensive Monte Carlo simulations on a simple cubic lattice, using Metropolis and Wolff algorithms. Field-mixing and multidimensional histogram techniques were employed in order to compute the probability distribution function of the extensive conjugate variables of interest and, using finite-size scaling analysis, the first-order transition line of the model was precisely located. The fourth-order cumulant of the order parameter was then calculated along this line and the bicritical point located with good precision from the cumulant crossings. The bicritical properties of this point were further investigated through the measurement of the universal probability distribution function of the order parameter. The results lead us to conclude that the studied bicritical point belongs in fact to the three-dimensional Heisenberg universality class.

We study the phase diagram of the three-dimensional classical ferromagnetic Heisenberg model with an easyplane crystalline anisotropy and an easy-axis exchange anisotropy through Monte Carlo simulations. We employ the Metropolis algorithm together with single-histogram techniques in order to characterize the transitions in each region of the phase diagram. Our results reveal, besides the disordered phase, the existence of Ising-like and XY-like ordered phases which are separated by a first-order transition line.

Neste trabalho, estudamos, através de simulações Monte Carlo, as transições de fases que ocorrem em dois modelos tridimensionais distintos, obtendo o diagrama de fases de cada um deles, para diferentes valores dos parâmetros do hamiltoniano correspondente. No caso do modelo de Heisenberg anisotrópico submetido a um campo cristalino, a competição entre anisotropias de troca e cristalina resulta em um diagrama de fases de topologia rica, que revela duas fases ordenadas, separadas por uma linha de transição de primeira ordem.Essas fases, por outro lado, encontram-se separadas da fase desordenada por meio de duas linhas de transição de segunda ordem. As três linhas de transição terminam em um ponto bicrítico. Para a versão XY do modelo vetorial de Blume-Emery-Griffiths, o diagrama de fase obtido reproduz, para uma faixa de parâmetros do modelo, a topologia do diagrama de fases de misturas de 3He - 4He. Esse diagrama revela uma fase superfluida, rica em 4He, que sofre uma transição para uma fase normal. Essa transição é de primeira ou segunda ordem, dependendo da região do diagrama examinada.Uma das características mais importantes desse diagrama é a existência de um ponto tricrítico, o qual, para certos valores dos parâmetros do modelo, se decompõe em um ponto crítico terminal e um ponto crítico duplo terminal. Esses pontos foram localizados com boa precisão. O estudo desse modelo proporciona a base necessária para a investigação futura de propriedades dinâmicas de misturas de 3He-4He.

Extensive Monte Carlo simulations are used to locate the bicritical point of the ferromagnetic anisotropic Heisenberg model on a simple cubic lattice in the presence of a crystal field. In order to achieve this goal, we employ a hybrid algorithm comprising Metropolis and Wolff algorithms along with field mixing. Multiple histogram techniques have also been used to obtain the probability distribution function of the conjugate extensive variables of interest. In conjunction with finite-size scaling analysis we computed the fourth-order cumulant of the order parameter along the first-order line and the corresponding bicritical point has been located from the crossings of the cumulants.

Using a single microscope objective lens to optically trap, illuminate, and collect backscattered light of a dielectric microsphere, we measure the temporal-intensity-autocorrelation functions (ACFs), and intensity profiles to obtain the trap stiffness and friction coefficient of the bead. This is an interesting study of an harmonically bound Brownian particle, with nanometer resolution. We extend the work of Bar-Ziv et al. [Phys. Rev. Lett. 78, 154 (1997)] to more general situations allowing for the use of our simpler geometry in other applications. As examples, we present measurements of the parallel Stokes friction coefficient on the trapped bead as a function of its distance from a surface and the entropic force of a single lambda-DNA molecule.

Neste trabalho estudamos o comportamento elastico entropico de uma molecula de DNA do bacteriofago , aliando as tecnicas de pincamento optico, videomicroscopia e correlacao de fotons. O metodo experimental permite amanipulacao de uma ´unica molecula por vez. A molecula de DNA é ancorada a superfcie de uma lamínula de vidro e ligada a uma microesfera depoliestireno de cerca de 3m de diametro. O pincamento optico dessa microesfera permite a manipulacao e o estiramento da molecula de DNA. A constante de forca da pinca ´optica é ´obtida pela tecnica de correlacao de fotons. Medindo-se tambem o deslocamento da microesfera em relacao `a sua posicao de equilíbrio no interior do poco da pinca, ´e possvel determinara forca exercida pelo DNA em funcao de seu estiramento. Essa forca ´é descrita pelo modelo Worm-Like Chain (WLC), que permite a obtencao do comprimento de persistencia da molecula de DNA. This work combines optical tweezer, videomicroscopy and photon correlation techniques in order to study the entropic elastic behaviour of single A-fage DNA molecules. A DNA molecule is attached by one end to the surface of a coverslip and by the other to the a 3m polystyrene bead. The optical trapping of this bead enables the manipulation and stretching of the DNA molecule. The photon correlation technique is used to measure the trapping force constant. By measuring also the displacement of the bead relative to its equilibrium position in the trapping potential well, it is possible to determine the force DNA exerts as a function of its extension. This force is theoretically described by the Worm-Like Chain (WLC) model and the persistence length of the DNA molecule is obtained.

Neste trabalho apresentamos um estudo computacional de magnetismo em nano-escala. Modelamos as propriedades magnéticas de nano-partículas usando um modelo de spins clássicos na rede. Em nosso modelo consideramos interações de troca e dipolar. Enquanto o termo de troca age somente entre primeiros vizinhos o segundo é de longo alcance e deve ser computado entre todos os spins do sistema. Neste trabalho nós discutimos modelos alternativos para o termo de dipolo. Nossos cálculos mostram que as interações dipolares não podem ser aproximadas por qualquer corte no potencial. A influência da geometria da nano-partícula e a aplicação de campos externos na formação de estruturas do tipo vórtice foi estudada em diversas situações. Nós determinamos as condições em que um vórtice aparece como o estado fundamental do modelo e como sua morfologia éafetada pela geometria da nano-partíucula. Pares de vórtice-antivórtice aparecem no sistema quando aumentamos a temperatura. Acredita-se que a criação e aniquilação desses pares é o mecanismo de inversão de polaridade do vórtice. Nossas simulações não corroboram estemecanismo. Avaliamos, tamb´em, o comportamento das nano-partículas na presença de um campo magnético externo. Obtivemos curvas de histerese típicas de nanoestruturas. Estudamos o efeito de Exchange-bias, considerando nano-discos depositados sobre substratos antiferromagnéticos. In this work we present a computational study of magnetism in nano-scale. We modeled the magnetic properties of nano-particles using a classical spins model defined in a lattice. In our model we consider exchange and dipolar interactions as well. The former acts only betweenfirst neighbors while the latter is long-ranged and must be computed between all the spins on the system. We have discussed alternative models for the dipole term. Our calculations show that the dipolar interactions cannot be approximated by any cutoff in the potential.The influence of the geometry of the nano-particle and the application of external fields in the formation of vortex structures were studied in several situations. We have determined the conditions under which a vortex appears as the ground state of the model and how theirmorphology is affected by the geometry of the nano-particle. Vortex-antivortex pairs appear in the system when the temperature is raised. It is believed that a vortex-antivortex creation and annihilation mechanism is responsible for the polarity switching of the vortex. Our simulations do not give support to this mechanism.We have also studied the behavior of nano-particles in the presence of an external magnetic field. Typical nanostructures hysteresis curves were obtained. The effect of exchange-bias in nano-disks deposited over antiferromagnetic substrates is also considered.