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Nonlinear energy sink (NES) refers to a typical passive vibration device connected to linear or weakly nonlinear structures for vibration absorption and mitigation. This study investigates the dynamics of 1-dof and 2-dof NES with nonlinear damping and combined stiffness connected to a linear oscillator. For the system of 1-dof NES, a truncation damping and failure frequency are revealed through bifurcation analysis using the complex variable averaging method. The frequency detuning interval for the existence of the strongly modulated response (SMR) is also reported . For the system of 2-dof NES, it is reported in a similar bifurcation analysis that the mass distribution between NES affects the maximum value of saddle-node bifurcation. To obtain the periodic solution of the 2-dof NES system with the consideration of frequency detuning, the incremental harmonic balance method (IHB) and Floquet theory are employed. The corresponding response regime is obtained by Poincare mapping, it shows that the responses of the linear oscillator and 2-dof NES are not always consistent, and 2-dof NES can generate extra SMR than 1-dof NES. Finally, the vibration suppression effect of the proposed NES with nonlinear damping and combined stiffness is analyzed and verified by the energy spectrum, and it also shows that the 2-dof NES system demonstrates better performance.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne disease caused by a phlebovirus in the Bunyaviridae family. Infection can result in systemic inflammatory response syndrome with a high fatality rate, and there are currently no treatments or vaccines available. The microbiota has been implicated in host susceptibility to systemic viral infection and disease outcomes, but whether the gut microbiota is implicated in severe fever with thrombocytopenia syndrome virus (SFTSV) infection is unknown. Here, we analysed faecal and serum samples from patients with SFTS using 16S ribosomal RNA-sequencing and untargeted metabolomics, respectively. We found that the gut commensal Akkermansia muciniphila increased in relative abundance over the course of infection and was reduced in samples from deceased patients. Using germ-free or oral antibiotic-treated mice, we found that A. muciniphila produces the β-carboline alkaloid harmaline, which protects against SFTSV infection by suppressing NF-κB-mediated systemic inflammation. Harmaline indirectly modulated the virus-induced inflammatory response by specifically enhancing bile acid-CoA: amino acid N-acyltransferase expression in hepatic cells to increase conjugated primary bile acids, glycochenodeoxycholic acid and taurochenodeoxycholic acid. These bile acids induced transmembrane G-protein coupled receptor-5-dependent anti-inflammatory responses. These results indicate the probiotic potential of A. muciniphila in mitigating SFTSV infection.

This paper presents a numerical approach to predict the thermal residual stresses in polymer nanocomposites reinforced with a periodic array of wavy carbon nanotubes. A three dimensional unit cell model is established to accurately account for the waviness of the nanotube. Periodic boundary conditions are determined for the unit cell with a pair of curved surfaces. Appropriate methods to evaluate the macroscopic stresses and strains are also determined for the unit cell model in which the interior pores of the nanotubes are explicitly included. It is demonstrated that the macroscopic behavior of the nanocomposites is orthotropic due to the symmetries manifested. By employing material properties of the two constituents, the thermal residual stresses and strains induced by high temperature curing and cooling-down are predicted for an epoxy/wavy-nanotube composite. It is also demonstrated that the curing process tends to increase the waviness of the nanotube and the waviness has a significant influence on the distribution of the microscopic residual stresses.

The effect of gravity and sliding friction on the dynamics around targeted energy transfer of an inclined vibro-impact nonlinear energy sink (VINES) is addressed. The non-smooth dynamics is first formulated in Signorini’s contact law and treated using a Moreau-Jean scheme adapted with an energy-conserving integration method. Exhaustive discussions are then carried out regarding the vibration damping performance of the VINES, as well as their dependence on the underlying vibro-impact induced response regimes, under both transient and steady-state responses. It is demonstrated that gravity is responsible for reducing the energy barrier required for achieving targeted energy transfer in the transient response, or enlarge the existing bandwidth of the effective strongly modulated response (SMR) regimes, which is an equivalent representation of TET in the steady-state response, to improve considerably the overall damping efficiency. Some adverse effects brought by gravity and friction are also reported, and it is found that, when the incline angle or friction coefficient grows large, the bandwidth of SMR reduces again and leads to a deterioration of efficiency. The results open a door for appropriately taking advantage of the gravity into the VINES design to improve its damping performance.

This paper presents the results of an investigation into the punch shear behaviour of polycarbonate. First, supported punch shear tests were carried out using injection moulded disk specimens. Ductile material behaviour was observed during the punch shear process which consisted of an elastic stage, a prolonged deformation stage, and an abrupt shear-out stage at a large displacement. The complex damage mode was investigated with the aid of scanning electron microscopy. Notably, it was found that a circumferential tensile crack existed in the punched-out specimen, indicating that tensile deformation played an important role in the punch shear process. An axisymmetric simple shear model was established assuming a uniform deformation mode. The kinematic analysis indicated that very large shear and normal strains occurred at high loads, validating the test observation that both tensile and shear damage can occur during punch shear. In addition, a finite element analysis accounting for the large deformation effect was performed. The predicted results of the average shear stress versus normalized displacement curves were in good agreement with the test results and the numerical results revealed that the punch tool clearance had a considerable influence on the punch shear response. Finally, the identification of the potential damage location and the evolution of the local plastic deformation were demonstrated. Copyright 2019. Used by the Society of the Advancement of Material and Process Engineering with permission., SAMPE 2019 Conference and Exhibition, May 20-23, 2019, Charlotte, NC

A 3D two-cell representation of laminates with periodic fiber arrays is established and the corresponding periodic boundary conditions are presented. The matrix is described by a nonlinear viscoelastic material model and the fiber by an elastic one. Matrix cracking is modeled by a "smeared crack" method. The modeling of material nonlinearity, crack initiation, and growth, with appropriate periodic boundary conditions, are incorporated into a finite element model (FEM). Numerical results for five different angle-ply laminates under tensile loading are presented. The results are found to be in fair agreement with the available test data. © 2013 Taylor and Francis Group, LLC.

In this work, a two-step method consisting of in situ polymerization of polymethyl methacrylate (PMMA) in the presence of single-walled carbon nanotubes (SWCNT), followed by the redispersion of the resulting compound in dimethylformamide (DMF), was used to fabricate SWCNT modified with PMMA (SWCNT-PMMA). Raman spectroscopy revealed that PMMA was merely wrapped around the SWCNT when raw SWCNT or purified SWCNT were used as the starting material. However, PMMA was covalently bonded to SWCNT when acid treated SWCNT (SWCNT-COOH) was used as the starting material. SWCNT-PMMA compounds were further diluted in pure PMMA by conventional melt compounding at large scale (several kilograms) to obtain transparent composites containing 0.09 wt % SWCNT. The micro- and nano-dispersion of the SWCNT in the composites were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thermal and mechanical properties of the composites were determined by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile testing, and Charpy impact testing. At the the low SWCNT loading studied, the tensile properties remain unchanged, whereas the impact strength improves by 20%.

Single-walled carbon nanotubes (SWCNT) exhibit exceptional mechanical, thermal and electrical properties, amongst the best of any known material. Combined with their very high aspect ratios, SWCNT are ideal fillers for the fabrication of ultralight multifunctional and structural composite materials with superlative properties. Therefore, over the past decade, a huge amount of effort has been expended exploring potential applications of SWCNT in different polymer matrices including PMMA (1), vinyl ester (2), Nylon (3), Epoxy resin (4) etc., to enhance their mechanical properties as well as their electrical and thermal conductivities. Unfortunately, as a result of strong van der Waals interactions, high crystallinity and aromaticity, SWCNT bundle together as ropes and have little affinity for, or compatibility with, any matrices and solvents. This makes the reinforcement of nanocomposites very challenging unless proper chemistry is employed. It is now widely accepted that chemistry is central to the proper integration of SWCNT into various matrices. Not only does chemistry address problems related to dispersion, it also resolves difficulties with interfacial compatibility. The latter is particularly important if one is seeking improvement of mechanical properties. Penicaud et al. reported that SWCNT bundles can be readily exfoliated by reduction with an alkali metal such as Na, Li or K in THF through electron transfer mediated by alkali-naphthalene-THF complexes (5). Furthermore, they concluded that the thus-reduced SWCNT dissolve spontaneously in polar aprotic solvents such as sulfolane, dimethyl sulfoxide (DMSO), N-methylformamide (NMF), dimethylformamide (DMF), and 1-methyl-2-pyrrolidone. Indeed, these solvents do appear to dissolve reduced SWCNT and therefore, they have been used to conduct chemistry (6-7). Our previous work demonstrated that care must be taken in choosing the proper solvent for reduced SWCNT, particularly in DMSO. Reduced SWCNT have increased nucleophilicity and are easily exfoliated in solution, thus allowing for versatile chemistries to be employed. However, reduced SWCNT can also act as initiators of radical reactions with radical scavengers such as DMSO. Our evidence has shown that DMSO cannot be used as a solvent with reduced SWCNT without first quenching their negative character (8). Therefore, care must also be taken to maintain this negative character and its nucleophilic ability. In this work, covalent chemistry is applied to integrate reduced SWCNT into a single component epoxy resin (MY0510), which was used as test bed, as well as in an epoxy resin system commercially used in industrial scale processes. Preliminary impact, compression after impact and fracture toughness test results for the modified resin and carbon fibre composites are reported and discussed.

A two-scale numerical approach to predict the effective in-plane properties of helical filament-wound thin-walled cylindrical tubes is provided. A meso-scale repeated unit cell (RUC) model for a filament-wound tube is established according to the manufacturing process, in which cross-overs, undulations and overlaps of fibre bundles are described using a bottom-up solid modelling technique. An approach to implement the general periodic boundary conditions in a finite element analysis scheme is also presented. As an application example, the effective in-plane elastic constants of glass fibre/epoxy filament-wound tubes are predicted and the influences of the number of the winding circuits and the shape of the fibre bundles are analyzed and discussed. In addition, the stress/strain distribution in the RUC is obtained which provides the essential information on the stress/strain concentration due to fibre cross-over/undulation/overlap. This then indicates the location where damage will initiate.

Single-walled carbon nanotubes (SWCNTs) were incorporated at low loading (up to ∼1 wt%) into an unfilled aerospace-grade epoxy system, to impart electrical conductivity while maintaining structural bonding capability, as a route for development of a structural and conductive adhesive. At these low SWCNT loadings the tensile properties were maintained or improved, while strength decreased in a higher loading case. The structural bonding performance of composite-to-composite joints, evaluated in lap-shear and peel tests, was reasonably maintained for adhesives containing 0.5 wt% or 1 wt% SWCNTs. In the case of the 0.5 wt% SWCNT–adhesive, peel and lap-shear strength were unchanged while the addition of 1 wt% resulted in 30% increase of peel strength but the lap-shear strength was reduced by 10–15%. For 1 wt% SWCNT–adhesives, conductivities as high as 10−1 S m−1 and typically ∼10−3 S m−1 were achieved. Joint electrical resistance measured between aluminum adherends was larger than predicted by the bulk conductivity, but was reduced by a post-treatment step resulting in apparent joint conductivities within one order of magnitude of the bulk samples.

This paper presents an experimental study of the mechanical behavior of polycarbonate modified with boron nitride nanotubes, an emerging type of nanotube with extraordinary mechanical performance that is optically transparent. Using molded disk specimens with the same geometry, equibiaxial flexural, supported punch shear, and low-speed impact tests were performed to determine the flexural stiffness, punch shear strength, and impact performance of nanocomposites. Polycarbonate nanocomposite samples with different nanotube content (up to 1 wt%) were investigated. Within the scope of the study, mechanical properties of the nanocomposite samples exhibited only very limited variations depending on the reinforcement content; however, noticeable improvement in flexural stiffness and dissipated energy in punch shear were observed, with the low-speed impact performance remaining essentially unchanged. In addition, the complex failure behavior in punch shear was studied with the aid of scanning electron microscopy and the preliminary results are reported., 30th Annual Technical Conference of the American Society for Composites, September 28-30 2015, East Lansing, MI, USA

Micro/meso-mechanical models for unidirectional and angle-ply laminates under an off-axis tensile loading are presented. Three-dimensional unit/multi-cell representations of laminates with periodical fiber arrays are established. The matrix is described by a nonlinear viscoelastic model, and the fiber is described by an elastic one. Matrix cracking is modeled by the smeared crack method, which permits a crack description in terms of stress-strain relations and stiffness reduction in particular orientations. The modeling of material nonlinearity, damage initiation and growth, and the multiaxial loading with appropriate periodic boundary conditions are successfully incorporated in a FEM analysis. As examples of this unified analysis procedure, numerical results for unidirectional, angle-ply, and crossply laminates are presented. Using the same set of material constants of the constituents, different response characteristics of the three types of laminates are predicted, and the results are found to be in good agreement with the test data.

A micromechanical model of unidirectional fibre-reinforced polymer laminates under off-axis loading is analyzed using finite element method. A three-dimensional periodic unit cell is established with the matrix described by a nonlinear viscoelastic model and the fibre by an elastic one. Matrix cracking is modeled by smeared crack method which permits a crack description in terms of stress–strain relations and stiffness reduction in particular orientations. The micromechanical analysis features material nonlinearity, damage initiation and growth, and multiaxial loading. Numerical results reveal the local and global response of the laminate including the damage mechanism. The predictions are in good agreement with the test results performed on a similar composite system.

This paper presents a meso/micromechanical study of fiber reinforced polymeric matrix cross-ply laminates during and after a high temperature curing process. The emphasis is on the generation and evolution of the curing induced residual stress/strain and their influence on the response under subsequent mechanical loading. The cross-ply laminate is modeled by a representative volume element and the constituent materials are described by an elastic model for the fiber and a non-linear viscoelastic model for the polymer matrix. The residual stress/strain fields at the ambient temperature immediately following the cool-down and their evolution with time are determined through the viscoelastic finite element discretization of the RVE with a special consideration given to the fiber/matrix interface region. The results indicate that the residual stresses decrease with time and tend to small asymptotic values after about 34 h. However, the remaining residual strains still have significant influence on the damage evolution of the cross-ply laminate under subsequently superimposed tensile loading.

An explicit unified form of boundary conditions for a periodic representative volume element (RVE) is presented which satisfies the periodicity conditions, and is suitable for any combination of multiaxial loads. Starting from a simple 2-D example, we demonstrate that the “homogeneous boundary conditions” are not only over-constrained but they may also violate the boundary traction periodicity conditions. Subsequently, the proposed method is applied to: (a) the simultaneous prediction of nine elastic constants of a unidirectional laminate by applying multiaxial loads to a cubic unit cell model; (b) the prediction of in-plane elastic moduli for [± θ ] n angle-ply laminates. To facilitate the analysis, a meso/micro rhombohedral RVE model has been developed for the [± θ ] n angle-ply laminates. The results obtained are in good agreement with the available theoretical and experimental results.

Carbon nanotubes (CNT) in their various forms have great potential for use in the development of multifunctional multiscale laminated composites due to their unique geometry and properties. Recent advancements in the development of CNT hierarchical composites have mostly focused on multi-walled carbon nanotubes (MWCNT). In this work, single-walled carbon nanotubes (SWCNT) were used to develop nano-modified carbon fiber/epoxy laminates. A functionalization technique based on reduced SWCNT was employed to improve dispersion and epoxy resin-nanotube interaction. A commercial prepregging unit was then used to impregnate unidirectional carbon fiber tape with a modified epoxy system containing 0.1 wt% functionalized SWCNT. Impact and compression-after-impact (CAI) tests, Mode I interlaminar fracture toughness and Mode II interlaminar fracture toughness tests were performed on laminates with and without SWCNT. It was found that incorporation of 0.1 wt% of SWCNT resulted in a 5% reduction of the area of impact damage, a 3.5% increase in CAI strength, a 13% increase in Mode I fracture toughness, and 28% increase in Mode II interlaminar fracture toughness. A comparison between the results of this work and literature results on MWCNT-modified laminated composites suggests that SWCNT, at similar loadings, are more effective in enhancing the mechanical performance of traditional laminated composites.

In this paper, a meso-scale repeated unit cell model for a general [±θ]n angle-ply laminate is developed and the implementation of periodic boundary conditions in a finite element analysis for this 3-D model is presented. A nonlinear viscoelastic constitutive model and a post-damage constitutive model based on the concept of a smeared crack are also incorporated in the finite element analysis. The stress-strain response and the damage initiation and propagation for different values of θ are predicted. © 2011 Published by Elsevier Ltd., 11th International Conference on the Mechanical Behavior of Materials, ICM11, 5 June 2011 through 9 June 2011, Como

The micro/meso-mechanical approach for composites is commonly based on the analysis of a representative volume element or a so-called repeated unit cell (RUC). Through analysis of the RUC model one can predict not only macroscopic mechanical properties but also microscopic damage initiation and its propagation in composites. In this paper, we present an overview of our contributions in the following three essential areas in the micro/meso-mechanical analyses: (1) a unified form of periodic boundary conditions for the RUC modelling; (2) a nonlinear viscoelastic constitutive model for polymer matrix materials; and (3) a post-damage constitutive model based on the concept of smeared crack. Application examples combining the above three topics are presented, in which three types of glass/epoxy laminates are analyzed using finite element method. The predicted results are compared with experimental data and they are in good agreement.

The evolution of thermal residual stresses induced during curing processing has been investigated. A micro-mechanical model of a cross-ply laminates is analyzed using viscoelastic finite element method. The polymer matrix is represented by a non-linear viscoelastic constitutive model. To study possible bounds of the residual stresses two types of analyses were carried out: one with temperature-independent and the other with temperature-dependent material constants. The results indicate that a higher cooling rate results in higher residual stresses in the laminates. However, due to the viscosity of the polymer matrix, the residual stresses relax with time and tend to an asymptotic small value irrespective of the different cooling rates. The average shrinkage strains and the coefficients of thermal expansion (CTE) of the composites, correspondingly, are also time-dependent and approach an asymptotic value after the stress relaxation process.Copyright © 2002 by ASME

The 24th Annual Technical Conference of American Society of Composites, Newark, Delaware, USA, September 10-13, 2009

18th International Conference on Composites Materials, ICCM 2011, 21 August 2011 through 26 August 2011, Jeju