Elastic Composites Research Articles

Statistical homogenization of polycrystal composite materials with thin interfaces using virtual element method

Polycrystalline materials with inter-granular phases are modern composite materials extremely relevant for a wide range of applications, including aerospace, defence and automotive engineering. Their complex microstructure is often characterized by stochastically disordered distributions, having a direct impact on the overall mechanical behaviour. In this context, within the framework of homogenization theories, we adopt a Fast Statistical Homogenization Procedure (FSHP), already developed in Pingaro et al. (2019), to reliably grasp the constitutive relations of equivalent homogeneous continua accounting for the presence of random internal structures. The approach, combined with the Virtual Element Method (VEM) used as a valuable tool to keep computational costs down, is here successfully extended to account for the peculiar microstructure of composites with polycrystals interconnected by thin interfaces. Numerical examples of cermet-like linear elastic composites complement the paper.

Effective Complex Properties for Three-Phase Elastic Fiber-Reinforced Composites with Different Unit Cells

The development of micromechanical models to predict the effective properties of multiphase composites is important for the design and optimization of new materials, as well as to improve our understanding about the structure–properties relationship. In this work, the two-scale asymptotic homogenization method (AHM) is implemented to calculate the out-of-plane effective complex-value properties of periodic three-phase elastic fiber-reinforced composites (FRCs) with parallelogram unit cells. Matrix and inclusions materials have complex-valued properties. Closed analytical expressions for the local problems and the out-of-plane shear effective coefficients are given. The solution of the homogenized local problems is found using potential theory. Numerical results are reported and comparisons with data reported in the literature are shown. Good agreements are obtained. In addition, the effects of fiber volume fractions and spatial fiber distribution on the complex effective elastic properties are analyzed. An analysis of the shear effective properties enhancement is also studied for three-phase FRCs.

Elastic composites with PDMS matrix and polysulfone-supported silver nanoparticles as filler

Polysulfone-supported silver nanoparticles (Ag@PSF) were obtained by a combination of polymer nanoprecipitation and redox synthesis of silver, in the presence of two siloxane-based stabilizing agents, and were incorporated into a PDMS matrix. Soft elastic composites (PDMS-Ag@PSF) were obtained and were investigated by dielectric relaxation spectroscopy and tensile measurements, in comparison with similar materials containing either PSF particles or AgNPs. The mechanical properties of the composites are tunable by composition, depending quasi-linearly on the Ag content. A PDMS-Ag@PSF film with 0.8 wt% Ag, having Y = 0.12 MPa, maximum strain 550% and EBD = 36 V μm−1 exhibited electrical actuation of 8% lateral strain at 22 V μm−1. Apparent piezoelectric coefficient d33 values of ~15 pm V−1 were measured by piezoresponse force microscopy for the composites with Ag@PSF, showing a synergistic effect of the fillers’ components. In addition, the materials exhibited antibacterial and antifungal activity and low bioadhesion. Based on their properties, the highly stretchable composites can be regarded as candidates for applications like actuators, sensors or micro/nanogenerators in biomedical field.

A stress-driven computational homogenization method based on complementary potential energy variational principle for elastic composites

Based on the complementary potential energy variational principle, in this work, we proposed a stress-driven homogenization procedure to compute overall effective material properties for elastic composites with locally heterogenous micro-structures. We have developed a novel incremental variational formulation for homogenization problems of both infinitesimal and finite deformations where the macro-stress-based complementary potential energy is obtained for hyperelastic materials for a global minimization problem with respect to fine-scale displacement fluctuation field. The point of departure of our approach is a general complementary variational principle formulation that can determine material responses of elastic composites with heterogeneous micro structures. We have implemented the proposed stress-driven computational homogenization procedure with the finite element method. By comparing the numerical results with the analytical method and the strain-driven homogenization method, we find that the stress-driven homogenization offers the lower bound estimate of materials properties for elastic composites.

The inclusion-based boundary element method (iBEM) for virtual experiments of elastic composites

This paper introduces the inclusion based boundary element method (iBEM) to calculate the elastic fields and effective modulus of a composite containing particles for both three dimensional (3D) and two dimensional (2D) cases. Considering a finite bounded domain containing many inclusions, the isotropic Green’s function has been used to obtain the elastic field caused by source fields on inclusion domains and applied loads on the boundary. Based on Eshelby’s equivalent inclusion method (EIM), the material mismatch between the particle and matrix phases is simulated with a continuously distributed source field, namely eigenstrain, on particles. Because explicit integrals can be obtained for ellipsoidal particles, no mesh is needed for those particles, which enables virtual experiments of a composite containing a large number of particles. The classic Eshelby’s tensor is extended from a constant eigenstrain for the single particle in the infinite domain to a form of a Taylor series for particle-boundary interaction and particle-particle interactions. Using the Hadamard regularization, the 2D formulation is derived from the 3D case by the integral of the elastic solution in the third direction together with an analytical circular harmonic potential integral scheme. The iBEM is particularly suitable to conduct virtual experiments for studying the local elastic field with the integrals of all sources and calculating the effective material properties by the volume average of local fields. A parametric study of accuracy on stress field for uniform, linear, quadratic eigenstrain fields was performed and case studies have been presented to demonstrate the capability of the iBEM for virtual experiments of composites. Some interesting discoveries of microstructure-dependent material behavior are reported with the aid of virtual experiments.

Effect of water/cement ratio on micro-nanomechanical properties of the interface between cementitious matrix and steel microfibers in ultra-high performance cementitious composites

abstract: Ultra-high performance concretes with steel microfibers have been studied in depth with the aim of producing more efficient and durable structures. The performance of these materials depends on the characteristics of the interface between microfibers and cementitious matrix. This research investigates the micro-nanomechanical properties of the interfacial transition zone between the steel microfibers and the matrix of ultra-high performance cementitious composite. The effect of the water/cement ratio and distance from the microfiber were analyzed. The results confirm the formation of high-density calcium-silicate-hydrate (HD C-S-H) matrix at higher concentrations than low-density calcium-silicate-hydrate (LD C-S-H) for w/c ratios of 0.2 and 0.3. The properties in cementitious matrix interface with steel microfibers were very similar to that measured for the cement paste, and no significant difference was observed regarding the distance to the microfibers in relation to the elastic modulus, hardness and chemical composition. Thus, the authors can conclude that the formation of a less resistant region does not occur at the interfacial transition zone cement paste/microfibers.

Elastomers based on epoxyurethane oligomers, cured at room temperature

New epoxyurethane oligomers (oligotetraurethane diepoxides) were synthesized on the basis of oligotetramethylene oxide diols of different molecular weight, various diisocyanates, as well as glycidol, and aminoethylpiperazine. On their basis, frost-resistant elastic cold-curing compositions were obtained. It was shown that varying the molar ratio between epoxyurethane oligomers and amine hardeners makes it possible to obtain elastomers with the deformation and strength properties that could be controlled over a wide range. It was proved by IR-spectroscopy that a various supramolecular structure was formed when different diisocyanates were used. High degree of microphase separation between soft and hard 50 blocks in elastomers synthesized on the basis of isophorone diisocyanate contributes to their higher strength properties. Methods of differential scanning calorimetry and mechanical analysis proved the possibility of the developed elastomers application in the Arctic and the Far North.

4D Printing Elastic Composites for Strain-Tailored Multistable Shape Morphing.

Three-dimensional (3D) morphing structures with multistable shapes that can be quantitatively and reversibly altered are highly desired in many potential applications ranging from soft robots to wearable electronics. In this study, we present a 4D printing method for fabricating multistable shape-morphing structures that can be quantitatively controlled by the applied strains. The structures are printed by a two-nozzle 3D printer that can spatially distribute phase change wax microparticles (MPs) in the elastomer matrix. The wax MPs can retain the residual strain after the prestrained elastomer composite is relaxed because of the solid-liquid phase change. Thanks to high design freedom of the 3D printing, spatial distribution of the wax MPs can be programmed, leading to an anisotropic stress field in the elastomer composite. This causes the out-of-plane deformations such as curling, folding, and buckling. These deformations are multistable and can be reprogrammed because of the reversible phase change of the wax MPs. What's more, characteristics of deformations such as curvatures and folding angles are linearly dependent on the applied strains, suggesting that these deformations are quantitatively controllable. Finally, the applications of the strained-tailored multistable shape morphing 3D structures in the assembly of 3D electronics and adaptive wearable sensors were demonstrated.

Magneto-mechanical properties of elastic hybrid composites

Abstract The paper gives an overview of tunable elastic magnetic composites based on silicon rubber matrix highly filled with a magnetic soft and hard filler. The magnetic soft phase, which is represented by iron microparticles, allows active control of the physical properties of the composites, while the magnetically hard phase (e.g. neodymium–iron–boron alloy microparticles) is mainly responsible for passive adjustment of the composite. The control is performed by the application of an external magnetic field in situ, and passive adjustment is performed by means of pre-magnetization in order to change material remanent magnetization, i.e. the initial state. The potential and limits of active control and passive tuning of these composites in terms of their magneto-mechanical behavior are presented and discussed.

Preparation of flexible and elastic thermal conductive nanocomposites via ultrasonic-assisted forced infiltration

Flexible and elastic thermal conductive polymer composites are of urgent demand in the microelectronics, thermal management systems, and energy industry. In this work, high thermal conductive carbon nanotubes (CNTs) buckypapers were prepared using vacuum filtration technique. Polyurethane (PU) matrices with different isocyanates and isocyanate index R were then polymerized by semi-prepolymer method. Finally, flexible and elastic PU/CNTs nanocomposites were fabricated via ultrasonic-assisted forced infiltration. Owing to the forced infiltration effect of high power (2 kW) and high frequency (20 kHz) ultrasonication, PU matrix with high viscosity can complete infiltrate the compact CNTs buckypaper within seconds. The CNTs are evenly embedded in the PU matrices and still overlap with each other to keep the 3D network after forced infiltration. PU/CNTs nanocomposite film, with a thickness of 70 μm, presented excellent mechanical stability (211.7% elongation at break, 5.97 MPa tensile strength, and 360° bending available) and superior thermal conductivity (6.028 W/mK), which can dissipate the heat accumulation effectively. This facile and robust method of ultrasonic-assisted forced infiltration provides a new strategy to design and fabricate highly thermal conductive composites and other functional polymer composites.

Abnormal Aortic Wall Properties in Women with Turner Syndrome.

Background Turner syndrome (TS) is associated with aortic dilatation and dissection, but the underlying process is unclear. The aim of this study was to investigate the elastic properties and composition of the aortic wall in women with TS. Methods In this cross-sectional study, 52 women with TS aged 35 ± 13 years (50% monosomy, 12 with bicuspid aortic valve [BAV] and 4 with coarctation) were investigated using carotid-femoral pulse wave velocity (CF-PWV) by echocardiography and ascending aortic distensibility (AAD) and aortic arch pulse wave velocity (AA-PWV) by magnetic resonance imaging (MRI). As control group, 13 women with BAV without TS and 48 healthy patients were included. Results Women with TS showed a higher AA-PWV (β = 1.08, confidence interval [CI]: 0.54–1.62) after correcting for age and comorbidities compared with controls. We found no significant difference in AAD and CF-PWV. In women with TS, the presence of BAV, coarctation of the aorta, or monosomy (45, X) was not associated with aortic stiffness. In addition, aortic tissue samples were investigated with routine and immunohistochemical stains in five additional women with TS who were operated. The tissue showed more compact smooth muscle cell layers with abnormal deposition and structure of elastin and diminished or absent expression of contractile proteins desmin, actin, and caldesmon, as well as the progesterone receptor. Conclusion Both aortic arch stiffness measurements on MRI and histomorphological changes point toward an inherent abnormal thoracic aortic wall in women with TS.

Effective balance equations for electrostrictive composites

This work concerns the study of the effective balance equations governing linear elastic electrostrictive composites, where mechanical strains can be observed due to the application of a given electric field in the so-called small strain and moderate electric field regime. The formulation is developed in the framework of the active elastic composites. The latter are defined as composite materials constitutively described by an additive decomposition of the stress tensor into a purely linear elastic contribution and another component, which is assumed to be given and quadratic in the applied electric field when further specialised to electrostrictive composites. We derive the new mathematical model by describing the effective mechanical behaviour of the whole material by means of the asymptotic (periodic) homogenisation technique. We assume that there exists a sharp separation between the micro-scale, where the distance among different sub-phases (i.e. inclusions and/or fibres and/or strata) is resolved, and the macro-scale, which is related to the average size of the whole system at hand. This way, we formally decompose spatial variations by assuming that every physical field and material property are depending on both the macro-scale and the micro-scale. The effective governing equations encode the role of the micro-structure, and the effective contributions to the global stress tensor are to be computed by solving appropriate linear-elastic-type cell problems on the periodic cell. We also provide analytic formulae for the electrostrictive tensor when the applied electric field is either microscopically uniform or given by a suitable multiplicative decomposition between purely microscopically and macroscopically varying components. The obtained results are consistently compared with previous works in the field, and can pave the way towards improvement of smart active materials currently utilised for engineering (possibly bio-inspired) purposes.

Constrained Amorphous Interphase in Poly(l-lactic acid): Estimation of the Tensile Elastic Modulus.

The mechanical properties of semicrystalline PLLA containing exclusively α′- or α-crystals have been investigated. The connection between experimental elastic moduli and phase composition has been analyzed as a function of the polymorphic crystalline form. For a complete interpretation of the mechanical properties, the contribution of the crystalline regions and the constrained amorphous interphase or rigid amorphous fraction (RAF) has been quantified by a three-phase mechanical model. The mathematical approach allowed the simultaneous quantification of the elastic moduli of (i) the α′- and α-phases (11.2 and 14.8 GPa, respectively, in excellent agreement with experimental and theoretical data reported in the literature) and (ii) the rigid amorphous fractions linked to the α′- and α-forms (5.4 and 6.1 GPa, respectively). In parallel, the densities of the RAF connected with α′- and α-crystals have been measured (1.17 and 1.11 g/cm3, respectively). The slightly higher value of the elastic modulus of the RAF connected to the α-crystals and its lower density have been associated to a stronger chain coupling at the amorphous/crystal interface. Thus, the elastic moduli at Troom of the crystalline (EC), mobile amorphous (EMAF), and rigid amorphous (ERAF) fractions of PLLA turned out to be quantitatively in the order of EMAF < ERAF < EC, with the experimental EMAF value equal to 3.6 GPa. These findings can allow a better tailoring of the properties of PLLA materials in relation to specific applications.

Effective modules of a layered elastic creep medium with power creep kernels

The paper considers the question of constructing the effective characteristics of a layered composite material when periodically repeating layers consist of an isotropic elastically creeping material. For the elastic composites, such characteristics are well known and can be obtained explicitly. In this paper, the effective characteristics of the layered elastic-creeping composite are obtained explicitly when the creep kernel are the power-law functions. The elements of the compliance matrix are presented as the sum of the terms corresponding to the instantaneous elastic compliance, unlimited and limited as the loading time increases due to the creep of the composite individual phases.

Stretchable piezoelectric elastic composites for sensors and energy generators

The search for a piezoelectric elastomer that generates an electrical signal when pressed and stretched has increased significantly in the last decade as they hold great promise in harvesting energy from human motion and monitoring human activities. Here, the excellent elasticity of polydimethylsiloxane-based elastomers and the piezoelectric properties of lead zirconate titanate (PZT) were combined and, using a thermally activated poling process, elastic piezoelectric composites were obtained. For this, two polydimethylsiloxane (PDMS) matrices with a molar mass of 139 kDa and 692 kDa and PZT fillers with particle sizes of 2 and 20 μm were used. For the same poling conditions, an increase in the piezoelectric response with increasing amount of filler, filler size and molar mass of the polymer matrix was observed. Overall, d*33 and d*31 values of 2.7–40 pC N−1 and 16–48 pC N−1 were achieved in this work with filler contents ranging from 37–72 vol%. A composite material with a PZT filler content of 38 vol% (20 μm particle size) in a commercially available PDMS with a Mw = 139 kg mol−1 exhibited high flexibility, good elasticity with long-term mechanical stretchability and high longitudinal and transverse piezoelectric coefficients of 3.6 pC N−1 and 30 pC N−1, respectively. The higher transverse piezoelectric constant d*31 can be explained by an additional capacitor effect of the composite film structure. These properties are interesting features for energy conversion from human motion, monitoring human activities, and stretchable electronics. The functionality of the newly developed material is demonstrated in a pressed sensor.

Minimum energy bounds on longitudinal elastic constants of transversely isotropic unidirectional composites.

We consider the n-component transversely isotropic unidirectional elastic composites, the longitudinal axis of which is parallel to those of the transversely isotropic components as well as the generators of the cylindrical phase boundaries between them. From the minimum energy and complementary energy principles, with appropriate constant strain and piece-wise constant stress trial fields, optimization and iteration techniques, a set of bounds for the macroscopic (effective) longitudinal elastic constants of the composites (including the simple lower arithmetic average estimate for longitudinal Young modulus E eff ≥ E V ) are constructed. Numerical examples are provided to illustrate the obtained results.

Liquid Metal Composites with Anisotropic and Unconventional Piezoconductivity

Summary Anisotropic elastic composites—that is, elastomers containing aligned fillers—often have enhanced properties in the direction of alignment. Depending on the fillers, these composites can have desirable electrical, thermal, or mechanical properties. Here, a silicone composite filled with both solid magnetic metal microparticles and liquid metal microdroplets has been developed. Aligning the solid particles within a magnetic field during curing imparts anisotropy in several properties. Thus, the composite is called an anisotropic liquid metal-filled magnetorheological elastomer (ALMMRE). Compared with isotropic liquid metal-filled composites, the conductivity of the ALMMRE is significantly enhanced in all directions. The ALMMRE also exhibits anisotropic piezoconductivity and a significantly enhanced electrical anisotropy under mechanical deformation; these properties are not observed in conventional anisotropic composites. The ALMMRE also shows anisotropic mechanical, thermal, and magnetic properties and demonstrates its several proof-of-concept applications. The sensitivity of the ALMMRE's properties to strain may help advance future flexible sensors and soft electronics.

Experimental and Numerical Investigation on Indentation of Orthotropic Microplates with Finite Thickness

Abstract This article presents comparative experimental and numerical studies to investigate the deformation and contact force of orthotropic microplates under indentation test. A simple model is developed to obtain the 3D homogenized elastic properties of orthotropic materials. In the FE simulation, the microplate is assumed to be a fully elastic orthotropic composite, and the indenter is a rigid body. Experimental indentation test is performed to obtain load-displacement curve, indentation profile, and maximum indentation depth. The numerical studies are performed to investigate the effect of indenter radius, material orthotropy, and microplate thickness on deformation of microplate under indentation load and contact pressure. The numerical model is applied to simulate two different load conditions: microplate rested on a fully supported rigid fixture, and microplate rested on a hollow support rigid fixture with circular cut-out. Numerical and experimental studies achieve excellent agreement and good correlation proving the validity of the proposed homogenization model. These parametric studies proved that the indentation behavior of orthotropic laminated structures is material independent and it depends greatly on the plate thickness.

Modeling intracranial aneurysm stability and growth: an integrative mechanobiological framework for clinical cases

We present a novel patient-specific fluid-solid-growth framework to model the mechanobiological state of clinically detected intracranial aneurysms (IAs) and their evolution. The artery and IA sac are modeled as thick-walled, non-linear elastic fiber-reinforced composites. We represent the undulation distribution of collagen fibers: the adventitia of the healthy artery is modeled as a protective sheath whereas the aneurysm sac is modeled to bear load within physiological range of pressures. Initially, we assume the detected IA is stable and then consider two flow-related mechanisms to drive enlargement: (1) low wall shear stress; (2) dysfunctional endothelium which is associated with regions of high oscillatory flow. Localized collagen degradation and remodelling gives rise to formation of secondary blebs on the aneurysm dome. Restabilization of blebs is achieved by remodelling of the homeostatic collagen fiber stretch distribution. This integrative mechanobiological modelling workflow provides a step towards a personalized risk-assessment and treatment of clinically detected IAs.

Fabrication and analysis of radiation dose for elastic lead polyester composites as a glass coating

Radiation affecting the human body can cause harm to workers from the lightest to the most fatal. As a result of these interactions, cells can undergo structural changes. Therefore a radiation shield is needed for radiation protection purposes. In this research, a polyester composite sample was made with a variation of lead doping concentration and illumination strength test using a flux meter to determine the permeability of the sample to light, as well as an X-ray radiation absorption test of the sample as desired by the lead polyester being able to be an alternative from the constituent materials of lead glass as a radiation shield that has the advantage of a high level of elasticity so as to minimize the risk of damage (break). Obtained the best mixture composition of 30% Lead Acetate and 70% Lead Polyester, with a radiation exposure test of 35 µSv the results of the radiation absorbing dose on the thickness of the 15mm lead resin layer and Transparency test with an inhibitory value of light of 394 Lux from an initial light source of 700 lux and the rest of the light that is able to penetrate a sample of 306 Lux