Micromechanics modeling of smart composites

This paper discusses a summary of analytical modeling as applied to selected smart composites which include piezoelectric composites, shape memory alloy (SMA) fiber composites, and piezoresistive composites. First we discuss the definition of `smart materials' which exhibit coupling among mechanical, thermal and electromagnetic behavior, then the Eshelby's formulations based on a simple algebraic method for predictions of two types of smart composite properties are stated; piezoelectric and SMA composites, followed by the percolation model which is applied to obtain the strain–electric conductivity relations of elastomer composites. The predictions based on these models are shown to be in good agreement with limited experimental results.     

Characterization of TEMPO-oxidized bacterial cellulose scaffolds for tissue engineering applications

Introduction of active groups on the surface of bacterial cellulose (BC) nanofibers is one of the promising routes of tailoring the performance of BC scaffolds for tissue engineering. This paper reported the introduction of aldehyde groups to BC nanofibers by 2,2,6,6-tetramethylpyperidine-1-oxy radical (TEMPO)-mediated oxidation and evaluation of the potential of the TEMPO-oxidized BC as tissue engineering scaffolds. Periodate oxidation was also conducted for comparison. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses were carried out to determine the existence of aldehyde groups on BC nanofibers and the crystallinity. In addition, properties relevant to scaffold applications such as morphology, fiber diameter, mechanical properties, and in vitro degradation were characterized. The results indicated that periodate oxidation could introduce free aldehyde to BC nanofibers and the free aldehyde groups on the TEMPO-oxidized BC tended to transfer to acetal groups. It was also found that the advantageous 3D structure of BC scaffolds remained unchanged and that no significant changes in morphology, fiber diameter, tensile structure and in vitro degradation were found after TEMPO-mediated oxidation while significant differences were observed upon periodate oxidation. The present study revealed that TEMPO-oxidation could impart BC scaffolds with new functions while did not degrade their intrinsic advantages.     

Micromechanics of hemp strands in polypropylene composites

The present paper investigates micromechanics of hemp strands. The main objective of the present work has been the determination of the intrinsic strength of hemp strands. Hemp strands have been used as reinforcement of Polypropylene composites. Different percentages of hemp strands and coupling agents (MAPP) have been tested to obtain a map of the mechanical properties of that kind of composites and the effect of the components on the final properties. Mechanical properties of the different specimens have been tested using standard experimental methods and equipment. Micromechanics of the strands have been obtained using Hirsch model, Bowyer–Bader methodology and Kelly-Tyson model.     

Micromechanics of local viscoelastic buckling in thick composites

The time-dependent, local microbuckling of multilayered viscoelastic media is modeled with emphasis on bifurcation and imperfection sensitivity. The instability or ‘growth-of-waviness’-type analysis is carried out for initially straight fibers (bifurcation), as well as for initially wavy fibers (imperfection-sensitivity), assuming a perfect-slip condition at the fiber-matrix interface. The concept of dominant wavelength (i.e. the fastest growing wavelength within the Fourier spectrum), previously defined for viscoelastic bifurcation (Biot, M. A. ‘Mechanics of Incremental Deformations’, John Wiley and Sons, Inc., New York, 1965), is extended from the single fiber analysis (Bhalerao, M. and Moon, T., On the growth-of-waviness in fiber-reinforced polymer composites: viscoelastic bifurcation and imperfection sensitivity, ASME J. Appl. Mechanics, in press, 1995) to multiple fiber analysis for a multilayered medium. A parametric study is presented which covers a wide range of physically relevant values. The results for the multiple fiber analysis are found to be analogous to those for single fiber analysis. The bifurcation dominant wavelength depends negligibly on matrix properties, yet highly on the applied load. On the other hand, the imperfection-sensitive dominant wavelength is predicted to depend strongly on matrix properties, while negligibly on load. The imperfection-sensitive wavelength is the one important in composites processing and in-service behavior.     

Characterization of conductive composite films based on TEMPO-oxidized cellulose nanofibers and polypyrrole

Abstract  

In this article, conductive composite films based on TEMPO-oxidized cellulose nanofibers (TOCN) and polypyrrole (PPy) were synthesized in situ by a Chemical Polymerization Induced Adsorption Process of pyrrole on the surface of TOCN in aqueous medium. Resulting composite films were investigated by X-ray photoelectron spectroscopy, scanning, and transmission electron microscopy, N₂ gas adsorption analysis, thermogravimetric analysis, mechanical tests, and conductivity measurements in the ambient air. Our results showed a stable, flexible, and highly electrically conductive composite film in which PPy nanoparticles coated the surface of the TOCN network. In addition, the advantage in using the famous material, TOCN, is clearly due to the presence of carboxylate (COOH/COO⁻Na⁺) and hydroxyl (OH) moieties on the surface of TOCN. These reactive moieties could enhance the adsorption process of positively charged PPy backbone during polymerization. TEM observations demonstrated the formation of a PPy coat along the surface of the cellulose nanofibers having a diameter of about 90 nm which is relatively higher compared to the initial diameter of pure TOCN (~9 nm). Despite the physical and chemical treatment of TOCN during polymerization, the micrometric length of the cellulosic nanomaterial was maintained. In addition, the incorporation of polyvinyl alcohol as an additive in the TOCN/PPy composite seems to enhance the flexibility of composite films (bent up to 180°) without losing the high electrical conductivity. Finally, because of the high conductivity and good mechanical properties of the TOCN/PPy composite films obtained in this work, they can be used as a promising material in applications of sensors, flexible electrodes, and other fields requiring electrically conductive flexible films.     

Toughening mechanisms in poly(lactic) acid reinforced with TEMPO-oxidized cellulose

The mechanical properties of poly(lactic) acid (PLA) were modified by the addition of small amounts of cellulose, prepared from the mechanical disintegration of birch Kraft pulp following oxidation of the primary alcohol groups mediated by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). The TEMPO-fibrillated cellulose (TOFC) was subsequently acetylated in acetic anhydride to degrees of substitution (DS) of 0.4 and 0.6 to enhance the compatibility between the polar cellulose and the non-polar polymer. The fracture behaviour of tensile specimens prepared from PLA film containing weight fractions of 1, 2 and 5 % of TOFC was considerably altered. The strain-to-failure of PLA modified by the incorporation of 1 wt% TOFC acetylated to a DS of 0.6 increased approximately 25-fold and the work of fracture by order of magnitude. The increase in the fracture properties were, nevertheless, accompanied by a reduction in Young’s modulus of around 60 % at both DS levels. At the higher TOFC addition levels, no toughening was observed, with the strains-to-failure and works of fracture both decreasing compared to pure PLA film. On the other hand, the Young’s modulus and tensile strength of films prepared from PLA incorporating TOFC esterified to a DS of 0.6 was found to be greater than that of pure PLA film. Possible mechanisms explaining the increase in toughness at 1 wt% are postulated.     

Micromechanics analysis of particulate-reinforced composites and their failure mechanisms

Stress fields and failure mechanisms have been investigated in composites with particles either surface treated or untreated under uniaxial tension. Previous experimental observation of failure mechanisms in a composite with untreated particles showed that tensile cracks occurred mostly at the polar region of the particle and grew into interfacial debonding. In a composite with surface-treated particles, however, shear yielding and shear cracking proceeded along the interphase-matrix interface at the polar area of the matrix and thus may improve the mechanical behaviour of the material. The finite element calculations showed that octahedral shear stress at the polar and longitudinal areas of the particle treated by coupling agents is much larger than that of materials with untreated particles, and the shear stress distribution around the interface is sensitive to the interphase property. The results suggest that a three-phase model can describe the composites with surface-treated fillers.     

Micromechanics of piezoelectric composites with improved effective piezoelectric constant

This paper is concerned with the derivation of a micromechanics model of a new type of piezoelectric fiber reinforced composite (PFRC) materials. A continuum mechanics approach is employed to determine the effective properties of these composites. The piezoelectric fibers of these composites are considered to be electroded at the fiber–matrix interface such that the electric fields in the fiber and matrix become equal in the direction transverse to the fiber direction. The model has been verified with the existing models. The present model also predicts that the effective piezoelectric coefficient of these PFRC which accounts for the actuating capability in the fiber direction due to the applied field in the direction transverse to the fiber direction improves over the corresponding coefficient of the material of the piezoelectric fibers if the fiber volume fraction exceeds a critical fiber volume fraction.     

Micromechanics of interfacial thermal stresses in fiber reinforced composites

This paper develops a generic 2D micromechanical model that deal with the interfacial thermal stresses in fiber reinforced composites with randomly packed fibers of different sizes and thermoelastic properties by successive superposition of the solution of a single fiber embedded in a matrix. This approach reduces the complicated micromechanical model to a system of linear algebraic equations using the perturbation technique. The accuracy of current approach is verified by comparing it with the existing analytical solution. The micromechanics model is then applied to analyze periodically and randomly packing fiber reinforced composites. Analysis results show that the effect of a single fiber is localized and there is an optimal elastic mismatch of fiber and matrix where the interfacial hoop stress is insensitive to the fiber packing arrangement and the fiber volume fraction.     

Micromechanics of stress transfer through the interphase in fiber-reinforced composites

A micromechanical model to predict the interphasial/interfacial stress transfer in a three-phase fiber-reinforced composite is presented. The axisymmetric system consists of a fiber embedded in a compliant matrix having an interphase between them. Each constituent of the composite is regarded as a linear elastic continuum. The matrix is treated as an isotropic material while the fiber and interphase are considered as a transversely isotropic material. Traction-free boundary conditions are strictly enforced. It is assumed that the interfaces are perfect and strong. A pair of uncoupled governing partial differential equations is obtained in terms of unknown displacements. Furthermore, assuming that the Eigenvalues exist for this system of equations, Eigenfunction expansion method is employed to derive an exact solution in terms of the Bessel functions. Analytical solutions are obtained for free boundary conditions at the external surface of the matrix cylinder to model a single fiber pull-out problem, and for fixed boundary conditions to approximately model a hexagonal array of fibers in the matrix material. This formulation provides an analytical framework for the analysis of interphasial and interfacial stresses as well as displacements in the entire 3D axisymmetric system. Finite element (FE) analysis was also performed to simulate stress transfer from the fiber to the matrix through the interphase. Analytically obtained stress fields are verified with FE results. Shear and radial interphasial stresses provide insight into the design of engineered interfaces/interphases.     

Micromechanics and damping properties of composites integrating shear thickening fluids

Damping is an important parameter for vibration control, noise reduction, fatigue endurance or impact resistance of composite materials. In this study, a micromechanical model was used to predict the damping of a composite material containing shear thickening fluids (STFs) at the fibre–matrix interfaces. Predictions of the model and dynamical mechanical analysis results are in concert. The damping of the composites was improved significantly. The dynamic properties exhibited a strong dependence on both frequency and applied external load amplitude. Damping peaks appeared which coincided with the thickening of the STF at the fibre–matrix interface. The location of the peaks depends on the onset of thickening and post-thickening rheological behaviour of the STF. This work shows that a micromechanics approach can be useful for an appropriate choice of microstructural design and properties of STFs in order to control the stiffness and damping behaviour of composites. STFs can be integrated at the microscale of polymer composites to create new materials with load-controlled adaptive dynamic stiffness-damping properties.     

炭黑增强橡胶复合材料的大变形细观力学模型

为考察炭黑对橡胶复合材料超弹性力学行为的影响,首先,利用不同填充体积分数的炭黑增强橡胶复合材料的准静态力学试验数据,对现有的基于均质化方法的"变形放大"细观力学模型的大变形表征能力进行了评估。其次,在此基础上提出了新的"第一不变量放大"关系,并获得了较为合理的预测结果。最后,利用随机序列吸附算法建立了较接近材料真实细观结构的球形颗粒填充数值模型,进行了大变形情况下的三维数值模拟;为考察颗粒聚集效应的影响,还设置了颗粒均匀随机分布和团聚随机分布两种形式。计算结果与试验数据的对照表明:提出的三维细观数值模型已经能在一定程度上预测填充橡胶的大变形宏观力学行为,且颗粒团聚随机分布模型的预测能力更好一些。试验结果验证了该模型的合理性,所建模型为进一步的相关研究提供了参考。     

Micromechanics approach to the indentation hardness of glass matrix particulate composites

A theoretical analysis is made of the indentation hardness of glass matrix, particulate composites. It is hypothesized that glass is an elastic-plastic solid on a microscopic scale. Based upon the Marsh theory of indentation, expressions are formulated for indentation hardness of two-phase composites containing spherical particles. When hard particles are dispersed in a soft glass matrix, the overall hardness depends upon the matrix hardness, the volume-fraction of dispersed phase, the elastic properties of the two phases and also the matrix flow stress. On the other hand, when soft particles are dispersed in a hard glass matrix, the hardness and the elastic moduli vary in parallel with the volume-fraction of dispersed phase. Furthermore, the present analysis indicates that the hardness of a composite is independent of the particle size and interparticle spacing if the volume-fraction of the particles is kept constant. Experimental results of the Vickers hardness of phase-separated glasses as well as published hardness data for a glass-ceramic are used for the verification of the theory. The proposed theory explains well the hardness behaviour of such glass matrix composites in terms of the properties and amounts of the individual phases and the microstructural effects.     

Micromechanics models for the effective nonlinear electro-mechanical responses of piezoelectric composites

The nonlinear behavior of piezoelectric composites becomes prominent when the composites are subjected to high electric fields, which is often the case in actuator applications. Understanding the nonlinear behavior of piezoelectric composites is crucial in designing structures comprising of these materials. This study presents micromechanics models for predicting nonlinear electro-mechanical responses of polarized piezoelectric composites, comprising of a linear non-piezoelectric homogeneous medium (matrix) reinforced by either nonlinear piezoelectric fibers or particles, subjected to high electric fields. The maximum electric field applied is within the coercive electric field limit. The constitutive relations for the polarized piezoelectric inclusions consist of the third- and fourth-order electro-mechanical coupling tensors and the second- and third-order electric permeability tensors. The Mori–Tanaka micromechanics and simplified unit-cell micromechanics models are formulated to predict the effective nonlinear electro-mechanical responses of piezoelectric fiber reinforced and particle reinforced composites, respectively. Linearized micromechanical relations are first used to provide trial solutions followed by iterative schemes in order to correct errors from linearizing the nonlinear responses. Numerical results are presented to illustrate the performance of each micromechanics model.     

Micromechanics based ply level material degradation model for unidirectional composites

The damaged response of a composite lamina depends on various mechanisms that take place at the microlevel, i.e., at the level of the fiber and matrix. The present work focuses on developing a ply level continuum damage model for point-wise stiffness degradation through simplified representation of the microlevel damage. A three dimensional micromechanical analysis of a single cell representative volume element is carried out for various volume fraction, and levels of damage. The model brings out the coupled effect of damage on the effective point-wise ply level stiffness. Further, the numerical results are employed to develop a functional continuum representation of stiffness degradation as a function of the damage parameters and fiber volume fraction perturbations. The micromechanics model is consistent with experimentally observed stiffness degradation, i.e., a strong influence of fiber breakage and fiber matrix debond, and a weak influence of normal cracking of matrix. The proposed model can be considered as an improved version of the widely accepted diffused (meso) damage models, i.e., DML. The study also gives a generalized and consistent definition for the free energy, which can be used for modeling growth of damage.     

Micromechanics and acoustic emission analysis of the failure process of thermoplastic composites

The finite element method (FEM) and acoustic emission technique (AE) were applied to the micromechanics analysis of the failure process of composites with thermoplastic matrix materials. FEM calculations to local stress-strain distribution and the influence of very different intermediate layer properties are interpreted with regard to microscopic failure mechanisms in composite materials. The strongly differing AE behaviour of both chalk-filled Polyvinylchloride and high density polyethylene and short-glass-fibre reinforced polypropylene, polyamide, PBTP, SAN and ABS in tensile test experiments is demonstrated. Representative loading limits are derived from the nature and extent of the dominating failure mechanisms by comparison of theoretical and experimental results. The influence of critical strain, shear strength and fracture toughness properties of the modified matrix as well as the composite morphology and phase adhesion on significant deformation and failure stages is discussed. Finally some conclusions are drawn about a possible critical long-term strain of composites.     

Diverse nanocelluloses prepared from TEMPO-oxidized wood cellulose fibers: Nanonetworks,nanofibers, and nanocrystals

When wood cellulose fibers are oxidized with NaClO and catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and NaBr in water at pH 10, significant amounts of sodium carboxylate groups (≤1.7 mmol/g) are introduced into the oxidized celluloses. The original fibrous morphologies and cellulose I crystal structures are unchanged by oxidation. The TEMPO-oxidized cellulose fibers can be converted to partially fibrillated nanonetworks, completely individualized cellulose nanofibers with high aspect ratios, and needle-like cellulose nanocrystals with low aspect ratios by controlling the conditions of mechanical disintegration in water. It is therefore possible to prepare diverse nanocelluloses with different morphologies and properties from the same TEMPO-oxidized cellulose fibers, for various end uses and applications. All TEMPO-oxidized nanocelluloses contain large amounts of carboxylate groups. These provide scaffolds for versatile surface modification of nanocelluloses by simple ion exchange of sodium for other metal ions and alkylammonium ions.     

Micromechanics and effective moduli of elastic composites containing randomly dispersed ellipsoidal inhomogeneities

Summary A micromechanical framework is proposed to investigate effective mechanical properties of elastic multiphase composites containing many randomly dispersed ellipsoidal inhomogeneities. Within the context of the representative volume element (RVE), four governing micromechanical ensemble-volume averaged field equations are presented to relate ensemble-volume averaged stresses, strains, volume fractions, eigenstrains, particle shapes and orientations, and elastic properties of constituent phases of a linear elastic particulate composite. A renormalization procedure is employed to render absolutely convergent integrals. Therefore, the micromechanical equations and effective elastic properties of a statistically homogeneous composite are independent of the shape of the RVE. Various micromechanical models can be developed based on the proposed ensemble-volume averaged constitutive equations. As a special class of models, inter-particle interactions are completely ignored. It is shown that the classical Hashin-Shtrikman bounds, Walpole's bounds, and Willi's bounds for isotropic or anisotropic elastic multiphase composites are related to the noninteracting solutions. Further, it is demonstrated that the Mori-Tanaka methodcoincides with the Hashin-Shtrikman bounds and the noninteracting micromechanical model in some cases. Specialization to unidirectionally aligned penny-shaped microcracks is also presented. An accurate, higher order (in particle concentration), probabilistic pairwise particle interaction formulation coupled with the proposed ensemble-volume averaged equations will be presented in a companion paper.     

Study of the hydrophobization of TEMPO-oxidized cellulose gel through two routes: amidation and esterification process

In this paper, we studied the hydrophobization of TEMPO-oxidized cellulose gel (TOCgel) by covalent coupling of long carbon chains via esterification and amidation processes. In this context, amidation process was achieved by covalent coupling of stearylamine (SA) on the carboxyl moieties of TOCgel using carbodiimide and hydroxysuccimide as catalyst and amidation agent. In parallel, esterification process was realized by grafting of alkyl ketene dimer (AKD) on the hydroxyl groups of TOCgel in the presence of 1-methylimidazole as a promoter. The grafting state of the final products obtained under heterogeneous conditions was confirmed by fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), transmission and scanning electron microscopy, and contact angle measurement (CAM). The hydrophobic behavior of the obtained products was discussed based on the results of CAM and absorption rate of water drop in their film surface. FTIR and XPS results indicated the formation of amide bonding for the SA-g-TOCgel (amidation), and β-keto ester linkages for the AKD-g-TOCgel (esterification). As confirmed by CAM, the both chemical treatments enhanced the transition hydrophilic/hydrophobic behavior of the TOCgel fibers. It appeared also that CA values of grafted samples showed a slightly greater hydrophobicity of AKD-g-TOCgel (115° ± 2°) relatively to SA-g-TOCgel (102° ± 2°). However, the absorption rate of water drop seems to be relatively faster for AKD-g-TOCgel than for SA-g-TOCgel. Indeed, the water resistance of amidation product could be due to the high graft efficiency obtained (46.3 %) in comparison with that of the esterification product (30 %). In parallel, this result was confirmed by the dispersion test of modified TOCgels in hexane solvent which indicated clearly the high stable dispersion of SA-g-TOCgel obtained through the amidation process. Moreover, TGA result demonstrated that the thermal stability was found to be slightly higher for SA-g-TOCgel than for AKD-g-TOCgel. Finally, the excellent hydrophobic properties of modified TOCgel material could be suitable to be used as reinforcement for nonpolar polymer matrices in industrial applications.