Modeling of Moisture Diffusion in Permeable Fiber-Reinforced Polymer Composites Using Heterogeneous Hybrid Moisture Element Method

This study proposes a two-dimensional heterogeneous hybrid moisture element method (HHMEM) for modeling transient moisture diffusion in permeable fiber-reinforced polymer composites.
The HHMEM scheme is based on a heterogeneous hybrid moisture element(HHME), with properties determined through an equivalent hybrid moisture capacitance/conductance matrix. This matrix was calculated using the conventional finite element formulation in space discretization as well as the θ-method in time discretization with similar mass/stiffness properties and matrix condensing operations. A coupled HHME-FE scheme was developed and implemented in computer code MATLAB in order to analyze the transient moisture diffusion characteristics of composite materials containing multiple permeable fibers. The analysis commenced by comparing the performance of the proposed scheme with that of conventional FEM to model the moisture diffusion process. Both hexagonal and square fiber arrangements were studied. Having validated its performance, the scheme was then employed to investigate the relationship between the volume fraction of the permeable fibers in the resin composite and the rate of moisture diffusion. It was found that the moisture diffusion was significantly retarded as the volume fraction of the fibers increased. The HHMEM approach proposed in this study provides a straightforward and efficient means of modeling transient moisture diffusion in composite materials containing multiple permeable fibers. This is because only one HHME moisture characteristic matrix of fibers requires calculation for all HHMEs sharing the same characteristics. Furthermore, varying volume fractions can be modeled without modifying the original model simply by controlling the size of the inter-phase region within the HHME domain.     

考虑纤维/基体界面相的复合材料湿扩散模型

通过类比复合材料湿扩散与热传导的控制方程以及边界条件,以Halpin & Tsai模型为基础,发展了一个考虑了纤维/基体界面相的三相复合材料湿扩散模型,并研究了纤维界面随机损伤对湿扩散的影响。建立了纤维周期排布、随机排布、界面相损伤随机分布3种细观有限元模型。用上述模型分析了单向复合材料横向有效湿扩散系数（TEMDUC）随纤维和界面相体积分数、湿扩散性能以及界面相损伤率变化的规律,理论预测与有限元计算结果一致。研究发现：界面相或纤维相的扩散系数存在一个临界值,当扩散系数小于该临界值时,TEMDUC随纤维体积分数的增大而减小;反之,TEMDUC随纤维体积分数的增大而增大,此临界值的大小与纤维体积含量无关。研究还发现纤维界面损伤率相同的条件下,其分布的随机性对复合材料的有效湿扩散系数影响不大。     

Moisture absorption by epoxy/montmorillonite nanocomposite

The peculiarities of moisture absorption of epoxy–nanoclay composite are estimated in the paper. Second Fick’s law of diffusion was used to predict moisture diffusivity and equilibrium moisture content using accelerated analytical procedure. It was experimentally confirmed that sorption process in NC passes more slowly than in pure epoxy resin, for the highest filler content diffusivity reduces about half of diffusivity as for epoxy resin. The deviation from mixture rule was obtained for the equilibrium moisture content and the estimation of interphase content in composite was undertaken. It was determined that the higher content of interphase consistently leads to greater moisture absorption.     

纤维聚合物复合材料的有效湿扩散系数

根据纤维聚合物复合材料的微观结构, 建立了基于复合材料单胞模型的湿扩散计算方法, 研究了不同温度和不同体积分数下纤维聚合物复合材料的湿扩散性能。假设纤维是不可渗透的, 并在聚合物基体中均匀分布, 计算了不同温度不同体积分数下复合材料的有效湿扩散系数。结果表明: 复合材料的有效湿扩散系数随温度的升高而增大, 随纤维体积分数的增大而减小; 在相同温度、 相同体积分数下, 正六边形排列的纤维复合材料的湿扩散系数比正方形的略大。计算结果及经验公式与Gueribiz曲线基本一致, 说明用单胞模型计算复合材料的湿扩散性能是非常有效的, 有助于理解纤维复合材料的湿扩散机制和性能。      

羰基铁/有机硅环氧树脂复合材料的电磁性质及微波吸收性质

为了解决无线电通信技术高速发展带来的日益严重的电磁污染和干扰,电磁波吸收材料的研究变得极为重要。制备了羰基铁粉分散在不同基质(石蜡,环氧树脂和有机硅环氧树脂)的复合材料的样品。在3种不同基质样品(羰基铁粉体积分数40%)中,以有机硅环氧树脂为基质样品的反射损耗最大,相对带宽最大,所以把有机硅环氧树脂选为最优基质,并且制备了不同体积分数的羰基铁/有机硅环氧树脂复合材料,研究了它们的电磁性质和反射损耗性质。其中体积分数为50%的样品通过调节样品厚度,反射损耗可在稍低频范围(1～3GHz)达到-10dB(90%的吸收率)。     

Thermo-mechanical response of heterogeneous materials with randomly distributed transforming particles

A hybrid finite element method is developed for thermo-mechanical response analysis of heterogeneous material where inclusions with stress-induced transforming property and (or) distinct thermal expansion coefficients are randomly distributed. The residual stresses due to thermal expansion coefficient mismatch between matrix and inclusions are calculated, and the accuracy and efficiency of the proposed method is verified by comparing the obtained results with those by the theoretical elastic solution and conventional finite element method. The developed method is applied to discrete modeling of transformation toughening. It is shown that the predictions based on the discrete modeling match those by a continuum type approach if particles are assumed to transform uniformly within the transformation zone. When particles are assumed to transform linearly within the transformation zone, the discrete modeling predicts results closer to experimental results.     

Moisture absorption by cyanate ester modified epoxy resin matrices. Part III. Effect of blend composition

The moisture absorption of thermally cured cyanate ester modified epoxy resin matrices has been studied under constant hygrothermal conditions and with a series of thermal spikes. Various resin blends of differing composition were studied, so that the effect of resin structure on moisture absorption could be assessed. It was found that intermittently applied thermal spikes can enhance the moisture absorption by cyanate esters and their blends with epoxy resin. The equilibrium moisture concentration was found to increase with the fraction of cyanate ester in the blend. The results of the desorption study on both control and spiked specimens showed that some of the water molecules remain entrained in the resin, probably as a result of hydrolysis of the polymer matrix. Up to a thermal spiking temperature of 160 °C, the crosslink density remained constant but at higher temperatures tended to decrease, as observed by examination of the modulus above the glass transition temperature.     

A hybrid numerical and imaging approach for characterizing defects in composite structures

In this study, a hybrid approach coupling hyperspectral near infrared imaging with a progressive finite element method is proposed for characterization of the elastic and failure response of composites with non-uniform variations of the wrinkles profile through the thickness and across the structure dimensions. In this approach, hyperspectral near infrared spectroscopy is used to create a 3D profile of the surface resin pockets with the capability of measuring resin thickness from approximately 125 to 2500 μm. These resin pockets are directly correlated to underlying ply level wrinkling as confirmed by optical microscopy. The 3D mapped resin plane obtained from the hyperspectral imaging is used to morph a ply-by-ply finite element model of a carbon-fiber/epoxy resin laminated plate using a progressive damage failure methodology. The results show the capability of the hybrid method to predict the structural response in laminated composites containing spatially distributed and non-uniform ply-level wrinkling.     

A novel combined molecular dynamics–micromechanics method for modeling of stiffness of graphene/epoxy nanocomposites with randomly distributed graphene

In this paper, by combining molecular dynamics and micromechanics methods, a new approach for prediction of the stiffness of the nanocomposites with randomly distributed nanoparticles in the macro level is presented. The molecular dynamics method is used to model the stiffness of the graphene/epoxy nanocomposites containing one layer of an aligned nano graphene embedded in epoxy resin. By considering the large sizes of the length and width of the nano graphene in comparison with its thickness and the shortcomings of the available hardware and software for simulation purposes, a new approach for modeling is also developed. This new approach, by using the moduli of different graphene sheets with different sizes embedded in a representative volume element, can predict the moduli of a real size graphene embedded in the matrix along the longitudinal, transverse and normal directions in the nano-scale. In order to consider the effect of the random distribution of graphene sheets in epoxy resin, a micromechanical approach is used. The results obtained by the molecular dynamics method are used by the micromechanics approach and the stiffness of graphene/epoxy nanocomposites with randomly distributed graphene in the macro-scale is predicted. An experimental program is conducted to evaluate the capability of the model. The result of the modeling is in a very good agreement with the experimental data.     

Diffusion of moisture into two-phase polymers

The moisture absorption and desorption behaviour of a styrenated isophthalic unsaturated polyester resin has been examined. This resin system is believed to have a two-phase structure comprising particles of the dense phase embedded in a less dense matrix. The model developed in Part 1, which describes the diffusion behaviour of a two-phase resin system, has been applied to this system and the diffusion coefficients for both the dense and less dense phases calculated. The diffusion coefficient of the dense phase is found to be 30 times lower than that of the less dense matrix. The volume fraction of the former was estimated at 0.16, which was lower than expected. It is postulated that the microgel particles have a dense core and become progressively less dense towards the periphery.     

Moisture absorption of unidirectional hybrid composites

Unidirectional hybrid composite rods were conditioned in humid air to investigate the sorption kinetics and the effects of moisture on mechanical and physical properties. Sorption curves were obtained for both hybrid and non-hybrid composite rods to determine characteristic parameters, including the diffusion coefficient (D) and the maximum moisture uptake (M_∞). The moisture uptake for the hybrid composites generally exhibited Fickian behavior (no hybridization effects), behaving much like non-hybrid composites. A two-dimensional diffusion model was employed to calculate moisture diffusivities in the longitudinal direction. Interfaces and thermally-induced residual stresses affected the moisture diffusion. In addition, the effect of hygrothermal aging on glass transition temperature (T_g), short beam shear strength (SBS), and tensile strength was determined for hygrothermal exposure at 60 °C and 85% relative humidity (RH). Property retention and reversibility of property degradation were also measured. Microscopic inspection revealed no evidence of damage.     

三维编织碳纤维/环氧复合材料的吸湿特性及外应力的影响

采用真空浸渍法制备了三维编织碳纤维增强环氧树脂(C_3D/ER)复合材料,研究了该复合材料的吸湿特性及外应力的影响。讨论了应力对C_3D/ER复合材料吸湿行为的作用机理。 结果表明,与环氧树脂不同,C_3D/ER复合材料的吸湿行为不能用Fick第二定律描述。外应力可加速吸湿初期复合材料的吸水和力学性能的下降,但可降低复合材料平衡吸湿量和最终力学性能的下降幅度。分析表明,吸湿过程中C_3D/ER复合材料力学性能的高低取决于其吸湿量的大小。     

吸湿后复合材料层合板快速加热分层扩展数值模拟

为研究复合材料层合板吸湿后的分层现象,首先建立了吸湿后复合材料层合板快速加热导致分层损伤的有限元模型,并对ABAQUS有限元软件进行二次开发,通过UAMP子程序模拟吸湿后复合材料快速加热时水分汽化引起的局部高压载荷作用下层合板分层扩展与载荷施加过程;然后,采用该模型预测了饱和吸湿T650-35/HFPE-II-52碳纤维聚酰亚胺复合材料层合板快速加热至310 ℃时产生的分层现象,并将数值模拟与文献实验结果对比;最后,运用该模型分析了树脂吸湿量和富脂区树脂聚集体积对层合板分层损伤面积的影响。结果表明:建立的有限元模型有效;快速加热后,层合板的分层损伤面积随树脂吸湿量的增加而增加;当富脂区树脂聚集体积较小时,其对层合板快速加热后分层损伤面积影响较小,但当富脂区树脂聚集体积增加到一定值后,层合板分层损伤面积随富脂区树脂聚集体积的增加而显著增加。所得结论表明,使用ABAQUS的UAMP子程序建立的有限元模型可以有效分析吸湿后复合材料层合板快速加热导致的分层现象。      

粒子空间分布与复合材料导热性能关系的模拟研究

深入研究了粒子空间分布对材料导热性能的影响, 探索了有效导热通路形成的必要条件。为了解决任意体积分数、指定空间构型的代表体积元(RVE)建模难题, 用空间分布势能函数来描述目标空间分布构型, 设计了Monte Carlo可控空间分布算法, 该算法能够有效生成包含团簇和网链结构的任意空间构型的RVE。模拟研究结果表明: 相同体积分数下, 网链构型较团簇构型更能有效地形成导热通路, 具有更高的热导率; 体积分数对有效导热通路能否形成有重要影响, 仅当体积分数大于20%之后, 才具备形成有效导热通路的条件; 粒子间距只有小于一定水平时, 导热通路才能有效形成, 随着粒子间距的增加, 热导率成指数衰减。一定量的体积分数和较有效的粒子分布是形成有效导热通路的两个必要条件, 二者缺一不可。      

Numerical simulation of the guided Lamb wave propagation in particle reinforced composites

This paper deals with the investigation of the Lamb wave propagation in particle reinforced composites excited by piezoelectric patch actuators. A three-dimensional finite element method (FEM) modeling approach is set up to perform parameter studies in order to better understand how the Lamb wave propagation in particle reinforced composite plates is affected by change of central frequency of excitation signal, volume fraction of particles, size of particles and stiffness to density ratio of particles. Furthermore, the influence of different arrangements is investigated. Finally, the results of simplified models using material data obtained from numerical homogenization are compared to the results of models with heterogeneous build-up. The results show that the Lamb wave propagation properties are mainly affected by the volume fraction and ratio of stiffness to density of particles, whereas the particle size does not affect the Lamb wave propagation in the considered range. As the contribution of the stiffer material increases, the group velocity and the wave length also increase while the energy transmission reduces. Simplified models based on homogenization technique enabled a tremendous drop in computational costs and show reasonable agreement in terms of group velocity and wave length.     

烟盒多层包装对水分阻隔影响研究

目的 卷烟包装的阻隔性能是卷烟产品品质稳定性的关键影响因素,研究干燥环境中卷烟多层包装对水分阻隔的作用。方法 利用自主研发测试系统——动态水分分析气候箱,考察不同盒包在特定温湿度条件下水分扩散过程,计算水分扩散通量,并对包装阻湿性能进行量化表征。结果 建立的烟盒多层包装水分阻隔性能评价方法可准确量化表征包装的阻湿性能。低湿条件下,水分从烟盒包装各层空隙及材料的透过量各有差异,对于Bopp膜,60%以上的水分主要从材料扩散,对于不同商标纸和内衬纸,水分从材料及空隙扩散的分配比不同。对于整体包装水分阻隔性能,三层材料中Bopp膜对烟支水分稳定性作用的占比达90%以上,硬包优于软包。准确预测了烟支在不同盒包中水分的散失速率,低湿条件下,硬包卷烟的水分散失速率明显低于软盒卷烟的。结语 文中建立了多层烟盒包装阻湿性测试及评价方法,量化表征了卷烟盒包各包装层及不同部位对水分的阻隔作用,明晰了烟盒包装中影响卷烟水分稳定性的重要因素。     

Moisture absorption in unfilled and glass-filled, cross-linked polyester

The moisture uptake characteristics of unfilled and filled lyester resin at 60°C are reported. The objective was to determine the influence of filler on the moisture uptake properties of unfilled polymer. Diffusivity in the filled polyester is lower than in the plain resin. However, the filled resin absorbs significantly more water than unfilled polyester. Tensile load was found to have no effect on the water uptake in plain polyester. However, moisture absorption was affected by tensile load in 20% (by volume) bead-filled polyester and in SMC-R65. Increasing load increased the diffusion coefficient in SMC-R65. Combining rules predicted the diffusion coefficient in unloaded SMC-R65, but could not account for changes in diffusivity caused by tensile loading. In bead-filled polyester, the diffusion coefficient was accurately predicted when microstructural effects were accounted for.     

Finite element modeling of the filament winding process

A finite element model of the wet filament winding process was developed. In particular, a general purpose software for finite element analysis was used to calculate the fiber volume fraction under different process conditions. Several unique user defined subroutines were developed to modify the commercial code for this specific application, and the numerical result was compared with experimental data for validation. In order to predict the radial distribution of the fiber volume fraction within a wet wound cylinder, three unique user defined subroutines were incorporated into the commercial finite element code: a fiber consolidation/compaction model, a thermochemical model of the resin and a resin mixing model. The fiber consolidation model describes the influence of the external radial compaction pressure of a new layer as it is wound onto the surface of existing layers. The thermochemical model includes both the cure kinetics and viscosity of the resin. This model analyzes the composite properties and tracks the viscosity of the resin, which is a function of the degree of cure of the resin. The resin mixing model describes the mixing of “old” and “new” resin as plies are compacted. Validations were made by comparing image analysis data of fiber volume fraction in each ply for filament wound cylinders with the FEM results. The good agreement of these comparisons demonstrated that the FEM approach has can predict fiber volume fraction over a range of winding conditions. This approach, then, is an invaluable tool for predicting the effects of winding parameters on cylinder structural quality.     

Meso-Scale Modeling to Characterize Moisture Absorption of 3D Woven Composite

For polymer-matrix composites, moisture is expected to degrade their mechanical properties due to matrix plasticization and moisture introduced micro-scale defects. In this study, the moisture absorptions of bulk epoxy, unidirectional composite (UD) and 3D woven composite (3D WC) were tested. Two-stage features have been observed for all these three materials. Moisture properties for UD and 3D WC were found not in simple direct proportion to their matrix volume fractions. The moisture approach of UD was modeled including the effect of fiber/matrix interphase which promotes the moisture uptake. Then, meso-scale FE model for 3D WC was established to characterize the inhomogeneous moisture diffusion. The moisture properties of resin-rich region and fiber bundle in 3D WC were determined from water uptake experiments of bulk epoxy and UD, respectively. Through homogenizing moisture properties of surface and interior weave structures, a simplified theoretical sandwich moisture diffusion approach was established. The moisture weight gains of 3D WC predicted by both meso-scale FE model and simplified sandwich approach were well agreed with the experimental data.     

Heterogeneous and homogenized models for predicting the indentation response of particle reinforced metal matrix composites

In this study, three-dimensional heterogeneous and homogenized finite element models are used to predict the indentation response of particle reinforced metal matrix composites (PRMMCs). The matrix is assumed to have elasto-plastic behavior whereas the particles (uniform in size and spherical in shape) are assumed to be harder than the matrix, and possess linear elastic behavior. The particles (25 % by volume) are randomly distributed in the metal matrix. Two modeling approaches are used. In the first approach, the PRMMC is fully replaced by an equivalent homogenous material, and its material properties are obtained through homogenization using representative volume element approach under periodic boundary conditions. In second approach, a small cubical volume under the indenter is modeled as heterogeneous material with randomly distributed particles, whereas the remaining domain is assigned equivalent material properties obtained through homogenization. The elastic material properties obtained through simulations are found within Hashin–Shtrikman bounds. A suitable size cubical volume consisting of heterogeneities under the indenter is established by considering different cubical volumes so as to capture the actual indentation response. The simulations are also carried out for different particle sizes to establish a suitable particle size. These simulations show that the second modeling approach yields harder indentation response as compared to first modeling approach due to the local particle concentration under the indenter.