Mechanical properties of polypropylene matrix composites reinforced with natural fibers: A statistical approach

This work presents a systematic and statistical approach to evaluate and predict the properties of random discontinuous natural fiber reinforced composites. Different composites based on polypropylene and reinforced with natural fibers were produced and their mechanical properties are measured together with the distribution of the fiber size and the fiber diameter. The values obtained were related to the theoretical predictions, using a combination of the Griffith theory for the effective properties of the natural fibers and the Halpin-Tsai equation for the elastic modulus of the composites. The relationships between experimental results and theoretical predictions were statistically analyzed using a probability density function estimation approach based on neural networks. The results show a more accurate expected value with respect to the traditional statistical function estimation approach. In order to point out the particular features of natural fibers, the same proposed method is also applied to PP–glass fiber composites.     

Bridging stresses and R-curves in ceramic/metal composites

Incorporation of metal into brittle ceramics results in an increase in fracture toughness, which can lead to an increase in strength, reliability and thermal shock resistance of the composite compared to monolithic ceramics. The basic material specific property, which controls the enhancement of the mechanical properties, is the bridging stress relation of the metal reinforcements. This relation was calculated from measured profiles of loaded cracks (COD) for fiber reinforced model composites and interpenetrating network composites in the system Al₂O₃/Al. Results are compared with directly measured bridging stress relations for the model materials. The bridging relations are further used to model the R-curve behavior of the composites which are compared with experimentally measured ones. Limitations of the applied procedure are discussed as well as the influence of specimen geometry and flaw size.     

On the Effect of Fiber Shape and Packing Array on Elastic Properties of Fiber-Polymer-Matrix Composites

Abstract

In this study, three-dimensional finite element simulations on the base of the cell model and micromechanics are made to predict effective elastic properties of fibrous composites. The effects of fiber shape, packing array and volume fraction on the overall elastic behavior of an epoxy resin containing unidirectional glass fibers are examined. The geometrical structure includes three types of periodic fiber arrangements in cubic, hexagonal and rectangular cells. The fibers are assumed to be of four shapes; square, circular, elliptic and rectangular. The numerical results indicate that the overall transverse elastic properties are rather sensitive to both fiber shape and packing array while fiber geometry has no effect on the apparent overall Young's modulus in the longitudinal direction of the fibrous composite.     

Effects of fly ash in composites fabricated by injection molding

In this study, the effects of fly ash in composites fabricated by injection molding are examined. Taguchi design of experiment was first utilized to estimate the effects different injection molding conditions and content ratios of fly ash have on a linear low‐density polyethylene (LLDPE)‐fly ash composite. The results reveal that the content of fly ash is highly significant and contributive to the shrinkage ratio and bending strength. For these reasons, LLDPE and polypropylene (PP) composites with different size particles of fly ash were fabricated and the mechanical properties were investigated. The particle size was changed by grinding fly ash with a planetarium ball mill. The shrinkage ratio, bending strength and flexural modulus of LLDPE composites containing raw fly ash were found to improve. The shrinkage ratio and flexural modulus of PP composites containing ground fly ash were also found to improve. Homogenization analysis using the finite element method was then used to calculate the Von Mises stress distributions and homogenized elastic matrix of PP composites containing ground fly ash. The homogenized elastic matrix was used to validate the experimental flexural modulus. The results show that the homogenized elastic matrix is in good agreement with the experimental flexural modulus. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Modeling and characterization of the electrical conductivity of carbon nanotube-based polymer composites

In this research, we investigate both analytically and experimentally the electrical properties of carbon nanotube (CNT)-based polymer composites. An analytical model was developed to predict the electrical conductivity of CNT-based composites. The micro/nanoscale structures of the nanocomposites and the electrical tunneling effect due to the matrix material between CNTs were incorporated within the model. Electrical conductivity measurements were also performed on CNT/polycarbonate composites to identify the dependence of their electrical transport characteristics on the nanotube content. The analytical predictions were compared with the experimental data, and a good correlation was obtained between the predicted and measured results. In addition, the effect of nanotube geometry on the nanocomposite electrical properties at the macroscale was examined.     

The effect of graphene flake size on the properties of graphene-based polymer composite films

In this work, the role of graphene flake size on the properties of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) composites was studied. Graphene flakes were added to PVDF-HFP using a solution mixing and molding process. By increasing graphene particle size and its concentration in the composites, higher electrical conductivity, in-plane thermal conductivity, and elastic modulus were achieved. Maximum tensile strength was obtained for the composites with average graphene flake size of 2, 5, and 7 μm at graphene concentrations of 10 wt%, 5 wt%, and 20 wt%, respectively. Thick flexible composite films (0.2–0.4 mm) with ultra-high in-plane electrical conductivity (~4500 S/m), in-plane thermal conductivity (~26 W/m/K), and tensile strength (~50 MPa) were obtained for the samples containing the graphene flakes with a larger average particle size of 7 μm. To our knowledge, the first two values are larger than any other values reported in the literature for PVDF-based composites.     

Development of a ceramic-based composite for direct bonded copper substrate

In the present paper, a computational approach is presented to design alumina-based composite with tailored properties that could replace commercial alumina used in Direct Bonded Copper (DBC) substrates for applications in power electronic modules. A mean-field homogenization and effective medium approximation (EMA) using an in-house code is used for predicting potential optimum thermal and structural properties for DBC substrates by considering the effect of filler type, volume, and size in the alumina matrix. The primary goal for designing such alumina-based composites is to have enhanced thermal conductivity for effective heat dissipation and spreading capabilities together with a coefficient of thermal expansion (CTE) value that is close to the silicon chips in electronic circuits in order to avoid interface layers. At the same time, other functional properties like elastic modulus and electrical conductivity have to be maintained. Our strategy incorporates thermal and structural properties of composites as a constraint on the design process. Among various metallic and carbon-based fillers, chromium, silicon carbide and diamond fillers were found suitable candidates that could enhance the thermal and structural performance of the alumina-based substrates. As a validation, we developed alumina-silicon carbide (Al₂O₃-SiC) composites in line with the designed range of filler size and volume fraction using Spark Plasma Sintering (SPS) process. Thermal and structural properties including thermal conductivity, CTE, and elastic modulus are measured to complement the computational design. It is found that the developed computational design tool is accurate enough in predicting the desired properties of composite materials for DBC substrate applications.     

Hierarchical material modeling of carbon/carbon composites

Unidirectional, long fiber carbon/carbon composites fabricated by chemical vapor infiltration (CVI) consisting of carbon fibers in a pyrolytic carbon matrix are anisotropic materials. It is practically impossible to identify experimentally the elastic properties (modules) of this anisotropic material. The aim of this investigation is to predict the elastic properties of this composite theoretically. The study of this material with the help of microscopy gives information about the very complicated anisotropic structure of this composite at each length scale. That is the reason that a hierarchical model for this material is developed, which consists of four length levels. A methodology for identification of the elastic properties for such composites is proposed. The problem is solved with the help of a homogenization procedure for each level.     

强界面陶瓷层状复合材料优化设计的最佳层厚比探讨

针对提高材料整体强度的目标，通过对材料中应力状态与几何结构因素关系的分析，给出了强界面层状材料优化设计的最佳层厚比原则。通过实验研究，探讨了A12O3—14．31％ZrO2／A12O3—39．17％ZrO2(质量分数，下同)层状材料的强度、断裂功和弹性模量等随着材料中残余应力的存在而发生变化的规律。A12O3—14．31％ZrO2／A12O3—39．17％ZrO23层材料的最佳层厚比理论计算值为4．14。实验结果表明，最佳层厚比在2．49～4．36之间。在最佳层厚比处，抗拉强度、弹性模量、断裂功和Vickers硬度均达到最佳值。实验进一步印证了理论分析的正确性。     

Mechanical properties of cellular materials. I. Linear analysis of hexagonal honeycombs

The purpose of this series of studies is to develop finite element computer models of the mechanical properties of cellular polymers, especially open cell foams. Using finite element methods, both the properties of the material making up the struts, as well as the geometrical structure of the cell, can be readily varied. The series of studies begins with two-dimensional hexagonal honeycombs because of their ease of analysis and comparison with previous works. Comparison of the present solutions and analytic ones have been conducted, and excellent agreement is obtained. The effects of cell dimensions, such as strut length, strut depth, and cell height, of irregular hexagons on the effective Young's modulus of foams were studied in the low strain and elastic regime. Load direction and cell geometry anisotropy effects are also investigated. In addition, the effects of friction model and the specimen size on the effective Young's modulus of the foam are studied. Nonuniform strut thickness was also a variable. The modulus effects of these variations in geometry ranged from minimal to highly significant and provide an understanding of geometry effects on foam performance. © 1997 John Wiley & Sons, Inc.     

The elastic properties of random-in-plane short fiber reinforced polymer composites

Four types of random-in-plane short fiber reinforced polymer composites were manufactured by the prepreg route using carbon or glass fiber tissue and 913 or 924 epoxy resin. The in-plane Young's modules and in-plane shear modulus of the composites were measured over the temperature range − 100 to + 200°C by dynamic mechanical analysis using three point bend and rectangular torsion testing geometries. Theoretical predictions of the elastic properties of the composites were determined over the same temperature range and compared with the experiment. Of particular interest was the use of the “S mixing rule” of McGee and McCullough to determine a single theoretical estimate for the composite elastic properties. Excellent agreement between experiment and theory was found for the four composites over the majority of the measured temperature range.     

Improved mechanical properties of carbon nanotube/polymer composites through the use of carboxyl-epoxide functional group linkages

Single-walled and multi-walled carbon nanotubes (CNTs) were functionalized with carboxyl groups and dispersed in a polymer containing an epoxide group. We have then observed experimentally that mutual chemical reaction between the functional groups on the CNTs with the polymer epoxide group can enhance, two-fold, both the tensile strength and elastic modulus, E, of single walled CNT/polymer composites. A simple model was formulated to understand the variation of E with CNT volume fraction, considering agglomeration effects as well. An increase in the work of fracture, obtained from the experimental stress-strain curves, was seen at low nanotube filling fractions and is presumably due to crack bridging of the polymer matrix by CNTs. The influence of CNT length and geometry on mechanical properties, along with the influences of electrical and mechanical percolation thresholds was considered.     

PP/甘蔗皮纤维复合材料的拉伸性能

采用热压工艺制造聚丙烯(PP)/甘蔗皮纤维复合材料,并研究其拉伸性能。研究热压温度为175℃、压力为2 MPa、时间15 min工艺条件下纤维粒径大小和质量分数对复合材料拉伸强度和拉伸弹性模量的影响。结果表明:在甘蔗皮纤维质量分数为40%条件下,复合材料拉伸性能随着粒径减小呈现先增加后减少的趋势,当纤维粒径为40~60目(0.45~0.3 mm)时材料拉伸强度最大,为8.58 MPa,此时弹性模量为2.44 GPa;在相同纤维粒径40~60目条件下,纤维质量分数为40%时PP复合材料拉伸强度最大,纤维质量分数为50%时PP复合材料拉伸弹性模量最大,达到2.65 GPa。根据实验结果,甘蔗皮纤维增强PP复合材料在纤维粒径为40~60目、质量分数在40%时综合拉伸性能最佳。     

Kenaf‐filled biodegradable composites: rheological and mechanical behaviour

Biodegradable polymer composites, typically based on biodegradable polymer matrices and natural‐organic fillers, are gaining rising interest and importance over the last few years. Several natural‐organic fillers can be used but the most widespread so far is wood, in the form of fibres or flour. Alternative cellulosic fillers can ensure advantages in terms of resource utilization and properties of the final composite. In this work, Mater‐Bi^® based biodegradable composites were prepared with two kinds of wood flour, and directly compared with alternative composites containing kenaf fibres. The use of kenaf fibres allowed improved elastic modulus, tensile strength and interaction with the polymer matrix to be obtained, although the filler dispersion was worse. Rheological measurements evidenced higher viscosity and an increasingly elastic behaviour of the melt. Copyright © 2012 Society of Chemical Industry     

Effects of droplet size on photorefractive properties of polymer dispersed liquid crystals

Effects of liquid-crystal droplet size on orientational photorefractive properties of the polymer dispersed liquid crystals are investigated experimentally. The composites consist of the same chemical components, but the liquid crystal droplet size was varied by controlling the fabrication process. Particular attention is given to the observation and qualitative and/or quantitative modeling of the resolution, dependence of the applied dc field, dynamics of grating generation and photocurrents, which is strongly dependent on the liquid crystal droplet size.     

The tensile behavior of off‐axis loaded plant fiber composites: An insight on the nonlinear stress–strain response

Composites in load‐bearing applications are often exposed to off‐axis loads. For plant fiber composites (PFCs) to be seriously and readily considered in structural applications, knowledge and reliable prediction of their response to off‐axis loads is critical. This article (i) characterizes the stress–strain response, (ii) investigates the tensile properties, and (iii) analyses the fracture modes, of unidirectional flax‐polyester composites subjected to off‐axis tensile loading. A key finding of this study is that due to the nonlinear stress–strain response of PFCs, the apparent stiffness of the composite reduces by ∼30% in the strain range of 0.05 to 0.25%. In addition, through cyclic tests on the composites, the elastic strain limit is found to be only ∼0.15%. This has major implications on the strain range to be used for the determination of the composite elastic Young's modulus. Consequently, it is proposed that the tensile modulus for PFCs should be measured in the strain range of 0.025 to 0.100%. Through comparison with experimental data, conventional composite micromechanical models are found to be adequate in quantitatively describing the tensile behavior of off‐axis loaded PFCs. The application of such models has also enabled the determination of, otherwise difficult to measure, material properties, such as fiber shear and transverse modulus. Off‐axis loaded PFCs fail by three distinct fracture modes in three different off‐axis ranges; each fracture mode produces a unique fracture surface. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers     

三维编织复合材料力学性能研究现状

对三维编织复合材料的力学性能研究现状进行了综述,研究内容大致归纳为细观结构特征研究、有限元仿真研究及实验研究。细观结构研究主要是研究单胞几何模型的建立和编织工艺与单胞结构模型的关系。有限元研究主要集中在利用有限元软件对细观结构模型进行力学分析、刚度强度性能预测。实验研究是运用实验的方法对材料的拉伸性能、弯曲性能及疲劳性能进行研究,并分析编织工艺参数和温度对其力学性能的影响。最后,对目前研究中存在的问题和今后的发展趋势进行了展望。     

The large deformation elastic response of woven kevlar fabric

The large deformation elastic response of a plane woven Kevlar fabric is investigated analytically and experimentally. The analysis assumes the undeformed geometry to be a sequence of interlaced arcs of circles that reverse at each yarn midpoint, and each yarn is modeled as an extensible elastica subject to certain compatibility conditions. Deflection-force relations for the fabric are determined in terms of the initial weave geometry and the elastic properties of the individual yarns. The theoretical results agree well with the results of experiments performed on a fabric woven from 400 denier Kevlar yarns under conditions of uniaxial loading in both warp and fill directions.     

Properties of potassium titanate whisker reinforced polytetrafluoroethylene-based friction materials of ultrasonic motors

Ultrasonic motors (USMs) are driven by friction forces between a stator and rotor. So the output properties and life of USMs are strongly related to the properties, such as the mechanical and tribological properties, of the frictional materials. In this study, the effects of the content of potassium titanate whiskers (PTWs) on the mechanical and tribological properties of polytetrafluoroethylene (PTFE)-based friction materials and the performances of the corresponding USMs were studied. The morphology of worn surfaces of PTFE composites were observed with a scanning electron microscope. The experimental results show that the PTWs not only increased the hardness and elastic modulus of the PTFE composites but also increased the friction coefficient and wear resistance of the PTFE composites. On the whole, the PTFE-based friction materials filled with 5 wt % PTWs were the preferable friction materials for USMs. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Determination of elastic constants of epoxy resin/biochar composites by ultrasonic pulse echo overlap method

This study is carried out in order to determine the elastic properties of Epoxy Resin (ER) Composites reinforced with various mixtures of China Poplar Char (CPC) and Pine Cone Char (PCC) as biochars by ultrasonic wave velocity measurement method. The prepared chars are mixed with epoxy resin matrix at weight percentages of 10%, 20%, and 30% for preparing the ER/Biochars (BC) composites. The effect of biochar amounts on the elastic properties of the ER/BC composites are investigated by ultrasonic pulse echo overlap method. The morphologies of the samples are investigated by scanning electron microscopy. Based on the findings obtained from the present study, forming of the ER/CPC composites gives better values of elastic properties compared to forming of the ER/PCC composites. According to the obtained results, the composition ratio of 70:30 is the most appropriate composition ratio for both of the ER/CPC and the ER/PCC composites. POLYM. COMPOS., 37:2907–2915, 2016. © 2015 Society of Plastics Engineers