随机植物短纤维复合材料界面性能对有效模量和拉伸行为的影响

研究了界面性能对随机短云杉纤维增强聚丙烯(PP)复合材料宏观拉伸性能的影响。采用双线性内聚力模型(CZM)描述随机短云杉纤维和PP基体间非理想界面的力学行为,建立了含非理想界面的随机短纤维增强复合材料代表性单元(RVE)的二维有限元模型,考虑了纤维含量、长细比、随机分布和随机各向异性弹性以及PP基体弹塑性的影响;模拟了不同纤维含量复合材料的实验拉伸应力应变曲线。结果表明,短云杉纤维/PP基体间非理想界面刚度与复合材料有效弹性模量之间有单调递增的曲线关系,即E-K曲线;同一复合材料不同纤维含量的E-K曲线簇有一个临界交点。在交点右侧强界面刚度区复合材料有效模量随着纤维含量的增加而提高,在交点左侧弱界面刚度区有效模量随着纤维含量的增加而减少。三种不同体积含量10%、20%和49%的云杉/PP复合材料的非理想界面刚度可用E-K曲线和实验测得的宏观有效弹性模量确定,云杉/PP界面初始破坏位移和界面完全破坏位移也可根据模拟拉伸应力应变曲线确定。数值分析结果能用非理想界面刚度来解释和理解随机短植物纤维体积含量对复合材料宏观有效模量的影响。     

基于复合材料层合箱梁改进解析模型计算等效刚度

提出了基于复合材料层合箱梁改进解析模型的等效刚度计算方法。在考虑三维应变效应的同时用复合材料单层的二维折算模量分量来表示三维折算模量分量,简化了复合材料层合箱梁等效刚度系数的计算,得到了由梁横截面几何尺寸和层合板刚度系数表达的等效抗弯刚度和等效抗扭刚度的解析式。该解析式适用于环向刚度一致的复合材料层合箱梁,并充分考虑了弯曲-剪切耦合和扭转-拉伸耦合效应对等效刚度的影响。通过三点弯试验和扭转试验,验证了解析式的正确性;通过与分层等效叠加法、有限元法进行对比,分析了解析式的计算精度。结合经典层合板理论,研究了铺层方式对等效刚度产生的影响及原因,预测了不同铺层复合材料层合箱梁等效刚度的变化规律。      

基于内聚力模型的形状记忆合金短纤维增强树脂基复合材料的模拟分析

通过单纤维拔出实验和单轴拉伸实验, 测定了形状记忆合金(SMA)增强树脂基复合材料的界面脱粘剪切强度和单向随机分布SMA短纤维增强复合材料的拉伸强度。根据蒙特卡罗法和边界条件控制方程, 编写了适于软件调用的单向随机分布短纤维增强复合材料的APDL语言生成程序, 建立数值模拟模型。基于指数型内聚力模型, 对SMA纤维与环氧树脂基体界面分离(即界面脱粘)过程进行了有限元模拟。结果表明: 相同纤维体积分数下, 随着纤维长细比的减小, 复合材料整体弹性模量逐渐降低; 温度驱使SMA纤维弹性模量发生变化, 可以有效提高复合材料整体弹性模量。     

高温环境下纤维增强复合材料等效参数预测

为提高复合材料热结构动力学建模与动态特性分析效率,从复合材料等效性能物理意义出发,建立了弹性性能与热膨胀系数预测方法:在细观有限元模型基础上,分别施加周期性位移边界条件与温度边界条件,然后根据均匀化方法获得等效力学性能。以复合材料梁、板结构为对象开展仿真研究,通过等效前精细化模型和等效后均质模型热模态分析结果比较,验证预测方法的有效性。此外,研究了不同类型单胞、不同纤维排布方式对复合材料宏观弹性常数、热膨胀系数的影响,结果表明:不同纤维排布方式下,正方形排布纤维束预测结果比六边形排布纤维束预测精度高;同一纤维排布方式下,整个单胞的等效预测精度较缩减后的代表性体积单元(RVE)精度高。     

学术前沿

正随机植物短纤维复合材料界面性能对有效模量和拉伸行为的影响《材料研究学报》第30卷,2016年第9期作者:沈珉;孙晓翔;刘洋作者单位:天津大学机械工程学院摘要:研究了界面性能对随机短云杉纤维增强聚丙烯(PP)复合材料宏观拉伸性能的影响。采用双线性内聚力模型(CZM)描述随机短云杉纤维和PP基体间非理想界面的力学行为,建立了含非理想界面的随机短纤维增强复合材料代表性单元(RVE)的二维有限元模型,考虑了纤维含量、长细比、随机分布和随机各向异性     

立体多向编织结构对复合材料性能的影响

本文采用1×1,1×2和1×3三种不同的编织结构对轴向增强和非增强三维多向编织复合材料的性能进行了研究。对编织复合材料的拉伸强度、刚度和弯曲强度、刚度进行了实验分析。结果表明;三维编织复合材料具有良好的性能。编织结构对复合材料性能有较大的影响。纤维表面编织角和纤维体积比是影响复合材料性能的重要结构参数。通过轴向加入非编织增强纤维,使编织复合材料的拉伸强度和模量,弯曲强度和模量有了较大改善。      

短纤维增强弹性体复合材料的强度预测

探讨了纤维的取向、长度和含量对短纤维增强弹性体复合材料强度的影响,在Fukuda预测模型的基础上,考虑了末端纤维的脱粘、纤维临界长度和高纤维体积含量等因素,建立了三维随机分布短纤维弹性体复合材料强度预测模型,并对预测结果与现有理论计算得到的结果进行了分析比较.     

立体多向编织结构对复合材料性能的影响

本文采用1×1,1×2和1×3三种不同的编织结构对轴向增强和非增强三维多向编织复合材料的性能进行了研究。对编织复合材料的拉伸强度、刚度和弯曲强度、刚度进行了实验分析。结果表明;三维编织复合材料具有良好的性能。编织结构对复合材料性能有较大的影响。纤维表面编织角和纤维体积比是影响复合材料性能的重要结构参数。通过轴向加入非编织增强纤维,使编织复合材料的拉伸强度和模量,弯曲强度和模量有了较大改善。     

玻纤增强注塑件的均匀化弹性力学参数研究

基于均匀化方法,根据长玻纤增强聚丙烯(LGFR-PP)的微观特征,建立了非连续长玻纤增强复合材料的代表性体积单元(RVE),通过有限元方法模拟预测了复合材料的宏观等效弹性力学参数,与注塑样条拉伸性能测试结果进行了比较。研究表明,通过在玻纤两侧增加聚丙烯(PP)分布,所采用的RVE较传统连续纤维的有限元模型更为合理;当玻纤成单一取向时,玻纤增强聚丙烯为一种横观各向同性材料;改变玻纤取向与拉伸方向之间的角度,拉伸方向的等效模量先微幅减小,再迅速降低,而后趋于稳定。利用均匀化方法预测非连续长玻纤增强注塑件的等效弹性力学性能具有较高的工程可行性,能进一步为玻纤增强注塑件的结构服役性能分析提供科学依据。     

纤维体积分数对K3D/MCPA复合材料力学性能的影响

本文研究了纤维体积分数对三维编织芳纶纤维增强铸性尼龙(简称K3D／MCPA)复合材料力学性能的影响。研究表明，K3D／MCPA复合材料有优异的抗冲击性能，冲击强度比三维编织芳纶纤维增强铸性尼龙(简称C3D／MCPA)和纯基体均有大幅度的提高，且随着纤维体积的提高而升高。K3D／MCPA复合材料剪切强度随纤维体积比的增大而增大，其纵向剪切强度低于纯基体和C3D／MCPA复合材料，但其横向剪切强度高于它们。K3D／MCPA复合材料弯曲强度与弯曲模量随纤维体积比的提高而提高，但与相同体积比的C3D／MCPA相比，K3D／MCPA的弯曲强度与弯曲模量均较低。     

Effect of fiber arrangement on mechanical properties of short fiber reinforced composites

The present paper developed a three-dimensional (3D) “tension–shear chain” theoretical model to predict the mechanical properties of unidirectional short fiber reinforced composites, and especially to investigate the distribution effect of short fibers. The accuracy of its predictions on effective modulus, strength, failure strain and energy storage capacity of composites with different distributions of fibers are validated by simulations of finite element method (FEM). It is found that besides the volume fraction, shape, and orientation of the reinforcements, the distribution of fibers also plays a significant role in the mechanical properties of unidirectional composites. Two stiffness distribution factors and two strength distribution factors are identified to completely characterize this influence. It is also noted that stairwise staggering (including regular staggering), which is adopted by the nature, could achieve overall excellent performance. The proposed 3D tension–shear chain model may provide guidance to the design of short fiber reinforced composites.     

亚麻落麻纤维增强可降解复合材料的拉伸强度预测

采用非织造结合热压成型工艺制备了亚麻落麻纤维增强聚乳酸(PLA)基可降解复合材料(亚麻落麻/PLA),研究了纤维体积分数对材料拉伸强度的影响,并利用 Kelly-Tyson拉伸强度预测模型及相关修正理论,提出了非连续植物纤维增强可降解复合材料(D-NFRBC)强度预测模型,该模型考虑了纤维长度、取向角、直径、强度概率分布及材料界面剪切强度与材料中纤维临界长度、纤维极限拉伸强度三者间制约关系对复合材料强度的影响。结果表明;亚麻落麻/PLA拉伸强度在纤维体积分数为39.6%时达到最大,应用本文建立的强度预测模型所得亚麻落麻/PLA拉伸强度预测值与实验值吻合良好。     

Effects of the carbon nanotube distribution on the macroscopic stiffness of composite materials

Having extremely high stiffness and low specific weight, carbon nanotubes (CNTs) have been known recently as perfect reinforcing fibers in nanotechnology. They can improve the stiffness and strength of nanocomposites by being used as reinforcing elements for example in polymer matrices. The corresponding properties of the fibers and matrix, such as volume fraction and aspect ratio are some of the significant factors in the characterization of mechanical properties of CNT reinforced composites. In recent years, the way in which fibers are distributed inside the matrix, in terms of randomness, has introduced another important factor in characterizing the mechanical properties of such composites. Based on this factor, composites can be classified into two types namely, aligned and randomly distributed. This research has studied the effect of random distribution of fibers in the matrix on the elastic modulus and initial yield stress of the nanocomposite. Therefore, several models of composites, with different distribution of fibers, were considered while holding the volume fractions and aspect ratio constant. As a result, the effect of randomness on the effective modulus of CNT reinforced composites was estimated. The finite element method (FEM), using the MSC.Marc software, was employed to predict the effective modulus of CNT reinforced composites and the results were successfully validated by comparison with the analytical Halpin-Tsai method.     

Method of continuous dipoles for modeling of materials reinforced by short micro-fibers

Method of continuous dipoles for modeling of the interaction of composites reinforced with fibers is presented in the paper. The dipoles are distributed along the fiber axis. All three components of dipoles are used to satisfy the boundary conditions along the fiber boundaries. The boundary conditions in strain components instead of displacements are used in the model, which simplifies the numerical solution. In resulting quasi-hyper-singular integral equations the integration is performed analytically. The dipole fields decay in infinity and they accurately simulate the near and far field interaction. The presented models are applicable to solution of problems of composites reinforced by straight fibers with large aspect ratio, which are most significant for practical purposes, as the fibers have large stiffness in tension, but not in bending. The fiber is considered to be rigid in the fiber axis direction.     

Mean-field based micro-mechanical modelling of short wavy fiber reinforced composites

Eshelby based Mean-Field homogenization is an effective method for modelling the mechanical response of short fiber reinforced composites. These models and especially the Mori–Tanaka model have been successfully used in previous studies for predicting the overall composite mechanical response. The present work describes a method for extending Mean-Field methods to discontinuous wavy fiber reinforced composites, including calculating local stresses in the fibers. The method involves discretization of a wavy fiber into smaller segments and replacing the original segments with an equivalent ellipsoids system which can be solved with Eshelby concept. The focus of this work is the validation of the local stress fields in fibers using Finite Element benchmarks of original Volume Elements (VEs) of wavy fibers. This validation is an essential basis for further accurate modelling of the damage behavior (i.e. debonding, fiber fracture) of discontinuous wavy fiber composites.     

Poisson ratio can play a crucial role in mechanical properties of biocomposites

Analytical models and complementary finite element (FEM) calculations are developed to evaluate the influence of Poisson ratio on the mechanical properties of biocomposites with staggered mineral platelets in parallel alignment in a soft protein matrix. As the Poisson ratio of the soft matrix is increased towards the incompressibility limit 0.5, the staggered biocomposites are significantly stiffened in both longitudinal and transverse directions, with the transverse stiffness increasing by more than two orders of magnitude. Since the Poisson ratio of most soft biological tissues is very close to 0.5, it is concluded that the Poisson ratio can play a crucial role in the mechanical properties of biocomposites. Based on the theoretical analysis, some discussions are made on several mechanisms that are responsible for the superior properties of biocomposites.     

A design approach for the mechanical properties of polypropylene discontinuous fiber reinforced cementitious composites by extrusion molding

Polypropylene discontinuous fiber reinforced cementitious composites were prepared by extrusion molding and tested in uniaxial tension to determine the mechanical properties such as ultimate composite strength and strain, and the critical volume fraction for multiple cracking. It was shown that the experimentally determined critical fiber volume fraction reasonably agreed with the theoretical value predicted by a micromechanics model. The extruded fiber composites yielded the ultimate composite strength of 9.0 MPa and composite strain of 0.55% at the fiber volume fraction of 7.4%. Our experimental results suggest that there is an optimal fiber aspect ratio and fiber volume fraction for enhancing the fracture properties.     

有一定取向性的碳纤维增强尼龙6复合材料弹性模量的实验和理论研究

纤维增强热塑性聚合物基复合材料注塑成型后往往被认为是各向同性复合材料。然而,注塑成型后纤维会具有一定的取向性,从而使复合材料试样呈现各向异性的特点。为了合理预测此类复合材料的弹性模量,本文对碳纤维增强尼龙6复合材料注塑试样内部的纤维长度和取向分布情况进行了测试和分析,得出了纤维取向的分布规律。随后结合单向纤维增强聚合物基复合材料力学模型和层叠理论,构造出了适用于有一定取向性的纤维增强树脂基复合材料弹性模量预测理论模型,其理论结果和拉伸实验结果吻合较好,表明该预测模型的准确性比较高。      

Al2O3f+Cf/ZL109混杂复合材料高温拉伸强度的理论分析

利用挤压铸造法制备了Al₂O_3f+C_f/ZL109短纤维混杂金属基复合材料,对该混杂复合材料的高温(300℃)强度性能进行了实验和理论分析。在综合考虑纤维长度变化规律、热应力诱发位错强化和纤维弥散硬化等因素对复合材料强度影响的基础上,对复合材料强度预测的混合律模型加以发展和修正,建立了Al₂O_3f+C_f/ZL109短纤维混杂复合材料的高温(300℃)强度预测模型。利用该模型得到的高温强度理论值所反映的规律与实验值所反映的规律吻合良好,该模型具有一定的正确性和实用性。