加捻植物纤维增强树脂基复合材料偏轴拉伸非线性多层次角度融合建模

针对加捻植物纤维增强树脂基复合材料偏轴拉伸非线性力学行为,以宏、细观相结合的思路,建立了融合植物纤维微纤丝角、纱线加捻表面捻转角和复合材料偏轴拉伸角的多层次角度融合本构模型,在实际计算时,由于微纤丝角的随机性、难于测量和对复合材料偏轴拉伸性能相对表面捻转角和偏轴拉伸角的较小影响,假定微纤丝角是常量,计算时仅考虑表面捻转角和偏轴拉伸角,适当简化。最后,利用单向亚麻纤维织物增强环氧树脂基复合材料拉伸实验数据和文献数据验证模型的有效性,模型计算结果与实验结果吻合较好。     

Transverse elastic moduli of unidirectional fiber composites with fiber/matrix interfacial debonding

An elastic contact model is developed to predict the transverse Young's modulus, Poisson's ratio and shear modulus of unidirectional fiber composites with interfacial debonding. The elastic deformation formulae of the fiber under contact pressure are derived by the use of elasticity theory. These results are then used in the formulation of an analytic boundaryelement method for solving the interfacial debonding problem. The two extreme cases of perfect bonding and the fiber-like void are also studied. On the basis of this theory, the upper and lower bounds of the transverse moduli for unidirectional fiber composites with imperfect fiber/matrix interfaces are provided. Numerical calculations of parametric studies are conducted for four composites, and some basic characteristics of the transverse elastic moduli of unidirectional fiber composites with interfacial debonding are presented.     

On contribution of pores into the effective elastic properties of carbon/carbon composites

In this paper, we discuss the effect of porosity on the effective elastic properties of unidirectional carbon/carbon composites (carbon fibers in pyrolytic carbon matrix) densified by chemical vapor infiltration (CVI). It is shown that CVI treatment results in formation of irregularly shaped pores randomly oriented in the plane perpendicular to the direction of fiber (transverse plane). These pores are analyzed using numerical conformal mapping procedure, and their contribution to the effective elastic properties is expressed in terms of the cavity compliance contribution tensor. Components of this tensor are found for a variety of typical pores shapes.     

Analytical study of strength and failure behaviour of plain weave fabric composites made of twisted yarns

A two-dimensional analytical method is presented for the failure behaviour of plain weave fabric composites made of twisted yarns. The studies have been carried out on laminates with different configurations under on-axis uni-axial tensile loading. The cross-sectional area of the yarn was taken to be elliptical and the yarn path was taken to be sinusoidal. Different stages of failure are considered in the analysis. It has been observed that there is no significant reduction in tensile strength properties of plain weave fabric composites as a result of twisting of yarns. For E-glass yarns, twisting of yarns up to 5°, can facilitate ease of fabrication without significantly compromising the strength properties of the woven fabric composites.     

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。     

不规则孔隙对复合材料横向拉伸力学性能的影响

本文主要研究分析了不规则形状孔隙对复合材料单向板横向拉伸力学性能的影响。首先通过C++编写不规则孔隙随机分布算法。然后通过Python参数化生成包含随机分布纤维和不规则孔隙的重复胞元（Repeating unit cell,RUC）。最后使用有限单元法（Finite element method）分析研究了不规则孔隙对单向板横向拉伸性能（横向弹性模量和横向拉伸强度）的影响。研究结果显示,孔隙的形状会影响单向板的初始损伤、损伤扩展和最终破坏。随着孔隙率的增大,横向弹性模量和横向拉伸强度都减小。相对于横向弹性模量,孔隙率对横向拉伸强度的影响较大。      

固化温度对亚麻纤维及其增强复合材料力学性能的影响

研究热压成型过程中,不同固化温度对亚麻纤维及其增强复合材料力学性能的影响。结果表明:亚麻纤维在120,140℃和180℃分别处理2h后单纤维拉伸性能发生不同程度的下降。环氧树脂E-51在120,140℃和180℃下固化2h后拉伸性能未发生明显变化。基于环氧树脂的单向亚麻纱线增强复合材料分别在120℃和140℃固化成型时,拉伸强度和冲击强度变化不大。但当固化温度达到180℃时,由于亚麻纤维在高温环境下损伤较为严重,其增强复合材料的拉伸强度和冲击强度均发生明显的下降。然而复合材料的拉伸模量随着成型温度的升高有一定幅度的提升。     

基于纤维束/环氧树脂复合材料试验的单向层合板横向拉伸强度预测方法

实验研究表明,纤维束/环氧树脂复合材料试件的横向拉伸强度与工程上常用的单向层合板横向拉伸强度在趋势上具有很好的相关性,但是数值上存在一定差距。本文使用两种碳纤维和两种环氧树脂制备了三种纤维束/环氧树脂复合材料和单向层合板,并分别测量了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度,以及环氧基体的拉伸强度。在实验基础上,应用Griffith断裂强度理论建立了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度的关系模型,通过两种复合材料实验的结果拟合了该模型中的参数。利用第三种复合材料实验进行校验,发现该模型预测的单向层合板横向拉伸强度与实测强度之间达到很好的一致性,相对偏差为9%。采用本文提出的方法,可以用较为简单的纤维束/环氧树脂复合材料和环氧基体拉伸试验预测单向层合板的横向拉伸强度。     

双随机分布细观分析模型与复合材料性能预报

发展了一种细观力学有限元分析方法——拟真实的参数化双随机分布模型, 该模型综合考虑了纤维增强树脂基复合材料的真实微结构特点和纤维单丝综合力学性能测试结果的离散性特征, 模拟了复合材料中纤维排列和强度分布的随机性。借助移动窗口法研究了该参数化双随机分布模型的可靠性, 确定了其代表性体积单元的尺寸。基于能量法原理推导了单向复合材料的弹性模量预测公式, 结合能量法和渐进失效分析方法, 利用该细观力学有限元方法分别预测了单向纤维增强树脂基复合材料T300/5228的弹性模量和强度性能。数值模拟结果和大部分试验结果吻合良好, 表明发展的细观力学有限元方法能够较好地预测复合材料的力学性能。      

Studies of microstructural and mechanical properties of Nylon/Glass composite Part II The effect of microstructures on mechanical and interfacial properties

This is a part of a series of studies on the influence of thermal processing on microstructures and mechanical properties of thermoplastic composites. In this paper, the effect of cooling rate during thermal moulding processes on the mechanical properties of bulk unidirectional commingled yarn GF/PA6 composites (Iosipescu shear strength, transverse flexural tensile strength and elastic modulus) has been investigated. Cooling rate from fast to slow, –60°C/min, –3°C/min and –1°C/min, were achieved at 1.5 MPa pressure. Scanning electron microscopy (SEM) was used to analyse the damaging mechanisms of the fracture surfaces of the tested samples. The different dynamic responses of the samples were observed by polarised optical microscopy (POM) during the mechanical tests. The results indicated that when the cooling rate was varied from fast to slow, the interfacial tensile and shear strength were improved associated with enhanced elastic modulus. These results may be attributed to the slow cooling achieved a high transcrystallinity between the glass fibres and PA6 matrix, and high crystallinity of phase in the PA6 matrix.     

孔隙微观特征对碳纤维/环氧树脂复合材料横向拉伸强度的影响

通过有限元方法研究了相同孔隙率下孔隙的分布、尺寸和形状等微观特征对碳纤维增强环氧树脂复合材料单向板横向拉伸强度的影响。首先使用Matlab对复合材料微观图像进行处理,提取孔隙的半径分布。然后通过C++编写多种孔隙随机分布算法,包括可以生成不同分布孔隙、不同尺寸孔隙以及不同形状孔隙的随机分布算法。最后通过Python参数化生成代表性体积单元（RVE）,用有限元方法研究相同孔隙率下孔隙的分布、尺寸和形状对碳纤维/环氧树脂复合材料单向板横向拉伸强度的影响。研究结果显示,孔隙率相同时,碳纤维/环氧树脂复合材料的孔隙形状对横向弹性模量的影响较大,孔隙尺寸和形状对横向拉伸强度有较大的影响。     

混杂纤维复合材料的拉伸刚度

对单向和多向混杂纤维复合材料的拉伸刚度进行了研究。在混合定律的基础上考虑混杂比和分散度对混杂效应的影响, 提出了单向混杂纤维复合材料拉伸模量的估算公式。通过实验得到了多向混杂纤维复合材料的拉伸模量, 并且采用经典层合板理论进行了估算, 基于混杂比以及分散度对拉伸模量的影响规律, 对多向混杂纤维复合材料拉伸模量的估算公式进行了修正。结果表明: 混杂纤维复合材料的拉伸模量与混杂比和分散度相关, 分散度的增大在一定程度上可以提高单向混杂纤维复合材料的纵向拉伸模量。采用经典层合板理论所得的拉伸模量与实验值有一定的误差, 而本文所提出的公式能够更加准确地估算混杂纤维复合材料的拉伸模量。      

An evaluation of the elastic properties and thermal expansion coefficients of medium and high modulus graphite fibers

In this work, the elastic properties and coefficients of thermal expansion of T650-35, M40J and M60J graphite fibers were determined from the macroscopic properties of either unidirectional and/or woven composites of these fibers embedded in polyimide resins. The T650-35 fibers were embedded in a PMR-15 matrix, whereas the M40J and M60J fibers were embedded in a PMR-II-50 polyimide. The three-component oscillator resonance method was employed to determine the elastic properties of the unidirectional and woven composites and their neat resins. The macroscopic coefficients of thermal expansion of the composites and the neat resins were measured by length dilatometry. Subsequently, the fiber properties were calculated from the unidirectional composite macro-data using the Eshelby/Mori-Tanaka approach. For the woven composites, a finite element approach based on the concept of a representative volume element was employed to determine the elastic and thermal properties of the fibers. In the case of the T650-35 fibers, both the longitudinal and transverse elastic and thermal properties of the fibers determined from the unidirectional and woven composites agreed very well with each other. However, for the M40J fibers, noticeable differences were observed between the fiber properties determined from the unidirectional and woven system, which was attributed to the lack of transverse isotropy of the unidirectional system. Since the properties of the M60J fibers were evaluated only from the woven system no direct comparison could be made between the properties obtained from the unidirectional and woven composite architectures. Overall, the methodology was shown to be highly applicable for the accurate determination of fiber properties from both unidirectional and woven systems.     

Thermoelastic properties of fiber composites with imperfect interface

Imperfect interface conditions are defined in terms of linear relations between interface tractions and displacement jumps. All of the thermoelastic properties of unidirectional fiber composites with such interface conditions are evaluated on the basis of the generalized self consistent scheme (GSCS) model. Results for elastic interphase are obtained as a special case by evaluation of interface parameters in terms of interphase characteristics. Numerical evaluation has shown that imperfect interface may have a significant effect on transverse thermal expansion coefficient, transverse shear and Young's moduli and axial shear modulus, a moderate effect on axial Poisson's ratio, small effect on axial thermal expansion coefficient and an insignificant effect on axial Young's modulus.     

Degradation of yarns recovered from soft-armor targets subjected to multiple ballistic impacts

Post ballistic impact residual yarn mechanical properties were analyzed from two different as-received shoot packs composed solely of AuTx yarn possessing the 2/2 twill weave structure, one being impacted by 9-mm projectiles and the other by 2-grain projectiles. It was found that yarn mechanical properties from both shoot packs yielded similar results, regardless of yarn orientation, ply location, or penetrator size, which indicates that ballistic damage in the packets is very localized, producing little damage to the neighboring yarns. Mechanical properties of these woven, ballistically impacted, and then extracted yarns were compared to as-received native spooled AuTx yarn yielding a slight reduction in tensile strength, an increase in failure strain, and a reduction in elastic modulus, thereby yielding little variation in yarn toughness.     

Micromechanics models for the elastic constants and failure strengths of plain weave composites

In this paper, two models are presented for plain weave composites. One is finite element analysis (FEA) model for elastic constants, namely, sinusoidal yarn model. Another is analytical model for failure strengths, namely, sinusoidal beam model. The FEA model is generated by interfacing an in-house computer code with FEA software strand6, and the analytical model is developed using the theory of elasticity. Numerical studies are carried out using the present models to investigate the effects of some major geometrical parameters on the properties of plain weave composites. It is concluded that the failure strengths are closely related to the fiber volume fraction of a yarn, and the mechanical properties are closely related to the overall fiber volume fraction of the composites. An experimental testing program is conducted for T300/934 plain weave composites to validate the developed models. A good agreement exists between the predicted and measured results.     

Analytical modelling of the compressive and tensile response of woven composites

An analytical model to predict the compressive and tensile response of woven composites up to failure is proposed. The model is based on a beam supported by an elastic foundation. The elastic foundation provides both normal and shear support. Its characteristics are derived from kinematic models for the deformation of the weave and account for: (i) weave effect, (ii) support provided by the adjacent layers, and (iii) properties of matrix and transverse tows.     

Strength of unidirectional glass/epoxy composite with silica nanoparticle-enhanced matrix

A technique was developed to improve the strength of unidirectional composites by enhancing the matrix properties through nanoparticles infusion. A commercially available standard DGEBA epoxy with silica nanoparticles (Nanopox F 400) was used as the matrix to make fiber composites. The silica nanoparticles in Nanopox were grown in situ via a sol–gel process resulting in a concentration of 40 wt% which was later diluted to 15 wt% particle loading. TEM images showed very uniform dispersion of silica nanoparticles with a size distribution of about 20 nm. Compression test revealed a substantial improvement (40%) in elastic modulus of the modified epoxy. A modified vacuum assisted resin transfer molding process was used to fabricate unidirectional E-glass fiber reinforced silica/epoxy nanocomposites. Inclusion of silica nanoparticles dramatically increased the longitudinal compressive strength and moderately increased the longitudinal and transverse tensile strengths. A microbuckling model was used to verify the compression testing results.     

Characterization of thermoplastic natural fibre composites made from woven hybrid yarn prepregs with different weave pattern

This paper focuses on the effect of weave structure on mechanical behaviour and moisture absorption of the PLA/hemp woven fabric composites made by compression moulding. The unidirectional woven fabric prepregs were made from PLA (warp) and PLA/hemp wrapped-spun hybrid yarn (weft) with two different weave patterns; 8-harness satin and basket. Unidirectional composites with 30 mass% hemp content were fabricated from these prepregs, and compared to winded PLA/hemp hybrid yarn laminates with same composition. The composite from the satin fabric had significantly lowest porosities and best mechanical properties compared to the composite made from the winded hybrid yarn and basket fabric. The tensile, flexural, and impact strength were 88 MPa, 113.64 MPa, and 24.24 kJ/m², respectively. The effect of weave pattern on water absorption is significant. Although the composite from hybrid yarn laminate has larger water absorption than that of the pure PLA, it exhibits lower moisture absorption than both weaves.