二维编织陶瓷基复合材料偏轴拉伸力学性能预测

基于二维编织C/SiC复合材料的细观结构,建立了碳纤维丝/热解碳界面/SiC基体和纤维束/表层SiC基体两种尺度下的细观单胞模型,通过有限元方法计算碳纤维丝/热解碳界面/SiC基体模型的等效弹性常数和强度,然后代入纤维束/表层SiC基体模型中计算,并引入Tsai-Wu失效准则,考虑不同失效模式的损伤,建立了二维编织C/SiC复合材料的渐进损伤模型,模拟了其偏轴拉伸应力-应变行为。针对该模型,阐述了二维编织C/SiC复合材料单胞模型在复杂应力状态下其纤维束的损伤过程。数值模拟结果与实验数据吻合较好,验证了模型的有效性,为该种材料的力学性能分析提供了一种有效方法。     

连续石墨纤维增强铝基复合材料准静态拉伸损伤演化与断裂力学行为

针对连续石墨纤维增强铝基(CF/Al)复合材料,采用三种纤维排布方式的代表体积单元(RVE)建立了其细观力学有限元模型,采用准静态拉伸试验与数值模拟结合的方法,研究了其在轴向拉伸载荷下的渐进损伤与断裂力学行为。结果表明,采用基体合金和纤维原位力学性能建立的细观力学有限元模型,对轴向拉伸弹性模量和极限强度的计算结果与实验结果吻合良好,而断裂应变计算值较实验结果偏低。轴向拉伸变形中首先出现界面和基体合金损伤现象,随应变增加界面发生失效并诱发基体合金的局部失效,最后复合材料因纤维发生失效而破坏,从而出现界面脱粘后纤维拔出与基体合金撕裂共存的微观形貌。细观力学有限元分析结果表明,在复合材料制备后纤维性能衰减而强度较低条件下,改变界面强度和刚度对复合材料轴向拉伸弹塑性力学行为的影响较小,复合材料中纤维强度水平是决定该复合材料轴向拉伸力学性能的主要因素。     

单向短纤维增强复合材料应力传递模型及其细观力学分析

建立了含单纤维和基体的双圆柱复合材料细观力学模型,采用基体剪应力的Lame形式推导了纤维轴向应力和纤维基体间界面剪应力计算方程。分析了纤维/基体模量比、纤维长径比和纤维体积分数等细观结构参数对纤维轴向应力和界面剪应力分布的影响,并对结果进行了验证。     

三维四步编织复合材料刚度和强度的数值模拟

为了研究三维四步法编织复合材料的力学性能,利用ANSYS有限元软件对材料的细观体胞模型进行数值模拟,计算三维编织复合材料的宏观弹性常数,讨论了纤维编织角和体积比对弹性常数的影响。采用不同的强度准则分别对纤维束和基体材料进行强度校核,从而得到材料发生破坏时失效单元的体积百分比。根据失效单元的分布情况分析材料的破坏机理,进而预报材料的拉伸强度。模拟计算结果与实验值吻合较好。     

纤维束增强复合材料的横向弹塑性性能研究

利用细观力学模型和有限元法研究纤维束复合材料的横向弹塑性力学性能。假设弹性的纤维束均匀地分布于幂硬化弹塑性基体材料中, 并假设纤维束内的纤维接触是光滑的。通过与单丝纤维复合材料的相应结果比较, 研究了纤维成束对复合材料宏观弹塑性性能的影响。通过对硼/铝复合材料的数值研究表明: 纤维的成束对复合材料的横向弹性刚度影响很小, 而对复合材料的塑性变形和切线刚度有显著的影响。      

圆管状立体机织复合材料的多尺度分析

采用多尺度耦合的数值模型研究了圆管状立体机织复合材料的力学性能。建立了反映纤维束中纤维/基体二相材料的微观尺度单胞和反映周期性编织结构的细观尺度扇形单胞,并重点讨论了扇形单胞的周期性边界条件。通过逐级计算微观单胞、细观单胞的平均弹性常数,得到了圆管状立体机织复合材料的刚度参数,实现了由组分材料性能及编织参数预测圆管的宏观弹性性能,模型预测刚度与试验结果吻合。另一方面,研究了从大到小各尺度耦合的应力分析,对于圆管环向应力非周期分布的情况,建立了嵌入细观单胞的环状模型,进行了复杂荷载下从宏观圆管结构、到细观纤维束尺度、再到微观纤维尺度之间的逐级应力分析。     

纤维增强复合材料考虑损伤的温度胀缩细观力学模型

采用细观力学方法,建立了纤维增强复合材料（FRC）包含基体微裂纹和纤维/基体脱粘的热胀/冷缩理论模型。模型考虑了基体、界面中不同分布取向的微裂纹在升温和降温过程中张开、闭合情况的差异,及其对复合材料平均热胀/冷缩系数（CTE/CTC）的影响,同时还考虑了细观应力分布不均匀的因素。建立了细观有限元模型对理论模型进行验证。研究发现：复合材料损伤后CTE和CTC不一致,且取决于损伤模式：基体微裂纹损伤使得复合材料的横向CTE高于无损材料,而横向CTC低于无损材料,但对纵向CTE/CTC影响不大;纤维界面脱粘能较明显地减小复合材料的纵向CTC,但对横向CTC的影响可忽略。     

基于细观仿真建模的CFRP细观破坏

碳纤维增强树脂基复合材料(CFRP)在细观上呈现纤维、树脂及界面组成的混合态,其切削加工过程的实质为刀具作用下材料细观层面的破坏至切屑宏观形成的演化过程。为了揭示CFRP切削加工过程中材料的细观破坏,建立了CFRP切削的细观有限元模型。该模型在几何上包含了纤维、基体及界面等组成相,而不是使用传统的等效均质建模方法。各组成相不仅考虑了各自不同的材料本构,而且为了能够模拟材料破坏,还将各组成相材料的失效及演化准则考虑其中。该模型可从细观层面更真实地模拟不同纤维角度CFRP单向板切削过程中纤维/基体断裂、界面开裂及演化的过程。仿真结果表明:不同纤维角度下CFRP细观破坏不同,切削0℃FRP时以界面开裂和纤维弯断为主;切削45°/90℃FRP时主要是刀具侵入工件,纤维基体被压断;切削135℃FRP时则以纤维弯曲断裂为主,断裂面往往在加工面以下。通过实验显微在线观测手段验证了模拟结果的正确性。      

基于细观有限元方法的复合材料横向力学性能分析

横向断裂是制约复合材料结构设计的关键点,传统细观模型因为不能充分考虑组分性能、体积分数和纤维形状及分布情况而不能有效预测材料横向力学性能。采用改进的随机序列吸收算法建立具有随机纤维分布的复合材料代表性体积单胞模型,考虑基体破坏和界面脱粘两种失效模式和固化过程中产生的残余应力,对模型在横向拉、压、剪3种载荷下的力学行为进行仿真计算。分析了不同界面强度对复合材料力学性能的影响规律。仿真结果与实验数据对比表明:横向模量预测误差在7%以内,压缩和剪切的强度误差在8%以内,结果一致性较好,表明该模型能够有效预测复合材料横向力学性能。     

三维机织复合材料的一种梁单元细观力学模型

根据三维机织复合材料中纤维束排列和变形的周期性特点,推导了一种细观梁单元模型。该模型考虑了纤维束的拉 (压) 弯耦合效应和纤维束之间的相互作用,可以描述纤维束和基体中的细观应力分布,并得到宏观的力学性能。针对一种典型的三维机织复合材料,首先根据编织参数,确定其细观几何结构,取最小周期的一段纤维束作为分析胞元,用上述细观梁单元分析了该段纤维束在面内拉伸荷载下的细观应力分布,计算出平均模量, 并用材料试验和细观实体有限元对本模型进行了检验,结果与本文的预测吻合良好。研究表明,拉、弯耦合效应引起的纤维束中的细观弯曲应力同平均轴向应力相比,不可忽略。      

Analytical study of tensile behaviors of UHMWPE/nano-epoxy bundle composites

Ultra-high molecular polyethylene (UHMWPE) fiber reinforced nano-epoxy and pure epoxy composites in bundle form were prepared and tested for tensile properties. UHMWPE fiber composites are well known for their superior tensile performance, and this work was conducted to assess the effect of adding nanoadditives to the resin and to evaluate possible enhancements or degradations to that attribute. The results showed that tensile tests on various types of UHMWPE fibers/nano-epoxy bundle composites resulted in an increase in modulus of elasticity due to the addition of small amounts of reactive nanofibers (r-GNFs) to epoxy matrix. It was observed that the modulus of elasticity of the composite bundles depended on both volume fractions of the matrix and the weight percent (wt%) of r-GNFs in the matrix. A non-linear relationship was established among them and an optimal modulus was determined by calculation. A three-dimensional surface plot considering these two parameters has been generated which gives an indication of change in modulus of elasticity with respect to volume fraction of matrix and wt% of r-GNFs in the matrix. A Weibull analysis of tensile strengths for the various bundle composites was performed and their Weibull moduli were compared. The results showed that presence of r-GNFs in the composites increased the strength effectively, and 0.3 wt% r-GNFs based composites showed the highest strength. An important ancillary finding is that optimum tensile values are a function not only of the above parameters, but also strongly influenced by the addition of diluents which control the viscosity of the blend.     

Effect of on-axis tensile loading on shear properties of an orthogonal 3D woven SiC/SiC composite

The present study examines in-plane and out-of-plane shear properties of an orthogonal 3D woven SiC fiber/SiC matrix composite. A composite beam with rectangular cross-section was subjected to a small torsional moment, and the torsional rigidities were measured using an optical lever. Based on the Lekhnitskii’s equation (Saint–Venant torsion theory) for a orthotropic material, the in-plane and out-of-plane shear moduli were simultaneously calculated. The estimated in-plane shear modulus agreed with the modulus measured from ±45° off-axis tensile testing. The effect of on-axis (0°/90°) tensile stress on the shear stiffness properties was also investigated by the repeated torsional tests after step-wise tensile loading. Both in-plane and out-of-plane shear moduli decreased by about 50% with increasing the on-axis tensile stress, and it is mainly due to the transverse crack propagation in 90° fiber bundles and matrix cracking in 0° fiber bundles. It was demonstrated that the torsional test is an effective method to estimate out-of-plane shear modulus of ceramic matrix composites, because a thick specimen is not required.     

基于纳米压痕技术的碳纤维/环氧树脂复合材料各组分原位力学性能测试

基于纳米压痕技术对碳纤维/环氧树脂复合材料各组分的原位硬度、 弹性模量和蠕变性能进行了测试, 实验得到了基体、 纤维和微小厚度界面层的力学性能。结果表明, 从环氧树脂基体到碳纤维过渡过程中, 硬度和弹性模量有明显的梯度变化, 并且纤维和树脂基体的原位弹性模量平均值与其非原位性能有一定的变化, 实验得到纤维的原位弹性模量有所下降, 环氧树脂的弹性模量有所增加。试件制备过程中的机械研磨对其表面产生的残余应力和复合后两种材料的相互影响是组分材料原位性能变化的主要原因。各组分的蠕变性能呈现出明显的差异。     

Developing rigid gliadin based biocomposites with high mechanical performance

We aim to produce unidirectional fiber composites with high mechanical performance based on flax fibers and a rigid gliadin matrix. As a fraction from wheat gluten, gliadin is soluble in alcohol containing media. The fabrication process did not involve any further solvents or plasticizers. Finally, samples were cooled at different rates. Overall, the cooling rate does not strongly affect the mechanical properties although slowly cooled materials contain a higher amount of non-disulfide cross-links, next to disulfide bonds within the gliadin matrix. At 40% fiber volume fraction, flax/gliadin composites with a flexural modulus and strength of respectively 21.5 GPa and 240 MPa were obtained when loaded in the longitudinal direction. These high values demonstrate that in this composite fabrication process, a good impregnation of the polymer matrix in between the fiber bundles has been achieved. However, the fiber–matrix adhesion, as measured by transverse flexural and tensile tests, was still relatively modest.     

Biomimetic Study on Helical Fiber Composites

The innovative approach imparting strength and toughness to carbon fiber reinforced polymer composites (CFRP) is put fotward from the viewpoint of biomimetics. Helical fibers produced by twisting fiber bundle are arranged in the matrix to simulate the unique structure of natural fibers.The results on the CFRP with helical fiber bundles show that, under the same state of strong fiber/matrix interface, the impact fracture toughness can be improved substantially compared to conventional composites with plain fibers. The beneficial effect is attributed to the helical fiber bundles which act as single fibers of huge diameter effectively increasing the fIber eIastic energy.     

界面对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响

界面对复合材料蠕变性能的影响很大。在试验分析的基础上建立了硅酸铝短纤维增强AZ91D镁基复合材料理论分析模型,利用三维有限元分析方法,系统研究了界面特性、界面上应力应变分布和短纤维位向变化对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响。研究表明：界面特性,如厚度、模量,均对纤维最大轴应力和稳态蠕变速率有影响,当界面厚度增加,纤维最大轴应力减小而稳态蠕变速率增大;当界面模量增大,纤维最大轴应力增大而稳态蠕变速率减小,但当界面模量高于基体模量时,纤维最大轴应力和稳态蠕变速率均保持不变;纤维位向也影响轴应力分布和稳态蠕变速率,纤维在其末端界面上存在较大的应力和应变,此处容易产生微裂纹而使材料抗蠕变能力下降;界面对硅酸铝短纤维增强AZ91D镁基复合材料的蠕变曲线和蠕变断裂机制也有影响,其影响程度还与纤维位向有关。     

Thermal stability and thermo-mechanical properties of hemp-high density polyethylene composites: Effect of two different chemical modifications

This study investigates the effect of silane and NaOH treatments of hemp fiber on the thermal and thermo-mechanical properties of hemp-high density polyethylene composites. The results indicated that thermal stability of composites decreased with increase in fiber loading and treated composites had higher thermal stability in comparison to untreated hemp composites. Dynamic mechanical analysis revealed an increase in the storage modulus of the treated composites compared to untreated ones. The increase in storage modulus was observed up to 40% fiber volume fraction, but at 50%, it dropped drastically. Silane treated hemp composites exhibited higher storage modulus compared to NaOH treated ones suggesting a better fiber–matrix interface.     

Analysis of effect of fiber orientation on Young’s modulus for unidirectional fiber reinforced composites

Young’s modulus of unidirectional glass fiber reinforced polymer (GFRP) composites for wind energy applications were studied using analytical, numerical and experimental methods. In order to explore the effect of fiber orientation angle on the Young’s modulus of composites, from the basic theory of elastic mechanics, a procedure which can be applied to evaluate the elastic stiffness matrix of GFRP composite as an analytical function of fiber orientation angle (from 0° to 90°), was developed. At the same time, different finite element models with inclined glass fiber were developed via the ABAQUS Scripting Interface. Results indicate that Young’s modulus of the composites strongly depends on the fiber orientation angles. A U-shaped dependency of the Young’s modulus of composites on the inclined angle of fiber is found, which agree well with the experimental results. The shear modulus is found to have significant effect on the composites’ Young’s modulus, too. The effect of volume content of glass fiber on the Young’s modulus of composites was investigated. Results indicate the relation between them is nearly linear. The results of the investigation are expected to provide some design guideline for the microstructural optimization of the glass fiber reinforced composites.     

Influence of filler/reinforcing agent and post-curing on the flexural properties of woven and unidirectional glass fiber-reinforced composites

The aim of this study is to investigate the reinforcing effect of woven and unidirectional glass fibers and the effect of post-curing on the flexural strength and flexural modulus of glass fiber-reinforced composites. A series of composites containing 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)-phenyl]propane and triethyleneglycol dimethacrylate matrices and different reinforcements of unidirectional or woven glass fibers were prepared. The samples, 25 × 2 × 2 mm, were cured with a halogen curing lamp, followed by additional curing by thermal treatment at 135 ± 5 °C temperature and 60 psi pressure. Samples were tested before and after post-curing in order to determine the flexural strength and flexural modulus. The degree of reinforcement with glass fibers was varied between 14 and 57 wt% or 7.64 and 38.44 vol% by changing the number of unidirectional bundles or woven glass fiber bands in the composites, respectively. The obtained flexural strength values were in the range of 95.20–552.31 Mpa; the flexural modulus ranged between 2.17 and 14.7 GPa. The highest flexural strength and flexural modulus values were recorded for samples with unidirectional glass fibers. The mechanical qualities of the glass fibers-reinforced composites increased after post-curing treatment. Increasing of the glass fiber amount in the experimental composites improves both flexural strength and modulus. SEM micrographs of fractured composites indicate a strong interfacial interaction between the glass fibers and the polymer matrix.     

不同先驱体制备C/SiC复合材料及其浸渍行为

采用先驱体浸渍裂解工艺(PIP工艺)制备C/SiC复合材料, 研究了不同先驱体对复合材料浸渍行为的影响(三种先驱体分别为固态聚碳硅烷(PCS(s))、液态聚碳硅烷Ⅰ(PCS-Ⅰ(l))和液态聚碳硅烷Ⅱ(PCS-Ⅱ(l)), 制备的三种复合材料体系分别为C/SiC-0、C/SiC-Ⅰ和C/SiC-Ⅱ)。结合C/SiC复合材料的力学性能以及不同裂解周期C/SiC复合材料的微观形貌, 研究了不同先驱体制备的C/SiC复合材料对碳纤维织物浸渍行为的影响。研究结果表明: C/SiC-Ⅰ复合材料的室温弯曲强度最高, 达到336 MPa。不同裂解周期的微观形貌显示, C/SiC-0复合材料内部孔隙分布于碳纤维束间; C/SiC-Ⅰ复合材料内部较致密, 孔隙分布均匀; C/SiC-Ⅱ复合材料基体和束丝内部都存在孔隙, 说明三种聚碳硅烷浸渍液对C/SiC复合材料有不同的浸渍效果。凝胶渗透色谱(GPC)的分析结果显示, 由于浸渍液的分子量不同, 大分子无法浸渍到碳纤维束丝内部, 会造成裂解后的复合材料束内SiC基体较少, 造成其力学性能较低。