对二维编织复合材料弹性、热、强度及失效分析模型的评论

纺织结构作为聚合物复合材料的增强相在许多工业应用中变得非常重要,例如在航海与航空 领域中,由于其刚度及强度与其重量的高比值而倍受欢迎.由于工业及工程应用的需求, 研究纺织复合材料的弹性性质及失效行为变得非常重要.这篇文章重点评论了关于二维编织复合材料弹性、热、强度及失效分析的数值及解析模型,给出了主要的建模技术及方法. 同时也简单给出了编织复合材料从最初的一维模型到最近三维模型的发展.本文的目的不是对所 论模型的数学方法进行详细分析,而是告知研究者关于先前工作的主要思想. 这篇评论总共引用了122篇文献.      

Morphological and rheological properties of PET/clay nanocomposites

This work investigates the effects of clay chemistry and concentration on the morphology and rheology of polyethylene terephthalate (PET)/clay nanocomposites. The complex viscosity of the PET nanocomposites exhibited a more solid-like behavior, in contrast to the matrix that had a frequency-independent viscosity. In addition, at high frequencies where the behavior of the matrix should be dominant, a lower complex viscosity of the nanocomposites was observed due to PET degradation in the presence of the organoclays. The high-frequency data were used to estimate the matrix degradation using the Maron–Pierce equation. The apparent molecular weight of the PET matrix was found to decrease from 65 kg/mol for the neat PET to 30 kg/mol for a PET nanocomposite containing 8 wt% Cloisite®; 30B. The apparent yield stress in the nanocomposites was determined using the Herschel–Bulkley model. Yield stress increased with the level of exfoliation and clay concentration, from ～0 to 166 Pa when the clay concentration increased from 2 to 8 wt%.     

Estimation of the effective thermoelastic moduli of fibrous nanocomposites with cylindrically anisotropic phases

Recent developments in nanotechnology make it possible to fabricate nanofibers and identify their mechanical fibers. In particular, nanofibers are used as reinforcement in composites. The present work concerns unidirectional nanofibrous composites with cylindrically anisotropic phases and aims to analytically estimate their effective thermoelastic moduli. This objective is achieved by extending the classical generalized self-consistent model to the setting of thermoelasticity, to the case of cylindrically anisotropic phases, and to the incorporation of interface stress effect. Analytical closed-form estimations are derived for all the effective thermoelastic moduli, showing that these moduli depend on the fiber cross-section size. Numerical examples are provided to illustrate this size-dependent effect.     

三维四向碳／环氧编织复合材料剪切力学性能实验研究

讨论了编织复合材料剪切力学性能研究的实验方法,研制并分析了用于剪切力学性能研究的夹具,通过实验得到了不同编织角的三维四向碳／环氧编织复合材料剪切弹性模量的实验数据     

Hygric characterization of woven glass/epoxy composites

Hygric behavior of woven glass/epoxy composites is investigated by characterizing them using a novel experimental technique calledhunch-up technique. Based on one-dimensional diffusion theory, the through-thickness distribution of moisture concentration is derived and experimentally predicted for a laminate exposed to water. A procedure is prescribed to determine average axial hygric strain from the measured hunch-up distance of an axially constrained specimen exposed to water. The experimental results show that the hunch-up distance is extremely sensitive to moisture expansion. There results also give excellent confirmation to the theoretical predictions. An error analysis reveals that the hunch-up technique can reduce errors by an order of magnitude better than traditional measurement techniques.     

碳纤维织物增强PES-C/PTFE复合材料的摩擦学性能研究

利用热压成型方法制备了不同PES-C/PTFE含量的碳纤维织物增强复合材料,用LJ-500万能材料试验机和MRH-5A环块试验机分别考察了复合材料的力学性能和摩擦磨损性能,并研究了压制成型温度和等离子处理碳纤维织物对复合材料力学性能的影响.结果表明,碳纤维织物极大提高了PES-C/PTFE树脂弯曲强度,并且有效增加PES-C/PTFE树脂的耐磨性;PES-C/PTFE含量分别为42%和8%的碳纤维织物增强复合材料性力学及摩擦磨损综合性能最好.     

Processing and characterization of biodegradable polymer nanocomposites: detection of dispersion state

Nanobiocomposites of poly(lactic acid) (PLA) with 3–5 wt% organically modified montmorillonite (OMMT) were prepared by melt compounding in two different mixers, miniature twin-screw extruder and internal batch mixer, leading to different degrees of dispersion. The progress of dispersion was characterized by melt rheology coupled with light attenuation. Processed PLA/OMMT samples showed percolating networks in the melt, detected by a step increase in low-frequency elastic moduli. The melt elasticity of nanocomposites increased, while the light attenuation coefficient and the loss tangent decreased progressively with mixing energy and reached saturation that can be attributed to the maximum level of clay dispersion achieved in the present experimental conditions. Results showed that a combination of low-frequency loss tangent and light attenuation coefficient provides a potentially sensitive method for the characterization of the degree of clay dispersion. The direct correlation between light attenuation coefficient and loss tangent follows linear dependence and may open an approach for the rapid inline analysis of the degree of dispersion in melt-processed nanocomposites.     

石墨烯及其复合材料纳米力学研究进展

石墨烯是一种由碳原子构成的二维晶体,是目前已知最薄但却有着极高强度的纳米材料.由于在强度、导热性、电子输运和光学上显示出不同寻常的特性,石墨烯迅速成为材料科学、物理、化学和力学等学科的研究热点.与此同时,石墨烯复合材料的研究也迅速兴起.论文综述了近年来石墨烯及其复合材料的力学特性的研究进展.根据力学行为的差异,我们主要阐述了石墨烯面内力学特性、离面力学特性、原子尺度修饰和石墨烯复合材料力学特性的研究进展:石墨烯的面内拉伸力学特性通过纳米压痕等技术得到了测量,其断裂行为在微纳尺度下不能完全使用连续介质力学模型进行解释,在多层石墨烯情况下会出现超润滑现象;石墨烯的可控离面位移对于改变其物理特性有重要的意义,石墨烯上的屈曲受到手性和尺度的影响,在高频器件中存在着非连续性的离面响应;适当的原子尺度修饰可以改善石墨烯的拉伸和扭转力学特性;石墨烯可以改善复合材料的力学特性,如提高强度、韧性等,其主要强化效应是通过与基体材料的离面、面内力学行为结合产生的.最后,论文对石墨烯及其复合材料的力学研究进行了总结和展望.     

Rheological properties of PDMS/clay nanocomposites and their sensitivity to microstructure

The effect of microstructure on the rheology of clay/polymer nanocomposites is investigated using dispersions of organically treated clay in nearly Newtonian poly(dimethylsiloxane). Degree of dispersion and floc size are altered by using two different dispersion procedures and by changing the shear history. The scaling for dynamic moduli of attractive colloids applies, except for a possible relaxation mechanism at very low frequencies. The time to reach the crossover at a given frequency is found to be extremely sensitive to the dispersion procedure used. Hydrodynamic and elastic components of the steady state stress, on the other hand, evolve in a very similar fashion for the different systems. Although the relaxation times of the elastic stress components change drastically with flow-induced changes in structure, the dispersion process hardly has an effect at all. Intermittent start-up flows in the forward and reverse directions show that anisotropy persists long after the flow has been arrested, even at shear rates where no large reversible flocs are present. The degree of dispersion only had a limited effect on the anisotropy. Finally, the effect of shear on structure recovery has been studied. Very low shear rates are found to increase the rate of recovery, even for small strains.     

镍涂层对碳纳米管/镁复合材料界面结合强度影响的分子动力学模拟

使用分子动力学模拟方法研究了镍涂覆单壁碳纳米管(SWCNTs)增强镁基复合材料的力学行为.结果表明,镍涂覆SWCNT/Mg复合材料的杨氏模量显著大于未涂覆SWCNT/Mg复合材料的杨氏模量,在碳纳米管表面修饰的Ni涂层可有效传递碳纳米管和Mg基体之间的载荷.此外,还研究了Ni涂层数对SWCNT/Mg复合材料界面结合强度的影响.对于不同Ni涂层数,即无Ni涂层、1层Ni涂层和2层Ni涂层,涂覆1层Ni和2层Ni的SWCNT从Mg基体中完全拔出后的界面结合强度分别约为无Ni涂层SWCNT/Mg复合材料界面结合强度的3.9和11.9倍.     

Mechanical properties of unidirectional nanocomposites with non-uniformly or randomly staggered platelet distribution

Unidirectional nanocomposite structures with parallel staggered platelet reinforcements are widely observed in natural biological materials. The present paper is aimed at an investigation of the stiffness, strength, failure strain and energy storage capacity of a unidirectional nanocomposite with non-uniformly or randomly staggered platelet distribution. Our study indicates that, besides the volume fraction, shape, and orientation of the platelets, their distribution also plays a significant role in the mechanical properties of a unidirectional nanocomposite, which can be quantitatively characterized in terms of four dimensionless parameters associated with platelet distribution. It is found that, compared with other distributions, stairwise and regular staggering of platelets produce overall the most balanced mechanical properties, which might be a key reason why these structures are most widely observed in nature.     

Biaxial Mechanical Evaluation of Planar Biological Materials

A fundamental goal in constitutive modeling is to predict the mechanical behavior of a material under a generalized loading state. To achieve this goal, rigorous experimentation involving all relevant deformations is necessary to obtain both the form and material constants of a strain-energy density function. For both natural biological tissues and tissue-derived soft biomaterials, there exist many physiological, surgical, and medical device applications where rigorous constitutive models are required. Since biological tissues are generally considered incompressible, planar biaxial testing allows for a two-dimensional stress-state that can be used to characterize fully their mechanical properties. Application of biaxial testing to biological tissues initially developed as an extension of the techniques developed for the investigation of rubber elasticity [43, 57]. However, whereas for rubber-like materials the continuum scale is that of large polymer molecules, it is at the fiber-level (∼1 μm) for soft biological tissues. This is underscored by the fact that the fibers that comprise biological tissues exhibit finite nonlinear stress-strain responses and undergo large strains and rotations, which together induce complex mechanical behaviors not easily accounted for in classic constitutive models. Accounting for these behaviors by careful experimental evaluation and formulation of a constitutive model continues to be a challenging area in biomechanics. The focus of this paper is to describe a history of the application of biaxial testing techniques to soft planar tissues, their relation to relevant modern biomechanical constitutive theories, and important future trends. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structure-property relationships in polymer composites with micrometer and submicrometer graphite platelets

The objectives of this work were (a) to investigate the influence of micrometer and submicrometer scale graphite platelets of different aspect ratios and volume fractions on the effective and local quasi-static and dynamic properties of composites with micrometer and submicrometer scale reinforcement, and (b) to compare and evaluate mechanical property measurements of inhomogeneous materials via local (microscale) and bulk (macroscale) experimental methods. Small platelet volume fractions (0.5%) provided proportionally larger increase of the elastic and storage moduli compared to large volume fractions (3.0%). Randomly distributed 15 μm platelets provided marginally higher composite stiffness compared to 1 μm platelets while small volume fractions (0.5%) of 15 μm platelets had a pronounced effect on the effective Poisson's ratio. It was found that local property measurements of inhomogeneous materials conducted by nanoindentation are not representative of the bulk behavior even when the characteristic length of the inhomogeneity is an order of magnitude smaller than the indentation contact area. In this case, statistical averaging of data from a large number of indentations does not result in agreement with bulk measurements. On the other hand, for small aspect ratio platelets with dimensions two orders of magnitude smaller than the nanoindentation contact area, the nanoindenter-obtained properties agreed well with the effective material behavior. It was found that platelets residing at the specimen surface contribute the most to nanoindentation data, which implies that this technique is only valid for well-distributed nanoparticulate and microparticulate systems, and that nanoindentation cannot be used for depth profiling of microstructured composites.     

Theoretical prediction of stiffness and strength of three-dimensional and four-directional braided composites

Based on unit cell model, the 3D 4-directional braided composites can be simplified as unidirectional composites with different local axial coordinate system and the compliance matrix of unidirectional composites can be defined utilizing the bridge model. The total stiffness matrix of braided composites can be obtained by the volume average stiffness of unidirectional composites with different local axial coordinate system and the engineering elastic constants of braided composites were computed further. Based on the iso-strain assumption and the bridge model, the stress distribution of fiber bundle and matrix of different unidirectional composites can be determined and the tensile strength of 3D 4-directional braided composites was predicted by means of the Hoffman＇s failure criterion for the fiber bundle and Mises＇ failure criterion for the matrix.     

Effect of vinyl acetate content and silicate loading on EVA nanocomposites under shear and extensional flow

In this study, three EVAs (ethylene-vinyl acetate co-polymers) with different vinyl contents (VA) ranging from 9 wt% to 28 wt% (EVA9, EVA18 and EVA28) were melt blended with organo-clay to obtain polymer layered silicate nanocomposites. Filler intercalation and exfoliation were evidenced by X-ray diffraction. The melt state viscoelastic properties of EVA nanocomposites were studied to examine the influence of clay in altering the flow properties of these polymeric nanocomposites. The EVA18 and EVA28 nanocomposites exhibited remarkable difference in dynamic and steady shear properties compared to neat polymers. On the other hand, EVA9-5% nanocomposite did not exfoliate and exhibited rheological behaviour very similar to that of the neat polymer. Furthermore, the first normal stress difference was found to be dependent on the silicate loadings when measured at low shear stresses. The uniaxial extensional viscosity measurement indicated that the strain hardening was weaker in EVA nanocomposites compared to neat polymers. Environmental scanning electron (ESE)-microscopy elucidated a possible reason for reduced strain hardening in these systems.     

三维编程复合材料拉压性能的实验研究

本文通过对三维编程T300／QY9512复合材料纵向和横向的拉伸和压缩试验，研究了三维编程复合材料的刚度和强度特性，通过试验发现纵向拉压以及横向拉压应力应变曲线几乎为直线，具有脆性破坏的特点；三维编织T300／QY9512复合材料纵向拉压强度与横向拉压强度均不同，这主要是因为其破坏模式不同的缘故；另外，声发射数据表明了纵向拉伸试验中试件损伤的发展和破坏与声发射的敲击计数和能量相关。     

Experimental and analytical characterization of multidimensionally braided graphite/epoxy composites

This paper presents results of an investigation of a novel, through-the-thickness fiber-reinforced composite material. The generic name for this composite technology is multidimensional (X-D) braiding. X-D braided composites consist of a net-shaped, densely braided fiber skeleton which is rigidized with a structural epoxy-resin system. This material is an alternative to the conventional laminated composite and has the potential for being more resistant to delamination and matrix cracking. This paper describes results of the mechanical characterization of one graphite fiber system which was braided into panels in which two braid parameters could be investigated. The variables investigated included the effect of edge condition and braid pattern on the tensile, compressive and flexural properties of the braided panels. These properties were obtained in the braid direction only. The cutting of the specimen edges substantially reduced both tensile and flexural strengths and moduli. Of the three braid patterns investigated, 1×1, 3×1, and 1×1×1/2 F, the 3×1 braid pattern showed superior tensile performance, while the 1×1×1/2 F braid pattern exhibited superior flexural properties. The development of an analytical method for modeling the tensile performance of the multidimensionally (X-D) braided composite is also presented. The fiber geometry in X-D braids was modeled based on the braid parameters used in the construction of these composites. By the nature of the symmetry of the resulting braided structure, an analytical model based on classical lamination theory was used to determine the extensional stiffness in the three principal geometric directions of a braided composite. These analytical results are shown to compare favorably with those obtained experimentally. Finally, to further validate the ability of this material to contain damage, multidimensionally braided and conventionally laminated panels were impacted and the resulting damage was nondestructively determined. The multidimensionally braided material was shown to reduce the area of damage caused by impact by a factor of three for the energy levels tested.     

铜和石墨对铁基含油自润滑复合材料机械及摩擦学性能的影响

采用粉末冶金工艺制备了铁基含油自润滑复合材料,考察了Cu与石墨的含量对铁基含油自润滑复合材料的机模样性能、摩擦学性能及组织结构的影响,并利用X射线衍射仪、扫描电子显微镜及光学显微镜等对材料的组分、显微组织形态和结构以及磨损表面形貌等进行了系统的观察和分析,结果表明：添加适量Cu的Fe-Cu二元系材料的机械性能和摩擦学性能明显优于Fe系材料,这主要是因为Cu的加入改变了材料的微观结构。添加适量石墨的Fe-Cu-石墨三元系材料比Fe-Cu二元系材料具有更优异的摩擦学性能,但机械性能有所下降,这主要是由于石墨与油的协同润滑效应和石墨的加入改变了材料的微观结构所致。     

结晶特性与制造工艺对炸药件力学性能的影响

通过对塑料粘结炸药(PBX)在压制、热老化及贮存中炸药HMX、TATB和粘结剂F的性能变化规律的研究,揭示了加工和贮存条件对炸药件的力学性能的影响。得出如下结论:1)随着老化温度的提高,粘结剂F结晶度增加。TATB基PBX炸药经老化后力学性能没有明显变化,说明粘结剂结晶度和炸药颗粒度的变化对炸药总体性能影响不大。2)钢模压制的TATB基PBX药柱在经历多次温度循环后,TATB与粘结剂F界面的作用有所减弱,药柱内部产生由脱粘引起的缺陷,其力学强度下降。TATB基PBX药柱的力学强度与模量均随着环境温度的升高而呈下降趋势,而等静压成型能明显改善TATB基PBX的力学性能。