炭布/树脂复合摩擦材料的湿式摩擦学性能

基于炭布优异的摩擦磨损性能、自润滑性能以及低密度等特点,将其应用于湿式摩擦材料中,以适应高转速、大压力或润滑不充分等极端工况.分别以1K、3K和6K碳布为增强体,制备出三种炭布/树脂复合摩擦材料,研究了其湿式摩擦学性能.结果表明:随着纤维束内单丝数量的增加,摩擦材料的瞬时制动稳定性降低,动摩擦系数减小,但是耐磨性能提高.所有摩擦材料的磨损率小于1.10×10-5 mm3/J,表现出较好的耐磨性能,并且对偶材料的磨损率很小,仅为0.40×10-5 mm3/J.磨损主要表现为纤维断裂、拔出及树脂脱粘等形式,但是在磨损表面没有形成大尺寸磨屑和明显的“第三体”磨粒,导致摩擦材料和对偶材料的磨损率较小.     

On the modelling of complex anisotropic frictional contact laws

In this paper, the formulation of complex anisotropic frictional models with orthotropic friction condition and non-associated sliding rule is discussed. The friction law is described by a superellipse, which allow to consider a wide range of convex friction condition by simply varying the roundness factor affecting the shape of the limit surface. The sliding potential is also a superellipse but with a different semi-axis ratio, which lead to a non-associated sliding rule. For bodies in contact, the Signorini conditions can be formulated in terms of velocities and combined with the sliding rule to give the frictional contact law describing interfacial interactions. Its is shown that the frictional contact law as well as its inverse can be derived from the same scalar valued function called bi-potential. Due to the non-associated nature of the law, this relation is implicit. The advantage of the present formulation lies in the existence of stationary points of a functional, called bi-functional, that depends on both the displacements and the stresses.     

Potentiality of utilising natural textile materials for engineering composites applications

This paper gives an overview of utilising natural textile materials as reinforcements for engineering composites applications. The definition and types of textile materials are addressed to provide readers a thoughtful view on the role of these materials in a structural composite system. Available material properties of natural textile and their composites are critically reviewed here. In general, these materials are categorised into fibre, yarn and fabric forms. The load bearing capacity of natural textile fibre reinforced polymer composites is governed by the quantity, alignment and dispersion properties of fibres. It has been found that the natural fibre reinforced composites are limited to use in low to medium load bearing applications. However, a limited research work has been performed to date and there is a significant gap between the high performance textile fabric and their use as reinforcement in fibre reinforced composite materials.     

Predicting failure in textile composites using the Binary Model with gauge-averaging

First introduced over a decade ago, the Binary Model has evolved into a computationally efficient tool for predicting the properties of textile composites. Key to the formulation is the question of what details of the textile composite and the distributions of stress, strain, temperature, etc., are necessary and sufficient to represent the physics of the problem adequately and to ensure useful engineering predictions. This paper is concerned specifically with the prediction of the ultimate strength in cases where failure follows a single substantial local damage event, such as the rupture or kinking of a tow or the creation of a shear band mediated by matrix damage, without further increase in the external load. The accuracy of predictions is assessed for some triaxially braided carbon/epoxy composites. A gauge length is introduced that is suggested by the micromechanics of the failure mechanisms. Predictions are made by reference to strains that are averaged over a volume whose sides are commensurate with this gauge, but nevertheless retain spatial variations associated with the textile architecture. Failure criteria for tow rupture and matrix shear failure are taken from a single un-notched tensile test; the calibrated model then successfully predicts the failure mechanism (matrix shear or fiber rupture) and ultimate strength in un-notched and open-hole tension tests for any orientation of the textile fabric relative to the load axis, as well as bending and simple shear tests. The successful predictions are made using strains calculated for an entirely elastic representation of the material, which is possible because of the brittle character of the stress-strain curves. Predictions are also attempted using strains computed under the assumption that the textile material is homogeneous. These predictions are significantly inferior.     

Micromechanical modeling of imperfect textile composites

After fabrication the carbon-carbon (C/C) plain weave textile composites often show a certain degree of geometrical or material disorder including yarn waviness and misalignment or nesting of individual fiber tows together with intrinsic material porosity observed at all relevant scales. A brief survey of recently developed approaches for estimating overall elastic stiffnesses or thermal conductivities of such composite systems is presented in this paper. Depending on the source and type of available geometrical data the homogenization scheme usually relies either on finite element (FEM) simulations performed for a suitable Periodic Unit Cell (PUC) or employs one of the popular averaging techniques such as the Mori-Tanaka (MT) method. While existing applications of both methodologies are encouraging, there still exists a number of steps to be completed in the course of the future research.     

Mechanical properties of 3D fiber reinforced C/SiC composites

High toughness and reliable three dimensional textile carbon fiber reinforced silicon carbide composites were fabricated by chemical vapor infiltration. Mechanical properties of the composite materials were investigated under bending, shear, and impact loading. The density of the composites was 2.0–2.1 g cm⁻³ after the three dimensional carbon preform was infiltrated for 30 h. The values of flexural strength were 441 MPa at room temperature, 450 MPa at 1300°C, and 447 MPa at 1600°C. At elevated temperatures (1300 and 1600°C), the failure behavior of the composites became some brittle because of the strong interfacial bonding caused by the mis-match of thermal expansion coefficients between fiber and matrix. The shear strength was 30.5 MPa. The fracture toughness and work of fracture were as high as 20.3 MPa m^1/2 and 12.0 kJ·m⁻², respectively. The composites exhibited excellent uniformity of strength and the Weibull modulus, m, was 23.3. The value of dynamic fracture toughness was 62 kJ·m⁻² measured by Charpy impact tests.     

Mechanics of textile composites: Micro-geometry

Textile fabric geometry determines textile composite properties. Textile process mechanics determines fabric geometry. In previous papers, the authors proposed a digital element model to generate textile composite geometry by simulating the textile process. The greatest difficulty encountered with its employment in engineering practice is efficiency. A full scale fiber-based digital element analysis would consume huge computational resources. Two advances are developed in this paper to overcome the problem of efficiency. An improved contact-element formulation is developed first. The new formulation improves accuracy. As such, it permits a coarse digital element mesh. Then, a static relaxation algorithm to determine fabric micro-geometry is established to replace step-by-step textile process simulation. Employing the modified contact element formulation in the static relaxation approach, the required computer resource is only 1–2% of the resource required by the original process. Two critical issues with regards to the digital element mesh are also examined: yarn discretization and initial yarn cross-section shape. Fabric geometries derived from digital element analysis are compared to experimental results.     

Mapping of preform architecture for textile reinforced composite products

In this paper, a technique is developed to map the internal fabric preform structure of textile composite products. This technique is based on physical serial sectioning of the composite product. Image analysis and manual yarn recognition are used to identify yarn location in each section. Image stereology is utilized to reconstruct the fabric preform in 3-D space. Data regarding average yarn orientation throughout the composite part are then extracted. This technique is applied to map the internal fabric preform structure of a proprietary textile reinforced composite product. Experimental work points to locations of fabric distortions that occur during the composite product manufacture. This technique could be extended to evaluate the effect of these distortions on the final composite properties.     

Multiscale progressive failure analysis of textile composites

The experimental determination of stiffness and strength of textile composites is expensive and time-consuming. Experimental tests are only capable of delivering properties of a whole textile layer, because a decomposition is not possible. However, a textile layer, consisting of several fiber directions, has the drawback that it is likely to exhibit anisotropic material behavior. In the presented paper a finite element multiscale analysis is proposed that is able to predict material behavior of textile composites via virtual tests, solely from the (nonlinear) material behavior of epoxy resin and glass fibers, as well as the textile fiber architecture. With these virtual tests it is possible to make predictions for a single layer within a textile preform or for multiple textile layers at once. The nonlinear and pressure-dependent behavior of the materials covered in the multiscale analysis is modeled with novel material models developed for this purpose. In order to avoid mesh-dependent solutions in the finite-element simulations, regularization techniques are applied. The simulations are compared to experimental test results.     

A phenomenologically based damage model for textile composites with crimped reinforcement

A phenomenological damage model for textile composites with woven or braided reinforcement is presented and verified in this paper. Essential mechanical questions are clarified with regard to the dominant damage mechanisms and the definition of representative equivalent layers. Using the framework of continuum damage mechanics, damage is defined as the change of the tensor of elasticity. The initiation of damage is described using novel failure criteria based on the Failure-Mode-Concept. Damage variables and associated evolution laws are introduced to describe the subsequent degradation of the material stiffness. The capability of the proposed model is shown for woven thermoplastic composites made of hybrid glass–polypropylene yarns.     

Use of hypar-shell structures with textile reinforced cement matrix composites in lightweight constructions

The main goal of this paper is to define a design procedure for modular, lightweight and freeform structures by quantifying the relative importance of serviceability limit states and ultimate limit states. The modular building stones of the freeform structures under study are sandwich panels with a foamed polyurethane core and TRC (textile reinforced concrete) faces, shaped in the form of hyperbolic paraboloids (hypars). The shape of these modular building stones allows the production of structural elements on a reusable doubly-curved mould. For the dimensioning of the global modular structure, two states are important according to the Eurocodes: the ultimate limit states and the serviceability limit states. Due to the lightweight aspect of the modular structure, the serviceability limit states will gain in importance: stiffness and crack formation become important factors, as does the influence of repeated loading. These factors and their influence on the final design of the proposed structures will therefore be discussed in this paper.     

Effects of through-the-thickness stitches on the elastic behavior of multi-axial warp knit fabric composites

In order to improve the resistance to delamination and some in-plane and out-of-plane properties of composite materials for structural integrity, through-the-thickness reinforcement must be provided. The reinforcement is achieved by using the stitched multi-axial warp knit (MWK) fabrics as preforms for the fabrication of composite structures. In this study, the influence of stitches on the elastic behavior of MWK fabric composites under tensile and shear loadings was investigated by utilizing a unified micromechanical model. In the analysis, the in situ constituent properties and fiber volume ratios of insertion and stitching fibers determined from the geometric parameters set by the representative volume were used. The crucial step in the analysis was to correlate the averaged stress states in the constituents by adopting the bridging matrix. The experimental results were compared with the predicted results. It was found that the predicted results are in reasonably good agreement with the experimental results.     

The method of cells and the mechanical properties of textile composites

The method of cells has been gaining ground as a method for predicting the elastic properties of textile reinforced composite materials. This method deals away with the constant stress/strain assumption present in most analytical models allowing for a more accurate prediction of the elastic properties, with computational cost only a fraction in comparison with finite element analysis. A new implementation is presented here, which links this method to a mechanistic predictive geometry preprocessor allowing the presented model to deal with 2D and 3D textile reinforced composites. Numerical results for prediction of stiffness and strength of textile composites are generated and compared to other methods.     

C/SiC复合材料的氧化对其内耗行为的影响

通过分析C/SiC在高温(1250、1300和1350℃)空气氧化过程中质量、强度、物相、气孔率、微观形貌演变规律,并同时采用动态热机械分析仪测得内耗的变化趋势,研究了氧化对其内耗行为的影响规律,进而为以内耗表征复合材料的氧化行为奠定基础.为明确C/SiC各组元在氧化与内耗行为对应关系中所发挥的作用,进一步研究了SiC陶瓷在1300℃、空气中的氧化与内耗行为之间的对应关系.结果表明:SiC陶瓷氧化对其内耗行为的影响规律不明显且影响程度较弱;C/SiC在氧化过程中的内耗行为受C相的氧化损伤控制,且作用规律明显,其内耗保持率曲线均出现峰值,其中1250、1300和1350℃的峰值分别为6.65、3.48和1.59.     

Damage characterisation of glass/textile fabric polymer hybrid composites in sea water environment

The results of the experimental analysis carried out on the glass/textile fabric reinforced hybrid composites under normal condition and sea water environments have been reported. The critical stress intensity factor, interlaminar shear strength and impact toughness have been evaluated, both in interlaminar and translaminar directions. The specimen preparation and the experimentations were carried out according to the ASTM standards. Results have revealed that the damage in hybrid composite under sea water environment is entirely different. The characterising parameters have shown changes in their magnitudes with the variation in immersion time. The nature of fracture as a function of the reinforcement volume, loading and environmental conditions has been analyzed with the aid of scanning electron microscopy. The SEM analysis has shown that the fibers pull out, matrix cracking and also the nature of crack growth is different in sea water environment. The fracture in individual fiber has also been identified.     

高温处理对炭/炭复合材料摩擦性能的影响

利用热梯度化学气相沉积工艺制备了炭/炭复合材料,研究了热处理温度对炭/炭复合材料石墨化度、硬度和摩擦性能的影响.结果表明:随着热处理温度的升高,炭/炭复合材料的石墨化度增加,石墨片层的间距d002逐渐减小,石墨微晶的厚度LC逐渐增大,炭/炭复合材料的硬度降低.随石墨化度的增加,有更多的石墨微品碎片参与摩擦而形成摩擦表面膜,这些石墨碎片会填充摩擦表面的犁沟,使凹凸不平的摩擦面变得平整光滑,从而使摩擦系数减小、刹车力矩降低,刹车时间延长.     

2.5维机织复合材料经向拉伸弹性模量预测与试验验证

采用沿经向切片积分的方法计算2.5维机织复合材料的宏观经向拉伸弹性模量,与试验结果和传统的体积平均合成方法进行了对比,分析表明,本文中的方法能够有效提高传统体积平均法的预测精度.利用文中的方法分析了2.5维机织复合材料沿在机织循环过程中的经向拉伸弹性模量分布情况,并且对相关机织结构的2.5维机织复合材料进行了试验研究.研究发现,经纱弯曲程度较小的直联结构和添加平直经纱的衬经结构能够有效提高2.5维机织复合材料的经向拉伸弹性模量,并且能够改善经向拉伸应力-应变曲线的非线性.     

Fracture characterization of interfacial cracks with frictional contact of the crack surfaces to predict failures in adhesive-bonded joints

Anisotropic/isotropic interfacial cracks with frictional contact along bi-material interfaces are investigated to predict the load-bearing capacity of adhesive-bonded joints. An asymptotic solution for an interfacial crack in an anisotropic/isotropic bi-material is presented. The adhesion strength of composite/steel bonding is evaluated in terms of the energy release rate of an interfacial crack and the fracture toughness of the interface. The applicability of the finite crack-extension strain-energy release rate is validated by comparing the critical finite extension energy release rate of double-lap joints with the measured fracture toughness from bi-material end-notched flexure tests, and the asymptotic solution is compared with finite element analysis results to investigate the range of applicability.     

Bearing behavior of triaxially braided flat and tubular composites

An experimental study was carried out to evaluate the bearing behavior of glass fiber reinforced epoxy composites made from the triaxial braiding process. The bearing strengths of braided and machined holes in flat as well as tubular samples were compared. The effects of specimen geometry, braid angle, and specimen consolidation condition were investigated. Comparison between specimens with braided and machined holes indicated a higher bearing capacity for the braided joint hole compared with the machined one in tubular specimens whose thickness was not precisely controlled. For flat samples with uniform thickness, the braided hole showed similar or even lower bearing capacity compared with the machined hole, possibly due to the disturbance in fiber arrangement around the braided hole.