Statistical Model of the 3-D Braided Composites Strength

Based on the statistical model for the tensile statistical strength of unidirectional composite materials and the stress analysis of 3-D braided composites, a new method is proposed to calculate the tensile statistical strength of the 3-D braided compos- ites. With this method, the strength of 3-D braided composites can be calculated with very large accuracy, and the statistical parameters of 3-D braided composites can be determined. The numerical result shows that the tensile statistical strength of 3-D braided composites can be predicted using this method.

Investigation on the Thermal Conductivity of 3-Dimensional and 4-Directional Braided Composites

It is vital to choose a factual and reasonable micro-structural model of braided composites for improving the calculating precision of thermal property of 3-D braided composites by finite element method （FEM）. On the basis of new microstructure model of braided composites proposed recently, the model of FEM calculation for thermal conductivity of 3-dimennsional and 4-directional braided composites is set up in this paper. The curves of coefficient of effective thermal conductivity versus fiber volume ratio and interior braiding angle are obtained. Furthermore, comparing the results of FEM with the available experimental data, the reasonability and veracity of calculation are confirmed at the same time.

Tensile Properties of 3-Dimension-4-directional Braided C_f/Si C Composite based on Double-scale Model

The longitude tensile properties of 3-Dimension-4-directional（3D-4d） braided C/Si C composites（CMCs） were investigated with the help of a double scale model. This model involves micro-scale and unit-cell scale. In micro-scale, the tensile properties of fiber tows which involves matrix cracking, interfacial debonding, and fiber failure are studied. The unit-cell scale model can reflect the braided structure and simulate the tensile properties of 3D-4d CMCs by introducing the tensile properties of fiber tows into it. Quasi-static tensile tests of 3D-4d braided CMCs were performed on a PWS-100 test system. The predicted tensile stressstrain curve by the double scale model is in good agreement with that of the experimental results.

Effective Thermal Conductivity for 3D Five-Directional Braided Composites Based on Microstructural Analysis

A method for predicting effective thermal conductivities(ETCs) of three-dimensional five-directional(3D5D) braided composites is presented. The effective thermal conductivity prediction method contains a digital image processing technology. Multiple scanning electron microscopy(SEM)images of composites are analyzed to obtain actual microstructural features. These actual microstructural features of 3D5D braided composites are introduced into representative volume element(RVE) modeling. Apart from applying actual microstructural features,compression effects between yarns are considered in the modeling of RVE,making the RVE more realistic. Therefore,the ETC prediction method establishes a representative unit cell model that better reflects the true microstructural characteristics of the 3D5D braided composites. The ETCs are predicted with the finite element method. Then thermal conductivity measurements are carried out for a 3D5D braided composite sample.By comparing the predicted ETC with the measured thermal conductivity, the whole process of the ETC prediction method is proved to be effective and accurate,where a relative error of only 2.9 % is obtained.Furthermore,the effects of microstructural features are investigated,indicating that increasing interior braiding angles and fiber fill factor can lead to higher transverse ETCs. Longitudinal ETCs decrease with increasing interior braiding angles,but increase with increasing fiber fill factor. Finally,the influence of variations of microstructure parameters observed in digital image processing are investigated. To explore the influence of variations in microstructural features on variations in predicted ETCs,the actual probability distributions of microstructural features obtained from the 3D5D braided composite sample are introduced into the ETC investigation. The results show that,compared with the interior braiding angle,variations in the fiber fill factor exhibit more significant effects on variations in ETCs.

Effects of Fiber Volume Fraction on Damping Properties of Three-Dimensional and Five-Directional Braided Composites

The effects of fiber volume fraction on damping properties of carbon fiber three-dimensional and five-directional( 3D-5Dir)braided carbon fiber / epoxyres in composite cantilever beams were studied by experimental modal analysis method. Meanwhile,carbon fiber plain woven laminated / epoxy resin composites with different fiber volume fraction were concerned for comparison. The experimental result of braided specimens shows that the first three orders of natural frequency increase and the first three orders of the damping ratios of specimens decrease, when the fiber volume fraction increases. Furthermore,larger fiber volume fraction will be valuable for the better anti-exiting property of braided composites,and get an opposite effect on dissipation of vibration energy. The fiber volume fraction is an important factor for vibration performance design of braided composites. The comparison between the braided specimens and laminated specimens reveals that 3D braided composites have a wider range of damping properties than laminated composites with the same fiber volume fractions.

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.

Electrical⁃Mechanical Coupling Behaviors and Thermal⁃Resistance Effects of 3D Braided Composites

Electrical-mechanical coupling behaviors and thermal-resistance effects of 3D braided composites under external loads are important for structural health monitoring(SHM). Electrical conductivity and electrical-mechanical coupling behaviors of 3D braided carbon fiber/epoxy composites under uniaxial tension were reported. It was found that the transverse resistance decreased and the axial resistance increased with the increasing braiding angle. The fractional change in resistance increased linearly as the strain was below 1.0%, and the nonlinearity appeared when the strain exceeded 1.0%. The negative temperature coefficient(NTC) effect was observed before the glass transition temperature Tg of epoxy resin, while there was a positive temperature coefficient(PTC) effect after Tg.

Elastic Modulus Prediction of Three-dimension-4 Directional Braided C_f/SiC Composite Based on Double-scale Model

Double-scale model for three-dimension-4 directional（3D-4d） braided C/SiC composites has been proposed to investigate its elastic properties. The double-scale model involves micro-scale that takes fiber/ matrix/porosity in fibers tows into consideration with unit cell which considers the 3D-4d braiding structure. Micro-optical photographs of composites have been taken to study the braided structure. Then a parameterized finite element model that reflects the structure of 3D-4d braided composites is proposed. Double-scale elastic modulus prediction model is developed to predict the elastic properties of 3D-4d braided C/SiC composites. Stiffness and eompliance-averaging method and energy method are adopted to predict the elastic properties of composites. Static-tension experiments have been conducted to investigate the elastic modulus of 3D-4d braided C/SiC composites. Finally, the effect of micro-porosity in fibers tows on the elastic modulus of 3D-4d braided C/SiC composites has been studied. According to the conclusion of this thesis, elastic modulus predicted by energy method and stiffness-averaging method both find good agreement with the experimental values, when taking the micro-porosity in fibers tows into consideration. Differences between the theoretical and experimental values become smaller.

Effects of Microstructure on Quasi⁃Static Transverse Loading Behavior of 3D Circular Braided Composite Tubes

The effects of microstructure on quasi-static transverse loading behavior of 3D circular braided composite tubes were studied. Transverse loading tests were conducted. Transverse load-deflection curves were obtained to analyze the effects of braiding parameters including the braiding angle, the wall thickness, and the diameter on the transverse loading of 3D circular braided composite tubes. Breaking loads, moduli and strengths had also been used to describe the transverse loading behaviors. The failure morphologies were shown to reveal damage mechanisms. From the results, the increase in braiding angle, wall thickness and diameter increases the ability of anti-deformation and breaking load of braided tubes. The breaking load of specimen with a braiding angle of 45° is about 1.68 times that of specimen with a braiding angle of 15°. The breaking load of specimen with 4 layers of yarns is about 2.15 times that of specimen with 2 layers of yarns. The breaking load of the tube with a diameter of 25.5 mm is about 2.39 times that of the tube with a diameter of 20.5 mm.

Using Acoustic Emissions to Detect Damage in Three-Dimensional Braided Composites

Three-dimensional(3D)braided composites with better properties have been used in some particular industries.Some have had obvious signs of crack when they are braided.Others have had catastrophic failures occuring without warning.A new methodology for the analysis of failure modes in composite materials by means of acoustic emission techniques has been developed.The occurrence of fiber-breakage during tensile loading tests has been observed by the acoustic emission technology.Using acoustic emission technology is investigated as a means of monitoring 3D braided composites structures,detecting damage,and predicting impending damage.Some of the findings of the research project were presented.

Defects Recognition of 3D Braided Composite Based on Dual-Tree Complex Wavelet Packet Transform

Textile-reinforced composites,due to their excellent highstrength-to-low-mass ratio, provide promising alternatives to conventional structural materials in many high-tech sectors. 3D braided composites are a kind of advanced composites reinforced with 3D braided fabrics; the complex nature of 3D braided composites makes the evaluation of the quality of the product very difficult. In this investigation,a defect recognition platform for 3D braided composites evaluation was constructed based on dual-tree complex wavelet packet transform( DT-CWPT) and backpropagation( BP) neural networks. The defects in 3D braided composite materials were probed and detected by an ultrasonic sensing system. DT-CWPT method was used to analyze the ultrasonic scanning pulse signals,and the feature vectors of these signals were extracted into the BP neural networks as samples. The type of defects was identified and recognized with the characteristic ultrasonic wave spectra. The position of defects for the test samples can be determined at the same time. This method would have great potential to evaluate the quality of 3D braided composites.

On Completeness for a 3-D Linear Theory of Composite Laminate

In Ref [1] a model of 3-D composite laminate theory was described In the present paper,based on the basic equations about 3-D linear elasticity and classicalrariational principles,by the block matrix operation and the Lagrange's method of multipliers a series of basic equations and variational principles are obtained whichare more complere and more systematic than that in Ref [1]

A New Scheme and Microstructural Model for 3D Full 5-directional Braided Composites

Three-dimensional（3 D）braided composites are a kind of advanced ones and are used in the aeronautical and astronautical fields more widely. The advantages, usages, shortages and disadvantages of 3D braided composites are analyzed, and the possible approach of improving the properties of the materials is presented, that is, a new type of 3D full 5-directional braided composites is developed. The methods of making this type of preform are proposed. It is pointed out that the four-step braiding which is the most possible to realize industrialized production almost has no effect on the composites＇ properties. By analyzing the simulation model,the advantages of the material compared with the 3D 4-directional and 5-directional materials are presented. Finally, a microstructural model is analyzed to lay the foundation for the future theoretical analysis of these composites.

Numerical simulation on thermal expansion coefficient of 3D braided C/C composites

To effectively get the thermal expansion coef- ficient （CTE） of three-dimensional （3D） braided C/C composites and study the variations, a VC＋＋ program with graphical user interfaces was obtained, based on the yam unit model and numerical analysis. With the limited basic properties of carbon fibers and carbon matrix, CTE of 3D braided C/C composites is obtained at 85 ~C. The deviation between the simulated and exl^erimental axial CTE of 3D braided C/C composites is no more than 11%. The effects of different parameters （including the braiding angle of 3D braided preform, the fiber volume fraction and the porosity of 3D braided C/C composites, and the elastic modulus, Poisson＇s ratio and CTEs of carbon fibers and carbon matrix） were analyzed with the program. The results show that the axial CTE of C/C composites decreases with the increase of the braiding angle, the fiber volume fraction, and the porosity of 3D braided C/C composites. The transverse elastic modulus of carbon fibers has the greatest effect on the axial CTE among the studied mechanical parameters, followed by the elastic modulus and Poisson＇s ratio of carbon matrix.

Damage Simulation for 3D Braided Composites by Homogenization Method

In order to study the failure patterns and strength of 3D braided composites from the microscopic view, the damage propagation under tensile loading steps in three kinds of unit cells is simulated. The homogenization formula of micro-stress and the solving approach of finite element method are given firstly. A criterion is presented to determine the damage and its pattern of each element, and then the stiffness degradation method based on Murakami's geometric damage theory is used to simulate the status of damage under tensile loading steps for three kinds of unit cells. It can be seen that the damage percentage and damage pattern of damaged unit cell are totally different for different kind of unit cells. More damaged elements are observed for face cell and corner cell than that for body cell. It is also observed that the damage firstly occurs at the area of face cell, which agrees well with experimental results. It is verified that considering the effects of face and corner cells are important for the damage and strength analysis of 3D braided composites.

三维编织T300/环氧复合材料的弯曲性能及破坏机理

针对不同编织角、不同纤维体积含量、不同编织结构的三维编织T300/环氧复合材料进行了三点弯曲试验,获得了这些编织复合材料的主要弯曲力学性能,分析了不同编织工艺参数对材料弯曲力学性能的影响。对试件断口进行了宏观及扫描电镜观察,从宏、细观角度研究了材料的变形及其破坏机理。结果表明,三维编织T300/环氧复合材料具有良好的弯曲力学性能,弯曲载荷-挠度曲线呈现明显的线性特征;编织角、纤维体积含量及编织结构对复合材料的弯曲性能有较大影响;三维编织复合材料的弯曲破坏机理与编织工艺参数有关。

氧化处理对MC尼龙/C_(3D)复合材料摩擦磨损性能的影响

以己内酰胺单体和经氧化处理的碳纤维三维编织物(C3D)为原料,采用原位聚合方法制备了C3D增强浇铸尼龙(MC尼龙/C3D)复合材料。在磨损试验机上进行了滑动摩擦试验,采用扫描电子显微镜对磨痕和磨屑形貌进行观察和分析,研究了氧化处理对MC尼龙/C3D复合材料摩擦学性能的影响。结果表明,C3D经过氧化处理后所制MC尼龙/C3D复合材料的摩擦系数明显小于C3D未经氧化处理的MC尼龙/C3D复合材料。随着载荷的增加,材料的摩擦系数增大,而磨损率减小;在较高滑动速度下,摩擦系数和磨损率均较小;从磨痕和磨屑形貌观察到,C3D经氧化处理后与基体结合好,而未经氧化处理的C3D与基体剥离,但是C3D经氧化处理的复合材料的磨损率在较高载荷下略有增大。表明,C3D的氧化处理提高了碳纤维与基体间的结合强度,同时在一定程度上提高了复合材料的摩擦学性能。

复合材料齿轮弯曲强度仿真与试验研究

三维编织复合材料的性能与编织参数密切相关,为了研究碳纤维编织复合材料齿轮的弯曲性能及其与编织参数的关系,提出复合材料及齿轮弯曲性能预测模型。基于代表性体积单胞法和均质化思想建立复合材料的细观、宏观双尺度模型,采用体积平均法、借助有限元手段预测复合材料及齿轮的弯曲性能。并且,采用四步法编织及模压成型方法制备两对碳纤维编织复合材料齿轮,通过试验获得特定编织参数下复合材料及齿轮的弯曲性能。预测结果与试验结果吻合良好,验证了预测模型的准确性。最后,进行了多组编织参数下复合材料及齿轮弯曲性能的预测,得到了编织角和纤维体积分数对复合材料及齿轮弯曲性能的影响规律,并给出了使复合材料齿轮弯曲性能最佳的编织参数。

三维五向编织复合材料纵向性能的实验研究

通过对具有不同编织结构参数的三维五向编织复合材料试件的纵向拉伸和压缩实验,分析了该类材料的纵向拉、压刚度和强度随编织工艺参数的变化规律以及材料的失效形式。三维五向编织复合材料在破坏前基本保持线弹性,纵向拉、压破坏具有脆性特征,拉伸模量和压缩模量比较接近,但拉伸强度远大于压缩强度。编织角和纤维体积含量对材料性能的影响显著,纱线粗细的影响不大。提高第五向纱线的比例,可提高材料的纵向性能。此外,研究中采用短标距薄板试件,以避免试件产生整体屈曲和端部纤维束开裂破坏。

三维编织复合材料的疲劳性能

研究了三维编织复合材料的疲劳性能和编织结构对疲劳性能的影响.进行了应力比为0.1、实验频率为10 Hz的拉-拉疲劳性能测试.结果表明,三维编织复合材料的疲劳强度约为其抗拉强度的60%～80%,比金属材料的疲劳强度的相对值高.编织角是影响三维编织结构复合材料疲劳性能的一个主要因素.随着编织角的增大,疲劳过程中易出现各种损伤,而且伴随明显的升温现象.编织角大的试件在疲劳实验过程中模量变化明显,并且呈现逐渐升高的趋势,这与金属材料的双模量变化规律不同.在疲劳次数为100万次后,试件的剩余强度高于静载拉伸强度,这主要是由于在疲劳测试过程中,试件内编织纱线的取向更接近受力方向所致.