碳纤维的强度及离散的探讨

用韦伯理论评价了碳纤维的抗拉强度和分散,验证了形状参数m和强度离散的关系.同时,探讨了体积效应和碳纤维强度离散,证实了细度离散对强度离散的影响.     

The effect of surface modification with carbon nanotubes upon the tensile strength and Weibull modulus of carbon fibers

Carbon fibers are widely used as reinforcements in composite materials because of their high specific strength and modulus. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6?GPa), and ultrahigh modulus pitch-based (more than 900?GPa) carbon fibers have been commercially available. In contrast, carbon nanotube (CNT) with the extremely high tensile strength have attracted attention as reinforcements. An interesting technique to modify the carbon fiber is CNT grafting on the carbon fiber surface. CNT-grafted carbon fibers offer the opportunity to add the potential benefits of nanoscale reinforcement to well-established fibrous composites to create micro-nano multiscale hybrid composites. In the present study, the tensile properties of CNT grown on T1000GB PAN- and K13D pitch-based carbon fibers have been investigated. Single filament tensile test at gauge lengths of 1, 5, and 25?mm were conducted. The effect of gauge length on tensile strength and Weibull modulus of CNT-grafted PAN- and pitch-based carbon fibers were evaluated. It was found that grafting of CNT improves the tensile strength and Weibull modulus of PAN- and pitch-based carbon fibers with longer gauge length (≥5?mm). The results also clearly show that for CNT-grafted and as-received PAN- and pitch-based carbon fibers, there is a linear relation between the Weibull modulus and the average tensile strength on log–log scale.     

A Bootstrap Control Chart for Weibull Percentiles

The problem of detecting a shift of a percentile of a Weibull population in a process monitoring situation is considered. The parametric bootstrap method is used to establish lower and upper control limits for monitoring percentiles when process measurements have a Weibull distribution. Small percentiles are of importance when observing tensile strength and it is desirable to detect their downward shift. The performance of the proposed bootstrap percentile charts is considered based on computer simulations, and some comparisons are made with an existing Weibull percentile chart. The new bootstrap chart indicates a shift in the process percentile substantially quicker than the previously existing chart, while maintaining comparable average run lengths when the process is in control. An illustrative example concerning the tensile strength of carbon fibers is presented. Copyright © 2005 John Wiley & Sons, Ltd.     

Statistics of single‐fibre tensile strength incorporating spatial distribution of microcracks

The random distribution of single‐fibre tensile strength has been commonly characterized by the two‐parameter Weibull statistics. However, the calibrated Weibull model from one set of strength data at a given gauge length cannot accurately predicts the strength variation of the fibre at different gauge lengths. Instead of presuming the two‐parameter Weibull distribution or any other specific statistical distribution for the single‐fibre strength to begin with, this work proposes an approach to incorporating the appropriate spatial flaw distribution within a fibre and synchronizing multiple sets of tensile strength data to evaluate the single‐fibre strength distribution. The approach is examined and validated by published single‐fibre strength data sets of glass, ceramic and synthetic and natural carbon fibres. It is shown that the single‐fibre strength statistics does not necessarily always follow the two‐parameter Weibull distribution.     

熔融纺丝方法制备纳米碳管/聚丙烯复合纤维及其拉伸性能

采用传统的熔融纺丝技术大量制备了定向性良好的纳米碳管／聚丙烯复合纤维。扫描电镜观察证实了纳米碳管在纤维里的定向性以及分散性都得到了较大的改善。通过拉伸实验测试了纳米碳管／聚丙烯复合纤维的力学性能，采用weibull统计分析发现纳米碳管的添加显著提高了复合纤维的拉伸强度，当添加纳米碳管的质量分数达到3％时，纤维强度最高，达到61MPa，超过聚丙烯纤维强度120％。复合纤维拉伸断口的形貌特征也证实了纳米碳管添加对复合纤维拉伸性能影响存在临界现象。     

Cumulative Damage Models for System Failure With Application to Carbon Fibers and Composites

To estimate the probability distribution of the tensile strength of materials, specimens tested are typically of a variety of sizes, or “gauge lengths.” Here, a cumulative damage argument is used to develop new models for the strength of a general system that incorporate the size as a variable and are strong competitors to the commonly used Weibull model. These strength distributions can be represented as three-parameter versions of the Birnbaum-Saunders distribution and the inverse Gaussian distribution. Estimation is discussed and illustrations are presented for experimental strength data for carbon fibers and small composite specimens.     

Statistical Tensile Strength for High Strain Rate of Aramid and UHMWPE Fibers

Technora,madefromaramidfibersandsome whatsimilartothecommonlyknowKevlar,and Ultrahighlyorientedhighmolecularweightpolyeth ylene UHMWPEfibersarealloftenusedinflexible armourapplicationsandhencesubjectedtohighrates ofloading.Theutilizationoftheirreinforcedcom positesarmourincertainballisticapplicationsisin creasinglypreferredoverconventionalrigidmetalar moursystemsbecauseofitssuperiorstrength to weightratioandflexibility.Todate,thedesignand developmentofsuchfabricarmoursystemshave largelybeen…     

Mechanical strength of cold plasma treated PET fibers

In this work, the effects of cold plasma treatment on the mechanical strength of polyethyleneterephthalate (PET) fibers has been verified. Single fibers were treated with oxygen and a mixture of oxygen and tetrafluoroethylene in a cold plasma reactor for 30, 100 and 200 s. The single fibers were then tested in tensile mode and the mechanical strength was analyzed by using the Weibull distribution function.     

The effect of high-temperature vapor deposition polymerization of polyimide coating on tensile properties of polyacrylonitrile- and pitch-based carbon fibers

Carbon fibers are widely used as a reinforcement in composite materials because of their high-specific strength and modulus. Current trends toward the development of carbon fibers have been driven in two directions; ultrahigh tensile strength fiber with a fairly high strain to failure (~2 %), and ultrahigh modulus fiber with high-thermal conductivity. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6 GPa), and ultrahigh modulus pitch-based (more than 900 GPa) carbon fibers have been commercially available. In the present work, the tensile properties of polyimide-coated PAN-based (T1000GB, T300, and M60JB) and pitch-based (K13D and XN-05) carbon fibers have been investigated using a single-filament tensile test. The pyromellitic dianhydride/4-4′-oxydianiline polyimide coating was deposited on the carbon fiber surface using high-temperature vapor deposition polymerization (VDP_H). The Weibull statistical distributions of the tensile strength were characterized. The results clearly show that the VDP_H polyimide coating improves the tensile strength and the Weibull modulus of PAN- and pitch-based carbon fibers.     

Characterization of interfacial and mechanical properties of “green” composites with soy protein isolate and ramie fiber

Environment-friendly fiber-reinforced composites were fabricated using ramie fibers and soy protein isolate (SPI) and were characterized for their interfacial and mechanical properties. Ramie fibers were characterized for their tensile properties and the parameters for the Weibull distribution were estimated. Effect of glycerol content on the tensile properties of SPI was studied. Interfacial shear strength (IFSS) was determined using the microbond technique. Based on the IFSS results and fiber strength distribution, three different fiber lengths and fiber weight contents (FWC) were chosen to fabricate short fiber-reinforced composites. The results indicate that the fracture stress increases with increase in fiber length and fiber weight content. Glycerol was found to increase the fracture strain and reduce the resin fracture stress and modulus as a result of plasticization. For 10% (w/w) of 5 mm long fibers, no significant reinforcement effect was observed. In fact the short fibers acted as flaws and led to reduction in the tensile properties. On further increasing the fiber length and FWC, a significant increase in the Young's modulus and fracture stress and decrease in fracture strain was observed as the fibers started to control the tensile properties of the composites. The experimental data were compared to the theoretical predictions made using Zweben's model. The experimental results are lower than the predicted values for a variety of reasons. However, the two values get closer with increasing fiber length and FWC.     

碳纤维静、动态加载下拉伸力学性能的试验研究

利用岛津试验机和自行研制的旋转盘式击拉伸试验装置,对Ｔ３００和Ｍ４０Ｊ两种碳纤维实施了应变速率范围为０．００１－１３００ｓ＾－１的静、动态拉伸试验,获得了两种材料在不同应变速率下的完整的应力变曲线。     

The effect of gauge length on tensile strength and Weibull modulus of polyacrylonitrile (PAN)- and pitch-based carbon fibers

Carbon fibers are widely used as a reinforcement in composite materials because of their high specific strength and modulus. Current trends toward the development of carbon fibers have been driven in two directions; ultrahigh tensile strength fiber with a fairly high strain to failure (~2%), and ultrahigh modulus fiber with high thermal conductivity. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6 GPa), and ultrahigh modulus pitch-based (more than 900 GPa) carbon fibers have been commercially available. In this study, the tensile strengths of PAN- and pitch-based carbon fibers have been investigated using a single filament tensile test at various gauge lengths ranging from 1 to 250 mm. Carbon fibers used in this study were ultrahigh strength PAN-based (T1000GB, IM600), a high strength PAN-based (T300), a high modulus PAN-based (M60JB), an ultrahigh modulus pitch-based (K13D), and a high ductility pitch-based (XN-05) carbon fibers. The statistical distributions of the tensile strength were characterized. It was found that the Weibull modulus and the average tensile strength increased with decreasing gauge length, a linear relation between the Weibull modulus, the average tensile strength and the gauge length was established on log–log scale. The results also clearly show that for PAN- and pitch-based carbon fibers, there is a linear relation between the Weibull modulus and the average tensile strength on log–log scale.     

纳米SiO2改性乳液上浆剂对炭纤维抗拉强度的影响

利用Weibull统计分布分析了上浆对炭纤维单丝抗拉强度的影响。通过扫描电镜（SEM）观察和X射线能谱仪（EDS）分析了上浆前后炭纤维表面和截面的结构形貌及表面元素组成。结果表明：当纳米SiO2粒子与上浆树脂的质量比为0．5％时，上浆炭纤维单丝的抗拉强度最高，增幅达10．1％；Weibull参数m值最大，强度的分散性最小。EDS显示了改性上浆炭纤维表面Si元素含量增大。上浆炭纤维比未上浆的表面较均匀，纹理沟槽变浅；截面凹凸不平，呈现一定的韧性断裂特性。说明上浆剂中添加纳米SiO2粒子可以明显提高上浆炭纤维的强度。     

Tensile strength and fracture surface characterisation of sized and unsized glass fibers

The tensile strength of commercial glass fibers is examined by single fiber tensile tests. The fibers are analysed as received from the manufacturer (sized) and after a heat treatment at 500C (unsized). Weibull plots of the two series are used for comparison of the strengths of the sized and unsized fibers. It is shown that large sample sizes (over 60 tests) are required to lead to a reliable two-parameter Weibull distribution. The experimental tests clearly indicated that the unsized fibers were weaker in the low strength range, but had similar strength in the high strength range. An investigation of the fracture surfaces in the SEM showed distinct differences in the fracture patterns for high and low strength fibers. Fracture mechanics were applied to estimate the original flaw size and relate the observed fracture mirror surface to the fiber strength. Based on the observation of surface flaws, a healing mechanism by the sizing is considered likely for this type of fiber and sizing, thereby effectively increasing the strength of the fiber in the presence of larger surface flaws.     

玻纤和碳纤在低温下的强度统计特性

利用纤维强伸实验仪和自制的低温实验装置,研究了碳纤维与玻璃纤维在低温(77K)下的拉伸力学性能.由测试数据发现,低温下纤维的强度分布具有统计性,采用Weibull统计分布可以较好的进行描述.在低温下,两种纤维的强度Weibull分布尺度参数和平均强度都有不同程度的提高,而形状参数和弹性模量则基本保持不变.     

Tensile mechanical behavior of T300 and M40J fiber bundles at different strain rate

Tensile mechanical behavior of T300 fiber bundles and M40J fiber bundles have been studied in the strain rate range from 0.001 1/s to 1300 1/s and complete stress strain curves were obtained. Results show that both ultimate strength and failure strain of two materials are strain rate insensitive, and T300 fiber and M40J fiber can be regarded as strain rate insensitive materials. On basis of the fiber bundles model and the statistic theory of fiber strength, single Weibull distribution model and bimodal Weibull distribution model have been developed to describe mechanical behavior of fiber bundles. And a method for determine the statistic parameters of fibers by tensile tests of fiber bundles is established, too. The simulated stress strain curves from the model are in good agreement with the test data. Simulated results show that the strength of T300 fiber can be described by single Weibull distribution function, and the strength of M40J fiber should be described by bimodal Weibull distribution function.     

On Some Permissible Estimators of the Location Parameter of the Weibull and Certain Other Distributions

In this paper an estimator of the location parameter of the Weibull distribution is proposed which is independent of its scale and shape parameters. Several properties of this estimator are established which suggest a proper choice of three ordered sample observations insuring a permissible estimate of the location parameter. This result is valid for every distribution which has the location parameter acting as the origin or threshold parameter. Asymptotic properties of such an estimator of the location parameter of the Weibull distribution is discussed. Finally, the paper contains a brief discussion on a percentile estimator of the location parameter of the Weibull distribution and includes some numerical illustration.     

裂纹分布与Weibull模数关系的研究

讨论了纤维的裂纹分布与Ｗｅｉｂｕｌｌ模数ｍ之间的关系。用小角Ｘ－ｒａｙ散射测定四种粗细不同的粘胶基炭纤维的孔径大小及其分布，并分析比较了四种纤维的Ｗｅｉｂｕｌｌ模数ｍ和强度值，结果表明纤维越细，其ｍ值越大，纤维内所含的大尺寸孔洞越少，央纤维的强度也越高。     

Competing Weibull and stress-transfer influences on the specific tensile strength of a bonded fibrous network

We present the results of an experimental study into the influence of areal density on the tensile strength of paper. Our data illustrate a non-linearity in the relationship between tensile strength and areal density that gives rise to a weight dependent maximum of specific tensile strength. A model is derived showing that this maximum can be attributed to the competing effects of a non-linear increase in the efficiency of stress transfer and hence strength with network weight, and the increased Weibull probability of reduced strength with increasing cross-sectional area of the network. We propose that two Weibull moduli may be required to account for the orthotropic nature of network structure and its contribution to strength.     

The relationship between tensile and bending properties of non-linear composite materials

The effect of a Weibull distribution of flaws on the strength on non-linear composite materials is considered. The tensile strength applying in bending is compared with that in direct tension. This leads to an estimate of the ratio modulus of rupture to ultimate tensile strength for composite materials.