Weibull分布理论在硼纤维及B/Al复合材料中的应用

本文采用单一Weibull统计分布函数来描述硼纤维拉伸强度的分散性,发现具有不同断裂机制的缺陷所引发的断裂其拉伸强度分布分别符合Weibull分布,并具有各自独特的形态和参数;对热压复合前后的硼纤维的拉伸强度分布特征进行分析比较,结果表明热压复合过程对纤维的性能及拉伸强度分布特征有一定的影响,这主要是因为热压改变了纤维内部缺陷的类型和分布.本文还就以上研究结论在硼纤维的生产及其产品的性能表征方法、B/Al复合材料性能的非破坏性评估方法上的应用进行了讨论.     

实验条件对硼纤维强度的影响

本文研究了硼纤维强度的分布以及测试试样个数，湿度／标距长度等不同因素对硼纤维拉伸强度测量值的影响。结果表明湿度、标距长度对硼纤维拉伸强度有明显影响，认为Ｗｅｉｂｕｌｌ分布能够正确表征硼纤维强度的分布特征，Ｗｅｉｂｕｌｌ分布形状系数ｍ值的大小反映了硼纤维强度测量值的分散性，评定硼纤维强度时，在同等条件下测试４０至５０根试样，其结果就足以满足工程技术应用所要求的精度。     

BN和BN/SiC涂层对SiC纤维单丝拉伸性能的影响及其失效行为

采用化学气相渗透(CVI)工艺在两种国产典型SiC纤维表面沉积了BN和BN/SiC涂层，并对涂层的成分进行分析。利用Weibull分布评价SiC纤维的单丝拉伸强度，研究了沉积涂层前后纤维的拉伸断裂失效行为。结果表明，合适厚度(15 nm)的富碳层能够弥合SiC纤维表面的缺陷，减少纤维从表面开始失效的可能。采用CVI工艺制备的涂层厚度均匀，成分稳定；沉积BN和BN/SiC涂层后，两种SiC纤维的拉伸强度和弹性模量下降。BN涂层亦可修复纤维的表面缺陷，使纤维强度分布趋于集中。与无涂层纤维不同的是，沉积涂层后纤维拉伸断裂失效源分别为BN和SiC涂层的表面缺陷。     

不同针刺工艺的针刺复合材料面内拉伸强度分析

6种针刺工艺不同的碳纤维增强树脂基复合材料,其面内拉伸强度随着针刺密度和针刺深度的增大而降低,针刺处纤维的断裂使材料内的缺陷失稳扩展和面内纤维断裂,进而使材料整体拉伸失效。根据面内拉伸实验结果和纤维累计损伤理论并引入纤维体积折减系数,建立了分析针刺复合材料面内拉伸强度的理论模型。这个模型的预测结果与实验结果相符,并发现断裂纤维簇的个数与体积折减系数相关。用该模型可预报不同针刺工艺复合材料的面内拉伸强度,并指导设计针刺复合材料的预制体。     

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。     

基体合金对连续M40石墨纤维/Al复合材料纤维损伤及断裂机制的影响

选用M40石墨纤维为增强体,采用真空气压浸渗法制备了纤维体积分数为40%,基体合金分别为ZL102、ZL114A、ZL205A及ZL301合金的连续M40/Al复合材料,并用NaOH溶液萃取出M40纤维,研究了基体合金对连续M40/Al复合材料纤维损伤和断裂机制的影响。结果表明:不同的基体合金对M40纤维造成的损伤差异较大,从M40/ZL301复合材料中萃取的纤维拉伸强度最高,其拉伸强度为1 686 MPa,约为纤维原丝拉伸强度的38.3%;而从M40/ZL102复合材料中萃取的纤维拉伸强度最低,其拉伸强度仅为687 MPa,且纤维表面粗糙程度不一。不同M40/Al复合材料的断裂机制存在明显差别,M40/ZL102和M40/ZL114A复合材料断裂时无纤维拔出及界面脱粘,裂纹横向穿过纤维导致复合材料在低应力下失效;M40/ZL205A复合材料则表现为少量纤维拔出,界面轻微脱粘;同时,M40/ZL301复合材料表现为大量纤维拔出,裂纹沿界面纵向扩展,界面脱粘明显,纤维充分发挥其承载作用,复合材料的拉伸强度最高,达到了670.2 MPa。      

经编用钼丝纤维强力的概率分布分析

钼丝是用于制造空间可展开网状天线反射体的关键材料之一,具有高强度、低伸长、低热膨胀系数等特性。对钼丝纤维分别在3种不同拉伸隔距下进行强力测试与统计分析,发现钼丝纤维强力符合二参数和三参数的Weibull分布规律,并且更适合采用三参数Weibull分布来描述;随着拉伸隔距的增加,钼丝纤维的强度下降,强度分散性减小,表现出...     

硼纤维拉伸破坏的断裂特征EI

以光学显微镜和扫描电子显微镜为主要手段，观察和分析了用纤维拉伸破坏的断口表面，结果表明：硼纤维的断裂均起源于固有的缺陷和裂纹。将所观察到的断裂类型进行归纳和分类，并对沉积工艺进行相应的改进，从而避免或减少了缺陷和裂纹的出现，提高了硼纤维的拉伸强度。     

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.     

CFRP强度复合Weibull分布的物理特征分析

本文对ＣＦＲＰ材料疲劳后剩余强度的复合Ｗｅｉｂｕｌｌ分布的物理特征进行了初步分析。指出树脂基复合材料失效（破坏）机理的双重性（基体和纤维失效模型）决定了剩余强度（累积损伤程度）测试结果的分布特征。复合Ｗｅｉｂｕｌｌ分布曲线的特征与分析结果对设计、使用及材料研究都有重要参考意义。     

FRM用先进陶瓷纤维及其拉伸强度

陶瓷,其耐热性、耐腐蚀性、功能性等引起人们重视,作为一种新材料而得到开发,其有用性得到了公认。所谓新材料,或明显提高原料纯度,或使其形状极端微细化、微粉化或者薄膜化,或使原料组合、制成合金,从而使材料性能得到空前的发挥。碳化硅(SiC)、硼(B)、氧化铝(Al₂O₃)等陶瓷也可使其形状变成又细又长的纤维状,从而使拉伸强度提高,使陶瓷的使用变得容易起来。本稿将就最近开发的轻质高强陶瓷纤维的性能加以介绍。      

Mechanical properties and surface defects of boron fibres prepared in a closed CVD system

The tensile strength of boron fibres, prepared on a tungsten wire substrate suspended in a closed CVD system, has been investigated. The influence of strain-rate, gauge length, and fibre diameter on the tensile fracture stress of the fibres has been evaluated and compared to fracture stress data of fibres produced in continuous CVD processes. Moreover, the E-modulus of the prepared fibres has been measured. Finally the surface defects of the fibres have been examined and classified into fracture stress depressive surface defects and non-fracture stress depressive surface defects.     

Compression fracture of unidirectional carbon fibre-reinforced plastics

The effect of volume fraction and tensile strength of fibres, temperature and stress concentrators on the compression strength and fracture mode of unidirectional CFRP was studied. The cause of kinking is different for composites reinforced by low-(<3 GPa) and high-strength fibres. If fibre strength is high, the kink is initiated by composite splitting followed by fibre bend fracture in the tip of the split. In the case of low-strength fibres, kinking is initiated by compressive fracture of the fibres. The effect of stress concentrators on the CFRP compressive strength is described by linear fracture mechanics. In the presence of defects, fracture is a result of the emergence of splits near a hole. As the critical stress of splitting growth initiation reduces in proportion to the square root of the defect size, the Griffith criterion describes the composite compressive fracture. At elevated temperature, failure is caused by fibre buckling. The fracture band in this case is oriented perpendicular to the fibre direction. Carbon fibre compressive strength may be measured by the loop method. Bending a strand of carbon fibres glued to the elastic beam gives a fibre-controlled upper limit of the composite compressive strength.     

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.     

Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Probabilistic aspects of strength of short-fiber composites with planar fibre distribution

The study of the fracture of short-fibre composites must involve statistics as an integral part. Two components of composite strength, each with probabilistic aspects, are described in this paper: fibre crossover reinforcement, and fibre gap bridging before fracture. The fibre crossover density is proposed as a measure of mutual fibre strengthening. and simulations are performed to estimate this density. Several different fibre orientations are proposed which have identical elastic properties but different crossover densities, indicating that more information is required for strength prediction than for elastic property prediction. The crossover density is a random variable whose average increases roughly as a fibre length squared function, and whose coefficient of variation decreases with increasing fibre length. The phenomenon of fibres bridging microcrocks is also examined as a fracture mechanism for fibres whose length well exceeds their critical length. General probabilistic expressions are derived which give the distribution of the number of fibres bridging a gap perpendicular to the applied load. These formulae are applied to the distribution of strength of an aligned fibre system.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Deterministic method for predicting the strength distribution of a fibre bundle

Owing to the non-strain hardening plastic behaviour of the aluminium matrix and the weak fibre/matrix interface, it has been shown that the strength of a carbon fibre-reinforced aluminium matrix composite made by diffusion bonding of prepreg layers can be derived from the corresponding fibre bundle strength. Application of Coleman's model to predict bundle strength leads to the conclusion that the composite must break when 15% of the fibres are broken. This greatly overestimates the experimental composite strength. Overestimations made by using the Coleman model are due to some implicit assumptions which are not valid in the case under consideration and which may consequently not describe our material. A new approach is proposed for the calculation of the strength distribution of a fibre bundle, based on the same fracture mechanism (fibres fracture progressively until the catastrophic fracture) but without restrictive assumptions. The real interpolated experimental fibre strength distribution (and not the Weibull distribution) is taken into account to predict bundle strength. The proposed method clearly shows the limit of strength prediction, in term of bundle size (number of fibres and gauge length). The risk of making predictions following the Weibull distribution out of the range of the observations (through single-fibre tensile tests) is demonstrated.