Behavior of electrical resistance of SiC<Subscript>CVD</Subscript> fiber and development of micro-heater with SiC<Subscript>CVD</Subscript> fiber

Behavior of electrical resistance was examined in room temperature and elevated temperatures up to 1000 °C for two types of SiC_CVD fibers with diameters of 140 and 70 μm, respectively. The results showed that electrical resistance showed a good linear relationship with the length of fibers. Electrical resistance decreased as temperature increased, besides, temperature coefficient of electrical resistance was a minus constant, −5.2 × 10⁻⁴ °C⁻¹ except that in the first heating. In the first heating, electrical resistance and temperature coefficient increased and had a peak in the range of 550–700 °C owing to the burning of the carbon-rich layer on the fiber surface. It suggested that behavior of electrical resistance of the fibers depended mainly on the carbon core and the carbon-rich layer. It was confirmed that SiC_CVD fiber could be used as heating elements for micro-heater and finally a micro-heater using SiC_CVD fiber as heating elements was developed.     

在碳化硅基复合材料中碳纤维的就位强度

在对用断裂镜面法(fracture mirror method)评估纤维就位强度(in-situ strength)的方法进行了评述之后,对此方法进行了修正,即采用纤维束试验确定镜面常数, 将标距长度定义为两倍纤维拔出长度与两倍失效长度之和,采用分级平均统计分析方法确定纤维就位强度的Weibull参数和平均强度。采用修正的断裂镜面法,对聚合物先驱体转化法制备的M40JB纤维增强的碳化硅基复合材料中M40JB纤维的就位强度进行了分析,得到了M40JB纤维的Weibull参数和平均强度,并与未修正的断裂镜面法得到的结果进行了比较。      

A probabilistic approach on residual strength and damage buildup of high-performance fibers

An elementary, probabilistic model for fiber failure, developed by Coleman in the fifties of the last century, predicts a Weibull distributed time-to-failure for fibers subject to a constant load. This has been experimentally confirmed, not only for fibers but for load-bearing products in general. In this paper, we analyze residual strength, i.e., the strength after having survived a given load program. We demonstrate that the Weibull modulus, describing variability of time-to-failure, affects residual strength. It determines (a) how fast residual strength of fibers decays during their service life, (b) the residual strength variability, and (c) the fraction of surviving fibers during service life. Experiments show that residual strength of Twaron fiber (p-aramid fiber), exceeding predictions of Coleman’s model, remains unrelentingly high (close to virgin strength) during service life.     

Interfacial control of uni-directional SiCf/SiC composites based on electrophoretic deposition and their mechanical properties

Interfacial control of uni-directional SiC_f/SiC composites were performed by EPD, and their mechanical properties at room temperature were evaluated. The effect of the thickness of carbon interphase on SiC fibers by EPD on mechanical properties of uni-directional SiC_f/SiC composites was also investigated. The average thickness of carbon coating on SiC fibers increased from 42 nm to 164 nm with an increase in the concentration of colloidal graphite suspension for EPD. Dense SiC_f/SiC composites were achieved and their fiber volume fraction was 47–51%. The SiC_f/SiC composites had a bending strength of 210–240 MPa. As the thickness of carbon coating was below 100 nm, the SiC_f/SiC composites (SC01 and SC02) fractured in almost brittle manner. In contrast, the SiC_f/SiC composites (SC03) showed a pseudo-ductile fracture behavior with a large number of fiber pullout as the thickness of carbon coating was above 100 nm. The fracture energy of SC03 was 3–4 times as high as those of SC01 and SC02 and the value was about 1.7 kJ/m². In consideration of the results of mechanical properties, the thickness of carbon coating on SiC fibers should be at least 100 nm to obtain high-performance SiC_f/SiC composites. The fabrication process based on EPD method is expected to be an effective way to control the interfaces of SiC_f/SiC composites and to obtain high-performance SiC_f/SiC composites.     

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.     

Bounds on the probability of failure of composite materials

Bounds are obtained for the cumulative distribution function of the strength of composite materials. The analysis is based on the chain-of-bundles probability model for composite strength. The bounds are computed from probabilities for the chance failure of two or more adjacent fibers in a bundle. A Weibull distribution is assumed for fiber strength, and fibers adjacent to failed fibers are subjected to overloading according to a specified load sharing rule. As the composite grows in size, the bound approaches a Weibull distribution. The Weibull shape parameter for the composite is double that for the fiber, but the scale parameter is somewhat less. Features of the limiting Weibull bound are shown to be consistent with experimental observation, though numerically the bound is quite conservative.

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Résumé On obtient les bornes d'une fonction de distribution cumulative de la résistance des matériaux composites. L'analyse est basée sur un modèle probabiliste d'une série de bottes assurant la résistance d'un composite. Les bornes sont calculées au départ d'une probabilité que deux ou d'avantage de fibres adjancentes dans une botte puissent se romprent. On suppose que la résistance d'une fibre répond à une distribution de Weibull et que les fibres adjacentes à la fibre rompue sont sujettes à une surcharge correspondant à une règle spécifiée pour le partage des charges. Quand le composite augmente de dimension, la liaison atteint une distribution de Weibull. Le paramètre de forme de Weibull pour le composite est double de celui pour la fibre, mais le paramètre d'échelle est quelque peu inférieur. Les caractéristiques de la borne limite de Weibull sont établies comme étant consistantes avec l'observation expérimentale, encore que numériquement la borne se révèle tout à fait conservative.

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This research was supported by the Energy Research and Development Administration under Contract EY-76-S-02-4027.     

Mechanical properties of basalt fibers and their adhesion to polypropylene matrices

This work is aimed to study the mechanical properties of basalt fibers, and their adhesion to polypropylene (PP) matrices. Single filament tensile tests were used to calculate the strength of different types of fibers, characterized by different providers and surface treatment. Single fiber fragmentation tests (SFFT) were used to calculate the critical length of the fibers, in a homopolymer PP matrix and in a maleic anhydride modified PP matrix. It was shown that the tensile strength of the fibers is not significantly influenced by the origin or the surface treatment. Only fibers without any sizing show very reduced mechanical properties. On the other hand, the tensile strength was shown to be severely dependent on the filament length. Weibull theory was used in order to calculate the fitting parameters σ₀ and β, which were necessary in order to extrapolate the tensile strength to the critical length determined by SFFT. This allowed calculating the adhesion properties of the basalt fibers. It was shown that fiber–matrix adhesion is dependent on both the presence of sizing on the fiber surface, as well as on the modification of the matrix.     

High-temperature behavior and degradation mechanism of SiC fibers annealed in Ar and N<Subscript>2</Subscript> atmospheres

The thermal and mechanical stability of SiC fibers at elevated temperature is an important property for the practical application of SiC fiber-reinforced ceramic matrix composites and is related to the heat-treating atmosphere. In this study, the high-temperature behavior of KD SiC fibers with low oxygen content was investigated in both Ar and N₂ at temperatures from 1400 to 1800 °C through scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, resistivity measurements, and tensile tests in order to understand the effects of atmospheres on the degradation of the fibers. The results show that high-temperature treatment caused more severe strength degradation in Ar than in N₂. In particular, the fibers heat treated in N₂ at 1700 °C retained a relatively high strength of 1.52 GPa, 60 % of their original strength, while the fiber strength was completely lost after heat treatment in Ar. Fiber strength degradation was mainly caused by a combination of crystal growth and surface flaws. The formation of huge grains and porosity in the fiber surfaces, owing to the thermal decomposition of the SiC _x O _y N _z and SiC _x O _y phases, significantly degraded the strength for fibers heat treated in Ar. However, the suppressing effect of N₂ on the decomposition of the SiC _x O _y N _z phase in the fiber surfaces and nitrided case on the decomposition of the SiC _x O _y phase in the fiber cores, led to higher SiC fiber temperature stability in N₂ rather than Ar.     

石英纤维及其 Mini复合材料强度及分布规律

针对国产石英纤维强度，本工作采用液相浸渍法（硅溶胶、石英浆料和氮化硅浆料）制得Mini复合材料，利用两参数Weibull分布，对其强度及分布进行了研究，采用Kolmogrov非参数检验对其分布进行了检验．同时研究了热处理对纤维束强度分布的影响．结果表明，可用两参数Weibull分布表征纤维束及其Mini复合材料强度分布．热处理使纤维强度急剧下降，且强度分散性变大；相同处理温度下，由硅溶胶制得的Mini复合材料强度较高，分散性较小；而石英或氮化硅浆料制备的Mini复合材料强度较低．     

Effect of strain rate on tensile behavior of carbon fiber reinforced aluminum laminates

In this paper, the tensile behavior of carbon fiber reinforced aluminum laminates (CRALL) has been determined at a strain rate range from 0.001 s^− 1 to 1200 s^− 1. Experimental results show that CRALL composite is a strain rate sensitive material, and the tensile strength and failure strain both increased with increasing strain rate. A linear strain hardening model has been combined with Weibull distribution function to establish a constitutive equation for CRALL at different strain rates. The analysis of the model shows that the Weibull scale parameter, σ₀, increased with increasing strain rate, but Weibull shape parameter, β, can be regarded as a constant.     

Strength evaluation and failure analysis of unidirectional composites using Monte-Carlo simulation

Tensile strength and failure process of composite materials depend on the variation in fiber strength, matrix properties and fiber–matrix interfacial shear strength. A Monte-Carlo simulation considering variation in these factors has been widely used to analyze such a complicated phenomenon as a strength and simulate the failure process of unidirectional composites. In this study, a Monte-Carlo simulation using 2-D and 3-D (square and hexagonal array) model was performed on unidirectional graphite/epoxy and glass/polyester composites. The results simulated by using 3-D hexagonal array model have a good agreement with the experimental data which were tensile strength and failure process of unidirectional composites.     

Fabrication of SiCf/SiC composites by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites, employing KD-1 SiC fibers (from National University of Defense Technology, China) as reinforcements, were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and properties of the as prepared SiC_f/SiC composites were studied. The results show that the density and open porosity of the as prepared SiC_f/SiC composites are 2.1 g/cm³ and 7.7%, respectively. The SiC fibers are not severely damaged during the VSI process. And the SiC fibers adhere to the matrix with a weak interface, therefore the SiC_f/SiC composites exhibit non-catastrophic failure behavior with the flexural strength of 270 MPa, fracture toughness of 11.4 MPa·m^1/2 and shear strength of 25.7 MPa at ambient conditions. Moreover, the flexural strength decreases sharply at the temperature higher than 1200 °C. In addition, the thermal conductivity is 10.6 W/mk at room temperature.     

Investigation on reliability of nanolayer-grained Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> via Weibull statistics

Weibull modulus of bending strength of nanolayer-grained ceramic Ti₃SiC₂ was estimated with over 50 specimens, using the least square method, the moment method and the maximum likelihood technique, respectively. The result demonstrated that the m-value of this layered ceramic ranged from 25 to 29, which is much higher than that of traditional brittle ceramics. The reason of high Weibull modulus was due to high damage tolerance of this material. Under stress, delamination and kinking of grains and shear slipping at interfaces give this material high capacity of local energy dissipation and easy local stress relaxation, leading to the excellent damage tolerance of Ti₃SiC₂. The effect of amounts of specimens on the reliability of the estimated m-values was also investigated. It was confirmed that the stability of the estimated m-value increased with increasing numbers of specimens. The parameter obtained using the maximum likelihood technique showed the highest reliability than other methods. The ranges of failure probability were determined using the Weibull estimates calculated from the maximum likelihood technique.     

Tensile strength of windmill palm (Trachycarpus fortunei) fiber bundles and its structural implications

Trachycarpus fortunei (windmill palm) is one of the most widely distributed and widely used palms in East Asia. In order to find further uses for the palm’s fibers, however, more information on their mechanical and anatomical properties is needed. With this in mind, tensile strength and Young’s modulus of windmill palm fiber bundles were investigated and the structural implications considered. The anatomical features in cross-section, the fracture mode, and the microfibril angle (MFA) of natural fiber bundles were determined. The transverse sectional area occupied by fibers in a fiber bundles (S _F) contributes to mechanical strength in practice. It was found that the ratio of S _F to the transverse sectional area of a fiber bundle dramatically increases with a decrease in bundle diameter. Therefore, tensile strength and Young’s modulus of an individual fiber bundle in this species increase in parallel with a decrease in fiber bundle diameter. The observed MFA features might have a relationship with the biomechanical movements of fiber bundles in the windmill palm. The potential uses of windmill palm fibers have been discussed.     

Statistical analysis for strength and spatial distribution of reinforcement in SiC particulate reinforced aluminum alloy composites fabricated by die-casting

Statistical analysis for strength and spatial distribution of reinforcement in die-cast SiC_p/Al alloy composites was performed in order to predict the reliability of composites. Microstructural analysis was also done to determine the critical features of the composites. Die-casting was carried out using the preheated die at the casting temperature range of 620–750°C. It was found that the SiC pacticulates were homogeneously dispersed in die-cast Al matrix alloy, resulting from the refinement of dendritic cell size due to rapid cooling rate. The tensile strength of die-cast SiC_p/Al alloy composites was higher than that of die-cast Al matrix alloy. Also, the tensile strength was slightly increased with increasing SiC particulate volume fraction at the casting temperature range of 650–700°C. It was concluded that the die-cast temperatures of 750 and 700°C are optimum condition for the distribution of SiC particulates in consequence of good fluidity of melt for 10 and 20 vol.% SiC_p/Al alloy composites, respectively. However, the strength scattering of composites was increased with increasing SiC particulate volume fraction. For the statistical evaluation of strength, the maximum Weibull modulus of die-cast SiC_p/Al alloy composites, which was obtained at the cast temperature of 700°C, was 29.6 in Al matrix alloy, 22.2 in 10 vol.% SiC_p and 14.2 in 20 vol.% SiC_p, respectively.     

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。     

Compressive fracture behavior of 3D needle-punched carbon/carbon composites

Compressive fracture behavior under transverse and longitudinal compressive loading are determined for 3D needle-punched carbon/carbon (C/C) composites with single rough laminar (RL) pyrocarbon matrix or dual matrix of RL pyrocarbon and resin carbon. The results of Weibull statistics analysis indicate that scale parameter σ₀ of transverse and longitudinal compression of the composites with single matrix are 153.41 and 94.26 MPa, and σ₀ of the composites with dual matrix are 205.16 and 105.33 MPa, respectively. The mean compressive strength of both composites is nearly equal to σ₀ under each experimental condition. Failure modes of both composites under transverse and longitudinal compressive loading are shear and extension, respectively. Both composites exhibit quasi-ductile fracture behavior under transverse compression. Many small fragments of fibers and matrix carbon on the fracture surface of the composites are observed for single matrix composites. And the fiber bundle breakage with extensive debonding occurs for dual matrix composites. Under longitudinal loading, the composites with single matrix show quasi-ductile fracture behavior and delamination and splitting of non-woven long carbon fiber cloth layers are observed. The composites with dual matrix exhibit catastrophic failure behavior and crack runs through the composites along compressive loading direction.     

纤维涂层法制备SiCf/Cu复合材料及其性能分析

为研究纤维涂层法制备SiCf/Cu复合材料的性能特点,通过磁控溅射法先后将Ti6Al4V界面改性层和基体Cu涂层涂覆到SiC纤维表面,并通过真空热压法将被涂覆的纤维制备成SiCf/Cu复合材料.对Ti6Al4V涂层、Cu涂层以及复合材料进行了微观分析,并测试了复合材料的拉伸强度.研究表明,复合材料的Cu基体由致密而细小的晶粒组成;Ti6Al4V提高了纤维/基体界面结合强度,复合材料轴向抗拉强度高达500 MPa,界面脱粘主要发生在纤维表面的碳涂层与纤维之间.     

Experimental Study on Tensile Behavior of Carbon Fiber and Carbon Fiber Reinforced Aluminum at Different Strain Rate

In this study, dynamic and quasi-static tensile behaviors of carbon fiber and unidirectional carbon fiber reinforced aluminum composite have been investigated. The complete stress–strain curves of fiber bundles and the composite at different strain rates were obtained. The experimental results show that carbon fiber is a strain rate insensitive material, but the tensile strength and critical strain of the C_f/Al composite increased with increasing of strain rate because of the strain rate strengthening effect of aluminum matrix. Based on experimental results, a fiber bundles model has been combined with Weibull strength distribution function to establish a one-dimensional damage constitutive equation for the C_f/Al composite.     

Influence of matrix degradation on composite properties in fiber direction

The influence of matrix degradation on fiber-mode mechanical properties of unidirectional composites is considered. In this case the strength of the composite reduces to the strength of a bundle of fibers. The longitudinal Young's modulus is evaluated as a nonlinear function of applied stress. In the case of a degradated matrix all the failure criteria reduce to the maximum stress criterion. The statistical parameters of the Weibull distribution of the fiber strength are evaluated from commercially available experimental data for commonly used materials.