Effects of gage length and stress concentration on the compressive strength of a unidirectional CFRP

Compression tests for a unidirectional CFRP were carried out for a wide range of gage lengths, three different configurations of end tab edges and two different end tab materials under the conditions of constant specimen thickness. The relation of σ_c-L is to be divided into two parts, namely, a part where the compressive strength, σ_c, is nearly constant independent of gage length, L, and beyond that, a part where σ_c decreases with increasing L. The apparent compressive strength measured by Celanese test method was lower than the true compressive strength because of the stress concentration near the end tab edges of the specimen. The true compressive strength was obtained by using the specimen where the gage lengths of 3.2 mm and 6.4 mm, and the material of end tab is stainless steel.     

Fracture behaviour of high-strength concrete at a wide range of loading rates

Three-point bending tests on notched beams of a high-strength concrete have been conducted using both a servo-hydraulic machine and a self-designed drop-weight impact device. The fracture energy and the peak load were measured over a wide range of loading point displacement rates, spanning eight orders of magnitude. Under low displacement rates, from 10⁻⁴ to 10 mm/s, the tests were performed with the servo-hydraulic machine; from 10² to 10³ mm/s we used the drop-weight impact machine. The results show that the fracture energy and the peak load increase as the loading rate increases. Nevertheless, such a trend is relatively slight under low rates and can be attributed to viscous effects mainly originating from the presence of water in the pore structure. Under high rates the increases in the fracture energy and in the peak load are dramatic due to the effect of inertia.     

超声波作用下对位芳纶纤维的超微结构

用X射线衍射和扫描电子显微镜（SEM）对超声波处理的对位芳纶纤维的结晶结构和形态结构进行研究,以揭示对位芳纶纤维的超微结构特点。研究表明,对位芳纶纤维具有皮芯层结构和多重原纤结构特征,纤维表层大量原纤沿纤维轴向高度取向,直径约600 nm;纤维内部圆柱状的微原纤平行于纤维轴,直径为30 nm,且微原纤间存在缝隙和孔洞;...     

Experimental evaluation of the strength distribution of fibers under high strain rates by bimodal Weibull distribution

This paper proposes a bimodal Weibull distribution model for strain- rate- and temperature-dependent fiber strength. The relationships of the mechanical quantities between fiber and fiber bundles at different strain rates and temperatures under tensile impact are established. A method for determining mechanical parameters of fibers by tensile impact tests of fiber bundles is established. Experiments on E-glass bundles have been performed at six strain rates (90, 300, 800, 1100, 1300 and 1700 s⁻¹) at three different temperatures (−70, 14, 80°C). According to the statistical analysis and models, the mechanical parameters for the fiber and their relationships with strain rate and temperature are obtained from the tensile impact experimental results. The emulated stress/strain curves from the model are in good agreement with the test data. The theoretical model and test results show that the shape parameters, β^d₁ and β^d₂, are not only strain rate independent but also temperature independent. The scale parameters σ^d₀₁ and σ^d₀₂, which change with strain rate and temperature, are not constant.     

Effects of loading rate, notch geometry and loading mode on the local cleavage fracture stress of a C–Mn steel

In this work, notched specimens with two notch geometries were tested in two loading modes (four-point bending (4PB) and three-point bending (3PB)) at various loading rates at a temperature of − 110°C for a C–Mn steel. An elastic–plastic finite-element method (FEM) is used to determine the stress distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ _f is measured. The results obtained and combining with previous studies by the authors show that the local cleavage fracture stress σ _f is closely related to the cleavage fracture mechanism (critical events) in steels. The σ _f values do not change with loading rate, notch geometry and loading mode, as long as the critical event of cleavage fracture does not change at various testing conditions. The σ _f is mainly determined by the steel microstructure, and its scatter is mainly caused by the size distribution of the weakest constituent in steels (ferrite grain or pearlite colony with large sizes and large second phase particles) and the change of the critical events in cleavage process. The σ _f can characterize the intrinsic toughness of steels and may be used in a “local approach” model for assessing integrity of flawed structures. The σ _f values could be measured by both 4PB and 3PB tests.     

Tensile properties and microstructure characterization of Hi-NicalonTM SiC fibers after loading at high temperature

Loading tests were performed on the Hi-Nicalon^TM fiber yarns by applying different dead loads at elevated temperatures in Ar atmosphere. After each loading test, the room temperature tensile properties of single filaments as a function of load, exposure time and temperature, were evaluated on these fibers. Strength degradation of single filament occurred after loading test, and strength retention decreased with increasing the exposure temperature and load. The microstructure observation revealed that oxidation and loading accelerated new flaw nucleation and growth resulting in stress corrosion cracks. The stress corrosion cracks acted as critical flaws and could be mainly responsible for the strength degradation. The mirror size corresponding to the critical flaw size was measured and fracture mechanics was applied to analyze failure mechanism of single filament.     

A study of the loading path and crystal orientation effects on size-dependent limit strength

To better understand the loading path and crystal orientation effects on size-dependent material strength, molecular dynamics (MD) simulations are performed with the use of single crystal diamond (SCD) of various sizes under uniaxial tension and simple shear loading conditions. In the MD simulations, mechanical responses of SCD blocks with three different sizes under 〈1 0 0〉 and 〈1 1 0〉 tensions, and under {1 0 0}〈0 1 0〉, {1 0 0}〈1 1 0〉 and {1 1 0}〈0 0 1〉 shear slips at different loading rates are studied. Based on the simulation data, a power scaling law is proposed to predict the size effect on the material strength of pristine diamond under given loading conditions.     

Effect of loading rates on pullout behavior of high strength steel fibers embedded in ultra-high performance concrete

In this paper single fiber pull-out performance of high strength steel fibers embedded in ultra-high performance concrete (UHPC) is investigated. The research emphasis is placed on the experimental performance at various pullout rates to better understand the dynamic tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC). Based on the knowledge that crack formation is strain rate sensitive, it is hypothesized that the formation of micro-splitting cracks and the damage of cement-based matrix in the fiber tunnel are mainly attributing to the rate sensitivity. Hereby, different pull-out mechanisms of straight and mechanically bonded fibers will be examined more closely. The experimental investigation considers four types of high strength steel fibers as follows: straight smooth brass-coated with a diameter of 0.2 mm and 0.38 mm, half end hooked with a diameter of 0.38 mm and twisted fibers with an equivalent diameter of 0.3 mm. Four different pull out loading rates were applied ranging from 0.025 mm/s to 25 mm/s. The loading rate effects on maximum fiber tensile stress, use of material, pullout energy, equivalent bond strength, and average bond strength are characterized and analyzed. The test results indicate that half-hooked fibers exhibit highest loading rate sensitivity of all fibers used in this research, which might be attributed to potential matrix split cracking. Furthermore, the effect of fiber embedment angles on the loading rate sensitivity of fiber pullout behavior is investigated. Three fiber embedment angles, 0°, 20°, and 45°, are considered. The results reveal that there is a correlation between fiber embedment angle and loading rate sensitivity of fiber pullout behavior.     

高强度卡拉胶纤维的制备及性能测试

通过卡拉胶与环氧氯丙烷交联反应制得改性卡拉胶;并将改性卡拉胶溶于热水制得纺丝液,采用湿法纺丝制备高强度卡拉胶纤维。红外光谱测试及粘度提高证明交联反应的发生;用单因素实验法找到影响因素的最佳范围,通过正交实验法得到最佳合成条件为温度90℃、交联剂加入量为6.25%、反应时间2.0h、pH值为10.0;利用万能材料试验机测得纯卡拉胶纤维的断裂强度为2.17cN/tex,改性卡拉胶纤维的最大断裂强度为3.99cN/tex,交联后断裂强度提高85%。     

搭接长度和铺层方式对CFRP复合材料层合板胶接结构连接性能和损伤行为的影响

针对不同搭接长度和铺层方式的碳纤维增强树脂（CFRP）复合材料层合板单搭胶接结构进行了拉伸试验,观察了试件的受力过程和失效形态,获得了载荷-位移曲线;同时基于连续损伤力学模型和三维Hashin失效准则模拟了CFRP复合材料层合板的层内损伤形成和演化,并利用内聚力模型来模拟层间及胶层的失效损伤,对CFRP复合材料层合板单搭胶接结构在拉伸作用下的失效强度和损伤机制进行了预测,通过对比验证了该数值方法的有效性;通过数值试验比较不同搭接长度和铺层方式的单搭胶接结构及双搭胶接结构的连接强度和损伤行为,并提出了一种优化的CFRP复合材料层合板胶接结构。结果表明:CFRP复合材料层合板胶接结构的极限失效载荷随着搭接长度的增大逐渐增加并趋于稳定值,且结构的失效形式逐渐从胶层自身剪切失效过渡到邻近胶层的层合板层间分层失效;CFRP复合材料层合板胶接结构的连接强度和损伤行为随着铺层方式的不同而改变,通过对3种铺层方式的对比和分析,得到性能最好的铺层方式是[0₃/90₃]_2S;在搭接长度为5~20 mm时,通过对搭接长度进行优化,得到单搭胶接结构的最优搭接长度是17 mm,双搭胶接结构的最优搭接长度是19.3 mm,与搭接长度为20 mm相比,单搭胶接结构和双搭胶接结构的连接强度分别提高了13.26%和0.43%。      

高强度炭纤维微观结构分析及力学性能的比较

采用元素分析、SEM、XRD、Raman光谱等分析方法,研究了高强度PAN炭纤维T800H和T1000G的元素含量、表面形貌及微观结构参数等因素.结果发现,T1000G的含碳量高于T800H,含氮含量低于T800H;T1000G的表面光滑,而T800H的表面存在沿纤维轴方向的沟壑,表明前者的原丝可能是用湿纺纺丝,后者则可能是干喷湿纺纺丝.从两者的断面上均能看到颗粒状结构,T1000G比T800H的颗粒要小而紧凑;与T800H相比,T1000G的微晶尺寸(Lc、La)较大,取向性较好,微晶层间距(d002)和孔隙率(Vp)均较小,且表现出较好的致密性和均质性.因此T1000G具有较高的抗拉强度.     

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.     

Effect of ZnO nanowire morphology on the interfacial strength of nanowire coated carbon fibers

The ability to tailor interfacial shear strength for a particular fiber and resin system is critical to the development of composite materials that perform optimally in specific applications. One approach to tailor the interface is to introduce a secondary phase between the fiber and matrix, which can act to functionally grade the material properties and enhance load transfer across the interface. This approach has been applied in the past using nanowires, nanotubes, and whiskers and was demonstrated to significantly enhance interface performance. Unfortunately, these processes lack control over the interphase morphology to allow design of the interface for optimal properties. Recently, ZnO nanowires grown on the surface of carbon fibers have demonstrated more than a 110% increase in interfacial strength [1]. Unlike other treatments, this interfacial reinforcement allows precise morphology control. Here, we develop the parameters for the growth of nanowires with varying lengths and diameters and study the influence of the nanowire’s morphology on the interfacial shear strength. ZnO nanowire arrays are grown on carbon fibers, with nanowire diameters ranging from 50 to 200 nm and lengths up to 4 μm. The interfacial shear strength with varying nanowire dimensions is shown to increase by up to 228%, ranging from 45.72 to 154.64 MPa. Unlike existing whiskerization approaches, it is shown that the tensile strength of the ZnO nanowire coated fibers remains constant throughout all growth procedures. The development of an interphase offering control over the interface strength and toughness will provide a means to produce multifunctional composites with optimized performance for multiple applications.     

Effects of gamma radiation on perfluorinated polymer optical fibers

The paper presents the first complex study of gamma radiation effects on a low-loss perfluorinated polymer optical fiber (PF-POF) based on Cytop^® polymer. Influence of gamma radiation on fiber’s optical, mechanical and climatic performance is investigated. The radiation-induced attenuation (RIA) in the visible and near-infrared region (0.4 μm–1.7 μm) is measured and its origins are discussed. Besides attenuation increase, radiation is also shown to decrease the thermal degradation stability of the fiber and to increase its susceptibility to water. With regard to complex fiber transmission performance upon irradiation, the optimal operation wavelength region of PF-POF-based systems intended for use in radiation environments is determined to be around 1.1 μm. On the other hand, the investigated fiber holds potential for low-cost RIA-based optical fiber dosimetry applications with sensitivity as high as 260 dBm⁻¹/kGy in the visible region.     

LiCl处理对芳纶纤维表面结构与性能的影响

为提高芳纶纤维与复合材料基体间的界面强度,首先,使用LiCl乙醇溶液处理芳纶纤维一定时间;然后,对LiCl处理芳纶纤维表面的化学组成、微观形貌、单丝拉伸强度及芳纶纤维/环氧树脂复合材料的界面性能等进行了测试分析。结果表明:使用LiCl乙醇溶液处理芳纶纤维后,芳纶纤维表面的含氮官能团含量增加;处理后,芳纶纤维表面有刻蚀出的沟槽,表面粗糙度增大,进而改善了芳纶纤维与环氧树脂基体的界面粘接性能,使芳纶纤维/环氧树脂复合材料的层间剪切强度由处理前的21.75 MPa提升到37.98 MPa;最佳处理时间为3~4 h,而处理时间过长会导致芳纶纤维的单丝拉伸强度及复合材料的层间剪切强度下降。所得结论证实使用LiCl处理芳纶纤维是一种有效的表面改性方法。      

Short fibre elastomer composites: effect of fibre length,orientation, loading and bonding agent

A series of short fibre reinforced styrene butadiene rubber (SBR) composites were prepared by incorporating sisal fibres of different lengths and different concentrations. The vulcanization behaviour and the properties of the vulcanizates were measured by the relevant ASTM procedures. The processing characteristics such as green strength and mill shrinkage were improved by the addition of fibres. The properties like tensile strength, tear strength, moduli at 10% and 20% elongations were found to be maximum for composites containing longitudinally oriented fibres of length 6 mm. Mixes containing 35 phr loading of fibres and bonding agent showed better mechanical performance than others. The failure behaviour of tensile and tear specimens have been analysed by scanning electron microscopy techniques. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

聚碳硅烷纤维氧化交联机理的研究

对聚碳硅烷(PCS)原丝在不同氧化交联温度区间生成的逸出产物进行红外、核磁和GC-MAS分析,并结合交联丝的红外分析,推测氧化交联的机理。结果表明,PCS的氧化交联主要是其Si—H氧化生成Si—OH,后者进而彼此缩合生成Si—O—Si交联结构;氧化交联温度高于150℃时,其部分Si—CH3也开始氧化生成Si—OH并进而交联;同时,在氧化交联过程还发生PCS侧链的热裂解,所形成小分子也通过Si—OH彼此结合,形成较大分子,且其分子量随交联温度的提高而提高。因此,要及时排除氧化交联过程废气,以免逸出产物黏附在纤维表面而导致粘结。     

Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime

Very high cycle fatigue tests under axial loading at frequencies of 95 Hz and 20 kHz were performed to clarify the effect of loading type on fatigue properties of a high strength bearing steel in combination with experimental result of this steel under rotating bending. As a result, this steel represents the single P-S-N (probabilistic-stress-life) curve characteristics for surface-induced fracture and interior inclusion-induced fracture, just like that under rotating bending. However, fatigue strength is lower, where the run-out stress at 10⁹ cycles is evaluated to be 588 MPa, less than that under rotating bending with about 858 MPa. Occurrence probability of larger and deeper inclusion-induced fracture is much higher than that under rotating bending. Furthermore, the formation process of fine granular area (FGA) is independent of the type and frequency of loading, which is very slow and is explained as the crack nucleation process under the special dislocation mechanism. The stress intensity factor range at the front of FGA, ΔK_FGA, is approximately regarded as the threshold value controlling the stable propagation of interior crack. For the control volume of specimen under axial loading, the estimated value of fatigue limit by FGA is similar to experimental run-out stress value at 10⁹ cycles, but that by inclusion is larger. However, the corresponding estimated results under rotating bending are all conservative.     

Effects of road geometry and traffic volumes on rural roadway accident rates

This paper revisits the question of the relationship between rural road geometric characteristics, accident rates and their prediction, using a rigorous non-parametric statistical methodology known as hierarchical tree-based regression. The goal of this paper is twofold: first, it develops a methodology that quantitatively assesses the effects of various highway geometric characteristics on accident rates and, second, it provides a straightforward, yet fundamentally and mathematically sound way of predicting accident rates on rural roads. The results show that although the importance of isolated variables differs between two-lane and multilane roads, 'geometric design' variables and pavement condition' variables are the two most important factors affecting accident rates. Further, the methodology used in this paper allows for the explicit prediction of accident rates for given highway sections, as soon as the profile of a road section is given.     

Strengths of C/C composites under tensile, shear, and compressive loading: Role of interfacial shear strength

Various strengths of carbon–carbon composites (C/Cs) are comprehensively reviewed. The topics reviewed include tensile, shear, compressive, and fatigue strength as well as fiber/matrix interfacial strength of C/Cs. When data are available, high temperature properties, including creep behavior, are presented. Since C/Cs have extremely low fiber/matrix interfacial strength τ_d, the interfacial fracture plays important roles in all of the fracture processes dealt in this review. The low τ_d was found to divide tensile fracture units into small bundles, to seriously degrade both shear and compressive strength, and to improve fatigue performance. In spite of the importance of the interfacial strength of C/Cs, techniques for its evaluation and analysis are still in a primitive stage.