Mechanical properties of 3D carbon/carbon composites by nanoindentation technique

Nanoindentation tests were conducted to investigate the near-surface mechanical properties of the individual components (fiber and matrix) for three-dimensional reinforced carbon/carbon composites (3D C/C). Optical microscope and polarizing light microscope were used to characterize the microstructure of 3D C/C. The microscopy results show that large number of pores and cracks exist at both bundle/matrix interface and pitch carbon matrix. These defects have important effect on the mechanical behavior of 3D C/C. The in situ properties for components of 3D C/C were acquired by nanoindentation technique. Relative to the matrix sample, the fiber samples have more larger values for modulus, stiffness and hardness. However, there is no significant difference of modulus and stiffness among fiber samples with different directions.     

碳纳米管增强复合材料的界面破坏形态及改善措施

以碳纳米管增强树脂基复合材料的二维平面模型为对象,在考虑基体材料非线性的情况下,探讨碳纳米管增强复合材料的界面层特性对复合材料破坏形态的影响。通过对破坏端部的局部处理,提出两种提高复合材料抵抗损伤破坏能力的方法。第一种方法是改变纤维及界面层端部与基体的界面特性即软化界面,用内聚力模型模拟界面,通过内聚力阻扰裂纹的扩展;第二种方法是改变碳纳米管的端部形状即封闭碳纳米管及界面层,通过改变碳纳米管的应力分布状态来提高复合材料的抗破坏能力。     

Wet friction performance of C/C-SiC composites prepared by new processing route

C/C-SiC composites with SiC island distribution were prepared via a new processing route. The fabrication process mainly included silicon infiltration by ultrasonic vibration, chemical vapor deposition (CVD), siliconizing, liquid phase impregnation and carbonization. The wear and friction properties were tested by an MM-1000 wet friction machine. The results show that SiC phases are mainly distributed between carbon fibers and pyrocarbons as well as among the pryocarbons. The dynamic friction coefficient of the composites decreases gradually from 0.126 to 0.088 with the increase of the surface pressure from 0.5 to 2.5 MPa at the same rotary speed. Furthermore, under the constant surface pressure, the dynamic friction coefficient increases from 0.114 to 0.126 with the increase of the rotary speed from 1 500 to 2 500 r/min. However, the coefficient decreases to 0.104 when the rotary speed exceeds 4 500 r/min. During the friction process, the friction coefficient of C/C-SiC composite is between 0.088 and 0.126, and the wear value is zero after 300 times brake testing. Foundation item: Project(2006CB600901) supported by the Major State Basic Research and Development Program of China; Project(0991015) supported by Guangxi Science Found, China; Project(200808MS083) supported by Guangxi Education Department Found     

Preparation and properties of C/C-SiC brake composites fabricated by warm compacted-in situ reaction

Carbon fibre reinforced carbon and silicon carbide dual matrix composites(C/C-SiC) were fabricated by the warm compacted-in situ reaction.The microstructure,mechanical properties,tribological properties,and wear mechanism of C/C-SiC composites at different brake speeds were investigated.The results indicate that the composites are composed of 58wt%C,37wt%SiC,and 5wt%Si.The density and open porosity are 2.0 g.cm~(-3) and 10%,respectively.The C/C-SiC brake composites exhibit good mechanical properties.The ...     

非屈曲织物增强复合材料双轴压缩力学行为

在实际工程应用中,复合材料与构件往往处于复杂的应力状态,开展近似真实载荷环境下的力学实验分析,能够更准确地认识实际应用中材料的真实承载能力和失效机理.基于非屈曲单向碳纤维织物复合材料层合板的细观结构特征,设计了双轴压缩试样,开展了材料单轴、双轴压缩试验研究,对比分析了单向织物复合材料在不同压缩载荷下的力学行为.研究结果表明:非屈曲单向织物复合材料的单轴压缩行为表现为线性、脆性断裂;破坏模式整体表现为剪切屈曲破坏,与单轴压缩相比,双轴压缩载荷作用下材料整体表现为线性、脆性断裂,但其应力-应变曲线表现出一定的非线性特征;双轴1∶1等比例压缩对材料抵抗变形能力有一定强化效应,材料压缩模量增加;双轴2∶1非等比例压缩的结果与之相反,材料压缩模量大幅降低;双轴压缩强度均低于其单轴压缩强度;破坏模式主要表现为分层、基体开裂和纤维断裂,其中以分层现象尤为明显.     

Properties and microstructure of polymer emulsions modified fibers reinforced cementitious composites

The synthesis and characterization of a new class of cementitious composites filled with polymer emulsions were investigated, and their superior mechanical strength and durability properties compared to composites devoid of fi llers were reported. Polymer emulsions were utilized to mechanically reinforce the composite and bridge the cement, fly ash, aggregate and fibers. The results reveal that the epoxy emulsion and poly（ethylene-co-vinyl acetate） emulsion markedly enhance the mechanical and durability properties of cemetitious composites. The fi bers can be pulled out in the form of slip-hardening and the abrasion phenomenon can be observed clearly on the surface of the fibers. The hydration extent of cement is higher than that of the pristine composites. The polymer modified cementitious composites designed on micromechanics, have fl exibility and plasticity which could be applied for a novel form of multifunctional materials with a range of pipeline coatings applications.     

Behavior of pure and modified carbon/carbon composites in atomic oxygen environment

Atomic oxygen （AO） is considered the most erosive particle to spacecraft materials in low earth orbit （LEO）. Carbon fiber, car-bon/carbon （C/C）, and some modified C/C composites were exposed to a simulated AO environment to investigate their behaviors in LEO. Scanning electron microscopy （SEM）, AO erosion rate calculation, and mechanical property testing were used to characterize the material properties. Results show that the carbon fiber and C/C specimens undergo significant degradation under the AO bombing. According to the effects of AO on C/C-SiC and CVD-SiC-coated C/C, a condensed CVD-SiC coat is a feasible approach to protect C/C composites from AO degradation.     

碳/酚醛复合材料烧蚀过程热应力分析

为了评价防热材料的烧蚀性能,利用有限元软件ANSYS对碳/酚醛复合材料烧蚀过程中热应力的分布与演化规律进行了数值模拟.计算了恒定热流边界条件下材料的瞬态温度场和热应力场,采用蔡-希尔准则对材料热解区进行了破坏分析.数值计算结果与试验测试及图像分析结果的比较表明,随烧蚀过程的进行,材料热影响深度逐渐增大,温度梯度减小;材料热解区存在因热膨胀引起的热应力峰值,并随烧蚀过程逐渐后移,热应力是导致材料裂纹产生并扩展的主要原因.     

Wear behavior of Cu matrix composites reinforced with mixture of carbon and carbon nanotubes

In order to improve wear resistance and decrease the cost, carbon and carbon nanotubes reinforced copper matrix composites were fabricated by the power metallurgy method. The effects of carbon (carbon and carbon nanotubes) volume fraction and applied load on the friction coefficient and wear rate under dry sliding of the composites were investigated at room temperature. By scanning electron microscopy (SEM), the worn surfaces and debris were observed, and wear mechanism was also analyzed and discussed. The experimental wear process consists of the run-in, steady wear and severe wear process with the increasing of sliding distance. Both the friction coefficient and wear rate of the composites first decrease and then increase with the increasing of carbon volume fraction. The minimum friction coefficient and wear rate are obtained when carbon is 4.0vol%. The wear mechanisms of the composites change from the adhesive wear and delamination wear to abrasive wear with the increasing of carbon volume fraction. Funded by the National Natural Science Foundation of China (50873047) and the Natural Science Foundation of Gansu Province (3ZS061-A25-039)     

Modeling of CVI process in fabrication of carbon/carbon composites by an artificial neural network

C/C composites maintain unique intensity features at temperatures as high as 2600℃. Be-cause of their superior thermal and mechanical properties, which can persist at ultra-high tem-peratures, C/C composites are used in many areas including national defense, aviation aerospace, etc. At present, high performance C/C composites are mainly fabricated by using CVI proc-esses[1,2]. But it should be pointed out that the CVI process is controlled by many factors such as the infiltration temperat…     

医用碳/碳复合材料的制备及骨组织相容性研究

医用碳/碳复合材料的制备工艺主要由碳纤维三维立体织物浸渍糠醇树脂、碳化、化学气相渗透和石墨化等主要步骤构成.本文采用扫描电子显微镜、肖氏硬度计对碳/碳复合材料的表面微观形貌及显微硬度进行观察和测定.为进一步探讨该材料作为骨组织替换材料的可行性，开展了实验动物骨内种植实验，并通过X射线和组织病理切片观察对材料与骨组织相容性进行了研究.结果表明，采用本工艺制备的碳/碳复合材料组织结构均匀致密.该材料生物相容性良好，具有与正常人体皮质骨十分接近的表面硬度.     

医用碳／碳复合材料的制备及骨组织相容性研究

医用碳／碳复合材料的制备工艺主要由碳纤维三维立体织物浸渍糠醇树脂、碳化、化学气相渗透和石墨化等主要步骤构成．本文采用扫描电子显微镜、肖氏硬度计对碳／碳复合材料的表面微观形貌及显微硬度进行观察和测定．为进一步探讨该材料作为骨组织替换材料的可行性，开展了实验动物骨内种植实验，并通过X射线和组织病理切片观察对材料与骨组织相容性进行了研究．结果表明，采用本工艺制备的碳／碳复合材料组织结构均匀致密．该材料生物相容性良好，具有与正常人体皮质骨十分接近的表面硬度．     

The effect of applied stress on damage mode of 3D C/C composites under bend-bend fatigue loading

The bend-bend fatigue behavior of 3D integral braided carbon/carbon composites (3D C/C) was examined. Fatigue test was conducted under load control at a sinu-soidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limit of the C/C was found to be 203 MPa (92% of the static flexural strength), the lag loops of fatigue load-displacement were transformed from elasticity to anelasticity and the flexibility of specimens were enhanced with increase in applied stress. It is revealed that the interfacial sliding abrasion played an important role in the fatigue failure process, and the extent and speed of sliding abrasion were con-trolled by the level of applied stress.     

短纤维增强复合材料动态剪切模量及热膨胀系数预报

为了加强对短纤维增强复合材料的细观动态及热失配性能预报,利用Eshelby等效夹杂理论,通过三参数粘弹性本构模型,研究了Glass/ED6短纤维增强复合材料的粘弹性响应机理,并对该材料动态剪切模量及碳/环氧材料热膨胀系数随温度变化的规律进行了预报.结果表明,材料的剪切模量与组分间的体积分数以及加载频率间联系紧密,有效热膨胀系数与温度密切相关.通过曲线可以发现,当加载频率值较高时,动态剪切模量主要由实部决定;而当纤维的体积分数较大时,动态剪切模量的变化主要由虚部决定,这将使材料力学性能计算过程得到简化.针对碳/环氧材料,通过研究发现随着温差的增大,材料的热膨胀系数随之下降.     

Effects of microstructure modification on properties of AlN/BN composites

It is very difficult to prepare full-densified aluminum nitride-boron nitride （AIN/BN） composite ceramics with homogeneous microstructure and high thermal conductivity. Spark plasma sintering （SPS） was used to fully densify the AIN/BN composites in this work. Microstructure, mechanical properties and thermal conductivity of the SPS sintered AIN/BN composites with 5-30 vol% BN were investigated. The results show that the microstructure of composites is fine and homogenous, and the AIN/BN composites exhibit high mechanical properties. To promote the growth of AIN grains and modify the distribution of grain boundary in AIN/BN composites, a heat treating methodology was introduced through gas pressure sintering （GPS）. This processing was significantly beneficial to enhancing the thermal conductivity of the specimen. The thermal conductivity of AIN/BN composites with 5-30 vol% BN reached 60 W/m K after the samples were treated by GPS.     

SiC/Si-W-Mo coating for protection of C/C composites at 1873 K

In order to prevent carbon/carbon composites from oxidation at 1873 K, an efficient oxidation protective SiC/Si-W-Mo coating was prepared by a two-step pack cementation technique. The microstructures and the phase composition of the as-received multi-coating were examined by scanning electron microscopy （SEM） and X-ray diffraction （XRD）. It is seen that the compact multi-coating is composed of α-SiC, Si, and （WxMO1-x）Si2. Oxidation test shows that, after oxidation at 1873 K in air for 102 h and thermal cycling between 1873 K and room temperature for 10 times, the weight loss of the SiC/Si-W-Mo coated C/C composites is only 0.26%. The invalidation of the multi-coating is attributed to the formation of penetrable cracks in the coating. 2008 University of Science and Technology Beijing. All rights reserved.     

MMM testing and failure analysis of fastening bolts on reciprocating compressor cylinder cover

To avoid the serious accidents caused by the failure fastening bolts on reciprocating compressor cylinder cover,a new nondestructive testing(NDT) technology,metal magnetic memory(MMM) testing,was applied to safety evaluating and failure analyzing for the fastening bolts.Based on the dynamic stress calculation of the failure bolts,MMM testing was carried out at workshop.Given are the MMM stress distribution characteristics of the failure bolts and fracture faces.It has been found that the MMM signal variation amplitude of the crack transition zone in the fracture surface is minimal,that of the crack initiation zone is in the middle,and that of the tear fracture zone is maximal.The failure reasons were analyzed with MMM effect.The results of the metallographic examination showed that the validity and feasibility of MMM testing and failure analysis.This means MMM technology is a new,fast and validity method of failure analysis.     

煤矿砂岩双向加载下力学特性及破坏特征实验研究

随着开采深度的不断加深,开采活动在地下开挖体周边引起的压缩应力可能足以引起岩爆甚至作业人员伤害等灾害.地下开挖体周边岩石处于平面应力或平面压缩状态.为了研究地质材料的双轴强度.对从煤矿井下采出的100 mm立方体灰砂岩试件的双轴力学性能和强度进行了实验研究.双轴压缩的加载速度为1 MPa/s.对砂岩立方体试件和标准试件的单轴强度进行了测试并与双轴试验结果进行比较.实验采用了2种加载路径:一种是保持双向受力相等直至破坏;另一种是保持双向受力相等直至单轴抗压强度的1/2,然后保持约束荷载恒定,增加另一荷载直至破坏.给出了试验结果和所观察到的现象,探讨了砂岩双轴加载的破坏机理.试验结果表明,随着约束荷载的增加,岩石双轴强度显著增加,试件的破坏通常由加载初期开始,并持续地沿自由面片落,最终产生加载平面外的剪切破坏,同时伴随轻微的断裂声响.     

Electrical resistance and microstructure of latex modified carbon fiber reinforced cement composites

The electrical resistance,flexural strength,and microstructure of carbon fiber reinforced cement composites(CFRC) were improved greatly by adding water-redispersible latex powder.The electrical resistance of CFRC was investigated by two-probe method.The input range of CFRC based strain sensors was therefore increased,whereas electrical resistance was increased and remained in the perfect range of CFRC sensors.The analysis of scanning electron microscopy indicated that elastic latex bridges and a latex layer existed among the interspaces of the adjacent cement hydration products which were responsible for the enhancement of the flexural strength and electrical resistance.The formation mechanism of the elastic latex bridges was also discussed in detail.The continuous moving of two opposite interfaces of the latex solution-air along the interspaces of the adjacent hydrated crystals or colloids was attributed to the formation of the elastic latex bridges.     

Fractal characterization of pore microstructure evolution in carbon/carbon composites

A fractal characterization approach was proposed to research pore microstructure evolution in carbon/carbon (C/C) composites during the chemical vapor infiltration process. The data obtained from mercury porosimetry determinations were analyzed using the sponge fractal model and the thermodynamics relation fractal model, respectively. The fractal dimensions of C/C composites at different densification stages were evaluated. The pore microstructure evolution with densification time was studied by fractal dimension analysis. The results showed that C/C composites belong to porous fractal structure. The fractal dimensions increase on the whole with decreasing porosity as the densification proceeds. The fractal dimensions are influenced by the texture of pyrocarbon and decrease with increasing anisotropy from isotropic pyrocarbon to high textural one. Both the complicacy of pore structure and the textural morphology of pyrocarbon can be represented simultaneously by the fractal dimension. The pore evolution of C/C composites in the densification process can be monitored using fractal dimension. Supported by the National Natural Science Foundation of China (Grant No. 50742008)