Fatigue Hysteresis of Carbon Fiber-Reinforced Ceramic-Matrix Composites at Room and Elevated Temperatures

When the fiber-reinforced ceramic-matrix composites (CMCs) are first loading to fatigue peak stress, matrix multicracking and fiber/matrix interface debonding occur. Under fatigue loading, the stress–strain hysteresis loops appear as fiber slipping relative to matrix in the interface debonded region upon unloading/reloading. Due to interface wear at room temperature or interface oxidation at elevated temperature, the interface shear stress degredes with increase of the number of applied cycles, leading to the evolution of the shape, location and area of stress–strain hysteresis loops. The evolution characteristics of fatigue hysteresis loss energy in different types of fiber-reinforced CMCs, i.e., unidirectional, cross-ply, 2D and 2.5D woven, have been investigated. The relationships between the fatigue hysteresis loss energy, stress–strain hysteresis loops, interface frictional slip, interface shear stress and interface radial thermal residual stress, matrix stochastic cracking and fatigue peak stress of fiber-reinforced CMCs have been established.     

Stress and Failure Analysis of Brittle Matrix Composites. Part II: Failure Analysis

A linear elastic fracture mechanics analysis of a cylindrical element of matrix with a single fiber and two matrix annular cracks perpendicular to the fiber direction under longitudinal tensile load was undertaken. The order of singularity and the angular dependence of the stress field in the neighborhood of the crack periphery were determined by using the stress function approach proposed by Zak and Williams. The stress intensity factor was evaluated by combining the results of the local stress solution with a finite element analysis. The case of fiber debonding originating from the periphery of the annular cracks was also studied. For that problem both opening-mode and sliding-mode stress intensity factors and the strain energy release rate were determined. These results help to understand the various failure mechanisms including matrix cracking, debonding along interfaces and kinking of interface cracks into fibers in brittle matrix composites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal Expansion and Shrinkage of Unidirectional Composites at Elevated Temperatures

Strength of Materials - The model of multiphase media analyzes the effectiveness of describing thermal expansion and shrinkage of unidirectional polymer composites at their uniform heating by...     

Comparison of Fatigue Life Between C/SiC and SiC/SiC Ceramic-Matrix Composites at Room and Elevated Temperatures

In this paper, the comparison of fatigue life between C/SiC and SiC/SiC ceramic-matrix composites (CMCs) at room and elevated temperatures has been investigated. An effective coefficient of the fiber volume fraction along the loading direction (ECFL) was introduced to describe the fiber architecture of preforms. Under cyclic fatigue loading, the fibers broken fraction was determined by combining the interface wear model and fibers statistical failure model at room temperature, and interface/fibers oxidation model, interface wear model and fibers statistical failure model at elevated temperatures in the oxidative environments. When the broken fibers fraction approaches to the critical value, the composites fatigue fracture. The fatigue life S–N curves and fatigue limits of cross-ply, 2D and 3D C/SiC and SiC/SiC composites at room temperature, 550 °C in air, 750 °C in dry and humid condition, 800 °C in air, 1000 °C in argon and air, 1100 °C, 1300 °C and 1500 °C in vacuum, have been predicted. At room temperature, the fatigue limit of 2D C/SiC composite with ECFL of 20 % lies between 0.78 and 0.8 tensile strength; and the fatigue limit of 2D SiC/SiC composite with ECFL of 20 % lies between 0.75 and 0.85 tensile strength. The fatigue limit of 2D C/SiC composite increases to 0.83 tensile strength with ECFL increasing from 20 to 22.5 %, and the fatigue limit of 3D C/SiC composite is 0.85 tensile strength with ECFL of 37 %. The fatigue performance of 2D SiC/SiC composite is better than that of 2D C/SiC composite at elevated temperatures in oxidative environment.     

Fatigue Life Prediction of Carbon Fiber-Reinforced Ceramic-Matrix Composites at Room and Elevated Temperatures. Part I: Experimental Analysis

This paper presents an experimental analysis on the fatigue behavior in C/SiC ceramic-matrix composites (CMCs) with different fiber preforms, i.e., unidirectional, cross-ply and 2.5D woven, at room and elevated temperatures in air atmosphere. The experimental fatigue life S???N curves of C/SiC composites corresponding to different stress levels and test conditions have been obtained. The damage evolution processes under fatigue loading have been analyzed using fatigue hysteresis modulus and fatigue hysteresis loss energy. By comparing the experimental fatigue hysteresis loss energy with theoretical computational values, the interface shear stress corresponding to different peak stress, fiber preforms and test conditions have been estimated. It was found that the degradation of interface shear stress and fibres strength caused by oxidation markedly decreases the fatigue life of C/SiC composites at elevated temperature.     

Stress and Failure Analysis of Brittle Matrix Composites. Part I: Stress Analysis

Damage initiation in brittle matrix composites with ultimate strain of the matrix less than that of the fiber under longitudinal tensile load takes place with the formation of cracks in the matrix. This is followed by debonding at the fiber-matrix interface and fiber fractures depending on the properties of the interface. Study of the various failure mechanisms of the composite has a prerequisite the analysis of the stress field around matrix cracks, fiber-matrix disbonds, fiber failures, etc. In the present work stress analysis for a cylindrical element of matrix with a single fiber and two matrix annular cracks perpendicular to the fiber direction under longitudinal tensile load takes place. The results of the finite element stress analysis are used to predict the overall tensile stress-strain curve of the composite which is compared with the predictions based on a modified shear lag analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

FeAl金属间化合物的高温变形行为

对Ｆｅ－３６．５ａｔ％Ａｌ金属间化合物进行了高温拉伸试验研究。结果表明，变形温度在６００－１０００℃范围内，这种ＦｅＡｌ合金的延伸率随温度的升高不断增加，在１０００℃时延伸率最高，达１１５％。经透射电镜分析，该合金高温变形后不仅有大量的滑移位错线，而且有一定数量的位错绻线存在。     

High Strain Rate Deformation Behavior of Zirconium at Elevated Temperatures

High temperature mechanical behaviors of zirconium at high strain rate of 103 s-1 were studied by split Hopkinson pressure bar (SHPB).The influences of strain rate and temperature on the micro-structural evolution,as well as the occurrence of shear localization and subsequent fracture,were also investigated.It s found that the compressive stress-strain response depends sensitively on the applied strain rate and test temperature.Micro-structural observations revealed that the density of the twinning grains r...     

Mechanical Behavior of Sic Fiber-Sic Matrix Composite and Correlation to In Situ Fiber Properites at Room and High Temperatures

Tensile mechanical properties of PIP-processed 2D, plain-woven fabric. BN-coated Hi-NicalonTM SiC fiber-reinforced SiC matrix composite at room temperature (298 K), 1400 and 1600 K in air have been studied.Young's modulus and tensile strength of the composite decrease with the increase of test temperature, especially at 1600 K a considerable reduction is observed. The tensile strength is found to be consistent with the value predicted by a global load sharing model, based on the actual in situ fiber strength properties which are obtained by a fracture mirror method.     

Mechanical Behavior of Sic Fiber-Sic Matrix Composite and Correlation to In Situ Fiber Properites at Room and High Temperatures

Tensile mechanical properties of PIP-processed 2D, plain-woven fabric. BN-coated Hi-NicalonTM SiC fiber-reinforced SiC matrix composite at room temperature (298 K), 1400 and 1600 K in air have been studied.Young's modulus and tensile strength of the composite decrease with the increase of test temperature, especially at 1600 K a considerable reduction is observed. The tensile strength is found to be consistent with the value predicted by a global load sharing model, based on the actual in situ fiber strength properties which are obtained by a fracture mirror method.     

整体毡碳/碳复合材料的高温剪切性能研究

运用双槽口剪切试验方法测试了碳纤维增强碳基复合材料(C/C)在室温、700℃、1000℃、1400℃下的剪切强度,并用扫描电子显微镜(SEM)观察了材料的原始组织形貌和断口微观形貌。结果表明:材料的剪切强度在一定范围内随温度的升高而增加,材料Z向的剪切强度优于XY向。通过SEM观察分析可知,材料XY向和Z向纤维的含量明显不同;室温下,C/C复合材料的破坏形式有纤维拔出及断裂,而高温下,失效形式主要为纤维的拔出、纤维/基体界面脱粘等。     

Creep Rupture Behavior of K40S Alloy at Elevated Temperatures

Creep testing was conducted on K40S alloy. The detailed creep deformation and fracture mechanisms under constant load were studied. The results show that the stress exponent ranges between 7 and 14.4 at elevated temperature 973~1173 K, and that the activation energy is approximately 449.1 kJ/mol. During creep, the grain boundary sliding cut off primary carbides at the boundary, generating the "O" model cracks. The creep failure mode of K40S alloy is transgranular ductile and cracks originate at the primary carbides. A long carbide and matrix interface is often a preferential path for crack propagation. The creep mechanism is discussed in light of the creep microstructure, the stress exponent and the activation energy.     

Characterization of the Tribological Behavior of Different Tool Coatings and Dry Lubricant for High-Strength Aluminum Alloys at Elevated Temperatures

The use of high-strength aluminum components in automotive manufacturing offers the opportunity to reduce vehicle weight significantly and provide new lightweight potentials. In the past, the so-called hot forming and quench process (HFQ) successfully demonstrates the potential for the production of complex-shaped components made out of age-hardenable high-strength aluminum alloys. Currently, no method permits wear-free quench forming without the use of lubricants. To fulfill the increasing ecological and economic requirements, it is necessary to identify wear-reducing techniques to promote this forming technology in the future. This contribution investigates the interaction of lubricant and tool coatings on the tribological performance during quench forming of the high-strength aluminum alloy AA7075 at elevated temperatures. For this purpose, the tribological behavior is investigated using both, flat strip drawing tests and deep drawing operations. Subsequently, the component quality is compared and discussed. The results demonstrate that tool coatings are effective for the production of high-strength components in the HFQ process with minimal or even no lubrication and thus provide ecological as well as economic advantages.     

Numerical Analysis of Stress Concentration for Brittle Matrix Composites

A FEM numerical model is constructed of SiCf/LASIII glass-ceramic uni-direction plate under tension loading. The smeared crack method is used to deal with the failure point of fiber and matrix elements. The solution of the model gives the distribution of meso-stress of SiCf/LASIII glass-ceramic uni-direction plate. The stress concentration factor K and the size of stress concentration effect zone are also analyzed. The average saturation cracking space is obtained by a numerical method. The mesh length of the elements is also studied.     

型砂高温力学行为的试验研究方法

研究了型砂高温力学性能微机数据采集处理系统并对ＳＱＷ－Ⅱ型砂高温强度仪进行改进，应用散体力学中的Ｍｏｈｒ－Ｃｏｕｌｏｍｂ破坏理论，确立了一套以定量描述型砂的高温力学行为为目的手段和方法。通过它们可以实现对型砂在不同温度下的应力－应变曲线，模量，抗压强度，粘降力，内磨擦角、型砂软化后的粘度系数以及热膨胀系数等基本参数的测算，从而基本上能满足型砂高温力学行为研究的需要。     

石墨的添加对NiCr-W-Ti自润滑复合材料高温摩擦学性能的影响

本文以Ni20Cr合金为基体添加稀有金属Ti、W粉末及石墨后,充分混合,采用机械合金化及热压烧结工艺制备了NiCr金属基复合材料,研究了石墨含量对NiCr金属基复合材料的组织结构和摩擦学性能的影响。在UMT-3高温摩擦试验机上进行了该复合材料同Al_2O_3陶瓷球的滑动摩擦磨损实验,利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)对复合材料及其摩擦测试后的形貌与结构进行观察分析,结果表明:当所添加石墨的质量分数为3wt.%时,复合材料具有相对较好的力学性能和摩擦学性能;当测试温度低于300℃时,摩擦表面未形成有效的摩擦膜,故磨损率较高;当温度高于500℃时,摩擦层中含有摩擦氧化物,摩擦表面被光滑氧化物所组成的润滑膜覆盖,对磨面具有很好的保护作用,因而磨损率降低。     

Interlaminar Fracture Toughness of CF/PEI and GF/PEI Composites at Elevated Temperatures

An experimental study has been conducted to assess temperature effects on mode-I and mode-II interlaminar fracture toughness of carbon fibre/polyetherimide (CF/PEI) and glass fibre/polyetherimide (GF/PEI) thermoplastic composites. Mode-I double cantilever beam (DCB) and mode-II end notched flexure (ENF) tests were carried out in a temperature range from 25 to 130°C. For both composite systems, the initiation toughness, G _IC,ini and G _IIC,ini, of mode-I and mode-II interlaminar fracture decreased with an increase in temperature, while the propagation toughness, G _IC,prop and G _IIC,prop, displayed a reverse trend. Three main mechanisms were identified to contribute to the interlaminar fracture toughness, namely matrix deformation, fibre/matrix interfacial failure and fibre bridging during the delamination process. At delamination initiation, the weakened fibre/matrix interface at elevated temperatures plays an overriding role with the delamination growth initiating at the fibre/matrix interface, rather than from a blunt crack tip introduced by the insert film, leading to low values of G _IC,ini and G _IIC,ini. On the other hand, during delamination propagation, enhanced matrix deformation at elevated temperatures and fibre bridging promoted by weakened fibre/matrix interface result in greater G _IC,prop values. Meanwhile enhanced matrix toughness and ductility at elevated temperatures also increase the stability of mode-II crack growth.     

Effect of Nb on Structure and Mechanical Properties of Chilled Cast Iron at Room and Elevated Temperatures

Effect of Nb on microstructure and mechanical properties of chilled cast iron at room and elevated temperatures isstudied in this research. The results demonstrate that the cast structure and mechanical properties of chilled castiron at room and elevated temperatures are improved with the addition of trace amount of Nb. However, if Nb wasadded too much, the cast structure and mechanical properties of chilled cast iron would deteriorate. The suitablecontent of Nb in chilled cast iron is about 0.05% (mass fraction). Except the dissolution in the matrix of cast ironthe excessive Nb will form Nb-rich phases in three morphologies. Those are lumpy NbC, complicated strip-like phaseand compound with pearlite structure.