Cansas-Ⅱ SiCf/SiC复合材料的高温拉伸蠕变行为

连续碳化硅纤维增强碳化硅复合材料(SiC_f/SiC)是发展先进航空发动机的关键材料,航空发动机长时服役要求材料具有优异的高温蠕变性能。本工作研究了平纹编织Cansas-Ⅱ碳化硅纤维增强碳化硅复合材料(2D-SiC_f/SiC)在空气中的高温蠕变行为,蠕变温度为1200～1400℃,应力水平为80～140MPa。利用扫描电子显微镜(SEM)观察了2D-SiC_f/SiC复合材料的微观组织和断口形貌,使用能谱分析仪(EDS)进行了成分分析。结果表明:当蠕变应力低于比例极限应力(σ_PLS)时, 2D-SiC_f/SiC的蠕变断裂时间超过500h,稳态蠕变速率为1×10^–10～5×10^–10/s,蠕变行为由基体和纤维共同控制。当蠕变应力高于σ_PLS时,复合材料的基体、纤维和界面均发生氧化,蠕变断裂时间显著降低,稳态蠕变速率提高一个数量级,蠕变行为主要由纤维控制。     

8090Al—Li合金蠕变断裂行为的研究

本文研究了8090型Al—Li合金在190～290℃以及30～200MPa的应力条件下的蠕变断裂行为和蠕变过程中的显微组织变化。实验结果表明,蠕变断裂行为与蠕变过程中发生的沉淀相粗化、平衡相在晶界的析出以及晶界无析出带的宽化等有关。和T_o处理的试样相比,T_8处理试样具有较高的蠕变断裂寿命。这是因为经T_8处理的试样中析出大量S＇相分散了共面滑移,同时能稳定亚晶界阻止动态回复过程因而提高了蠕变断裂阻力。     

原位合成TiB2/ ZL109 复合材料的高温蠕变行为

采用原位合成方法制备了TiB₂ 超细颗粒增强ZL109 复合材料, 对材料进行了高温拉伸蠕变实验。实验结果表明, 复合材料在高温恒应力条件下, 表现出高的名义应力指数和高的名义蠕变激活能, 优于纯Al 和ZL109 合金, 而且比常规外加颗粒复合材料具有更好的高温蠕变性能。引入门槛应力概念, 复合材料的蠕变实验结果能够用微观结构不变模型来解释, 说明复合材料的蠕变受到基体点阵扩散的控制。复合材料的蠕变断裂行为可以用Monkman2Grant 经验公式来描述, 蠕变断裂特征为延性断裂。     

Al_2O_3-SiO_(2(sf))/AZ91D镁基复合材料蠕变本构方程

利用常应力拉伸蠕变试验法对体积分数为25%的硅酸铝短纤维(Al2O3-Si O2(sf))增强AZ91D镁基复合材料及其基体合金AZ91D在温度为473 K和573 K、外加应力为30～100 MPa下进行蠕变测试。根据应变和应变速率曲线,计算出复合材料的真应力指数、真蠕变激活能、真门槛应力、载荷转移因子和蠕变本构方程。TEM分析结果表明,复合材料蠕变后的门槛应力来源于短纤维表面上的MgO颗粒和Mg17Al12析出相对可动位错的钉扎作用,短纤维具有承载和传递载荷作用,从而提高了复合材料的抗蠕变性能。     

非连续增强LY12复合材料蠕变机制研究

研究了应力、温度和热处理对22vol%莫来石短纤维增强LY12复合材料性能的影响,实验采用铸态、退火态和T4处理复合材料,三种状态下复合材料的抗蠕变性能均高于基体材料LY12铝合金.文中提出一种复合材料的蠕变机制:在低应力阶段,复合材料的蠕变受位错的攀移过程所控制;在高应力阶段,复合材料的蠕变受增强体与基体的界面所控制.用TEM分析了材料的界面.     

界面对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响

界面对复合材料蠕变性能的影响很大。在试验分析的基础上建立了硅酸铝短纤维增强AZ91D镁基复合材料理论分析模型,利用三维有限元分析方法,系统研究了界面特性、界面上应力应变分布和短纤维位向变化对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响。研究表明：界面特性,如厚度、模量,均对纤维最大轴应力和稳态蠕变速率有影响,当界面厚度增加,纤维最大轴应力减小而稳态蠕变速率增大;当界面模量增大,纤维最大轴应力增大而稳态蠕变速率减小,但当界面模量高于基体模量时,纤维最大轴应力和稳态蠕变速率均保持不变;纤维位向也影响轴应力分布和稳态蠕变速率,纤维在其末端界面上存在较大的应力和应变,此处容易产生微裂纹而使材料抗蠕变能力下降;界面对硅酸铝短纤维增强AZ91D镁基复合材料的蠕变曲线和蠕变断裂机制也有影响,其影响程度还与纤维位向有关。     

DD3单晶合金蠕变性能的实验研究

对三种不同试验条件下DD3合金的蠕变寿命、蠕变曲线、断口及不同蠕变阶段的显微组织和对应的筏化宽度进行了分析.结果表明:中温条件下应力对DD3合金蠕变寿命影响显著;较高温度时,温度或应力的增加均可促进微孔和韧窝的长大,此时的断裂模式为微孔聚集型断裂;蠕变过程是γ'相发生定向粗化、形成筏排和解筏的过程,筏化形成的快慢和解筏过程直接影响合金的蠕变寿命.     

颗粒增强Ti基复合材料的特性

本文研究了采用粉末冶金工艺制备的碳化钛颗粒(TiCp)增强 Ti-Al-V 复合材料的显微组织、弹性模量、高温强度、蠕变行为及断裂特征。研究结果表明,选用合适的不连续增强相及 Ti合金基体可以获得增强相与基体结合良好、具有高比强、高比模、良好高温性能的不连续增强 Ti基复合材料。这些 Ti 基复合材料具有各向同性,可用于复杂应力状态,可通过热压、热挤压等一般热加工工艺加工成型。     

玻璃短纤维增强树脂基复合材料的单轴时间相关棘轮行为实验研究

对玻璃短纤维增强树脂基复合材料(短纤维体积分数：28%,40%)的室温单轴循环棘轮行为进行了实验研究,讨论了复合材料在不同加载条件下的棘轮变形特征。结果表明：该复合材料在宏观层次上表现出与基体相类似的棘轮变形规律,即在非对称应力循环下也将产生明显的棘轮变形,并随应力幅值和平均应力的增加而增加;树脂基复合材料的棘轮行为具有明显的时间相关特性,棘轮应变值依赖于应力率和峰值保持时间。在建立玻璃短纤维增强不饱和聚酯树脂基复合材料棘轮行为的本构模型时必须考虑基体的黏性变形特征。     

基体特性对δ-Al2O3/Al合金复合材料力学行为的影响

基体行为是短纤维增强金属基复合材料中一个重要的因素.本文将通过实验分析方法和在一定的理论分析模型上,借助于弹塑性有限元分析方法,对基体特性的变化对δ-Al2O3/Al合金复合材料力学行为的影响作较为详细的研究,其中包括基体性能对应力传递、抗拉强度以及断裂机理的影响.研究表明,基体性能的变化显著影响基体与纤维间的应力传递,从而对复合材料的抗拉强度和断裂机理产生较大的影响.     

Creep Behavior in Interlaminar Shear of a SiC/SiC Ceramic Composite with a Self-healing Matrix

Creep behavior in interlaminar shear of a non-oxide ceramic composite with a multilayered matrix was investigated at 1,200 °C in laboratory air and in steam environment. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated Hi-Nicalon? fibers woven in a five-harness-satin weave. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied. The interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 16–22 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. In air and in steam, creep run-out defined as 100 h at creep stress was achieved at 16 MPa. Larger creep strains were accumulated in steam. However, creep strain rates and creep lifetimes were only moderately affected by the presence of steam. The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Creep of Nextel™720/alumina–mullite ceramic composite at 1200 °C in air,argon, and steam

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200 °C in laboratory air, in steam and in argon. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured at 1200 °C. The elastic modulus was 74.5 GPa and the ultimate tensile strength was 153 MPa. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in laboratory air for creep stress levels ?91 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, as well as damage and failure mechanisms were investigated.     

碳纤维增强树脂复合材料细观蠕变性能

碳纤维增强树脂复合材料以其优异的性能,在各领域得到广泛应用。由于树脂基体具有黏弹性,使其合成的复合材料也表现出黏弹性行为。蠕变是材料黏弹性行为中最典型的一类现象,因此对碳纤维增强树脂复合材料细观蠕变性能的研究具有重要意义。室温下利用纳米压痕技术对碳纤维增强树脂复合材料中的基体、界面及纤维相在不同峰值载荷下的细观蠕变行为进行分析。结果表明:在相同的蠕变时间下,最大载荷为2 mN和10 mN的纤维蠕变位移约为基体蠕变位移的1/3和1/2,界面的蠕变位移介于两者之间;稳态蠕变阶段的蠕变速率小于0.1%;基体、界面、纤维的蠕变应力指数分别为3.6、2.9和2.1。同时根据Kelvin-Voigt模型得到了基体、界面及纤维的第一、第二复数模量、黏度系数及蠕变柔量。      

Modeling of the influence of fibers on creep of fiber reinforced cementitious composite

An analytical model has been developed to study the influence of fibers on creep of fiber reinforced cementitious composites. The model is based on the assumption that shear stress is produced between fiber and surrounding matrix as the matrix deforms. This shear stress in turn influences the matrix creep behavior resulting in macroscopic creep strain lower than that of pure cement-based matrix. In the present paper, a creep strain expression in the form of matrix creep strain multiplying by a fiber influence factor, which reflects the influences of matrix and fiber properties as well as fiber orientation characteristics, is presented. A parametric study, including the influence of elastic moduli of fiber and matrix, fiber dimension and fiber content is carried out. The modeling results indicate that creep strain of fiber reinforced cement-based composite is significantly influenced by the elastic moduli of fiber and matrix as well as fiber length and thickness (i.e. diameter for fiber with circular cross-section). Model predictions compare favorably with experimental measurements of creep strain of fiber reinforced mortar and concrete under compressive load.     

Effects of steam environment on creep behavior of Nextel™720/alumina–mullite ceramic composite at elevated temperature

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1000 and 1100 °C in laboratory air and in steam. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel?720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. The presence of steam accelerated creep rates and dramatically reduced creep lifetimes. The degrading effects of steam become more pronounced with increasing temperature. At 1000 °C, creep run-out (set to 100 h) was achieved in all tests. At 1100 °C, creep run-out was achieved in all tests in air and only in the 87.5 MPa test in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.     

基于细观力学的纤维沥青混凝土有效松弛模量

为了研究纤维沥青混凝土的本构模型,将其视为以沥青混合料为粘弹性基体,纤维为弹性夹杂的两相复合材料。对基于复合材料细观力学理论建立的有效模量表达式进行了修正,提出了纤维沥青混凝土的割线有效松弛模量。以聚酯纤维沥青混凝土为例进行了有效松弛模量的解析分析和模拟蠕变实验的有限元分析,分析结果与试验数据的比较表明,该文提出的割线有效松弛模量模型对于纤维沥青混凝土粘弹性力学行为具有很好的预测能力。应用该模型对路面弯沉变形进行了有限元分析,结果表明:纤维的加入有效的改善了沥青混凝土路面的粘弹性性能。     

Comparative evaluation of fiber treatments on the creep behavior of jute/green epoxy composites

This work presents the short term creep behavior of novel treated jute fabric reinforced green epoxy composites. Jute fabric was treated with CO₂ pulsed infrared laser, ozone, enzyme and plasma. The treated jute fibers were characterized by scanning electron microscopy (SEM). Composites were prepared by hand layup method and compression molding technique. The creep and dynamic mechanical tests were performed in three-point bending mode by dynamic mechanical analyzer (DMA). The creep strain was experiential to increase with temperature. The treated composites exhibited less creep strain than untreated one at all temperatures. The best result in terms of creep deformation is presented by laser treated composite which dominantly exhibited elastic behavior rather than viscous behavior, especially at higher temperatures. The Burgers four parameters model was used to fit the experimental creep data using R statistical computing software. A good agreement between experimental data and theoretical curves were obtained. Dynamic mechanical analysis results revealed the reduction in the tangent delta peak height of treated composites, might be due to improvement in fiber/matrix interfacial adhesion. The degree of interfacial adhesion between the jute fiber and green epoxy was also anticipated using adhesion factor obtained through DMA data and laser treated composite revealed the better interlocking of fibers and matrix at the interface.     

Creep crack growth in fiber reinforced composites with rate-dependent bridging - Role of a viscous fiber matrix interface

A model is developed for creep crack growth in continuous fiber reinforced composites wherein a growing matrix crack is subjected to rate-dependent bridging by unbroken fibers. The rate-dependence in this model arises as a result of the presence of a viscous fiber/matrix interfacial layer. Under load this layer undergoes shear flow causing time-dependent pull-out of bridging fibers from the crack surfaces. The mechanics of time-dependent bridging is combined with a failure criterion based on secondary failure in a crack-tip creep process zone. The dependence of the matrix creep crack growth rates on flaw size and crack wake parameters as well as on composite microstructure is derived. It is shown that the crack wake plays a predominant role in influencing not only the magnitude of creep crack growth rates but also the relationship of growth rates to the crack sizes. A closed form expression is derived for the dependence of crack growth rates on loading, creep and bridging parameters in the regime wherein crack growth rates are independent of crack size.     

Effects of environment on creep behavior of two oxide/oxide ceramic–matrix composites at 1200 °C

The tensile creep behavior of two oxide/oxide ceramic–matrix composites (CMCs) was investigated at 1200 °C in laboratory air, in steam, and in argon. The composites consist of a porous oxide matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, have no interface between the fiber and matrix, and rely on the porous matrix for flaw tolerance. The matrix materials were alumina and aluminosilicate. The tensile stress–strain behavior was investigated and the tensile properties were measured at 1200 °C. Tensile creep behavior of both CMCs was examined for creep stresses in the 80–150 MPa range. Creep run-out defined as 100 h at creep stress was achieved in air and in argon for stress levels ≤100 MPa for both composites. The retained strength and modulus of all specimens that achieved run-out were evaluated. The presence of steam accelerated creep rates and reduced creep life of both CMCs. In the case of the composite with the aluminosilicate matrix, no-load exposure in steam at 1200 °C caused severe degradation of tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated. Poor creep performance of both composites in steam is attributed to the degradation of the fibers and densification of the matrix. Results indicate that the aluminosilicate matrix is considerably more susceptible to densification and coarsening of the porosity than the alumina matrix. The views expressed are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense or the U.S. Government.     

Creep Behavior of Mullite Short Fiber Reinforced ZL109 Alloy Composites at High Temperature

The creep behavior of AI203.SIO2 fiber reinforced ZL109 composites has been investigated at four temperatures ranging from 553 to 623 K. The results show high stress exponent and highapparent creep activation energy. A good correlation between the normalized creep rate and normalized effective stress means that the true stress exponent of minimum creep strain rate of the composite is very close to 5, and the minimum creep strain rate is matrix lattice diffusion