论文标题:3D-C/SiC的高温拉伸蠕变性能
Tensile Creep Behavior of a 3D-C/SiC Composite at Elevated Temperature
论文作者 杨忠学
论文导师 乔生儒,论文学位 硕士,论文专业 材料学
论文单位 西北工业大学,点击次数 157,论文页数 70页File Size5095k
2002-03-01论文网 http://www.lw23.com/lunwen_65794467/ 3D-C/SiC复合材料,高温蠕变,蠕变激活能,应力指数,持久寿命,蠕变机理
The Three-Dimensional C/SiC composite,High temperature creep,Creep activation energy,Stress exponent,Creep rupture life,Creep mechanics.
本文研究了3D-C／SiC复合材料的高温拉伸蠕变行为。3D-C／SiC复合材料由三维编织的纤维预制体经化学气相渗透(CVI)SiC基体制成。基纤维体积分数约为40％。蠕变持久实验的应力范围为180 MPa～290MPa，温度范围为1100℃～1500℃。 研究表明：3D-C／SiC复合材料具有较高的蠕变抗力，其稳态蠕变速率大约为10~(-8)-10~(-9)/s。在实验过程当中没有观察到蠕变的加速阶段，这和大多数陶瓷基复合材料的高温蠕变现象是一致的。蠕变的第一阶段也就是减速蠕变阶段的蠕变变形可用ε—ε_0=At~m来描述。可以唯象地解释稳态蠕变速率和蠕变应力和温度之间的关系。 用应力经验公式，Monkman-Grant关系和Larson-Miller参数这三种方法来估算3D-C／SiC复合材料的蠕变持久寿命。 3D-C／SiC的高温蠕变过程也是其组织内部损伤不断演化的过程，选择合适的损伤变量可以很好地描述这个过程。本文分别用电阻的变化率和弹性模量的变化率作为损伤因子，测定在高温蠕变实验过程当中的损伤演变规律。 传统的蠕变理论在解释3D-C／SiC这种非匀质，不连续材料的蠕变机理时遇到了困难，显示出其局限性。通过扫描电镜，透射电镜等对碳纤维，基体，界面等的微观组织在蠕变前后的变化的分析，试图用一种蠕变-损伤机制来解释3D-C／SiC的高温蠕变。 高温下，承受恒定载荷的陶瓷基复合材料的破坏是由于各组元(纤维、基体和界面)的蠕变和随时间演变的损伤二者机理共同作用的结果。在宏观层次上，可以把通过基体微裂纹和界面滑动所形成的损伤积累看作是一种裂纹的缓慢生长过程。一旦基体开裂达到一种饱和状态，裂纹在界面处被止住，然后裂纹会随时间在蠕变实验中缓慢地张开。而3D-C／SiC中界面处热解碳层的各向异性结构会促进界面层的脱粘和滑动。由于在所测定的温度区间和应力范围内，碳纤维和基体本身都不大可能发生蠕变。因此，基体开裂和界面的滑动是3D-C／SiC复合材料宏观变形的主要贡献者。
The tensile creep and rupture behavior of a Three-Dimensional C/SiC composite has been investigated in this thesis. The Three-dimensional C/SiC composite, containing 40 Vol% T300 carbon fiber, was fabricated by Chemical Vapor Infiltration. Tensile creep and rupture tests have been performed under vacuum for temperatures ranging from 1100ε to 1500ε and for stresses from 180MPa to 290MPa.The study of the macroscopic creep of the Three-Dimensional C/SiC has revealed a good creep resistance compared to other ceramic matrix composites with the steady-state strain rate in the 10-8~ 10-9s-1 range.Throughout the experiments, there is no accelerated creep regime, which is in accordance with most of the ceramic matrix composite. It has been discovered that the deformation in the primary creep regime can well described byeqution: ε -ε0 = Atm, Moreover, the eqution: ε = Aσn exp(-Q/RT) can be used tointerpret phenomenally the relationship between the steady-state creep rate and stress, temperature.The empirical relation, the Monkman-Grant relationship and the Larson-Miller parameter can be used for creep rupture life prediction for the Three-Dimensional C/SiC composite,The damage can be accumulated during the tensile creep tests at elevated temperature. A suitable damage parameter is needed to fully describe the process. The variation of fractional electrical resistance increase and the variation of elastic module were adopted as damage parameter to identify the tendency of damage evolution.The limits of classical creep mechanics were evidenced when it wan applied to analysis the mechanics of the Three-Dimensional C/SiC composite, which is seriously inhomogeneous and anisotropic.The rupture of ceramic matrix composite submitted to a constant load, at elevated temperature, generally results from the interaction of time-dependent damage mechanisms together with creep of the constituents (ie. Fiber, matrix and/orinterphase ). At the macroscopic scale, it can be considered that we are in presence of a damage accumulation via matrix microcracking coupled with interfacial sliding phenomenon which may be settled according to a slow crack growth mechanism. Once matrix microcracking has reached the saturation state, microcracking opening due to a time-dependent mechanism, which is encountered at the fiber/matrix interface, has been evidenced. Decohesion and sliding are enhanced thanks to the anisotropy if the graphite sheets at the interphase/matrix interface. It seemed that the fiber and the matrix did not creep themselves in the range of the testing stress and temperature, so the matrix microcracking and interfacial sliding will be the main contributer to the macroscopic deformation of the Three-Dimensional C/SiC composite.
|
