考虑孔隙和微裂纹缺陷的C/C-SiC编织复合材料等效模量计算

通过观察C/C-SiC复合材料组元分布的扫描电子显微镜(SEM)照片 , 获得了C/C-SiC复合材料化学气相渗透(CVI)制备过程中产生孔隙和微裂纹的几何信息。在此基础上 , 建立了包含孔隙和微裂纹的C/C-SiC微结构有限元模型 , 并利用均匀化等效计算方法预测了平纹编织C/C-SiC复合材料的模量。针对CVI沉积方式制备的2组不同的C/C-SiC复合材料 , 实验测试与等效计算结果表明 : 基于 SEM照片建立的C/C-SiC纤维束和复合材料微结构有限元模型 , 能够反映CVI工艺制备C/C-SiC中孔隙和微裂纹的分布状况; 计算结果与实验数据有良好的一致性 , 数值计算可有效预测C/C-SiC编织复合材料的模量。      

二维编织C/SiC复合材料的热膨胀系数预测

根据二维编织 C/ SiC复合材料的细观结构及其制备工艺特点 , 提出了一种预测该材料面内热膨胀系数的单胞模型。模型充分考虑了编织结构复合材料中的纤维束弯曲和 CVI工艺制备陶瓷基复合材料产生的孔洞对热膨胀系数的影响。利用单胞模型预测了二维编织 C/ SiC的结构参数、 纤维体积含量、 孔洞含量对复合材料热膨胀系数的影响规律 , 结果表明 : 随着纤维束扭结处产生间隙与纱线宽度比值的增大 , 热膨胀系数增大 ; 当其它参数不变时 , 随着纤维体积含量的增大 , 热膨胀系数反而下降; 随着孔洞含量的增加 , 热膨胀系数也出现了下降的趋势。利用 DIL402C热膨胀仪测试了二维编织 C/ SiC复合材料纵向热膨胀系数 , 试验结果与模型预测结果吻合较好。     

C/C-SiC缎纹编织复合材料孔隙缺陷的建模及其拉伸性能仿真

主要研究了随机孔隙缺陷在C/C-SiC缎纹编织复合材料中的有限元建模方法及其对拉伸性能的影响。基于C/C-SiC缎纹编织复合材料的细观结构和实验观察所得的微观形貌,得出孔隙缺陷具有随机分布特征,提出了一种三维随机碰撞算法模拟孔隙在复合材料中的分布,建立了含随机孔隙缺陷的C/C-SiC缎纹编织复合材料的有限元模型。采用有限元软件ABAQUS模拟了其在拉伸载荷下的力学行为,讨论了孔隙缺陷的尺寸和分布形式对材料拉伸性能的影响,并对试样进行了单轴拉伸实验测试,验证了数值模拟的有效性。结果表明,用本文方法建立的有限元模型符合含孔隙缺陷C/C-SiC缎纹编织复合材料的真实细观结构,相应的数值模拟结果也与试验数据吻合较好。本文的研究结果为含孔隙缺陷的缎纹编织复合材料及具有相似结构特征的复合材料的力学分析与优化设计提供了一种有效的方法。      

二维编织C/SiC陶瓷基复合材料的热传导系数预测

根据二维编织C/SiC复合材料的细观结构及其制备工艺特点,提出了一种预测该材料热传导系数的单胞模型。模型简化了编织结构纱线的实际构型,充分考虑了编织结构复合材料由于化学气相渗透(CVI)工艺制备陶瓷基复合材料产生的孔洞对热传导系数的影响。利用单胞模型预测了二维编织C/SiC的结构参数、纤维体积含量、孔洞体积含量对复合材料热传导系数的影响规律。结果表明: 随着纤维束扭结处产生间隙与纱线宽度比值的增大,热传导系数减小;当其它参数不变时,热传导系数随着纤维体积含量和孔洞体积含量的增加而下降。利用Hot Disk热测量仪采用瞬变平面热源法测试了二维编织C/SiC复合材料面内的热传导系数,试验结果与模型预测结果吻合较好。     

C/SiC复合材料等温化学气相浸渗过程的数值模拟研究

根据C/SiC复合材料的结构以及等温化学气相浸渗法的工艺特点, 建立了ICVI过程中C纤维预制体结构变化的多尺度孔隙模型和C/SiC复合材料ICVI致密化过程的数学模型. 利用该模型对ICVI法制备C/SiC复合材料进行了数值计算和分析. 模拟结果与实验结果呈现出相同的规律并且两者之间误差较小, 表明本文所建立的模型可以很好地描述C/SiC复合材料的ICVI致密化过程. 利用该模型计算出C/SiC复合材料孔隙率的分布情况以及总体孔隙率在浸渗过程中的演化规律, 对ICVI工艺的优化具有一定的指导意义.     

孔隙缺陷对三维编织C/C复合材料等效弹性性能的影响

弹性常数是工程材料结构设计的重要参数.为了研究孔隙缺陷对三维编织C/C复合材料弹性性能的影响,考虑了纤维增强相、基体相和界面相的孔隙缺陷,建立了孔隙缺陷随机分布的纤维丝尺度和纤维束尺度的双尺度单胞模型,采用均匀化方法预报了含孔隙缺陷的三维编织C/C复合材料等效弹性常数,预报结果与实验结果吻合良好.在此基础上研究了孔隙率...     

SiC涂层C/C复合材料在1300℃湿氧环境中的疲劳行为

在半圆柱型氧化模式的基础上,建立了具有SiC涂层C/C复合材料在湿氧环境中的疲劳失效模型。在高温湿氧环境下,C/C复合材料的疲劳失效主要受到氧化深度的控制,而影响氧化深度的主要参数是涂层表面裂纹的宽度。对此疲劳失效模型,讨论分析实验温度以及外加应力对裂纹宽度的影响,考虑了疲劳载荷引起的材料模量变化对裂纹宽度的影响。在 90、105、120、135和150 MPa疲劳应力下的材料在1300℃ 湿氧环境中的平均寿命分别为48660、32645、22078、12332和4786循环。进行了实验数据与模型计算值的对比,实测寿命与预测结果吻合良好。另外,本模型在外加应力较大、实验时间较短条件下的预测数据与实验结果最为接近。     

C/SiC-SiB4复合材料的制备及氧化行为

在多孔C/SiC中渗入SiB4微粉后,采用先驱体浸渍裂解(PIP)结合化学气相渗透(CVI)法进行致密化制备C/SiC-SiB4复合材料.利用XRD、EDS、SEM分析了材料的组分及微结构.研究了材料在500～1000℃静态空气的氧化行为,并与致密C/SiC复合材料的氧化行为进行了比较.结果表明,SiB4主要渗入到纤维束间,它与随后PIP及CVI法引入的SiC较好地结合在一起.在氧化过程中,SiB4起自愈合作用,它能减缓碳纤维和界面的氧化.在600～900℃氧化10h后,C/SiC-SiB4的失重率均比致密C/SiC小,抗弯强度没有明显降低,且均比致密C/SiC高.     

丙烯CVI致密C/C复合材料的模拟研究

通过模拟法对丙烯化学气相渗透(cvI)制备炭/炭复合材料(C/c)的工艺过程进行研究.基于16种反应物,建立了由34个基元反应组成的均相反应模型.引用双孔隙演化模型、非均相反应模型,对整个丙烯CVI模型进行耦合模拟.模拟结果表明,CVI法C/C复合材料的密度均匀性受沉积气体滞留时间、致密温度和致密时间等因素影响;致密化过程可分为微孔隙快速致密和大孔隙致密两个阶段,丙烯初级热解产物的浓度分布与这两个阶段关系密切;其中大分子气体苯的浓度分布对CVI沉积速度和C/C复合材料密度均匀性的影响明显,当苯浓度分布最高区严重变窄时,表明致密化过程结束,致密表面将出现结壳现象.验证实验与模拟数据吻合较好.     

二维编织陶瓷基复合材料偏轴拉伸力学性能预测

基于二维编织C/SiC复合材料的细观结构,建立了碳纤维丝/热解碳界面/SiC基体和纤维束/表层SiC基体两种尺度下的细观单胞模型,通过有限元方法计算碳纤维丝/热解碳界面/SiC基体模型的等效弹性常数和强度,然后代入纤维束/表层SiC基体模型中计算,并引入Tsai-Wu失效准则,考虑不同失效模式的损伤,建立了二维编织C/SiC复合材料的渐进损伤模型,模拟了其偏轴拉伸应力-应变行为。针对该模型,阐述了二维编织C/SiC复合材料单胞模型在复杂应力状态下其纤维束的损伤过程。数值模拟结果与实验数据吻合较好,验证了模型的有效性,为该种材料的力学性能分析提供了一种有效方法。     

Simulation of Degraded Properties of 2D plain Woven C/SiC Composites under Preloading Oxidation Atmosphere

In this paper, a numerical model which incorporates the oxidation damage model and the finite element model of 2D plain woven composites is presented for simulation of the oxidation behaviors of 2D plain woven C/SiC composite under preloading oxidation atmosphere. The equal proportional reduction method is firstly proposed to calculate the residual moduli and strength of unidirectional C/SiC composite. The multi-scale method is developed to simulate the residual elastic moduli and strength of 2D plain woven C/SiC composite. The multi-scale method is able to accurately predict the residual elastic modulus and strength of the composite. Besides, the simulated residual elastic moduli and strength of 2D plain woven C/SiC composites under preloading oxidation atmosphere show good agreements with experimental results. Furthermore, the preload, oxidation time, temperature and fiber volume fractions of the composite are investigated to show their influences upon the residual elastic modulus and strength of 2D plain woven C/SiC composites.     

Modeling the effects of reactor wall reaction on isothermal CVI process of C/SiC composites

Two comparative mathematical models were proposed to simulate the effects of reactor wall reaction on isothermal CVI process of C/SiC composites. Transport phenomena of momentum, energy and mass in conjunction with infiltration induced changes of preform structure were taken into account in both models. The difference between the two models is that one model bases on the consumption that reaction on ICVI reactor wall is neglected while another model is treated as a case that reactor wall reaction is taken into account. The integrated models were solved by finite element method. Calculation results show that the difference of mean molarities of methyltrichlorosilane in preform for the two cases varies less than 2.5% and that of global porosities of C/SiC composites for the two cases varies less than 0.6% during ICVI process. The trivial differences imply that ignorance of reactor wall reaction is reasonable and acceptable for numerical simulation of ICVI process of C/SiC composites.     

Predictions of elastic property on 2.5D C/SiC composites based on numerical modeling and semi-analytical method

In this paper, the predictions of elastic constants of 2.5D (three-dimension angle-interlock woven) continue carbon fiber reinforced silicon carbide (C/SiC) composites are studied by means of theoretical model and numerical simulation. A semi-analytical method expressing elastic constants in terms of microstructure geometrical parameters and constitute properties has been proposed. First, both the geometrical model of the 2.5D composite and the representative volume element (RVE) in both micro- and meso-scale are proposed. Second, the effective elastic properties of the RVE in 2.5D C/SiC composites are obtained using finite element method (FEM) simulation based on energy equivalent principle. Finally, the remedied spatial stiffness average (RSSA) method is proposed to obtain more accurate elastic constants using nine correction factor functions determined by FEM simulations, also the effects of geometrical variables on mechanical properties in 2.5D C/SiC composites are analyzed. These results will play an important role in designing advanced C/SiC composites.     

2D-SiC/SiC复合材料拉伸加卸载行为

为了研究国产2D-SiC/SiC复合材料的拉伸损伤行为以及低周循环载荷作用下的力学性能,通过试验和建立加卸载细观力学模型,对其拉伸加卸载行为进行了探讨。建立了单向连续纤维增强陶瓷基复合材料加卸载细观力学模型,得到了初始加载、卸载和重新加载时的应力-应变关系;利用断裂统计方法得到了基体裂纹数随应力变化的关系和复合材料失效判断条件。经过应力转化,将该模型应用于国产二维编织SiC/SiC复合材料。对单向加载试件,采用正交试验方法和最小二乘法得到基体Weibull模量和界面剪切阻力,通过控制材料失效强度与试验结果一致,得到纤维Weibull模量。由上述参数确定的2D-SiC/SiC复合材料拉伸循环加卸载应力-应变曲线与实测曲线吻合很好。通过Matlab编程得到2D-SiC/SiC复合材料单向加载时基体开裂过程图。结果表明,2D-SiC/SiC复合材料失效时,基体裂纹分布相对比较均匀;基体裂纹数随应力单调增加,未出现持平段,表明材料失效时,基体裂纹还没有达到饱和。     

CVI结合浆料浸渍法制备2D C/SiC复合材料的微观结构和力学性能

采用CVI结合浆料浸渍工艺制备2D C/SiC复合材料。研究了SiC微粉对复合材料微结构和力学性能的影响。结果表明,当碳纤维预制沉积SiC80h后,微粉主要渗入到纤维束间。复合材料的力学强度随着渗微粉前CVI时间的增加及渗入浆料浓度的降低而增加。微粉的渗入大大降低了材料的层间剪切强度,而对材料的拉伸强度影响较小。     

平纹编织SiC/SiC复合材料多尺度建模及强度预测

连续SiC纤维增强SiC基体复合材料（SiC/SiC）具有优异的高温力学性能、辐照稳定性及较低的氚渗透率,在核工程结构领域具有良好的应用前景,掌握其承载状态下的损伤演化和强度性能,对SiC/SiC复合材料的应用具有重要指导意义。本文基于平纹编织SiC/SiC复合材料的制备过程和组分材料分布的多尺度特性,考虑复合材料微观结构的局部近似周期性,建立了纤维丝尺度和纤维束尺度单胞模型。使用有限元分析软件对纤维丝尺度模型的弹性性能和强度性能进行预测,将这些性能参数代入纤维束尺度模型,引入Tsai-Wu失效准则,根据材料的不同失效模式并对失效单元进行方向性刚度折减,模拟了平纹编织SiC/SiC复合材料在单轴拉伸载荷下的渐进损伤过程。数值模拟曲线与试验曲线吻合较好,实现了对平纹编织SiC/SiC复合材料强度的有效预测。      

Laser ablation behaviors of C/SiC composites in air简

Ablation behaviors of silicon carbide (SiC) coated 2D carbon fiber reinforced silicon carbide matrix (designated as 2D C/SiC) composites prepared by chemical vapor infiltration (CVI) were investigated by a continuous wave CO₂ laser. The 2D C/SiC specimens were exposed under laser for 0.5 s in ambient air, and the laser powers varied from 500 W to 1500 W. A 3D finite element model was established to calculate the temperature distribution in the laser ablation process. The ablation depth, width and profile were measured by Laser Confocal Microscope (LCM). The results indicate that the increase of ablation depth follows a linear relation with the increase of laser power, and the increase of ablation width follows a similar trend with the increase of the isotherm diameter of 1712 ℃. The ablated surface can be distinguished into three different regions by scanning electron microscope (SEM) observation, including ablation center, transition zone and ablation fringe. The ablation behaviors of carbon fibers and SiC matrices in different regions were presented and discussed in the study.