真空压力浸渗制备CF/Al复合材料热收缩与残余应力数值模拟与实验研究

针对连续碳纤维增强铝基复合材料(CF/Al复合材料),采用细观力学数值模拟与热性能试验结合的方法,研究了真空压力浸渗制备过程中的热收缩行为和热残余应力分布。结果表明,复合材料的横向热收缩应变量远大于轴向热收缩应变量,且具有横观各向同性,纤维随机分布的单胞有限元模型能够准确地预测复合材料轴向与横向热收缩行为曲线;复合材料制备完成后纤维和基体合金分别处于压应力和拉应力状态,基体和纤维的横向残余应力均小于其轴向残余应力,且均表现出横观各向同性;基体合金在轴向残余拉应力作用下会出现不同程度的损伤现象,特别是纤维间距较小部位过高的残余应力会引发界面的局部失效,从而不利于发挥复合材料承载性能,减少纤维局部偏聚是进一步改善提高复合材料力学性能的重要技术手段。     

Rolling consolidation of metal matrix composites

Transverse rolling of metal matrix composite precursor wires is proposed as a consolidation technique for making sheets. Rolling in the transverse direction to the fiber orientation is analytically shown to be feasible, and longitudinal rolling results in fiber breakage. Plasticity analysis is conducted using Hill's general yield criterion for an isotropic materials and the associated Levy-Miles equations modified for plane strain conditions. The slab method is used to calculate the stresses in the material, and the effects of rolling parameters on the principal stress ratio are investigated.At the microscopic level, an elastic-plastic finite element formulation and a computation procedure are presented. Individual fibers are modeled to determine the stress state around each fiber. The principal stress ratio is suggested as a parameter that determines the tendency for void formation due to debonding and fiber breakage; finite element analysis is used to determine the effects of the principal stress ratio on the fiber-matrix interfacial stresses in the micromechanics model. The analysis determines the deformed mesh, plastic zone propagation and the stresses at the interface as a function of volume fraction and principal stress ratio. Interfacial stresses are assumed to be responsible for debonding during the deformation of metal matrix composites. This assumption and the results of the analysis provide guidelines for defining, the level of the biaxial stress field in the plane transverse to the fibers during rolling that will minimize interfacial fiber-matrix stresses.     

Residual Stresses in Thermal Spray Coatings and Their Effect on Interfacial Adhesion: A Review of Recent Work

An overview is presented of the development of residual stresses in thermal spray coatings and their ef-fects on interfacial debonding. The main experimental techniques for measurement of residual stresse are briefly described, with particular attention given to the method of continuous curvature monitoring. Boundary conditions satisfied by all residual stress distributions are identified and expressions derived for the curvatures and stress distributions arising from a uniform misfit strain between coating and sub-strate.It is noted that stress distributions in thick coatings rarely correspond to the imposition of such a uniform misfit strain, so that recourse to numerical methods becomes essential for quantitative predic-tion of stress distributions. Relationships are presented between residual stresses and corresponding strain energy release rates during interfacial debonding. The effect on this of superimposing stresses from an externally applied load is outlined. The initiation of debonding is then considered, covering edge effects and other geometrical considerations. Finally, some specific case histories are briefly outlined to illustrate how the various theoretical concepts involved relate to industrial practice.     

Stress Distribution in the Cruciform Specimen under Transverse Tension Stress for SiC/Ti-6Al-4V Composites

The cruciform specimen was selected to obtain the transverse tensile behavior of SiC fiber reinforced titanium matrix composites. Moreover, the means of combining the unilaterally coupled finite element method with the transverse tensile test was developed to evaluate the interfacial normal bond strength of composites. The results showed that the initial non-linearity in the transverse stress-strain curve of SiC/Ti-6Al-4V occurs at the stress of 350 MPa. The means of combining the unilaterally coupled finit...     

采用顶出方法研究纤维增强钛基复合材料的界面断裂韧性

为了估计单向SiC纤维增强钛基复合材料的界面断裂韧性G_IIc,本文提出了一个关于单根纤维顶出试验的新模型,在本模型中,界面脱粘开始于试样的底端面。本文以断裂力学为基础推导出了G_IIc 的理论公式,并且讨论了几个关键因素对G_IIc的影响,如裂纹扩展所需的外加应力,裂纹长度以及界面的摩擦剪切应力。并且运用此模型预测了复合材料 Sigma1240/Ti-6-4, SCS/Ti-6-4, SCS/Timetal 834 and SCS/Timetal 21s 的界面断裂韧性,并与以前的有限元结果进行了比较。结果显示,对于脱粘起始于试样的底端面的顶出试验,本模型能较可靠地预测钛基复合材料的界面断裂韧性。     

陶瓷基复合材料结构固有频率与内部损伤分析

通过室温条件下的循环加卸载试验,研究编织SiC/SiC复合材料固有频率特性及内部损伤演化过程。结果表明:固有频率随循环加卸载过程中峰值应力的增大而减小,通过分别定义频率衰退参数Φ与模量衰退参数D表征了复合材料固有频率与弹性模量的折减程度。基于细观力学理论对编织陶瓷基复合材料循环加卸载迟滞回线进行理论计算,理论模拟结果与试验数据良好吻合。同时,分析计算结果还发现,随着循环加卸载过程中Φ的增加,材料弹性模量衰退参数D、界面脱粘比2l_d/l_c和纤维断裂概率q均呈明显上升趋势,当陶瓷基复合材料结构固有频率衰退1%时,基体出现开裂,界面发生脱粘;当固有频率衰退4.17%时,2l_d/l_c上升至1（即完全脱粘）,q增加到2.5%,这表明复合材料固有频率的变化可以反映出材料内部的损伤失效过程。     

Interface effects on crack deflection and bridging during fatigue crack growth of titanium matrix composites

The effect of the interface on the crack deflection and crack bridging behavior of continuous fiber-reinforced titanium matrix composites has been investigated using three interfaces with significantly different mechanical characteristics. Each of these composites exhibited stress ranges in which fiber bridging was present and stress ranges in which stable fiber bridging was not present. The fatigue crack growth rate for all composites, even for the ones that did not exhibit fiber bridging was significantly below that of the matrix. This phenomenon, believed to be an effect of elastic crack shielding, was most significant for composites with the strongest interfacial bond. Interface failure ahead of the crack tip and its influence on the local stress intensity factor is believed to be responsible for the decrease in the shielding effect of low strength interfaces. Interface debonding was observed in all three composites, and damage to the interface ahead of the crack tip was seen in two of the three composites. A stress-based criterion for predicting debonding appeared to effectively explain the crack deflection behavior for the three composites. Evidence of crack deflection even for the strongest interface suggests that there is scope to increase the interface bond strength in SiC/Ti-alloy system for improved transverse properties without compromising the fatigue life.     

用十字形试样评定钛基复合材料界面法向结合强度(英文)

采用单向耦合的有限元方法预测钛基复合材料界面法向结合强度以及十字形试样内的应力分布。有限元模拟共包括两步：复合材料冷却过程和横向拉伸过程。详细分析了界面杨氏模量和残余热应力对界面径向应力分布的影响,并评定得出SiC／Ti-6Al-4V界面法向结合强度为300MPa。     

Mechanical properties of bone-shaped-short-fiber reinforced composites

Short-fiber composites usually have low strength and toughness relative to continuous fiber composites, an intrinsic problem caused by discontinuities at fiber ends and interfacial debonding. In this work a model polyethylene bone-shaped-short (BSS) fiber-reinforced polyester–matrix composite was fabricated to prove that fiber morphology, instead of interfacial strength, solves this problem. Experimental tensile and fracture toughness test results show that BSS fibers can bridge matrix cracks more effectively, and consume many times more energy when pulled out, than conventional straight short (CSS) fibers. This leads to both higher strength and fracture toughness for the BSS-fiber composites. A computational model was developed to simulate crack propagation in both BSS- and CSS-fiber composites, accounting for stress concentrations, interface debonding, and fiber pull-out. Model predictions were validated by experimental results and will be useful in optimizing BSS-fiber morphology and other material system parameters.     

纤维排布方式对SiC/Ti材料残余应力影响的有限元分析

采用三维有限元法模拟SiC/Ti-6Al-4V复合材料界面的残余应力分布,分析纤维排列方式对纤维一侧界面残余应力的影响。结果表明,纤维排列方式对纤维一侧界面径向、轴向和周向残余应力均有较大影响,其中纤维六方排列时纤维一侧界面残余应力沿纤维周向分布均匀,且周向残余应力小,不易在界面形成径向裂纹,是较为理想的纤维排列方式。     

Evaluation of interface fracture toughness in SiC fiber reinforced titanium matrix composite

The finite element method based on the equivalent domain integral technique was developed to simulate the push out test and evaluate the interfacial fracture toughness of SiC reinforced titanium matrix composites. A special subroutine was introduced while modeling the push-out test to control interfacial failure process. In addition, the residual stresses, Poisson ratio and friction stresses were all considered in the finite element analysis and the interface debonding was described as a continuous process. The results show that the interfacial fracture toughness of SiC/Timetal-834 is about 50 Jim2. Moreover, the effects of various parameters on the interfacial fracture toughness and the variations of energy release rates at both ends of the specimen were analyzed in detail.     

纤维排布方式对SiC/Ti材料残余应力影响的有限元分析

采用三维有限元法模拟SiC/Ti-6Al-4V复合材料界面的残余应力分布,分析纤维排列方式对纤维一侧界面残余应力的影响。结果表明,纤维排列方式对纤维一侧界面径向、轴向和周向残余应力均有较大影响,其中纤维六方排列时纤维一侧界面残余应力沿纤维周向分布均匀,且周向残余应力小,不易在界面形成径向裂纹,是较为理想的纤维排列方式。     

An evaluation of residual stresses in graphite/PMR-15 composites by X-ray diffraction

In this work, a method based on X-ray diffraction measurements of internal stresses in embedded metallic ellipsoidal inclusions is briefly described. The method has been recently developed for the determination of residual thermal stresses in high temperature graphite/polyimide composites. The effects of external bending loads and aging on the measurements of the internal stress in unidirectional and woven graphite fiber (T650-35)/polyimide (PMR-15) composites were examined in addition to several other factors, which could influence the accuracy of the stress measurements. Such factors as the volume fraction of inclusions, their aspect ratios and the interaction between individual embedded inclusions were also evaluated in this study. It has been shown that despite its complexity the proposed method can be successfully applied to the evaluation of residual stresses in high temperature polymer matrix composites subjected to external loads and aging.     

循环变形提高SiC纤维增强铝基复合材料强度及塑性Ⅱ.理论分析

采用断裂力学方法获得了纤维增强复合材料强度与脱粘长度、纤维临界长度以及纤维体积分数的定量关系。该公式较好地预测了纤维的临界长度以及强度与纤维体积分数的关系，并再现了复合材料混合定则。该公式也较好地解释了丝状复合材料强度随短期循环变形载荷与周次增加而增加的现象。其原因是在循环变形中，纤维与基体界面结合强度发生变化，导致纤维临界长度与脱粘长度发生变化。从而使复合材料强度增加，但这种增加是有限的和有范围的。循环变形的发展最终导致强度下降。     

The mechanisms of interfacial failure for lateral force-sensing microindentation test: finite element analysis

The interfacial bond strength for coatings and composites can be quantitatively determined using a newly developed lateral force-sensing microindentation method. In this study, a finite element analysis was made to investigate the interfacial failure mechanisms for Cu–ceramic and Al alloy–ceramic interfaces. The model is validated by comparing obtained results of the finite element analysis with analytical solutions. Two different interfacial failure mechanisms, depending on material properties and microindentation positions, are proposed. As demonstrated, interfacial debonding may result from shear stress or a coupling of tensile stress and shear stress at the interface, corresponding to material “pile-up” deformation or “sink-in” deformation. In addition, the high sensitivity of the lateral force response to interfacial debonding, associated with two different interfacial failure mechanisms, is also examined.     

SiC纤维增强钛基复合材料的横向力学性能

采用十字形试样测试分析有C涂层和无C涂层两种SiC纤维增强钛基复合材料的横向力学性能,以横向载荷作用下应力-应变曲线上的非线性拐点计算界面的强度.结果表明,有C涂层的界面横向开裂强度为53 MPa,低于无C涂层的界面开裂强度196 MPa,并且前者在横向载荷作用下沿C涂层与纤维之间开裂,而后者沿反应生成物与基体间开裂;体积分数为30%的多根纤维钛基复合材料的非线性拐点应力低于单根纤维复合材料,这主要是由于残余应力的减少引起,界面强度并没有明显变化.     

纤维增强金属基复合材料中轴向热残余应力分析

采用ANSYS有限元分析软件,通过显微复合材料模型,系统地分析了纤维直径、TiC 中间层厚度、纤维体积分数和制备温度等因素对SiC纤维增强Cu基复合材料轴向热残余应力的影响特征.结果表明,纤维直径的变化并不影响热应力的人小,但直径越小应力梯度越大;TiC中间层的厚度变化对自身及基体中的应力影响较小:但制备温度的变化对TiC层中的热应力影响较大;纤维体积分数的变化对纤维和基体中的热应力也有着非常显著的影响.     

TiO2纳米管薄膜的脱层现象（英文）

分别通过曲率法和基体拉伸应变测试方法研究TiO2纳米管薄膜的残余应力和脱层行为。结果表明:TiO2纳米管薄膜的内残余应力为-54MPa。TiO2纳米管薄膜室温样、250°C退火样和400°C退火样的脱层出现点的应变依次为2.6%、5.1%和8.6%,半径依次为27.5、17.1和19.4μm。TiO2纳米管薄膜室温样、250°C退火样和400°C退火样的真实临界脱层应力为220.4、394.5和627.9MPa。在脱层条件下,对界面剪滞模型进行修订,并对TiO2纳米管薄膜界面剪切强度进行多项式拟合。由于拟合结果与裂纹密度的分析结果能很好吻合,因此界面剪滞模型的修订方程和多项式拟合方程均为可信的。     

Raman Strain Sensing and Interfacial Stress Transfer of Hierarchical CNT-Coated Carbon Fibers

Hierarchical reinforcements of carbon fibers (CFs) coated with carbon nanotubes (CNTs) were produced via two different methodologies, namely chemical vapor deposition (CVD) and wet chemical treatment. These advanced smart structural materials are possible candidates for inherent strain sensing and improved interfacial properties when incorporated in the volume of structural fiber-reinforced polymer composites. The morphology and wetting properties of the hierarchical CFs were evaluated via SEM and contact angle measurements. Enhanced values in the strain-induced Raman sensing ability of the hierarchical reinforcement produced via CVD in comparison with the uncoated fibers were measured at single fiber level. The residual stress transfer profiles of both the hierarchical and the uncoated carbon fibers were also evaluated using the Raman spectroscopy in order to evaluate the phase interaction in the hierarchical structure. Compressive residual stresses in the order of 1.5 GPa were found to act on the CF surfaces, while tensile residual stresses were measured for both the CNT-coated systems. The effects of the preparation process of the CNT coatings on the residual stress profiles were also prominent, with the CVD-grown coatings resulting in higher tensile residual stresses, 1.76 GPa instead of 1.22 GPa for the coatings produced via wet chemical treatment. The change in the residual stress profiles established the interaction between the CFs and the CNTs via an anchoring mechanism which significantly altered the stress field at the vicinity of the interface.     

Visco-elastic stress distributions and elastic properties in unidirectional composites with large volume fractions of fibers

In this work, the visco-elastic stress distributions and elastic properties of unidirectional graphite/polyimide composites have been examined as a function of the volume fraction of fibers. The stress distributions were determined by employing two different methods, namely the finite element method (FEM) assuming either hexagonal or square fiber arrangements, and the Eshelby method modified by Mori and Tanaka to account for the presence of multiple fibers. It has been presented in this research that the Eshelby/Mori-Tanaka approach can be used for the calculations of the stresses inside and outside graphite fibers provided the volume fraction of the fibers does not substantially exceed 35% in the case of the square fiber array and 50% for the hexagonal fiber distribution. It has also been shown that the elastic properties of unidirectional graphite/polyimide composites can be accurately determined using the analytical Eshelby/Mori-Tanaka method even for large volume fractions of fibers.