基于VCCT的复合材料低周疲劳性能研究

基于VCCT建立复合材料低周疲劳模型,对层合板结构分层损伤进行疲劳寿命预测。采用ABAQUS软件通过直接循环法计算复合材料低周疲劳分层扩展情况,在模拟中指定分层扩展所沿的界面,基于VCCT可以计算界面单元裂纹尖端的断裂能量释放率,通过Paris准则来判断疲劳裂纹的产生和扩展。     

微螺旋碳纤维/高密度聚乙烯复合材料电渗流行为及机制

以微螺旋碳纤维(CMCs)作为复合材料的导电填料,以高密度聚乙烯(HDPE)作为复合材料的基体树脂,研究了CMCs/HDPE基复合材料的电渗流行为、渗流阈值附近复合材料的导电机制。结果表明:CMCs/HDPE复合材料存在明显的渗流行为,其渗流阈值为6.28%(CMCs的体积分数)或9.77%(CMCs的质量分数);当CMCs的体积分数接近于渗流阈值时,CMCs/HDPE复合材料的导电方式由隧道效应所致。     

炭黑/橡胶复合材料多尺度微观结构分布模型

拟采用数值模拟微观结构的方法来推导炭黑/橡胶复合材料的多尺度分布模型。事实上,基于统计特性的多尺度方法,使用同质化技术,使我们能够估计非均匀介质的整体性能界限。本文采用此法的目的是建立一个立体炭黑/橡胶复合材料微观结构形态的三维数学模型。这种多尺度模型,包括一些主要的模型组合,对应于微观结构的物理尺度模型。它可被识别为一种从实验透射电子显微镜(TEM)图像数据和瞬变电磁法获得统计矩数值模拟的原始方法。此法使复杂的粒子团聚体的微观结构的三维数学模型有效。最后,确定建立形态学模型满足实验材料中的炭黑团聚体的碎石渗流率。     

基于随机算法的纤维微观结构渗透率预报及树脂流动数值模拟研究

树脂基纤维增强复合材料的液体成型工艺制件的质量与制备过程中树脂流动浸渍情况紧密相关,微观尺度纤维结构和微观渗透率作为介观尺度和宏观尺度的基石,对树脂的流动浸渍具有直接影响,然而微观尺度难以有效开展试验,数值模拟成为更有效的研究手段。本文采用Monte Carlo随机算法建立了微观尺度纤维束内单丝随机排布结构和纤维束内、束外介观尺度结构模型;基于有限元方法对纤维束内结构进行渗透率预报,并创造性地采用Phase Field相场方法考虑表面张力和毛细力进行树脂流动数值模拟。结果表明:本文所建立的纤维单丝随机结构渗透率预报方法较传统经验公式预报精度显著提高;本文模拟的微观纤维束内孔隙的形成过程,对从微观尺度系统阐述树脂流动浸渍过程和微观气泡的产生机理具有重要意义。     

高性能复合材料弯曲疲劳性能研究

用湿法缠绕技术制作了CF／5228预浸料,对热压罐固化的CF／5228复合材料的力学性能和弯曲疲劳性能进行了研究,并用扫描电镜、电子显微镜等对复合材料的疲劳损伤机理进行了微观表征和理论探讨。研究表明,M40J／5228复合材料比M40／5228具有更为优异的耐疲劳性能。复合材料的疲劳损伤主要有纤维断裂、基体开裂和界面剪切破坏3种表现形式,通常复合材料构件的疲劳破坏多为3种形式的综合表现。基体增韧、选用高强高模碳纤维、界面强化和铺层优化是提高复合材料构件耐疲劳性能有效手段。     

含损伤复合材料加筋板强度分析及修补研究

对含损伤复合材料加筋板进行了强度分析及修补研究。建立了复合材料层合加筋壁板的有限元分析模型,该模型采用界面单元以有效模拟筋条和壁板之间的连接界面及层板分层界面,连接界面和复合材料层板分别采用Quads和Hashin失效准则作为失效判据,引入材料刚度退化模型,采用非线性有限元方法,研究了复合材料加筋壁板在压缩载荷下的破坏过程。建立了筋条脱粘面积、层板分层面积与结构承载能力之间的关系,对不同损伤加筋板进行了修补研究,研究结果可为合理制定复合材料构件缺陷验收标准和结构修理容限提供分析依据。     

三维角联锁机织复合材料弯曲抗疲劳性能有限元分析

使用三维绘图软件PRO/E 5.0绘制出三维角联锁机织复合材料结构模型,借助有限元软件ANSYS Workbench对该结构模型的弯曲疲劳性能进行分析。在复合材料弯曲静力学分析的基础上,添加疲劳工具对复合材料的抗疲劳性能进行分析,通过复合材料纤维、树脂各自的寿命、损伤分布云图分析复合材料的抗疲劳性能。结果表明:弯曲载荷作用下,复合材料与弯曲压头接触的位置表现出更大的弯曲应力;这些位置在较小循环载荷作用下较早发生破坏;与测试方向平行的纬纱较经纱发生更严重的破坏。     

仿生石墨烯增强纳米复合材料力学性能的跨尺度数值模拟和实验研究

为了从微观层面解释仿珍珠层的增强机理,本文设计了一种石墨烯交错排布增强聚甲基丙烯酸甲酯的纳米复合材料,从分子动力学基础上建立有限元力学模型,模拟研究了大尺寸下石墨烯的几何形状和排布尺寸对仿生石墨烯增强聚合物复合材料拉伸和弯曲性能的影响,进一步对比宏观和微观尺度下模型的变形机制与性能变化的关系。最后,结合有限元仿真和实体实验方法研究了微结构对仿珍珠层结构材料力学性能的影响。模拟和试验结果表明：硬质层的形状、厚度、排列方式都会影响材料的强度。通过改变这些参数能够得到最优的力学性能。     

高性能高分子/无机粉体复合材料制备技术

介绍了在高分子/无机粉体复合材料研究中有代表性的工作。从高分子/无机粉体复合体系中各种微观相界面设计与调控角度出发,提出了按无机粉体、偶联剂、助偶联剂及基体树脂之间的作用机制对体系中微观相界面进行分类的原则,系统地研究了各种微观相界面设计与调控方法、所需相界面的形成条件、及其对材料结构、性能的影响与作用机制,并在此基础上建立了种种实现高分子/无机粉体复合材料高性能化或功能化的技术方法。     

外贴FRP加固钢筋混凝土梁抗弯疲劳性能研究进展

纤维增强复合材料(Fiber Reinforced Polymer,简称"FRP")因其轻质、高强、抗疲劳性能好等优点被广泛用于加固工程中。在疲劳荷载作用下,外贴FRP加固钢筋混凝土梁中的各材料性能以及FRP-混凝土界面粘结性能不断劣化,既有损伤不断累积,从而大大降低了加固梁的使用安全性。本文从试验研究、数值模拟以及理论分析三方面对外贴FRP加固钢筋混凝土梁的抗弯疲劳性能研究进行了详细总结,并对FRP-混凝土粘结界面和锈蚀钢筋混凝土加固梁的疲劳性能进行了介绍,分析了现有研究中存在的不足,并为今后的研究方向提出一些建议。     

In Situ Reacted Rare-Earth Hexaaluminate Interphases

A novel in situ reaction between a ceria-doped zirconia interphase coating on Saphikon fibers and an outer alumina coating has resulted in the formation of oriented hexaaluminate platelets which can act as a low fracture energy interface barrier for crack deflection in oxide-oxide ceramic-matrix composites (CMCs). The reaction proceeds only in reducing environments where the reduction of the cerium and zirconium ions to their 3+ valent state causes a destabilization phenomenon consistent with previously reported findings. The diffusion of the cerium from the zirconia into solid solution with the alumina can stabilize the layered hexaaluminate structure. Preferred orientational growth of the hexaaluminate parallel to the coating interface was observed which is the required orientation for enhanced debonding at the fiber/matrix interface in long-fiber-reinforced CMCs.     

Tensile behavior of ceramic matrix minicomposites with engineered interphases fabricated by chemical vapor infiltration

Ceramic matrix composites (CMCs) exhibit quasi-ductile behavior beyond the initial elastic region driven by a weak fiber-matrix interface that can be further engineered by introducing a finite thickness interphaseleading to enhanced strength and toughness. The current work explores the engineering of interphases in CMCs by a controlled variation of fabrication process parameters. C/BN/SiC minicomposite configurations have been fabricated by chemical vapor infiltration (CVI) with the intent of varying interphase thickness and constituent volume fractions by varying the interphase and matrix infiltration durations. The effect of processing durations on the resulting microstructure, tensile response, and damage mechanisms up to and during ultimate failure of CMC minicomposites have been investigated. The presented results highlight the significant influence of processing duration on the tensile and failure behavior of CMC minicomposites thereby providing an insight into the processing-microstructure-tensile response relationship in CMCs.     

A progressive damage model for ceramic matrix mini-composites with thick interphases

Toughness enhancement in ceramic matrix composites (CMCs) with brittle matrix and fiber phases is often accomplished by introducing a weak finite-thickness interphase between the fiber and matrix. The current work presents a progressive damage model to predict the tensile response of single tow CMCs (mini-composite) representative of a unidirectional composite at the microscale. Implementation of a 3-phase shear-lag model for a geometrically accurate representation of the underlying microstructure in CMCs with finite thickness interphase has been highlighted. A probabilistic progressive modeling approach has been adopted, accounting for multiple matrix cracking, interfacial debonding, and fiber failure in 3-phase mini-composites. The predicted tensile response of CMCs from the progressive damage modeling approach agrees with experimental results obtained for C/BN/SiC mini-composites validating the approach.     

Effect of Oxidation on Crack Deflection in SiC/Al2O3 Laminated Ceramic Composites

Laminated composites consisting of SiC and a thin porous alumina interphase were exposed to air at 500°C to produce a persistent, nearly uniform oxidation product layer. Crack deflection at the interface was then studied using a four-point bend testing procedure and interfacial fracture resistances were found to decrease with increasing oxidation times. Electron microscopy observations of the fractured interface show a complex multi-phase microstructure. These results show that oxidation can produce a sufficiently weak interface in a SiC-porous alumina interphase composite, in contrast to most other SiC composites where interface oxidation produces a strongly bonded interface which inhibits crack deflection.     

Mechanical hysteresis and damage evolution in C/SiC composites under fatigue loading at room and elevated temperatures

In this paper, the mechanical hysteresis and damage evolution in C/SiC ceramic matrix composites (CMCs) under cyclic tension-tension fatigue loading at room and elevated temperatures in air and in inert atmosphere and different loading frequencies are investigated. The fatigue hysteresis loops models considering multiple matrix cracking modes are developed to establish the relationships between fatigue hysteresis loops, fatigue hysteresis dissipated energy, and fiber/matrix interface shear stress. The evolution of fatigue hysteresis dissipated energy and interface shear stress vs applied cycles is analyzed. It was found that the interface shear stress degradation rate increases with fatigue peak stress, and loading frequency from 40 to 375 Hz.     

Microstructure and mechanical properties of Si3N4f/BN composites with BN interphase prepared by chemical vapor deposition of borazine

The boron nitride (BN) interphase of silicon nitride (Si₃N₄) fiber-reinforced BN matrix (Si₃N_4f/BN) composites was prepared by chemical vapor deposition (CVD) of liquid borazine, and the microstructure, growth kinetics and crystallinity of the BN coating were examined. The effects of coating thickness on the mechanical strength and fiber/matrix interfacial bonding strength of the composites were then investigated. The CVD BN coating plays a key role in weakening the interfacial bonding condition that improves the mechanical properties of the composites. The layering structure of the BN coating promotes crack propagation within the coating, which leads to a variety of toughening mechanisms including crack deflection, fiber bridging and fiber pull out. Single-fiber push-out experiments were performed to quantify the fiber/matrix bonding strength with different coating thicknesses. The physical bonding strength due to thermal mismatch was discussed.     

Microstructure and tensile behavior of (BN/SiC)n coated SiC fibers and SiC/SiC minicomposites

Interphase plays an important role in the mechanical behavior of SiC/SiC ceramic-matrix composites (CMCs). In this paper, the microstructure and tensile behavior of multilayered (BN/SiC)_n coated SiC fiber and SiC/SiC minicomposites were investigated. The surface roughness of the original SiC fiber and SiC fiber deposited with multilayered (BN/SiC), (BN/SiC)₂, and (BN/SiC)₄ (BN/SiC)₈ interphase was analyzed through the scanning electronic microscope (SEM) and atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. Monotonic tensile experiments were conducted for original SiC fiber, SiC fiber with different multilayered (BN/SiC)_n interfaces, and SiC/SiC minicomposites. Considering multiple damage mechanisms, e.g., matrix cracking, interface debonding, and fibers failure, a damage-based micromechanical constitutive model was developed to predict the tensile stress-strain response curves. Multiple damage parameters (e.g., matrix cracking stress, saturation matrix crack stress, tensile strength and failure strain, and composite’s tangent modulus) were used to characterize the tensile damage behavior in SiC/SiC minicomposites. Effects of multilayered interphase on the interface shear stress, fiber characteristic strength, tensile damage and fracture behavior, and strength distribution in SiC/SiC minicomposites were analyzed. The deposited multilayered (BN/SiC)_n interphase protected the SiC fiber and increased the interface shear stress, fiber characteristic strength, leading to the higher matrix cracking stress, saturation matrix cracking stress, tensile strength and fracture strain.     

Damage development and lifetime prediction of cross-ply ceramic-matrix composites subjected to cyclic loading at room and elevated temperatures

ABSTRACT

In this paper, the damage development and lifetime prediction of fibre-reinforced ceramic-matrix composites subjected to cyclic loading at elevated temperatures in oxidising atmosphere have been investigated. Considering the damage mechanisms of matrix cracking, interface debonding, interface wear and interface oxidation, the damage evolution of fatigue hysteresis dissipated energy, fatigue hysteresis modulus, fatigue peak strain, interface shear stress and broken fibres fraction have been analysed. The relationships between damage parameters and internal damage of matrix cracking, interface debonding and slipping, and fibres fracture have been established. The experimental fatigue hysteresis, interface slip lengths, peak strain, and the fatigue life curves of cross-ply CMCs under cyclic loading at elevated temperature have been predicted. The different fatigue behaviour in unidirectional and cross-ply CMCs at room and elevated temperatures subjected to low-cycle and high-cycle fatigue has been discussed.     

Boron Modified Phenol Formaldehyde Derived C<Subscript>f</Subscript>/SiBOC Composites with Improved Mechanical Strength for High Temperature Applications

Ceramic matrix composites (CMCs) are potential thermo-structural materials for use in space applications. Fiber/matrix (F/M) interface plays a key role in determining the mechanical properties of CMCs. Present study focuses on the optimization of F/M volume ratio and the influence of Pyrocarbon (PyC) interphase coating on the mechanical properties of CMCs derived from precursor route. CMCs are fabricated using phenol formaldehyde (PF) resin and boron modified PF (BPF) resin as precursor slurries, 2D carbon fabric (Toray, T300 3K, 8H, satin weave) as reinforcement and PyC as interphase. The deposition of PyC interphase was done by chemical vapor infiltration on the carbon fabric followed by densification of the matrix using reaction bonded silicon carbide method. In CMCs prepared from PF resin, without interphase the flexural strength improves from 25 ± 3.9 MPa (fiber content-40) to 63 ± 9.9 MPa (fiber content-60) on increasing the fiber vol%. In the second part of the investigation, the effect of PyC interphase was studied using CMCs prepared from BPF resin with fiber volume ratio of 60 %. The CMCs with PyC interphase shows an improvement in flexural strength (102 ± 11.5 MPa) compared to that of CMCs prepared without interphase (38 ± 4.4 MPa). The fractography of CMCs with and without interphase was closely evaluated under a scanning electron microscope. CMCs without interphase show no fiber pull-out, indicating the strong fiber-matrix bonding. While CMCs with interphase show fiber pull-out phenomenon and hence fails in a ductile manner.