Hysteresis loops of fiber-reinforced ceramic-matrix composites under in-phase/out-of-phase thermomechanical and isothermal cyclic loading

In this paper, the stress?strain hysteresis loops of fiber-reinforced ceramic-matrix composites (CMCs) under in-phase/out-of-phase thermomechanical and isothermal cyclic loading have been investigated. The thermomechanical hysteresis loops models have been developed considering synergistic effects of thermal temperature cycling, stress levels and fiber/matrix interface debonding. The relationships between thermal cyclic temperatures, peak stress, fiber/matrix interface shear stress and stress?strain hysteresis loops under in-phase/out-of-phase thermomechanical and isothermal cyclic loading have been established. The effects of fiber volume fraction, peak stress, matrix crack spacing, interface frictional coefficient, interface debonded energy and temperature range on the stress?strain hysteresis loops under in-phase/out-of-phase thermomechanical and isothermal cyclic loading have been analyzed. The hysteresis loops of cross-ply SiC/magnesium aluminosilicate (MAS) composite under in-phase/out-of-phase thermomechanical and isothermal fatigue loading have been predicted.     

Damage monitor and life prediction of carbon fiber-reinforced ceramic-matrix composites at room and elevated temperatures using hysteresis dissipated energy-based damage parameter

In this paper, the damage monitor and life prediction of carbon fiber-reinforced ceramic-matrix composites (C/SiC CMCs) have been investigated using the hysteresis dissipated energy-based damage parameter. The evolution of the interface shear stress, hysteresis dissipated energy, hysteresis dissipated energy-based damage parameter and the broken fibers fraction vs. cycle number, the fatigue life S?N curves of unidirectional, cross-ply and 2.5D C/SiC composites at room temperature and 800 °C in air atmosphere have been analyzed. For unidirectional C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter first increase and then decrease with cycle number, and the fatigue limit stress decreases from 88% tensile strength at room temperature to 20% of the tensile strength at 800 °C in air atmosphere; for cross-ply C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter decrease with increasing applied cycles, and the fatigue limit stress decreases from 85% tensile strength at room temperature to 22% tensile strength at 800 °C in air; and for 2.5D C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter increases with cycle number, and the fatigue limit stress decreases from 70% tensile strength at room temperature to 25% tensile strength at 800 °C in air.     

Interfacial debonding and slipping of carbon fiber-reinforced ceramic-matrix composites under two-stage cyclic loading

In this paper, the interface debonding and frictional slipping of carbon fiber-reinforced ceramic-matrix composites (CMCs) under two-stage cyclic fatigue loading have been investigated using micromechanics approach. Under cyclic fatigue loading, the fiber/matrix interface shear stress degrades with increasing cycle number due to interface wear. The synergistic effect of interface wear and fatigue loading sequence on interface debonding and frictional slipping has been analyzed. Based on the fatigue damage mechanism of fiber slipping relative to matrix, in the interface debonded region, upon unloading and subsequent reloading, the interface debonded length and interface slip lengths, i.e. interface counter-slip length and interface new-slip length, are determined using the fracture mechanics approach. The relationships between interface debonding, interface slipping, interface wear, cycle number, and different loading sequences are determined. There are two types of fatigue loading sequences considered, i.e. (1) cyclic loading under low peak stress for N₁ cycles, and then high peak stress; and (2) cyclic loading under high peak stress for N₁ cycles, and then low peak stress. The effects of peak stress level, interface wear, cycle number, and loading sequence on interface debonding and frictional slipping of fiber-reinforced CMCs have been analyzed. The fatigue hysteresis loops of cross-ply carbon fiber-reinforced silicon carbide composite corresponding to different cycle number under two-stage cyclic fatigue loading have been predicted.     

Comparison of fibre/matrix interface strength for a 3D woven SiC/SiC composite

The fibre/matrix interface shear strength, τ _P, was determined by analysis of fibre pullout length distributions for a 3D woven SiC/SiC-based composite that had undergone tensile testing between room temperature and 1300°C in vacuum and air. Data was compared with the fibre/matrix interface shear strength, τ _S, obtained previously for this system by analysis of in situ fibre strength distributions. τ _P was found to follow the same general trend as that of τ _S and this was explained in terms of the carbon-rich fibre/matrix interface region. However, τ _P was smaller than τ _S by a factor of 3-4 for all cases, but the reason for this remains unclear although several tentative suggestions have been put forward.     

高速气流对C/SiC复合材料激光烧蚀行为影响的实验研究

为了明确高速气流对C/SiC复合材料激光烧蚀行为的影响机制,开展了不同环境下强激光对C/SiC复合材料的烧蚀对比实验研究。利用激光器与高速风洞联合实验平台,完成了静态以及Ma 1.8,Ma 3.0,Ma 6.0气流环境下2D与3DN C/SiC复合材料激光烧蚀实验。结果表明,与静态环境相比,高速气流对C/SiC复合材料的激光烧蚀行为产生了显著的影响,气流的冲刷使得烧蚀坑呈现出更宽、更深、更光滑的变化趋势。随着气流速度的增长,线烧蚀速率与质量烧蚀速率逐渐增大,主要原因为当地静压降低引起的升华速率增大,以及动压增大引起的剥蚀速率增大。此外,通过实验对比了不同构型对C/SiC激光烧蚀行为的影响。结果表明：2D C/SiC复合材料由于厚度方向更低的导热能力、更低的孔隙率等原因,其在不同环境条件下抗烧蚀能力均强于3DN C/SiC复合材料。     

SiC/C界面辐照性能的分子动力学研究

本文通过分子动力学模拟的方法,研究了5种含不同空间结构的SiC/C界面的材料受辐照后的缺陷分布随时间以及PKA位置的变化关系,并与单质SiC中缺陷分布情况进行对比.利用径向分布函数分析了辐照对界面原子排列情况的影响.研究结果表明,SiC/C界面的抗辐照能力明显低于SiC内部,不同的空间结构对界面缺陷数量存在一定影响.由径向分布函数推得界面区域石墨原子密度高则界面原子排列情况受辐照影响越大.     

The Effect of ZrO2 Interphase on Interfacial Frictional Stresses in SiC/ZrO2/SiCf Composites

Two types of SiC fiber tows (Hi-Nicalon? and Hi-Nicalon S?) were coated with stabilized ZrO₂ and composited using preceramic polymer impregnation pyrolysis to form SiC/SiC_f minicomposites. Properties of the fiber/matrix interface in composites were investigated using the indentation method in which a pyramidal indenter was used to push on an individual fiber and cause sliding at the interface. The interfacial frictional stresses were determined from the force–displacement relation. The composites reinforced by the ZrO₂-coated fibers have smaller interfacial frictional stresses than composites reinforced by the initial fibers and show fibers sliding relatively more easily with respect to the SiC matrix.     

Deposition of BN interphase coatings from B-trichloroborazine and its effects on the mechanical properties of SiC/SiC composites

Boron nitride thin films were deposited on silicon carbide fibers by chemical vapor deposition at atmospheric pressure from the single source precursor B-trichloroborazine (Cl₃B₃N₃H₃, TCB). The film growth and structure, as a function of deposition temperature, hydrogen gas flow rate, and deposition time, were discussed. The deposition rate reaches a maximum at 1000 °C, then decreases with the increasing of temperature, and the apparent activation energy of the reaction is 127 kJ/mol. Above 1000 °C, gas-phase nucleation determines the deposition process. The deposited BN films were characterized by Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of BN interphase on the mechanical properties of the unidirectional SiC fiber-reinforced SiC matrix (SiC/SiC) composites was also investigated. The results show that the flexural strength of SiC/SiC composites with and without coating is 276 MPa and 70 MPa, respectively, which indicates that BN interphase coating deposited from B-trichloroborazine precursor can effectively adjust the fiber/matrix interface, thus causing a dramatic increase in the mechanical properties of the composites.     

Micromechanical Behavior of Carbon Nanotube-Covered SiC Fibers in Polymer Matrix Composites

The incorporation of nanotube-covered fibers in continuous fiber/epoxy composites has been shown to influence the mechanical, electrical, and thermal properties of the composite. Increased interlaminar shear stress, flexural strength and modulus have been reported in such composites over composites containing bare fibers. In this study, the microstructure and interfacial shear strength (ISS) of continuous silicon carbide fiber/epoxy composites with and without nanotubes grown from the SiC fiber surface were investigated with micro-Raman spectroscopy (MRS) and microscopy. The fibers with nanotubes grown from the surface were found to have a reduced ISS compared with the bare fibers. Electron microscopy showed good wetting of epoxy in the nanotube forests, but poor attachment of the nanotube forests to the fibers. These results suggest that the mechanism leading to improvements in bulk composite properties is not due to an improvement in the fiber/matrix ISS.     

Structural effects of three-dimensional angle-interlock woven composite undergoing bending cyclic loading

This paper reports the structural effects of three-dimensional (3-D) angle-interlock woven composite (3DAWC) undergoing three-point bending cyclic loading from experimental and finite element analysis (FEA) approaches. In experiment, the fatigue tests were conducted to measure the bending deflection and to observe the damage morphologies. By the FEA approach, a micro-structural unit-cell model of the 3DAWC was established at the yarn level to simulate the fatigue damage. The stress degradation at the loading condition of constant deformation amplitude was calculated to show the degradation of mechanical properties. In addition, the stress distribution, fatigue damage evolution and critical damage regions were also obtained to qualitatively reveal the structural effects and damage mechanisms of the 3DAWC subjected to three-point bending cyclic loading.     

SiC纤维增强Ti17合金复合材料轴向残余应力的拉曼光谱和X射线衍射法对比研究

准确测量和分析SiC纤维增强Ti合金复合材料(SiC_f/Ti)中残余应力状态对优化复合材料的成型工艺和理解其失效模式具有重要意义,但其残余应力的实验测量和分析仍是一个挑战.石墨C涂层作为SiC纤维与Ti17基体合金之间必需的扩散障涂层,承载了由纤维与基体之间热不匹配引入的残余应力.本文采用显微拉曼光谱法对比测量纤维表面C涂层在复合材料中和去掉基体无应力态下G峰的峰位,通过石墨C涂层应力态下峰位移动计算出SiCf/C/Ti17复合材料中SiC纤维受到～705.0 MPa的残余压应力.采用X射线衍射方法测量了不同方向上该复合材料中基体钛合金的晶面间距以获取其空间应变,根据三轴应力模型分析了复合材料中基体钛合金沿轴向方向的残余应力为～701.3 MPa的张应力,并通过线性弹性理论转化为SiC纤维的残余压应力为～759.4 MPa.两种测试方法都确定了SiC纤维在成型过程中受到残余压应力,且获得的应力值较为接近,都可以用于对SiC_f/Ti复合材料的残余应力测量.     

Fabrication of composite structure of carbon fibers and high density SiC nanowires

Composite structure of carbon fibers and SiC nanowires was fabricated by a simple chemical vapor deposition process, using commercial silicon dioxide and graphite powders as raw materials. The analysis of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction indicates that the synthesized SiC nanowires distribute uniformly with high density in the used carbon fiber preform, which are perpendicular to and around each carbon fiber in a radial array. The SiC nanowires located at the interface of advanced composites is very favorable to the interfacial bonding between composites matrix and carbon fibers, thereby increasing the strength of composites greatly.     

Fabrication of composite structure of carbon fibers and high density SiC nanowires

Composite structure of carbon fibers and SiC nanowires was fabricated by a simple chemical vapor deposition process, using commercial silicon dioxide and graphite powders as raw materials. The analysis of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction indicates that the synthesized SiC nanowires distribute uniformly with high density in the used carbon fiber preform, which are perpendicular to and around each carbon fiber in a radial array. The SiC nanowires located at the interface of advanced composites is very favorable to the interfacial bonding between composites matrix and carbon fibers, thereby increasing the strength of composites greatly.     

基于第一性原理的Si/SiC、Al/SiC界面成键特性和结合强度对比研究

采用半固态搅拌铸造法制备Al Si7-Si C复合材料，并利用真空压铸工艺实现了其近净成形，结合第一性原理计算方法研究了共晶Si对Si C颗粒和基体界面结合强度的影响.结果显示，在Al Si7-Si C复合材料中，发现较为严重的共晶Si偏析现象，当Si C颗粒同时处于共晶Si和α-Al边界时，形成了少量的共晶Si夹杂、被大量共晶Si包裹、完全被共晶Si包裹三种典型的界面.第一性原理计算结果显示，在C端和Si端的Si/Si C界面中，弛豫后top Si¹配位方式具有最大的粘附功，与Al/Si C界面相比，Si/Si C界面具有更高的结合强度.Si偏析相提高了界面处的电荷密度，因而具有更好的界面结构稳定性.     

A review on the research progress of push-out method in testing interfacial properties of SiC fiber-reinforced titanium matrix composites

Experimental analysis of single-fiber push-out for SiC fiber-reinforced titanium matrix composites (TMCs) is complicated by the incorporation of large thermal residual stresses, strong chemical bond of the fiber/matrix interface and matrix plastic deformation. This paper summarizes the development of push-out test and the characteristics of push-out test for TMCs such as crack initiating at the bottom face and theoretical analysis of the test. Moreover, it deeply analyzes the progresses of interfacial shear strength and fracture toughness, and work focus is pointed out in future.     

Comparison of the properties of Nextel 720/SiC composites at room temperature and 1300°C

Continuous Nextel 720 fibers reinforced SiC composites with PyC interface are fabricated by LPCVI at 1000°C for 200 h using SiCH₃Cl₃ as precursor. The mechanical properties at RT and 1300°C are measured by three-point bending. The microstructures of the interface are characterized by TEM. The results indicate the composites have the metal-like behavior of fracture, whether they are at RT or high temperature. The RT and 1300°C strengths are 310 MPa and 140 MPa, respectively. The RT and 1300°C strains are 0.32% and 0.12%, respectively. The loss of flexural strength and strain of the Nextel 720/SiC composites at high temperature result from stronger residual thermal stress caused by the mismatch of CTE between fibers and matrix. A gap appears between fibers and PyC interface after the 1300°C test, which could be resulted from 7.7% compressive strain of PyC interface caused by the residual thermal stress and 0.1% sintering shrinkage of Nextel 720 fiber.     

SiC纳米纤维/C/SiC复合材料拉伸行为的分子动力学研究

本文采用分子动力学计算方法和Tersoff作用势研究了无定型碳(amorphous carbon, a-C) 涂层厚度对SiC纳米纤维/SiC纳米复合材料断裂方式及力学性能的影响. 分析结果发现, 随着涂层厚度的增加, 纳米纤维的平均应力集中系数下降, 即足够厚度涂层可以同时起到增强和补韧的作用. 当a-C涂层厚度t ≤ 0.3 nm时, 裂纹直接穿透纤维, 纳米复合材料表现出典型的脆性断裂方式; t = 4.0 nm时, 裂纹发生偏转, SiC纳米纤维发生拔出现象, 此时纳米复合材料的拉伸强度约为无涂层纳米复合材料的4倍, 断裂能则提高一个数量级. 计算结果表明, a-C涂层的厚度是SiC纳米纤维/SiC纳米复合材料中产生韧性机理的重要因素, 即传统微米级陶瓷基复合材料的增韧理论在纳米复合材料中仍适用. 研究结果可望为设计同时具有高强度、高韧性的陶瓷基纳米复合材料提供理论基础.     

Microstructure and thermal residual stress analysis of SiC fiber through Raman spectroscopy

SiC fiber‐reinforced metal matrix composite is an interesting material for aerospace industry because of its excellent properties. However, these properties are greatly influenced by fiber microstructure and thermal residual stresses introduced by the preparation of the composites. Due to complicated preparation technology, microstructure and thermal stress along SiC fiber radius varies, which makes characterization difficult. Raman spectroscopy is a non‐destructive technique which provides information, at micrometer scale, on the phase composition and the crystalline state (structure and texture) of materials. Line scanning was used to assess microstructure along SiC fiber radius embedded in Ti64. The SiC coating is subdivided into three concentric parts across the fiber diameter, according to the differences in intensity and width of SiC transverse optical phonon (TO) band. Part 2 is considered to be a buffer zone connecting Part 1 and Part 3 with different deposition conditions, respectively. Amorphous Si is detected throughout fiber radius, while crystalline Si is only detected in the outer part. Thermal residual stress along fiber radius in the composite was calculated by using SiC TO band shifts with a bare fiber as reference. A cylindrical model was also used to compare with the stress data obtained from Raman shift. Copyright © 2013 John Wiley & Sons, Ltd.     

Stress transfer efficiency in model composites under dynamic loading

The micromechanics of tension–tension fatigue loading in model single-fibre composite geometries is investigated in this paper. In an attempt to emulate the conditions encountered in full carbon fibre composites, the fibres were prestrained prior to the curing process to ensure that they were free of high residual compressive stresses as a result of resin shrinkage. The resulting specimens were grouped into two categories depending on the level of the initial fibre prestrain (case A low, case B high). The cyclic load is designed to be well below the endurance fatigue limit of the polymer matrix (∼0.6%), and to have a frequency low enough to avoid unwanted thermal post curing. Throughout the preparation procedure, as well as during fatigue loading, the fibre stress (strain) was constantly monitored by means of laser Raman spectroscopy. The fibre axial stress distributions at each fatigue step were converted to interfacial shear stress (ISS) distributions, from which important parameters such as the maximum ISS the system can accommodate, the transfer length for efficient stress built-up and the length required for the attainment of maximum ISS were obtained. The results showed that, up to 2×10⁶ loading cycles, the main parameters which affected the stress transfer efficiency at the interface were the fibre fracture process itself and the viscoelastic behaviour of the matrix material. Received: 7 November 2001 / Accepted: 22 March 2002 / Published online: 5 July 2002     

Features of film growth during plasma anodizing of Al 2024/SiC metal matrix composite

Plasma anodizing is a novel promising process to fabricate corrosion-resistant protective films on metal matrix composites. The corrosion-resistant films were prepared by plasma anodizing on SiC reinforced aluminum matrix composite. The morphology and microstructure of films were analyzed by scanning electron microscopy. Specifically, the morphology of residual SiC reinforcement particles in the film was observed. It is found that the most SiC reinforcement particles have been molten to become silicon oxide, but a few tiny SiC particles still remain in the film close to the composite/film interface. This interface is irregular due to the hindering effect of SiC particles on the film growth. Morphology and distribution of residual SiC particles in film provide direct evidence to identify the local melt occurs in the interior of plasma anodizing film even near the composite/film interface. A model of film growth by plasma anodizing on metal matrix composites was proposed.