Conditions for matrix crack deflection at an interface in ceramic matrix composites

This paper describes a numerical approach developed to simulate the mechanism of matrix crack deflection at the fibre/matrix interface in brittle matrix composites. For this purpose, the fracture behaviour of a unit cell (microcomposite) consisting of a single fibre surrounded by a cylindrical tube of matrix was studied with the help of a finite element model. A fracture mechanics approach was used to design a criterion for deflection at the fibre/matrix interface of an annular crack present in the matrix. The analysis of the fracture behaviour of SiC/SiC and SiC/glass ceramics microcomposites shows that the introduction of a low modulus and low toughness interfacial layer at the fibre/matrix interface (e.g. a carbon coating) greatly favours matrix crack deflection at the interphase/fibre interface.     

On the fracture toughness of ceramic matrix composites

Based on the resistance curve (R-curve) behaviour of ceramic matrix composites (CMCs) determined under either quasi-static or cyclic loading, the crack-face fibre bridging stress field is determined for the compact tension (CT) test specimen geometry. Two different methods have been used for the analysis of the bridging stresses. The first considers a compliance approach. Using the difference in compliance calibration curves with and without bridging and assuming a power-law relation between bridging stress and crack opening displacement, the bridging stress field was calculated. The second approach uses the existence of an invariant stress reversal point in the CT geometry and assuming that the material exhibits linear elastic fracture behaviour, yields a recurrence relation for the bridging stresses resulting in a piece-wise constant stress function. Both models are applied to the experimentally determined fracture behaviour of a 2D carbon/carbon (C/C) composite, and the resulting bridging stress distributions are discussed.     

Fracture in angle-ply ceramic matrix composites

An analytical/computational fracture mechanics scheme, combined with finite element computations, is presented to explain observed fracture patterns in angle ply fiber-reinforced ceramic composite laminates. Unlike polymeric composites, where microcracks are channeled parallel to the fiber directions, cracks in ceramic composites initiate perpendicularly to the load axis. The results presented herein provide rational explanations for the experimental observations and confirm the advantages bestowed by multi-directional reinforcements. Such reinforcements would still allow the development of multitudes of matrix cracks, bridged by intact fibers, that lead to the desirable gradual and `graceful' failure of the composite. At the same time, multidirectional reinforcement prevents premature failures by constraining the laminate against interfacial fiber/matrix debondings. Such debondings, which may otherwise grow rapidly along the abovementioned weak interfaces, may readily occur in unidirectionally reinforced off-axis laminates, as noted in a preceding article.     

A model for the transverse strain response of unidirectional ceramic matrix composites during tensile testing

A comprehensive micromechanical model relating the longitudinal stress and transverse strain of unidirectional fibre toughened ceramic matrix composites (CMCs) is presented. The model uses different cylindrical unit-cells to describe the composite throughout a tensile test and considers all relevant damage mechanisms. The proposed model takes into account the Poisson contraction of fibre and matrix, the relief of thermal residual stresses upon damage development, and the build-up of compressive radial stresses at the interface due to mismatch between fibre and matrix after debonding and sliding. Thus the modelled transverse strain response depends on a wide range of microstructural and micromechanical parameters. The approach is checked by comparing the experimentally observed and simulated response of a unidirectional SiC/CAS composite of which all constituent properties were determined experimentally. The agreement between experiment and theory is excellent.     

Ceramic thin film thermocouples for SiC-based ceramic matrix composites

Conductive ceramic thin film thermocouples were investigated for application to silicon carbide fiber reinforced silicon carbide ceramic matrix composite (SiC/SiC CMC) components. High temperature conductive oxides based on indium and zinc oxides were selected for testing to high temperatures in air. Sample oxide films were first sputtered-deposited on alumina substrates then on SiC/SiC CMC sample disks. Operational issues such as cold junction compensation to a 0 °C reference, resistivity and thermopower variations are discussed. Results show that zinc oxides have an extremely high resistance and thus increased complexity for use as a thermocouple, but thermocouples using indium oxides can achieve a strong, nearly linear response to high temperatures.     

High-temperature oxidation of ceramic matrix composites dispersed with metallic particles

Oxidation behavior of ceramic matrix composites dispersed with metallic particles is discussed to establish materials design for high-temperature applications. Oxidation kinetics of ceramic matrix composites dispersed with metallic particles is understood from the viewpoint of the diffusion properties and defect chemistry of matrix oxides. High-temperature oxidation of Ni(p)/partially stabilized zirconia, Ni(p)/Al₂O₃ and Ni(p)/MgO was described as examples.     

On the mechanical behaviour under cyclic loading of ceramic matrix composites

In this paper, a constitutive law is presented to model the mechanical behaviour of ceramic matrix composites. It allows matrix-cracking, interfacial debonding, sliding and wear to be accounted for in the framework of continuum mechanics. Based upon micromechanical studies, a 1D and 2D model was derived. An application was performed on a [0,90] SiC/SiC composite.     

Thermal cycling response behavior of ceramic matrix composites under load and displacement constraints

Thermal cycling response of a two-dimensional carbon fiber reinforced SiC matrix composite (2D C/SiC) to load constraint (LC) and to displacement constraint (DC) in an oxidizing environment was investigated. During thermal cycling between 700 and 1200 °C, a constraint strain with a 0.208% range and a constraint stress with a 180 MPa range were, respectively, generated on the composites in LC and DC. It was found that with increasing cycles, the constraint strain increased in LC and the constraint stress decreased in DC. After 50 cycles, in contrast to the as-received composite materials, the as-cycled composites suffered greater loss in mechanical properties: the residual strength and failure strain are 204 MPa and 0.49% for the LC tested samples, and 223 MPa and 0.64% for the DC tested samples, respectively. Microstructural observations indicated that the LC could develop thermal microcracks and assist in oxidizing the internal fibers, whereas the DC reduced crack propagations and fiber oxidation because of decreasing tensile and increasing compressive stresses.     

Defects in ceramic matrix composites and their impact on elastic properties

Defects created during the manufacture of an oxide/oxide and two non-oxide (SiC/SiNC and MI SiC/SiC) ceramic matrix composites (CMCs) were categorized as follows: (1) Intra-yarn defects such as dry fibers, (2) Inter-yarn defects such as those at crossover points, matrix voids, shrinkage cracks and interlaminar separation, and (3) Architectural defects such as layer misalignment. Their impact on elastic properties was analytically investigated using a stiffness averaging approach considering the defects to have volumetric and directional influences. In-plane tensile and shear moduli as well as the through-thickness compressive modulus were experimentally evaluated. Results of analytical model were around 7% on average from the mean value of the experimental data. It was observed that interlaminar separation drastically reduced the through-thickness modulus by about 63% for the SiC/SiNC, 40% for the MI SiC/SiC and around 32% for the oxide/oxide composites. Shrinkage cracks in oxide/oxide composite reduced the in-plane tensile and shear moduli by 14% and 8.8%, respectively.     

The yarn size dependence of tensile and in-plane shear properties of three-dimensional needled textile reinforced ceramic matrix composites

The yarn size scaling of tensile and in-plane shear properties is examined for three-dimensional needled textile reinforced ceramic matrix composites (3DN CMC) fabricated by chemical vapor infiltration. The results showed that large yarn size would cause the nonwoven yarn of 3DN CMC crimp and lower composite density, resulting in decrease of tensile and in-plane shear properties. The “modified lamina modeling” was presented to predict the tensile and shear elastic moduli of 3DN CMC with different yarn size. Other two methods were also proposed to evaluate the tensile and in-plane shear strengths of 3DN CMC with different yarn size, respectively. All predicted results showed consistent well with the experimental results.     

Interface damage of SiC/Ti metal matrix composites subjected to combined thermal and axial shear loading

A three-dimensional micromechanical finite element model is developed to study initiation and propagation of interface damage of unidirectional SiC/Ti metal matrix composites (MMCs) subjected to combined thermal and axial shear loading. Effects of various important parameters such as manufacturing process thermal residual stress, fiber coating and interface bonding are investigated. The model includes a representative volume element consists of a quarter of SiC (SCS-6) fibers covered by interface and coating, which are all surrounded by Ti-15-3 matrix. Appropriate boundary conditions are introduced to include effects of combined thermal and axial shear loading on the RVE. A suitable failure criterion for interface damage is introduced to predict initiation and propagation of interface de-bonding during shear loading. It is shown that while predictions based on perfectly bonded and fully de-bonded interface are far from reality, the predicted stress–strain curve for damaged interface demonstrates very good agreement with experimental data.     

Assessment of three oxide/oxide ceramic matrix composites: Mechanical performance and effects of heat treatments

Mechanical performance of three oxide/oxide ceramic matrix composites (CMCs) based on Nextel 610 fibers and SiOC, alumina, and mullite/SiOC matrices respectively, is evaluated herein. Tensile strength and stiffness of all materials decreased at 1000 °C and 1200 °C, probably because of degradation of fiber properties beyond 1000 °C. Microstructural changes in the composites during exposure at 1000 °C and 1200 °C for 50 h reduce their flexural strength, fracture toughness and work of fracture. A literature review regarding mechanical properties of several oxide/oxide CMCs revealed lower influence of fiber properties on composite strength compared with elastic modulus. The tested composites exhibit comparable stiffness and strength but higher fracture toughness compared with average values determined from a literature review. Considering CMCs with different compositions, we observed an interesting linear trend between strength and fracture toughness. The validity of the linear relationship between fracture strength and flexural toughness for CMCs is discussed.     

Influence of fibre properties on the mechanical behaviour of unidirectionally-reinforced ceramic matrix composites

Tensile tests on unidirectionally-reinforced ceramic matrix composites have been simulated using a micromechanical approach based on a shear-lag model and the chain-of-bundles theory. The model presented takes the probabilistic nature of the constituent properties into account. The results of the simulation have been compared with experimental data from literature in order to obtain information about the in-situ properties of the constituents and the influence of the fibre properties on the composite behaviour.     

Aluminium nitride-molybdenum ceramic matrix composites possessing high thermal shock resistance

Aluminium nitride-molybdenum ceramic matrix composites were produced by hotpressing a mixture of AlN and Mo powders. Thermal shock resistance of a composite (AM25) which contained 25 vol.% of metallic phase dispersed in the AlN matrix, was studied in order to verify the influence of metallic phase additional on the thermal properties. Results showed that AM25 possess a critical temperature difference (ΔT_c) for thermal shock of about 550°C compared to a mean value of 350°C measured in the case of pure hot pressed AlN. This improvement in thermal shock resistance was attributed to better mechanical properties and higher thermal conductivity of AM25 as compared to pure AlN.     

正交铺设陶瓷基复合材料单轴拉伸行为

采用细观力学方法对正交铺设陶瓷基复合材料单轴拉伸应力-应变行为进行了研究。采用剪滞模型分析了复合材料出现损伤时的细观应力场。采用断裂力学方法、 临界基体应变能准则、 应变能释放率准则及Curtin统计模型4种单一失效模型确定了90°铺层横向裂纹间距、 0°铺层基体裂纹间距、 纤维/基体界面脱粘长度和纤维失效体积分数。将剪滞模型与4种单一损伤模型结合, 对各损伤阶段应力-应变曲线进行了模拟, 建立了复合材料强韧性预测模型。与室温下正交铺设陶瓷基复合材料单轴拉伸应力-应变曲线进行了对比, 各个损伤阶段的应力-应变、 失效强度及应变与试验数据吻合较好。分析了90°铺层横向断裂能、 0°铺层纤维/基体界面剪应力、 界面脱粘能、 纤维Weibull模量对复合材料损伤及拉伸应力-应变曲线的影响。      

原位合成SiC-BN复合陶瓷设计与热力学研究

采用原位合成的方法设计并制备了SiC-BN复合陶瓷.通过热力学计算,选择了制备SiC-BN复合陶瓷的反应方程式,并通过进一步的计算,选择合适的烧结工艺保证了反应方程式Si3N4 B4C 2C=3SiC 4BN不会受到副反应的影响.通过实验验证此反应在1 700℃能够反应完成,生成β-SiC和h-BN.     

Fabrication of hybrid ceramic matrix composites

The desire to improve the transverse properties and microcracking stress of unidirectional continuous fiber reinforced ceramic matrix composites has led to development of the hybrid ceramic matrix composite (HCMC). This paper discusses the techniques we used in the fabrication of HCMC specimens used for mechanical characterization.     

连续纤维增韧陶瓷基复合材料可持续发展战略探讨

连续纤维增韧陶瓷基复合材料(CMC)是航空航天等高科技领域发展不可缺少的材料。其中, 连续纤维增韧碳化硅陶瓷基复合材料(CMC-SiC)是研究最多、 应用最成功的一种。本文作者以CMC-SiC为例, 介绍了该材料的战略需求与应用领域, 详细分析了国内外的应用研究现状及发展趋势, 并对我国陶瓷基复合材料面临的机遇与挑战, 以及发展战略进行了探讨。      

Fabrication and characterization of three dimensional woven carbon fiber/silica ceramic matrix composites

Carbon fiber reinforced fused silica composites exhibit the advantages of excellent mechanical properties, high heat resistance, low thermal expansion and low density, but low impact resistance or toughness. A novel modified slurry impregnation and hot pressing (SIHP) method was adopted to fabricate a new type of three dimensional orthogonal woven structure carbon fiber reinforced silica ceramic matrix composites (3D Cf/SiO₂ CMCs) with higher density and lower porosity. Physical characterization, flexural behavior, impact performance and toughening mechanism of the composites were investigated by three-point bending tests, impact tests, and scanning electron microscopy analysis. The 3D Cf/SiO₂ CMC showed a higher flexural strength in both warp (201.6%) and weft (263.6%) directions than those of pure SiO₂ and failed at a non-brittle mode due to the fiber debonding and pullout, and a delaminated failure of the 3D preform. The maximum impact energy absorption of the 3D Cf/SiO₂ CMC was 96.9 kJ/m², almost 4 times as much as those for typical other carbon fiber reinforced CMCs.     

Mechanical property degradation of a Nicalon fiber reinforced SiNC ceramic matrix composite under thermal shock loading

The degradation of mechanical properties of a SiC fiber reinforced SiNC ceramic matrix composite due to thermal shock by water quenching have been investigated. Post thermal shock tensile tests were performed to determine the degradation of mechanical properties of this composite. In situ acoustic emission (AE) tests were also conducted. The tensile tests data and acoustic emission data were correlated. The AE signal indicated a sudden increase in AE events at critical points in the stress–strain relationship. The effects of thermal shock temperature and the number of thermal shock cycles on the mechanical properties, and on the AE responses were also evaluated. It was observed that an increase in either factor resulted in more AE responses. Fracture damage in the tensile test specimens was examined by Scanning Electron Microscopy. It was observed that the failure mechanism changed as the thermal shock temperature increased. The fracture surfaces of the specimens tested without thermal shock indicated an extensive fiber pullout while the thermally shocked specimens showed reduced fiber pullout.