Modeling of fatigue failure for SiC/SiC ceramic matrix composites at elevated temperatures and multi-scale experimental validation

The fatigue failure of ceramic matrix composites at elevated temperatures was predicted using the micromechanics method. Multiple micro-damage models were developed to describe the evolution processes of matrix cracking, interface wear, and fiber fracture during fatigue loading. On this basis, the fatigue life was calculated. To validate the fatigue failure model, multi-scale experiments were conducted. In the macroscale, the S-N curve was obtained by the fatigue test. In the microscale, multiple in-situ measuring methods were developed through which the matrix crack density, the interfacial shear stress, and the percentage of fracture fibers were obtained. Both the macroscale and microscale experimental results were in good agreement with the predicted results. Therefore, the fatigue failure model developed in the present work is accurate.     

Development and evaluation of sub-element testing of SiC/SiC ceramic matrix composites at elevated temperatures

SiC/SiC ceramic matrix composites (CMCs) are being developed for use in aero-engines to replace nickel superalloy components. Sub-element testing acts as the key stepping stone in bridging understanding derived from basic coupon testing and more complex component testing. This study presents the development of high temperature C-shape sub-element testing with the use of digital image correlation to study damage progression. The specimen is designed with a bias towards a mixed mode-stress state more similar to what a CMC component may see in service. Both monotonic and fatigue tests were completed on C specimens and compared with predicted behaviour from modelling. Test data from both test types suggested that specimens were failing once they reached a critical radial stress level. However evidence from fractography of specimens showed that in both monotonic and fatigue tests radial cracks (driven by hoop stresses) are initiating prior to circumferential cracks.     

Damage assessment of alumina fibre-reinforced mullite ceramic matrix composites subjected to cyclic fatigue at ambient and elevated temperatures

The damage evaluation behaviour of alumina fibre-reinforced mullite ceramic matrix composites subjected to cyclic fatigue was investigated by means of acoustic emission (AE) monitoring and forced resonance techniques. AE technique provided sufficient information about the damage initiation and progression in real time whilst the forced resonance (FR) technique allowed the detection of changes in elastic modulus (E) and internal friction (Q⁻¹) that occurred with increasing number of cyclic fatigue at room temperature. From the two non-destructive detection techniques results combined with microstructural observations, it is concluded that the composite cyclic fatigue damage evolution begins with multiple crack formation within the matrix and is followed by delamination (interfacial failure). Final failure of the composite is caused by fibre fracture and extensive cyclic sliding along the fibre/matrix interface. The strong bonding between mullite matrix and alumina fibre caused by the glassy phase within the mullite matrix determined the fatigue performance of the composite at 1350°C. Regions with glassy phase failed catastrophically as a result of early fibre fracture.     

Monotonic tension behavior of 2D woven oxide/oxide ceramic matrix composites at ultra-high temperature

The research on mechanical behavior and failure analysis of oxide/oxide CMC at ultra-high temperatures can broaden its application scope. The present work studied monotonic tension behavior of the oxide/oxide CMC at 800 °C～1200 °C and two tensile rates (i.e. 5 mm/min and 0.5 mm/min). The uniaxial tensile test, fracture morphology characterization and finite element analysis were preformed to reveal the deformation and failure mechanisms of the oxide/oxide CMC at ultra-high temperature. The results show that the mechanical properties of the oxide/oxide CMC are sensitive to the temperatures and tensile rates. The stress-strain curves are almost linear at the high tensile rate and nonlinear at the low tensile rate. The ultimate tensile strength decreases significantly at low tensile rate and for temperatures higher than 1100 °C. The mechanical properties of the material are principally determined by oxide fiber/oxide matrix interface strength under low temperature and high-stress conditions, while by interlayer bonding strength under high temperature and low-stress conditions.     

Oxidation of SiC/BN/SiC ceramic matrix composites in dry and wet oxygen at intermediate temperatures

The oxidation behavior of SiC/BN/SiC ceramic matrix composites (CMCs) was evaluated from 400° to 800 °C in 100% O₂ and 50% H₂O/50% O₂ gas mixtures. Thermogravimetric analysis (TGA) was utilized to measure weight change during controlled environment exposures at elevated temperatures for 1 and 50 hours. Oxidized CMCs and their oxides were studied post-exposure with scanning electron microscopy and energy dispersive spectroscopy. The oxidation onset and composition transition temperatures were evaluated. Key observations include oxide composition, oxide wetting, oxygen solubility in Hi-Nicalon SiC fibers and BN fiber coating oxidation and volatility behavior as a function of temperature. Degradation in wet environments at 600 °C was most extensive due to the formation of a non-wetting, non-protective surface oxide, allowing oxidant access to the BN fiber coatings followed by oxidation and volatilization. Implications of the CMC oxidation behavior are discussed for CMCs in service.     

Analysis of failure mechanisms of Oxide - Oxide ceramic matrix composites

The failure mechanisms of Oxide-Oxide ceramic matrix composites AS-N610 were studied at both room temperature and high temperature using tensile and fatigue tests with and without lateral and laminar notches. The unnotched coupons had an average tensile strength of 423 MPa with elastic modulus of 97 GPa at room temperature showing a perfect elastic behaviour whereas the laminar notched samples shown similar strength of 425 MPa with elastic modulus (98 GPa) revealing pseudo-ductile behaviour. A reduction in tensile strength of the oxide ceramic matrix composites was observed at high temperatures. Thermal shock experiments revealed that the retained strength of the samples quenched from 1100 °C deteriorated by ～10 % (395 ± 15 MPa). In all samples, fracture origin was observed on the mid-plane showing a higher degree of fiber pull-out, delamination and pseudo ductile behaviour. Finite element analysis confirmed higher stress concentration on the areas of failures.     

Crack opening behavior in ceramic matrix composites

The evolution of matrix cracks in a melt‐infiltrated SiC/SiC ceramic matrix composite (CMC) under uniaxial tension was examined using scanning electron microscopy (SEM) combined with digital image correlation (DIC) and manual crack opening displacement (COD) measurements. CMC modeling and life prediction strongly depend a thorough understanding of when matrix cracks occur, the extent of cracking for given conditions (time‐temperature‐environment‐stress), and the interactions of matrix cracks with fibers and interfaces. In this work, strain relaxation due to matrix cracking, the relationship between CODs and applied stress, and damage evolution at stresses below the proportional limit were assessed. Direct experimental observation of strain relaxation adjacent to regions of matrix cracking is presented and discussed. Additionally, crack openings were found to increase linearly with increasing applied stress, and no crack was found to pass fully through the gage cross‐section. This calls into question the modeling assumption of through‐cracks for all loading conditions and fiber architectures, which can obscure oxidation mechanisms that are active in realistic cracking conditions. Finally, the combination of SEM with DIC is demonstrated throughout to be a powerful means for damage identification and quantification in CMCs at stresses well below the proportional limit.     

Modeling creep behavior in ceramic matrix composites

In this work, a three-dimensional viscoplasticity formulation with progressive damage is developed and used to investigate the complex time-dependent constituent load transfer and progressive damage behavior in ceramic matrix composites (CMCs) subjected to creep. The viscoplasticity formulation is based on Hill's orthotropic plastic potential, an associative flow rule, and the Norton-Bailey creep power law with Arrhenius temperature dependence. A fracture mechanics-informed isotropic matrix damage model is used to account for CMC brittle matrix damage initiation and propagation, in which two scalar damage variables capture the effects of matrix porosity as well as matrix property degradation due to matrix crack initiation and propagation. The Curtin progressive fiber damage model is utilized to simulate progressive fiber failure. The creep-damage formulation is subsequently implemented as a constitutive model in the generalized method of cells (GMC) micromechanics formulation to simulate time-dependent deformation and material damage under creep loading conditions. The developed framework is used to simulate creep of single fiber SiC/SiC microcomposites. Simulation results are in excellent agreement with experimental and numerical data available in the literature.     

Tribological performance of SiC coating for carbon/carbon composites at elevated temperatures

SiC coating was deposited on carbon/carbon (C/C) composites by chemical vapor deposition (CVD). The effects of elevated temperatures on tribological performance of SiC coating were investigated. The related microstructure and wear mechanism were analyzed. The results show that the as-deposited SiC coating consists of uniformity of β-SiC phase. The mild abrasive and slight adhesive wear were the main wear mechanisms at room temperature, and the SiC coating presented the maximum friction coefficient and the minimum wear rate. Slight oxidation of debris was occurred when the temperature rose to 300?°C. As the temperature was above 600?°C, dense oxide film formed on the worn surface. The silica tribo-film replaced the mechanical fracture and dominated the frication process. However, the aggravation of oxidation at elevated temperatures was responsible for the decrease of friction coefficient and the deterioration of wear rate. The SiC coating presented the minimum friction coefficient and the maximum wear rate when the temperature was 800?°C.     

An investigation on wear behavior of UHMWPE/carbide composites at elevated temperatures

Ultra-high molecular weight polyethylene (UHMWPE) is extensively used in frictional applications due to its advanced wear resistance. This advanced polymer is reinforced with hard particulate fillers for further developments against wear conditions. Since elevated temperatures prevail in the service conditions, wear behavior of UHMWPE composites is an important issue for the engineering applications. In the present work, UHMWPE-based composites including silicon carbide (SiC) fillers were fabricated in a compression molding chamber. In the specimen preparation stage, molding pressure, filler amount, and filler particle size were varied to investigate the influence of these variables. Upon deciding the optimum parameters from the wear tests conducted at room temperature, the wear experiments were repeated for the optimum specimen at elevated temperatures, such as 40 and 60°C. According to the results, the wear behavior of the SiC/UHMWPE composites is heavily changed by the effect of elevated temperature. Adhesive effect is pronounced at elevated temperatures while the wear characteristics possess the abrasive effect in the sliding path. In addition, the composites exhibit an accelerated material loss as temperature increases during the frictional system.     

Additive manufacturing of polymer-derived ceramic matrix composites

This study presents a fabrication method and identifies processing bounds for additively manufacturing (AM) ceramic matrix composites (CMCs), comprising a silicon oxycarbide (SiOC) ceramic matrix. A digital light projection printer was used to photopolymerize a siloxane-based preceramic resin containing inert ceramic reinforcement. A subsequent pyrolysis converted the preceramic polymer to SiOC. Particle reinforcements of 0 to 40% by volume in the green state were uniformly dispersed in the printed samples to study their effects on pyrolysis mass loss and shrinkage, and CMC notch sensitivity and strength. Both particle and whisker reinforcements toughened the glassy SiOC matrix (1 MPa m^1/2), reaching values >3 MPa m^1/2. Bending strengths of >300 MPa (>150 MPa (g cm⁻³)⁻¹) and a Weibull modulus of 10 were measured on AM samples without surface finish. We identified two pore formation mechanisms that placed processing bounds on sample size and reinforcement volume fraction. Methods for increasing these bounds are discussed. With properties commensurate to traditionally processed technical ceramics, the presented process allows for free-form fabrication of high-performance AM CMC components.     

Shear failure evolution and its dispersion of plain woven ceramic matrix composites z-pin structure

In this paper, the digital image correlation technique was applied to the shear test of 2D SiC/SiC composites z-pin with the purpose of analyzing the shear behavior of the plain woven pin and studying the structure factors of test results dispersion. After obtaining the strain–stress curves of the joint's connection region, the evolution of z-pin shear failure process was investigated. The z-pin's failure fractures were observed, and the main factors of its dispersion of shear mechanical properties were discussed. Changing the yarn parameter of model building and importing into Workbench for calculation, the average stress results in the shear plane were obtained. Different simulation results show that optimization of plain-woven pin structure parameters can effectively improve its shear strength.     

Crosslinked polyethylene at elevated temperatures

The behaviour of crosslinked polyethylene has been studied by differential thermal analysis (d.t.a.) up to 150°C, by stress relaxation up to 240°C and by thermogravimetry (t.g.) up to 350°C. No single technique is sufficient to characterize the polymer. D.t.a. gives a very rapid indication of the crystalline portion of the crosslinked polyethylene; t.g. shows the loss of degradation products some of which may be associated with crosslinking; stress relaxation gives information on crosslinking and degradation but is time consuming and does not provide absolute values. The results obtained agree with earlier work that showed marked differences between the properties obtained with low and high amounts of peroxide.     

Microstructural stability at elevated temperatures

The factors affecting the diffusion-controlled coarsening of microstructures at elevated temperatures are reviewed in this paper. Coarsening can occur in a variety of different ways. Perhaps the most familiar of these is Ostwald ripening of a dispersed phase, e.g. precipitates in a 2-phase alloy. However, there are other related Ostwald-ripening-types of processes, including coarsening of dispersed phases in 2-dimensions (e.g. precipitates in thin films, ‘fibers’ in directionally solidified eutectics) and coarsening of particles that grow in three dimensions in a 2-dimensional diffusion field (e.g. grain-boundary precipitates, particles on a substrate). Coarsening by fault migration, which is an important issue in the microstructural stability of directionally solidified rod and lamellar eutectics, and discontinuous coarsening, which affects the stability of cellular microstructures produced by discontinuous precipitation, eutectoid decomposition and eutectic solidification, are also discussed. The principal predictions of theories of these various types of coarsening mechanisms are described, and exemplified where possible by reference to published data. Emphasis is placed on ability of current theory to explain existing data obtained from real as well as computer simulation experiments. Possible reasons for the shortcomings of theory are discussed.     

Constitutive model and failure criterion for orthotropic ceramic matrix composites under macroscopic plane stress

To predict the nonlinear stress-strain behavior and the rupture strength of orthotropic ceramic matrix composites (CMCs) under macroscopic plane stress, a concise damage-based mechanical theory including a new constitutive model and two kinds of failure criteria was developed in the framework of continuum damage mechanics (CDM). The damage constitutive model was established using strain partitioning and damage decoupling methods. Meanwhile, the failure criteria were formulated in terms of damage energy release rate (DERR) in order to correlate the failure property of CMCs with damage driving forces, and the maximum DERR criterion and the interactive DERR criterion were suggested simultaneously. For the sake of model evaluation, the theory was applied to a typical CMC with damageable and nonlinear behavior, that is, 2D-C/SiC. The damage evolution law, strain response and rupture strength under incremental cyclic tension along both on-axis and off-axis directions were completely investigated. Comparison between theoretical predictions and experimental data illustrates that the newly developed mechanical theory is potential to give reasonable and accurate results of both stress-strain response and failure property for orthotropic CMCs.     

Interaction of fiber matrix bonding in SiC/SiC ceramic matrix composites

Non-oxide ceramic matrix composites (CMC) based on SiC fibers with SiC matrix were fabricated by polymer infiltration and pyrolysis (PIP) and characterized regarding their microstructural features and their mechanical properties. The fiber preform was made using winding technology. During the winding process, the SiC fiber roving was impregnated by a slurry containing SiC powder and sintering additives (Y₂O_3, Al₂O₃ and SiO₂). This already helped to achieve a partial matrix formation during the preform fabrication. In this way, the number of PIP cycles to achieve composites with less than 10% open porosity could be reduced significantly. Additionally, damage-tolerant properties of the composites were obtained by an optimal design of the matrix properties although only uncoated fibers were used. Finally, composites with a strength level of about 500 MPa and a damage-tolerant fracture behavior with about 0.4% strain to failure were obtained.     

碳纤维增强SiC陶瓷复合材料的研究进展

碳纤维增强SiC陶瓷基复合材料具有良好的高温力学性能，是航空航天和能源等领域新的高温结构材料研究的热点之一．本文回顾了增强体碳纤维的发展，对材料的成型制备工艺，材料的抗氧化涂层研究进展和现有的一些应用做了综述，并展望了碳纤维增强SiC陶瓷基复合材料以后的研究重点及发展前景．     

Enhancing thermal stability of oxide ceramic matrix composites via matrix doping

To overcome the main limitation of oxide ceramic matrix composites (Ox-CMCs) regarding thermal degradation, the use of matrix doping is analyzed. Minicomposites containing Nextel 610 fibers and alumina matrices with and without MgO doping were produced. The thermal stability of the minicomposites was evaluated considering their microstructure and mechanical behavior before and after thermal exposures to 1300 °C and 1400 °C for 2 h. Before heat treatment, both composite types showed very similar microstructure and tensile strength. After heat treatment, densification, grain growth and strength loss are observed. Furthermore, the MgO dopant from the matrix diffuses into the fibers. As a result, abnormal fiber grain growth is partially suppressed and MgO-doped composites show smaller fiber grains than non-doped composites. This more refined microstructure leads to higher strength retention after the heat treatments. In summary, doping the matrix can increase the overall thermal stability without impairing the room-temperature properties of Ox-CMCs.     

Microstructure and mechanical behaviors of 2D-Cf/ZrB2-SiC composites at elevated temperatures

Microstructure and mechanical behaviors of the 2D-C_f/ZrB₂-SiC composites at RT to 1800 ℃ were studied. It is indicated that the interface structure is critical, which affects the matrix phase composition and cracks deflection path in the intra-bundle area. It subsequently determines the load transmitting, cracks propagation behaviors and the mechanical properties of the composites. Flexural strength of the composites increases slightly at 1000 ℃ by the release of residual stress and self-healing of defects. Partial decomposition and crystallization of polycarbosilane (PCS) derived SiC result in the weakening of the SiC matrix at 1500 ℃. On the other hand, residual carbon reacts with ZrO₂ impurity above 1300 ℃. As a result, the matrix turns to porous structure at 1500 ℃, leading to the formation of short micro cracks. At higher temperature of 1800 ℃, quasi-creep mechanical behavior is presented in the composite.     

Temperature-induced fibre/matrix interactions in porous alumino silicate ceramic matrix composites

The thermal stability of alumino-silicate fibre (Nextel 720)/porous mullite matrix composites was investigated in the temperature range between 1300 and 1600°C. In the as-prepared state the fibres consist of mullite plus α-Al₂O₃, while the porous mullite matrix includes minor amounts of a SiO₂-rich glass phase. Temperature-controlled reactions between the silica-rich glass phase of the matrix and α-Al₂O₃ at the rims of the fibres to form mullite have been observed. At the end of this process, virtually all glass phase of the matrix is consumed. Simultaneously, alumina-free layers about 1 μm thick are formed at the periphery of the fibres. The mullite forming process is initiated above about 1500°C under short time heat-treatment conditions (2 h) and at much lower temperature (1300°C) under long-term annealing (1000 h). Subsequent to annealing below the thermal threshold, the composite is damage tolerant and only minor strength degradation occurs. Higher annealing temperatures, however, drastically reduce damage tolerance of the composites, caused by reaction-induced gradually increasing fibre/matrix bonding. According to this study, the thermal stability of alumino silicate (Nextel 720) fibre/mullite matrix composites ranges between 1500°C in short-term and 1300°C in long-term heat-treatment conditions.