Damage modeling of oxide/oxide ceramic matrix composites under cyclic loading conditions

Ceramic matrix composites exhibit optimal high temperature property and complex nonlinear behaviors including irreversible deformations and stiffness degradation under different mechanical loading conditions. In the present work, the damage evolution of the material under multi-axial loads considering effects of loading-unloading cyclic was studied and a novel continuum damage constitutive model was proposed. The material degradation was decomposed into monotonic damages and cyclic damages. The anisotropic hardening behavior of the material was considered by taking orientation dependence into account. Compared to the experimental results, the constitutive model could accurately predict the stress-strain response and stiffness degradations of the oxide/oxide ceramic matrix composites for monotonic loading and cyclic loading.     

Experimental and numerical study on creep behaviors of 2D twill woven quartz fiber/silica matrix composites

This study investigates the creep deformation, damage, and rupture behaviors of 2D woven SiO₂/SiO₂ composites via experimental and numerical methods. In situ monotonic tensile tests and creep tests were conducted at 900 °C using a self-designed experimental system and digital image correlation. The tested specimens were characterized by X-ray computed tomography and scanning electron microscopy to conduct quantitative analyses and fracture observations. The obtained creep strain–time curves consist of primary and secondary stages, similar to the creep strain–time curves of most ceramic matrix composites. The matrix at the intersection of fiber bundles cracked under tensile loading. During subsequent creep loading, the propagation of matrix cracks, interfacial debonding, and fiber breakage in longitudinal fiber bundles were observed. At the mesoscale, the creep rupture entails a mechanism analogous to that observed in the monotonic tensile tests. Overall, the SiO₂/SiO₂ composites employed in this study exhibit excellent potential for long-term operation under mechanical loads at high temperatures. Next, a micromechanics-based creep model was proposed to simulate the creep behavior of the composites. In this model, the primary creep law and rule of mixtures were combined to describe the stress redistribution of various constituents and predict the deformation of the composites. In addition, the rupture life was predicted based on the global load-sharing model, two-parameter Weibull model, and shear lag model. The degradation of the matrix modulus and fiber strength was also considered to improve the accuracy of the simulation. The predicted results were in good agreement with the experimental data.     

Effect of multiple loading sequence on time-dependent stress rupture of fiber-reinforced ceramic-matrix composites

In this paper, the effect of multiple loading sequence on time-dependent stress rupture of fiber-reinforced ceramic-matrix composites (CMCs) at intermediate temperatures in oxidative environment is investigated. Considering multiple damage mechanisms, a micromechanical constitutive model for time-dependent stress rupture is developed to determine damage evolution of matrix crack spacing, interface debonding and oxidation length, and fiber failure probability under single and multiple loading sequences. The relationships between multiple loading sequence, composite strain evolution, time, matrix cracking, interface debonding and oxidation, and fiber fracture are established. The effects of fiber volume, matrix crack spacing, interface shear stress in the slip and oxidation region, and environment temperature on the stress/time-dependent strain, interface debonding and oxidation fraction, and fiber broken fraction of SiC/SiC composite are analyzed. The experimental stress rupture of SiC/SiC composite under single and multiple loading sequences at 950°C in air atmosphere is predicted. Compared with single loading stress, multiple loading sequence affects the interface debonding and oxidation fraction in the debonding region, leading to the higher fiber broken fraction and shorter stress-rupture lifetime.     

A New Experimental Approach for In Situ Damage Assessment in Fibrous Ceramic Matrix Composites at High Temperature

High‐temperature applications of ceramic matrix composites necessitate a rigorous understanding of the fracture and damage mechanisms that occur under thermomechanical loading, requiring the development of advanced small‐scale characterization approaches. In this work, fiber‐reinforced SiC/SiC tensile specimens were loaded in a scanning electron microscope at 800°C, and full‐field deformation maps at the constituent length scale were generated using digital image correlation (DIC). A colloidal system containing mechanically milled titanium nanopowder, bicine, and water was developed for use as a DIC tracking pattern that is stable at 795°C. The resultant full‐field strain maps provide a constituent level characterization of damage evolution from crack initiation through final fracture. An analysis of strain along fiber lengths indicated that fiber mean strain and standard deviation reached a minimum at fiber fracture. In addition, multiple matrix cracks in the process zone ahead of a notch/crack tip were apparent and could falsely appear as a continuous region of high strain in DIC fields. Relatively large displacement (strain) error was attributed to noise and bias at these small length scales and small strain values, and approaches for mitigating this error are discussed.     

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.     

Damage and failure analysis of a SiCf/SiC ceramic matrix composite using digital image correlation and acoustic emission

The analysis of failure behaviors of continuous fiber-reinforced ceramic matrix composites (CMCs) requires the characterization of the damage evolution process. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso architecture, inherent defects, and loading conditions. In this paper, the in-plane tensile mechanical behavior of a plain woven SiC_f/SiC CMC was investigated, and damage evolution and failure process were studied in detail by digital image correlation (DIC) and acoustic emission (AE) methods. The results show that: the initiation of macro-matrix cracks have obvious local characteristic, and the propagation paths are periodically distributed on the material surface; different damage modes (matrix cracking and fiber fracture) would affect the AE energy signal and can be observed in real-time; the significant increase of AE accumulated energy indicates that serious damage occurs inside the material, and the macroscopic mechanical behavior exhibits nonlinear characteristic, which corresponds to the proportional limit stress (PLS) of the material.     

Damage analysis of a SiCf/SiC ceramic matrix composite under stepwise fatigue loading with acoustic emission

Continuous fiber-reinforced ceramic matrix composites (CMCs) exhibit different damage mechanisms at multiple scales under cyclic loading. In this paper, the tension-tension fatigue behavior of a plain woven SiC_f/SiC CMC was investigated, and damage accumulation and evolution process were studied in detail via acoustic emission (AE) method. With the increase of cycles, the material exhibits obvious hysteresis behavior affected by interfacial slip and wear mechanisms. Most of the fibers with radial fracture characteristic have relatively high strength, showing excellent toughening property. In the stepwise cyclic loading process, the Kaiser effect of AE determines the initiation of AE activities at each initial loading moment, which shows obvious nonlinear damage accumulation behavior of the material. High-energy events are related to significant matrix cracking and fiber fracture, and the evolution process of material damage initiation and propagation is monitored in real time.     

Temperature‐dependent tensile strength model for 2D woven fiber reinforced ceramic matrix composites

This paper presents a temperature‐dependent model for predicting the tensile strength of 2D woven fiber reinforced ceramic matrix composites. The model takes into account the combined effects of temperature, temperature‐dependent residual thermal stress, temperature‐dependent matrix strength, and fibers strength on the tensile strength of composites. To verify the model, the tensile strengths of 2D woven fiber reinforced ceramic matrix composites available are predicted at different temperatures. The model predictions agree well with the experimental data. This work could provide a practical technical means for predicting the temperature‐dependent tensile strength of 2D woven fiber reinforced ceramic matrix composites and uncovering the dominated mechanisms leading to the change of tensile strength and their evolution with temperature.     

A review of the research progress on the interface between oxide fiber and oxide ceramic matrix

Oxide ceramics have excellent high temperature performance, superior thermal and chemical stability, which can be used in high temperature oxidizing environments, while oxide ceramics generally have low toughness and are prone to catastrophic damage... Oxide ceramics can be reinforced by high performance ceramic fibers to improve the fracture strength and fracture toughness, thus expanding its application in high-temperature components such as aero-engine combustion chambers and tail nozzles. The interface between fiber and matrix is an important factor that determines the performance of the composites. By tailoring the interface, the energy dissipation mechanisms such as fiber debonding and fiber pull-out can be brought into play to avoid the catastrophic damage of the composites. This review paper summarizes the recent research progress of the oxide fiber/oxide ceramic matrix composites interface. The mechanical properties of the interface and the design principles of the interface engineering are discussed, and types of interfaces and coating preparation methods are reviewed.     

Development and characterisation of high-density oxide fibre-reinforced oxide ceramic matrix composites with improved mechanical properties

A low cost and reliable ceramic matrix composite fabrication route has been developed. It involves the coating of 2D woven ceramic fibres (Nextel? 720) with oxide nano-size ceramic particles by electrophoretic deposition (EPD) followed by impregnation of the coated fibres with ceramic matrix and warm pressing at 180 °C to produce the “green” component ready for pressureless sintering. The effects of two different weak interface materials, NdPO₄ and ZrO₂, on the thermomechanical properties of the composites are also examined. Damage mechanisms, such as debonding, fibre fracture, delamination and matrix cracking within the composite plates subjected to tensile loading are analysed using acoustic emission technique and correlated with microstructure. It is shown that the composites with NdPO₄ interface, 10% porosity and 40 vol.% fibre loading have superior themomechanical properties in terms of strength and damage-tolerant behaviour in multilayer plate form. The improved sinterability and microstructure stability at moderate temperatures ensure both the fibre integrity and load transfer efficiency resulting in high strength damage-tolerant composites. The final components produced are considered to be suitable for use as shroud seals and insulating plates for combustor chambers in aircraft engines.     

Study of material removal mechanisms in grinding of C/SiC composites via single-abrasive scratch tests

As one of the ceramic matrix composites (CMCs), carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are promising materials used in various engineering applications owing to their superior properties. Precision surface grinding has been widely applied in the machining of CMC composites; however, the material removal mechanisms of C/SiC composites have not been fully elucidated yet. To reveal the material removal mechanisms in the grinding of chemical vapor infiltration-fabricated C/SiC composites, novel single-abrasive scratch tests were designed and conducted in two typical cutting directions. The experimental parameters, especially the cutting speed, conformed to the actual grinding process. The results show that the grinding parameters (feed rate, spindle speed, depth of cut, and cutting direction) have significant influences on the grinding forces, surface integrity, and affected subsurface region. The tangential force is in general larger than the normal force at the same cutting depth. Furthermore, both the tangential and normal forces in the longitudinal cutting direction are larger than those in the transverse cutting direction. The impacts and abrasive actions at the tool tip mainly caused the material removal. The predominant material removal mode is brittle fracture in the grinding of unidirectional C/SiC composites, because the damage behaviors of the C/SiC composites are mainly the syntheses of matrix cracking, fiber breakage, and fiber/matrix interfacial debonding. These results are rationalized based on the composite properties and microstructural damage features.     

Stereological Observations of Platelet-Reinforced Mullite- and Zirconia-Matrix Composites

Recently, the effect of solid inclusions on the sintering of ceramic powders has been explained in terms of a "backstress" that opposes densiflcation. Several analyses havebeen proposed to describe this problem. However, little quantitative information exists concerning the effect of reinforcement on microstructural evolution. This study compares the microstructural development of zirconia and mullite matrices in the presence of alumina platelets. The effect of platelet loading on density is similar for both composites. Quantitative stereological examinations reveal that the average grain size and pore size are finer for the zirconiamatrix composite. The platelet loading does not have any noticeable effect on the average grain size of the matrix in either composite. However, the average pore size increases as the volume fraction of platelets increases for both materials. Contiguity measurements have detected some aggregation of platelets in the zirconia-matrix composite.     

Tensile properties and damage behaviors of glass‐bead‐filled modified polyphenylene oxide under large strain

Based on Continuum Damage Mechanics (CDM), a damage model for glass‐bead‐filled modified polyphenylene oxide (GB/PPO) has been proposed to describe its damage behavior at various levels of tensile strain by considering the reduction of effective loading area. Hence, an equation for prediction of effective elastic modulus of the damaged GB/PPO composites in terms of the three principal true strains was derived. The tensile properties and damage behaviors of the GB/PPO composites with different volume percentages of glass beads were investigated using standard tensile tests and load‐unload tests, respectively. The addition of glass beads increases Young's modulus of PPO but has a weakening effect on its tensile strength. A maximum value of tensile work to break and tensile strain at break was found when 5 vol% of glass beads with a mean diameter of 11 μm was blended with PPO. These results were justified through microscopic examination of the fracture surfaces of the tensile specimens by using a scanning electron microscope (SEM). In‐situ observations of the strain damage processes were made through the SEM equipped with a tensile stage to determine the strain at fully debonding of glass beads. The volumetric strain of GB/PPO composites increases because of microcavitation during strain damage. In general, the prediction for the effective elastic modulus of the damaged GB/PPO composites at different true strains is slightly higher than the experimental results. The damage evolution rates after fully debonding of glass beads from the matrix are close to those predicted by the proposed damage model.     

Compressive strength and damage mechanisms of 2D-C/SiC composites at high temperatures

Compressive strength of 2D-C/SiC composite was investigated from room temperature(RT) to 1600?°C at present work. Damage evolution was investigated by conducting loading/unloading tests at RT and the damage mechanisms were elucidated by observing the fracture morphology. It is found that compressive strength of 2D-C/SiC was retained until 1200?°C and then decreased with increasing temperature. The variation of compressive strength is closely related to the degradation in matrix modulus. The compressive damage of 2D-C/SiC starts at the buckling of 0° fiber and is followed by opening and closing of original pores, initiation and growth of longitudinal interbundle cracks, separation of 90° fiber bundles by longitudinal cracks, matrix cracking from intrabundle pores, propagation of matrix cracks into 0° fiber bundles, connection of cracks in 0° fiber bundles and longitudinal cracks in 90° fiber bundles.     

模压压力对SiC/SiC复合材料力学性能及力学行为的影响

采用热模压辅助聚合物先驱体浸渍裂解工艺制备了国产近化学计量比SiC纤维增强SiC陶瓷基复合材料,通过阿基米德排水法和SEM技术对SiC/SiC复合材料致密化过程进行表征,采用弯曲强度、拉伸强度和断裂韧性对SiC/SiC复合材料力学性能和力学行为进行评价。研究表明,热模压压力是影响材料结构和性能的重要因素,热模压在提升材料致密度的同时,亦造成纤维的损伤。随着热模压压力的增加,SiC/SiC复合材料力学性能先增加后降低。热模压压力适中时,致密度增加因素占优,材料力学性能较为优异;热模压压力较大时候,热模压操作对纤维性能的损伤因素逐渐凸显,基体致密化和纤维损伤两种作用机制相当。     

Finite element analysis of grinding process of long fiber reinforced ceramic matrix woven composites: Modeling,experimental verification and material removal mechanism

A Finite element analysis (FEA) modeling method for grinding wheel and long fiber reinforced ceramic matrix woven composites (LFRCWCs) specimen was integrated and proposed in this paper. This method was adopted to analyze the grinding process of a 2.5D woven quartz fiber reinforced silicon dioxide ceramic matrix (SiO₂/SiO₂) composite. Relevant grinding experiments were conducted, whose results verified the accuracy of the FEA method. Based on the FEA and experimental investigation, material removal mechanism in the grinding process of this material was discussed and the following conclusions can be drawn: 1) the modeling method that sets the interface of fibers and matrix with the same mechanical configuration as the matrix can obtain a pretty high simulation precision; 2) ceramic matrix can be easily broken and removed by fiber squeezing caused by grinding forces thus reducing the surface quality. To alleviate this damage, grinding directions should be selected as along with the fiber orientation which generates shear stress on fibers; 3) fiber debonds are caused by the inconsistent deformations of the warp and weft fiber bundles. Grinding across the axis of the wefts is a better choice to alleviate this damage.     

Study on interfacial debonding stress and damage mechanisms of C/SiC composites using acoustic emission

Among ceramic matrix composites (CMCs), carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are widely used in numerous high-temperature structural applications because of their superior properties. The fiber–matrix (FM) interface is a decisive constituent to ensure material integrity and efficient crack deflection. Therefore, there is a critical need to study the mechanical properties of the FM interface in applications of C/SiC composites. In this study, tensile tests were conducted to evaluate the interfacial debonding stress on unidirectional C/SiC composites with fibers oriented perpendicularly to the loading direction in order to perfectly open the interfaces. The characteristics of the material damage behaviors in the tensile tests were successfully detected and distinguished using the acoustic emission (AE) technique. The relationships between the damage behaviors and features of AE signals were investigated. The results showed that there were obviously three damage stages, including the initiation and growth of cracks, FM interfacial debonding, and large-scale development and bridging of cracks, which finally resulted in material failure in the transverse tensile tests of unidirectional C/SiC composites. The frequency components distributed around 92.5 kHz were dominated by matrix damage and failure, and the high-frequency components distributed around 175.5 kHz were dominated by FM interfacial debonding. Based on the stress and strain versus time curves, the average interfacial debonding stress of the unidirectional C/SiC composites was approximately 1.91 MPa. Furthermore, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) were used to observe the morphologies and analyze the chemical compositions of the fractured surfaces. The results confirmed that the fiber was completely debonded from a matrix on the fractured surface. The damage behaviors of the C/SiC composites were mainly the syntheses of matrix cracking, fiber breakage, and FM interfacial debonding.     

2D-SiC/SiC陶瓷基复合材料的拉伸本构模型研究

通过单向拉伸试验,研究了2D-SiC/SiC复合材料的应力-应变行为.结果表明,材料单向拉伸应力-应变曲线表现出明显的双线性特征,且线弹性段较长.通过试件断口照片,分析了2D-SiC/SiC复合材料单向拉伸破坏机理和损伤模式.基于对损伤过程的假设,建立了二维连续纤维增强陶瓷基复合材料的双线性本构模型,并将其应用于2D-SiC/SiC复合材料的应力-应变曲线模拟,模拟结果与试验值吻合很好.同时,分析计算表明,2D-SiC/SiC复合材料的单轴拉伸行为主要由纵向纤维柬决定,横向纤维对材料的整体模量和强度贡献很小.     

Monitoring damage evolution of ceramic matrix composites during tensile tests using electrical resistivity: Crack density-based electromechanical model

In order to further develop and understand the data provided by electrical resistance measurements, three ceramic matrix composites (CMCs) were characterized in tension. The observed changes of the electrical resistance were compared to the acoustic emission spectra, another commonly used damage monitoring method, as well as the classical interposed unloading/reloading cycles. A model was then proposed in order to predict the evolution of the resistance as a function of the damage state of the three composites. The proposed model provides accurate results for the three materials which, although they all belong to the CMC family, display different mechanical and physical behaviors.     

三维编织Cf/SiC复合材料的拉伸破坏行为

通过三维编织碳纤维(carbon fiber，Cf)／SiC复合材料样品单向拉伸以及单向拉伸加卸载实验．结合样品断口观察．从宏观上分析了三维编织Cf／SiC复合材料单向拉伸时的力学响应，为进一步描述三维编织Cf／SiC复合材料力学行为奠定了实验基础。实验结果表明：三维编织Cf／SiC复合材料单向拉伸时，卸载模量衰减与应力呈线性关系，残余应变的增加与应力呈二次函数关系。微裂纹主要在编织节点处萌生，沿纤维束界面扩展，最终在编织节点处汇合，导致样品发生破坏。