Matrix cracking of 2D SiC/SiC composite characterized by in situ SEM and nano-CT

Two-dimensional plain-woven silicon carbide (SiC) fiber-reinforced SiC matrix (2D SiC/SiC) composite was prepared by polymer infiltration-pyrolysis (PIP). Matrix cracking mechanisms of the composite were investigated by in situ SEM and nano-CT to grasp tensile damage evolution. Results showed that PIP-SiC matrix possessed low-fracture energy with non-homogeneous distribution, leading to simultaneous initiation of matrix cracking outside transverse fiber bundles and in unreinforced regions. Cracks then got deflected along weak fiber/matrix interface, which accelerated crack proliferation within the composite. With an increase in the stress, cracks subsequently deflected along plain-woven layers and converged to form longitudinal macrocracks. The composite was finally delaminated via sliding.     

Effects of Matrix Cracks on the Thermal Diffusivity of a Fiber-Reinforced Ceramic Composite

Effects of matrix cracks and the attendant interface debonding and sliding on both the longitudinal and the transverse thermal diffusivities of a unidirectional Nicalon/MAS composite are investigated. The diffusivity measurements are made in situ during tensile testing using a phase-sensitive photothermal technique. The contribution to the longitudinal thermal resistance from each of the cracks is determined from the longitudinal diffusivity along with measurements of crack density. By combining the transverse measurements with the predictions of an effective medium model, the thermal conductance of the interface (characterized by a Biot number) is determined and found to decrease with increasing crack opening displacement, from an initial value of ∼1 to ∼0.3. This degradation is attributed to the deleterious effects of interface sliding on the thermal conductance. Corroborating evidence of degradation in the interface conductance is obtained from the inferred crack conductances coupled with a unit cell model for a fiber composite containing a periodic array of matrix cracks. Additional notable features of the material behavior include: ( i ) reductions of ∼20% in both the longitudinal and the transverse diffusivities at stresses near the ultimate strength, ( ii ) almost complete recovery of the longitudinal diffusivity following unloading, and ( iii ) essentially no change in the transverse diffusivity following unloading. The recovery of the longitudinal diffusivity is attributed to closure of the matrix cracks. By contrast, the degradation in the interface conductance is permanent, as manifest in the lack of recovery of the transverse diffusivity.     

Toughening Behavior of a Two-Dimensional SiC/SiC Woven Composite at Ambient Temperature: II, Stress-Displacement Relationship in the Crack Process Zone

In this paper insight into the origin of the J_R -curve of a SiC/SiC woven composite was obtained by experimental characterization of the closure stress-crack opening displacement, sigma( u ), relationship in the process zone of the crack. This process zone included both a crack frontal zone and a crack wake damage zone so that quantitative estimates could be obtained of the magnitudes of toughening associated with these two separate zones. The research indicated that the closure stress-crack opening displacement curve has a positive slope in the crack frontal zone and a negative slope in the wake zone with a maximum stress capability on the order of 350 MPa. The toughness contributions from the crack wake and from the crack front were consistent with the J_R -curve results obtained in the previous paper. The stresses supported locally in the crack frontal zone were almost twice as large as those supported by tensile specimens even though this zone was considerably damaged by matrix cracks. This appears to be the result of stabilization of matrix cracks by arrest at fiber bundles. Application of a previously derived theoretical function, sigma_b( u ), solely based on crack bridging by continuous unidirectional fibers, suggested that the efficacy of bridging in the woven composite may in part be related to the woven fiber architecture. Such an architecture apparently induces greater sliding resistance of the SiC bundles against the surrounding SiC matrix.     

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 environmentally induced property degradation of SiC/BN/SiC ceramic matrix composites

The degradation of SiC‐based ceramic matrix composites (CMCs) in conditions typical of gas turbine engine operation proceeds via the stress rupture of fiber bundles. The degradation is accelerated when oxygen and water invade the composite through matrix microcracks and react with fiber coatings and the fibers themselves. We review micromechanical models of the main rate‐determining phenomena involved, including the diffusion of gases and reaction products through matrix microcracks, oxidation of SiC (in both matrix and fibers) leading to the loss of stiffness and strength in exposed fibers, the formation of oxide scale on SiC fiber and along matrix crack surfaces that cause the partial closure of microcracks, and the concomitant and synergistic loss of BN fiber coatings. The micromechanical models could be formulated as time‐dependent coupled differential equations in time, which must be solved dynamically, e.g., as an iterated user‐defined material element, within a finite element simulation. A paradigm is thus established for incorporating the time‐dependent evolution of local material properties according to the local environmental and stress conditions that exist within a material, in a simulation of the damage evolution of a composite component. We exemplify the calibration of typical micromechanical degradation models using thermodynamic data for the oxidation and/or volatilization of BN and SiC by oxygen and water, mechanical test data for the rate of stress rupture of SiC fibers, and kinetic data for the processes involved in gas permeation through microcracks. We discuss approaches for validating computational simulations that include the micromechanical models of environmental degradation. A special challenge is achieving validated predictions of trends with temperature, which are expected to vary in a complex manner during use.     

Mechanism of cumulative damage to short fiber type C/SiC under tension

Stress–strain relations at different degrees of peak stress were investigated using loading–unloading tests to elucidate cumulative damage mechanisms of short fiber type C/SiC under tension. Damage observations revealed their crack length, number, and angle characteristics. Furthermore, stress–strain relations were estimated by expanding Basista’s equations and by substituting measured damage characteristics into them, which revealed a nonlinear stress–strain relation. Cracks propagated in transverse fiber bundles without fiber fracture, connecting other cracks that had 75 ° – 90 ° orientation to the tensile axis. Stress–strain relations estimated qualitatively and quantitatively suggest that mixed mode I and mode II crack opening in transverse fiber bundles in the through-thickness plane caused the stress–strain nonlinear relations.     

Toughening Behavior of a Two-Dimensional SiC/SiC Woven Composite at Ambient Temperature: I, Damage Initiation and R-Curve Behavior

The damage initiation and R -curve behavior for a two-dimensional (2-D) SiC/SiC woven composite are characterized at ambient temperature and related to in situ microscopic observations of damage accumulation and crack advance. Matrix cracking and crack deflection/branching are observed and dominate fracture behavior in the early loading stage such that primary crack extension occurs at apparent stress intensity values as high as 12 MPam^1/2. Linear elastic fracture mechanics (LEFM), though questionable, was assumed to be valid in the early stages of damage initiation prior to primary crack advance, but was clearly invalid once primary crack extension had occurred. Such a high primary crack extension toughness value is confirmed by a renotch technique whereby the crack wake is removed and the fracture resistance drops close to the initial value. Based on microstructural observations, multiple matrix cracks are found to be arrested at fiber bundles. The key to toughening appears to be associated with the mechanics of crack arrest at fiber bundles in the woven architecture. Toughening mechanisms include multiple matrix cracking (similar to microcracking), crack branching, and crack deflection in the crack frontal zone. Application of models to evaluate toughening based on these mechanisms results in values comparable to experimental data. In the regime of primary crack extension, a J -integral technique was applied to investigate the R -curve behavior and results showed a rising J_R -curve which started at 1500 J/m² and reached 6150 J/m² after about 13 mm of primary crack extension. There was evidence of substantial crack bridging by fiber tows and fibrous pull-out in this regime of crack advance.     

Simulation of crack propagation in Alumina particle-dispersed SiC composites

Addition of alumina particles to silicon carbide results in strongly improved toughness values. In order to come to a better understanding of this phenomenon, crack propagation is simulated for a 20 vol% alumina particles-dispersed silicon carbide composite material using the Body Force Method. Special emphasis is paid to the influence of graded compositions. Numerically obtained crack paths are compared to crack paths generated experimentally by Vickers indentations. Moreover, mechanical properties of the investigated material were measured experimentally. Microstructural toughness variations as well as the direction of crack propagation are found to be strongly influenced by residual stresses due to the mismatch between thermal expansion coefficients of alumina and silicon carbide and by the actual crack location. According to tensile residual stresses in the radial direction cracks approaching a particle are deviated circumferentially in the matrix around the particle. Moreover, the failure behavior of cracks propagating into a zone of increasing or decreasing volume fraction of alumina particles is found to behave differently as residual stress fields superimpose in the case of particle clustering. ©.     

Stress rupture of SiC/SiC composite tubes under high-temperature steam

SiC-fiber–reinforced SiC matrix composite cladding for light water reactor fuel elements must withstand high-temperature steam oxidation in a loss-of-coolant accident scenario (LOCA). Current composite designs include an outer monolithic SiC layer, in part, to increase steam oxidation resistance. However, it is not clear how such a structure would behave under high-temperature steam in the case when the monolithic layer cracks and carbon interphases and SiC fibers are exposed to the environment. To fill this knowledge gap, stress-rupture tests of prototypic SiC composite cladding at 1000°C under steam and inert environments were conducted. The applied stress was ∼120 MPa, which was beyond the initial cracking stress. The failure lifetime under steam was 400–1300 s, while 75% of the composite specimens did not fail after 3 h of total exposure under inert gases. Microstructural observations suggest that steam oxidation activated slow crack growth in the fibers, which led to failure of the composite. The results from this study suggest that stress rupture in steam environments could be a limiting factor of the cladding under reactor LOCA conditions.     

High-temperature fatigue crack propagation mechanism of orthogonal 3-D woven amorphous SiC Fiber/SiC/YSi2-Si matrix composites

To elucidate degradation mechanisms attributable to high-temperature fatigue crack propagation, a study was conducted of 3-D woven SiC_f/SiC CMC in which amorphous SiC fiber was used as a reinforcement material and in which a matrix was formed through low-temperature melt infiltration. From a high-temperature fatigue test conducted at 1373 K in the atmosphere with stress of 142 MPa or more, the fracture lifetime of newly developed SiC_f/SiC CMC was found to be longer than that of SiC_f/SiC CMC, which uses crystalline SiC fiber. Furthermore, repeatedly applying high temperatures during high-temperature fatigue tests and using X-ray computed tomography, fatigue cracks were found to propagate in a direction across 0-degree fiber bundles that undergo stress. Electron mapping of regions with crack propagation revealed that oxidation eliminates boron nitride (BN), which has a crack deflection effect. The SiC fibers and matrix are fixed through the formation of oxides. Cracks propagate because of the consequent decrease in toughness of the SiC_f/SiC CMC. In regions without crack propagation, fracture surfaces were not covered with oxides. These regions underwent forcible fracture in the final stage of the high-temperature fatigue tests. From the test results presented above, SiC_f/SiC CMC is considered to undergo fracture when the effective cross-sectional area is reduced because of crack propagation accompanying oxidation and when the test load exceeds the tensile strength of the residual cross-sectional area. However, some cracks in the matrix produced by a low-temperature melt infiltration process were closed by oxides derived from YSi₂. Because of crack closing, crack propagation is presumed to be avoided. Also, LMI-CMC showed excellent high-temperature fatigue properties at pressures higher than 150 MPa, which exceeds the proportional limit.     

Stress Development and Fracture of Surface Nucleated Cristobalite on Silica Glass

Stress development and fracture of isolated cristobalite spherulites in amorphous silica matrix were observed. High purity bulk silica was annealed to produce partial surface crystallization consisting of isolated and impinged spherulites in an amorphous matrix. The stress state of the amorphous silica surrounding cristobalite spherulites was qualitatively examined using crossed‐polars microscopy. Fracture was observed to occur with many spherulites encircled by cracks in the matrix and other spherulites observed to self‐fracture in a “mud‐cracking” pattern. The fracture was found to be size dependent with encircling matrix cracks occurring as a minority phenomena in spherulites 20–70 microns in diameter and “mud‐cracking” self‐fracture to occur in all spherulites over 70 microns in diameter. The stresses develop as a result of the strain associated with the 4.9% volume reduction in the cristobalite on transitioning from beta‐to‐alpha phase at ~250°C. Observed fracture behaviors were modeled. Matrix cracks encircling spherulites were found to be consistent with a Weibull failure model of the glass under a stress field derived from the Eshelby inclusion model. Self‐fractured spherulite failure was found to be consistent with a failure model based on thin films under biaxial stress.     

Modeling the effect of damage on electrical resistivity of melt-infiltrated SiC/SiC composites

Electrical resistivity (ER) measurements are a possible health monitoring technique for ceramic matrix composite components in future aerospace applications. In order to use ER measurements to detect and identify damage, it is necessary to understand how each specific damage state will affect the ER response. In this study, finite element models are developed and applied to quantify the effect of specific damage states on the ER response in a melt-infiltrated silicon carbide (SiC) fiber-reinforced SiC composite. The ER of several damage states are calculated by simulating the electric current flow through the damaged microstructure. This is achieved by performing the numerical solution of the steady-state conservation of charge density equation. Numerical results reveal the effect of various cracking features on the ER response such as type of cracking, extent of cracking, crack density and fiber/matrix debonding.     

Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites

The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.     

Abnormal mechanical hysteresis behavior of Tyranno-ZMI SiC/SiC containing Ti3Si(Al)C2

This work studied the effect of tough phase Ti₃Si(Al)C₂ on the mechanical hysteresis behavior of SiC/SiC. Different from continuous fibers reinforced brittle ceramic matrix composites, the mechanical hysteresis behavior of SiC/SiC containing Ti₃Si(Al)C₂ shows some abnormal phenomena: as peak applied stress increases during cyclic loading-unloading-reloading tests, the thermal residual stress values exhibit highly dispersion and the thermal misfit relief strain shows abnormally slow growth. These abnormal phenomena are caused by the reduction of transvers cracks (perpendicular to loading fibers) and the generation of hoop cracks (parallel to loading fibers). The plastic deformations of Ti₃Si(Al)C₂ prevents the transverse cracking of modified matrix, while promoting the hoop cracking of SiC matrix prepared by chemical vapor infiltration (CVI-SiC). Hoop cracking occurs within the transition zone containing amorphous SiO₂ layer and carbon layer in CVI-SiC matrix. The combination of weak transition zone and strong modified matrix finally leads to the occurrence of hoop cracking.     

Static fatigue and compressive stress generation in an aged crack

In classic experiments by Michalske and others, it was found that cracks aged statically below the fatigue limit acquired a temporary strength increase compared to the non‐aged crack. In our previous publication we observed that cracks growing near the fatigue limit exhibited a time dependent slowing down of crack growth. Both of these phenomena are related to a toughening of the crack tip that we attribute to a water‐assisted surface stress relaxation mechanism. To test this hypothesis, the K‐v crack growth curves have been measured using the double cantilever beam (DCB) experimental technique for two commercial glasses, a sodium aluminosilicate, and a potassium aluminosilicate, both of which exhibit clear fatigue limits in air. Using polarimetry, it is shown that the stress state near an unloaded but previously aged crack tip is opposite in sign to the stress state near the tip of a crack held in Mode I loading. These results clearly indicate that a stress relaxation mechanism is occurring at the crack tip.     

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.     

Effects of gradual matrix crack closure on the constitutive behavior of SiC/SiC composites upon unloading

Gradual matrix closure and its effects on the constitutive behavior of SiC/SiC composites are examined in the present study. Real-time matrix crack detection and a macroscopic loading–unloading tensile test are performed on SiC/SiC minicomposites. To verify the effects of matrix crack closure, stress-strain responses under loading and unloading with and without crack closure are discussed. The experimental and numerical results show that matrix cracks close gradually upon unloading, and gradual matrix closure greatly reduces the unloading stiffness.     

Interface Design for Oxidation-Resistant Ceramic Composites

Fiber-reinforced ceramic composites achieve high toughness through distributed damage mechanisms. These mechanisms are dependent on matrix cracks deflecting into fiber/matrix interfacial debonding cracks. Oxidation resistance of the fiber coatings often used to enable crack deflection is an important limitation for long-term use in many applications. Research on alternative, mostly oxide, coatings for oxide and non-oxide composites is reviewed. Processing issues, such as fiber coatings and fiber strength degradation, are discussed. Mechanics work related to design of crack deflecting coatings is also reviewed, and implications on the design of coatings and of composite systems using alternative coatings are discussed. Potential topics for further research are identified.     

Damage Development and Moduli Reductions in Nicalon—Calcium Aluminosilicate Composites under Static Fatigue and Cyclic Fatigue

Unidirectional and cross-ply Nicalon fiber-reinforced calcium aluminosilicate (CAS) glass-ceramic composite specimens were subjected to tension–tension cyclic fatigue and static fatigue loadings. Microcrack densities, longitudinal Young's modulus, and major Poisson's ratio were measured at regular intervals of load cycles and load time. The matrix crack (0° plies) density and transverse crack (90° plies) density increased gradually with fatigue cycles and load time. The crack growth is environmentally driven and depends on the maximum load and time. Young's modulus and Poisson's ratio decreased gradually with fatigue cycles and load time. The saturation crack densities under fatigue loadings were found to be comparable to those under monotonic loading. A matrix crack growth limit strain exists, below which matrix cracks do not grow significantly under fatigue loading. This limit coincides with the matrix crack initiation strain. Linear correlations between crack density and moduli reductions obtained from quasi-static data can predict the moduli reductions under cyclic loading, using experimentally measured crack densities. A logarithmic correlation can predict the Young's modulus reduction in a limited stress range. A fatigue crack growth model is proposed to explain the presence of two distinct regimes of crack growth and Young's modulus reduction.     

Fatigue Mechanisms in Graphite/SiC Composites at Room and High Temperature

Some deductions have been made from fractographic evidence about mechanisms of low-cycle mechanical fatigue in plain woven graphite/SiC composites at room and high temperature in vacuum. At both room temperature and 830°C, fatigue appears to be confined to the crack wake, where attrition reduces the efficacy of bridging fibers. It is inferred that the crack tip advances at some critical value of the crack tip stress intensity factor, as in monotonic growth, rather than by any intrinsic fatigue mechanism in the matrix. However, the manifestations of attrition are very different at room and high temperatures. At high temperature, wear is greatly accelerated by the action of SiC debris within the crack. This distinction is rationalized in terms of the temperature dependence expected in the opening displacement of a bridged crack. This argument leads in turn to plausible explanations of trends in loadlife curves and the morphology of cracks as the temperature rises.