Mechanical hysteresis and damage evolution in C/SiC composites under fatigue loading at room and elevated temperatures

In this paper, the mechanical hysteresis and damage evolution in C/SiC ceramic matrix composites (CMCs) under cyclic tension-tension fatigue loading at room and elevated temperatures in air and in inert atmosphere and different loading frequencies are investigated. The fatigue hysteresis loops models considering multiple matrix cracking modes are developed to establish the relationships between fatigue hysteresis loops, fatigue hysteresis dissipated energy, and fiber/matrix interface shear stress. The evolution of fatigue hysteresis dissipated energy and interface shear stress vs applied cycles is analyzed. It was found that the interface shear stress degradation rate increases with fatigue peak stress, and loading frequency from 40 to 375 Hz.     

Effects of interface bonding properties on cyclic tensile behavior of unidirectional C/Si3N4 and SiC/Si3N4 composites

In this paper, the effect of fiber/matrix interface bonding properties on the cyclic loading/unloading tensile stress?strain hysteresis loops of 2 different ceramic‐matrix composites (CMCs), ie, C/Si₃N₄ and SiC/Si₃N₄, has been investigated using micromechanical approach. The relationships between the damage mechanisms (ie, matrix multicracking saturation, fiber/matrix interface debonding and fibers failure), hysteresis dissipated energy and internal frictional damage parameter have been established. The damage evolution processes under cyclic loading/unloading tensile of C/Si₃N₄ and SiC/Si₃N₄ composites corresponding to different fiber/matrix interface bonding properties have been analyzed through damage models and interface frictional damage parameter. For the C/Si₃N₄ composite with the weakest fiber/matrix interface bonding, the composite possesses the lowest tensile strength and the highest failure strain; the hysteresis dissipated energy increases at low peak stress, and the stress?strain hysteresis loops correspond to the interface partially and completely debonding. However, for the SiC/Si₃N₄ composite with weak interface bonding, the composite possesses the highest tensile strength and intermediate failure strain; and the hysteresis dissipated energy increases faster and approaches to a higher value than that of composite with the strong interface bonding.     

Tensile and Fatigue Behavior of Silicon Carbide Fiber-Reinforced Aluminosilicate Glass

The onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross-ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.     

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.     

Hysteresis loops of carbon fiber-reinforced ceramic-matrix composites with different fiber preforms

The hysteresis loops of C/SiC ceramic-matrix composites (CMCs) with different fiber preforms, i.e., unidirectional, cross-ply, 2D and 2.5D woven, 3D braided, and 3D needled at room temperature have been investigated. Based on fiber slipping mechanisms, the hysteresis loops models considering different interface slip cases have been developed. The effects of fiber volume fraction, matrix cracking density, interface shear stress, interface debonded energy, and fibers failure on hysteresis loops, hysteresis dissipated energy, hysteresis width, and hysteresis modulus have been analyzed. An effective coefficient of fiber volume fraction along the loading direction (ECFL) was introduced to describe fiber preforms. The hysteresis loops, hysteresis dissipated energy and hysteresis modulus of unidirectional, cross-ply, 2D and 2.5D woven, 3D braided and 3D needled C/SiC composites have been predicted.     

Fatigue behavior and damage evolution of SiC/SiC composites under high-temperature anaerobic cyclic loading

In the present study, the fatigue behavior and damage evolution of SiC/SiC minicomposites at elevated temperatures in oxygen-free environment are investigated which are important for their application and are still unclear. The high-temperature fatigue test platform is developed and the fatigue stress-life curve and the stress-strain response are obtained. The test result shows that the life of the material at elevated temperature is shorter than that at room temperature under the same stress level. Moreover, the hysteresis loop width and the residual strain increase with the increasing of the cycles while the hysteresis modulus decreases during the fatigue cycling. The evolution process of matrix cracks is observed using the real-time remote detection system. It is found that matrix cracking is insensitive to the cyclic loading which is similar to room temperature and is due to that the degeneration of the interfacial shear stress reduces the area of high stress in matrix. The fiber/matrix interfacial shear stress under different cycles is determined based on the fatigue modulus of each hysteresis loop. The result shows a fatigue enhancement phenomenon for the interface which is not observed at room temperature.     

Characterization of cyclic loading/unloading damage behavior in fiber-reinforced ceramic-matrix composites using inverse tangent modulus

Under cyclic loading/unloading, the mechanical hysteresis appears in fiber-reinforced ceramic-matrix composites (CMCs) due to multiple micro damage mechanisms. In this paper, the cyclic loading/unloading damage evolution in different CMCs is analyzed using the inverse tangent modulus (ITMs). Experimental micro damage mechanisms are observed using the X-ray computed tomography (XCT) and scanning electron microscopy (SEM). Based on the damage mechanisms’ analysis, a damage-based micromechanical constitutive model is developed to predict the cyclic loading/unloading curves and related damage parameters. Effects of composite’s constitutive properties, peak stress, damage state and interface properties on the cyclic loading/unloading damage evolution are discussed. For the 1D and 2D SiC/SiC, and 3D C/SiC composites, the evolution curves of ITMs can be divided into two regions. In region I, the increasing rate of the ITMs is constant and depends on the composite’s constitutive properties; and in region II, the increasing rate of the ITMs decreases as the interface slip range approaches the interface debonding tip.     

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.     

Tensile behavior of 2D-C/SiC composites at elevated temperatures: Experiment and modeling

In this paper, on-axis tensile behavior of a coated 2D-C/SiC composite at elevated temperatures was studied experimentally and theoretically. The measured data reveals that the tensile modulus and strength increase continuously with increasing temperature till 1273 K. Contrarily, the failure strains decrease sharply at high temperatures than the counterpart at room temperature, manifesting the significant influence of thermal residual stresses (TRS) on mechanical behavior of C/SiC composites. Simulation of stress-strain response is based on a two-scale analytical model, in which the plain-weave element is idealized as a cross-ply laminate and its macroscopic mechanical parameters are evaluated by shear-lag approach. The primary calculation was concentrated on TRS of the composite. And, a new crack evolution model was introduced to describe the stochastic cracking process. The total strain response including residual strain and elastic strain from the loading-unloading-reloading conception was finally formulated through micromechanical analysis involving the influence of TRS on matrix cracking and interface debonding. Additionally, a strength model was developed for plain-weave structures by using shear-lag theory, statistical theory and rule of mixture. Both of the proposed constitutive and strength models can give accurate predictions for 2D-C/SiC composites at elevated temperatures.     

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.     

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.     

A method for identifying the interface shear stress of unidirectional ceramic matrix composites

A method for identifying the interface shear stress of unidirectional ceramic matrix composites (CMCs) under fatigue loading was proposed. A functional relationship between the interface shear stress and the area of the hysteresis loop, the slip strain of the hysteresis loop was established based on the shear lag model by the method of symbolic-graphic combination. A fatigue test of SiC/SiC mini-composites was performed and the hysteresis curves were obtained. Then the interface shear stress under a different number of cycles was identified by inserting the experimental data of the area and slip strain of the hysteresis curves into the functional relationship. The identification results using previously introduced methods based on the area or the secant modulus of the hysteresis loop were also examined. To verify these methods, the interface shear stress identified by the three different methods was entered into the shear lag model to simulate the hysteresis curves for the different number of cycles. A comparison between the simulated results and experimental data shows that the proposed method is more appropriate for the composites used in this paper.     

Modeling matrix multicracking development of fiber-reinforced ceramic-matrix composites considering fiber debonding

In this paper, the effect of fiber debonding on matrix multicracking development of different fiber-reinforced CMCs is investigated using the micromechanical approach. The Budiansky–Hutchinson–Evans shear-lag model is adopted to analyze the fiber and matrix stress distributions of the damaged composite. The fracture mechanics approach is used to determine the fiber/matrix interface debonding length. Combining the critical matrix strain energy criterion and fracture mechanics fiber/matrix interface debonding criterion, the stress-dependent matrix multicracking development is analyzed for different fiber volume fraction, fiber/matrix interface properties and matrix cracking characteristic stress. The experimental matrix multicracking development of unidirectional C/Si₃N₄, SiC/Si₃N₄, SiC/CAS, SiC/CAS-II, SiC/SiC, SiC/Borosilicate and mini-SiC/SiC composites are predicted.     

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.     

Modeling deformation of a melt-infiltrated SiC/SiC composite under fatigue loading

Results from fatigue experiments done on a SiC/SiC composite are presented. A micromechanics-based model is used to study the observed behavior under cyclic loading. The model includes consideration of progressive damage, creep and oxidation of the fiber and matrix. Comparison of model predictions with test data showed that the deformation during fatigue in this material is explained primarily by damage in the form of matrix microcracking and interface debonding, in combination with creep under the cyclic load. Stiffness of the material was observed to not change significantly during fatigue indicating that the contribution of fiber fracture to deformation is limited. Fiber fracture however was found to determine final failure of the composite. Failure under cyclic fatigue loading was found to be affected by load transfer from the matrix to the fiber due to damage and creep, and by progressive degradation of the load-carrying fibers due to the combined effect of oxidation and load cycling.     

Relationships between the accumulative damage and dielectric properties of woven BN-coated Hi-Nicalon™ SiC fibre-reinforced SiC matrix composites

The accumulative damage behaviour of BN-coated Hi-Nicalon™ SiC fibre-reinforced SiC matrix composite was examined under tensile cyclic loading at room and elevated temperatures. The accumulative damage occurring during the cyclic loading was quantitatively characterised using the damage parameter obtained by the hysteresis loop curves. The damage parameter increased with increasing applied stress beyond the matrix cracking stress, and it subsequently retained a nearly constant value until just before fracture. Moreover, the dielectric constant, dielectric loss and loss tangent of the composite were measured before and after the fracture in the frequency range 1–1000 MHz. The dielectric properties had similar frequency dependency before and after the fracture. However, the dielectric constant, dielectric loss and loss tangent were lower in the post-fractured specimens than in the pristine ones. The reduction of the dielectric properties was associated with the accumulative damage stored in the specimens. In addition, the relationships between the dielectric properties and the damage parameter were described in detail.     

Microstructure and tensile behavior of (BN/SiC)n coated SiC fibers and SiC/SiC minicomposites

Interphase plays an important role in the mechanical behavior of SiC/SiC ceramic-matrix composites (CMCs). In this paper, the microstructure and tensile behavior of multilayered (BN/SiC)_n coated SiC fiber and SiC/SiC minicomposites were investigated. The surface roughness of the original SiC fiber and SiC fiber deposited with multilayered (BN/SiC), (BN/SiC)₂, and (BN/SiC)₄ (BN/SiC)₈ interphase was analyzed through the scanning electronic microscope (SEM) and atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. Monotonic tensile experiments were conducted for original SiC fiber, SiC fiber with different multilayered (BN/SiC)_n interfaces, and SiC/SiC minicomposites. Considering multiple damage mechanisms, e.g., matrix cracking, interface debonding, and fibers failure, a damage-based micromechanical constitutive model was developed to predict the tensile stress-strain response curves. Multiple damage parameters (e.g., matrix cracking stress, saturation matrix crack stress, tensile strength and failure strain, and composite’s tangent modulus) were used to characterize the tensile damage behavior in SiC/SiC minicomposites. Effects of multilayered interphase on the interface shear stress, fiber characteristic strength, tensile damage and fracture behavior, and strength distribution in SiC/SiC minicomposites were analyzed. The deposited multilayered (BN/SiC)_n interphase protected the SiC fiber and increased the interface shear stress, fiber characteristic strength, leading to the higher matrix cracking stress, saturation matrix cracking stress, tensile strength and fracture strain.     

Multiple Cracking of Unidirectional and Cross-PlyCeramic Matrix Composites

This paper examines the multiple cracking behavior of unidirectional and cross-ply ceramic matrix composites. For unidirectional composites, a model of concentric cylinders with finite crack spacing and debonding length is introduced. Stresses in the fiber and matrix are found and then applied to predict the composite moduli. Using an energy balance method, critical stresses for matrix cracking initiation are predicted. Effects of interfacial shear stress, debonding length and bonding energy on the critical stress are studied. All the three composite systems examined show that the critical stress for the completely debonded case is lower than that for the perfectly bonded case. For cross-ply composites, an extensive study has been made for the transverse cracking in 90° plies and the matrix cracking in 0° plies. One transverse cracking and four matrix cracking modes are studied, and closed-form solutions of the critical stresses are obtained. The results indicate that the case of combined matrix and transverse crackings with associated fiber/matrix interfacial sliding in the 0° plies gives the lowest critical stress for matrix cracking. The theoretical predictions are compared with experimental data of SiC/CAS cross-ply composites; both results demonstrated that an increase in the transverse ply thickness reduces the critical stress for matrix cracking in the longitudinal plies. The effects of fiber volume fraction and fiber modulus on the critical stress have been quantified. Thermal residual stresses are included in the analysis.     

A progressive damage model for ceramic matrix mini-composites with thick interphases

Toughness enhancement in ceramic matrix composites (CMCs) with brittle matrix and fiber phases is often accomplished by introducing a weak finite-thickness interphase between the fiber and matrix. The current work presents a progressive damage model to predict the tensile response of single tow CMCs (mini-composite) representative of a unidirectional composite at the microscale. Implementation of a 3-phase shear-lag model for a geometrically accurate representation of the underlying microstructure in CMCs with finite thickness interphase has been highlighted. A probabilistic progressive modeling approach has been adopted, accounting for multiple matrix cracking, interfacial debonding, and fiber failure in 3-phase mini-composites. The predicted tensile response of CMCs from the progressive damage modeling approach agrees with experimental results obtained for C/BN/SiC mini-composites validating the approach.     

Delamination Resistance of Two Hybrid Ceramic-Composite Laminates

Hybrid laminar composites that are comprised of alternating layers of ceramic sheets and fiber-reinforced ceramic-matrix composite (CMC) layers exhibit attractive mechanical properties, including a high first cracking stress and a high strain to failure. To achieve these properties, a strong bond must exist between the ceramic and CMC layers; otherwise, delamination will occur readily between the layers. The present study focuses on the delamination resistance of such laminates at ambient and elevated temperatures. The delamination resistance of interfaces that have been subjected to mixed-mode loading has been measured for two different hybrid composites by using edge-notched flexure specimens. At low temperatures, delamination occurs by a process that involves multiple matrix cracking within the CMC layers normal to the fibers, followed by cracking of the matrix parallel to the fibers at or near the ceramic/CMC interface. The corresponding fracture energies are typically in the range of ∼100-300 J/m²—comparable to the delamination resistance of the CMC itself. At elevated temperatures, delamination occurs via cavitation and rupture of the matrix within the CMC layers at or near the ceramic/CMC interface, with an attendant loss in toughness (to ∼10-30 J/m²). The loss in toughness occurs most rapidly at temperatures that are close to the strain point of the matrix phase; this represents the life-limiting temperature for this class of composites.