Strain field evolution of 2D needled C/SiC composites under tension

In this study, the digital image correlation (DIC) technique was applied to the tensile test of 2D needled C/SiC composites as a full-field measuring tool with the purpose of characterizing the tensile behavior of the material. The non-linear macroscopic tensile stress-strain curve was obtained. The relationship between the local non-linearity and the macroscopic non-linearity was investigated. The spot- and band-type strain field distributions were observed, and the evolution of the non-uniform strain field distribution was studied. The correlation between the strain field distribution and the structure of the needled preform of the material was also discussed.     

Mechanical response and damage mechanism of C/SiC composites impacted by high-velocity water jet

Rain erosion is a potential hazard for supersonic vehicles, with severe damage to materials that may be impacted by raindrops. In this paper, a series of impact tests of 413–572 m/s are carried out on a 3 mm-thick 2D C/SiC composite specimen using a single impact waterjet apparatus. The typical morphology of C/SiC specimen is obtained by single jet impact test. Under the multi-drop impact, the stress wave interaction is enhanced, and the internal damage of the specimen is severe, showing a funnel-shaped damage. Moreover, the C/SiC specimen is penetrated after 5 drops of impact. Quasi-static tensile tests were employed to quantify the post-impact strength of the specimen, during which the digital image correlation (DIC) method was used to obtain the strain value, at the same time acoustic emission (AE) signal was detected and processed by the K-Means to reveal the damage evolution.     

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.     

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.     

Failure analysis of C/SiC composites plate with a hole by the PFA and DIC method

The progressive failure analysis (PFA) and digital image correlation (DIC) method were used to study the strain distribution and failure evolution of C/SiC composites with a circular hole under tension. The nonlinear constitutive relation based continuous degradation strategy (NCRCDS) was combined with the PFA and compared with the sudden material property degradation strategy (SMPDS). The maximum strain criterion was adopted in the PFA to model local failure. The initiation and evolution of local failure around the hole were studied by PFA and DIC method. The results indicate that the local failure distribution simulated by PFA was close to that of the DIC measurement. The strain field was relevant to the surface topography and the microstructure of the material, and the strain distribution around the hole was affected by the arrangement and the damage of the fiber bundle. The simulated results of the strain distribution were in good agreement with the DIC measurements.     

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.     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.     

Determining the interlaminar tensile strength of a SiCf/SiC ceramic matrix composite through diametrical compression testing

Interlaminar tensile strength (ILTS) of a SiC_f/SiC Ceramic Matrix Composite (CMC) was determined through use of a diametrical compression test of disk geometries, with two geometries are investigated (Φ4.5 and Φ9 mm). Results are correlated with the fracture surface architecture, specifically relating to fibre tows. Due to the stochastic nature of ceramic material systems a Weibull distribution was implemented to understand the characteristic strength and distribution of the data sets for both disk geometries. Overall, a decrease in characteristic ILTS coupled with a narrowed distribution is observed for the Φ9 mm compared the Φ4.5 mm disk geometries due to the repeating unit cell size of the SiC_f/SiC CMC under investigation.     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Effect of a diffusion-enhancing hole on the densification of a thick-section 2D C/SiC composite

Diffusion-enhancing holes (DEHs) have been used to mitigate the large density gradients that are formed in the thick-section ceramic matrix composites (CMCs) fabricated by chemical vapor infiltration (CVI). However, the densification characters of the thick-section CMCs with DEHs through vapor infiltration remain a concern. Here, the densifications of a10-mm-thick two dimensional (2D) C/SiC composite with or without DEHs are investigated by experiments and calculations. Results showed both the measured densities, the predicted final densities, and the density growth rates (DGRs) for the composite with DEHs (diameters of 2 or 4 mm) are higher than those of the counterpart without DEHs, due to the forming of dense rings (DRs) around DEHs and the increased infiltration in the large pores (diameter > 52 μm). In addition, the diffusion increase in infiltration with DEHs is attributed to the increase of Knudsen diffusion resulted from the reopening of the blocked/sealed pores by DEH-machining.     

Multi-scale modelling of progressive damage and failure behaviour of 2D woven SiC/SiC composites

In this paper, a multi-scale modelling approach has been developed to predict the progressive damage and failure behaviour of 2D woven SiC/SiC composites. At the tow scale, non-linear tow properties have been determined by a micromechanics-based damage model, in which two scalar damage variables were introduced to characterize the fibre-dominated and matrix-dominated damage, respectively. Based on periodic boundary conditions, a meso-scale unit cell model has been established to simulate the macroscopic stress-strain responses and progressive damage processes of the composite under uniaxial tensile, compressive and in-plane shear loadings, respectively. In the numerical method, the non-linear properties of constituent materials have been implemented by the user defined subroutine, USDFLD of the finite element package, Abaqus. The numerical results and their comparisons with experimental stress-strain curves have been presented. The failure mechanisms of the composite under each loading have been also discussed. The high efficiency and prediction accuracy of the model make it possible to analyse large scale woven composites.     

Correlation between failure mechanism and rupture lifetime of 2D-C/SiC under stress oxidation condition based on acoustic emission pattern recognition

Creep tests of 2D-C/SiC in a wet oxidizing atmosphere were implemented for six samples. The loading process was monitored by acoustic emission (AE). Principal component analysis and a fuzzy clustering algorithm were used to perform pattern recognition of the AE data. All of the AE events were divided into four clusters and labelled as matrix cracking, interfacial damage, fiber breakage and fiber-bundle breakage respectively, according to their physical origin. It was found C/SiC has very scattered rupture lifetimes even under the same test conditions, and the evolution of AE events corresponding to fiber failure is quite different. With increasing rupture lifetime, the AE energy of fiber-bundle breakage is higher, while the number of these events is less. Thus, it is concluded that local oxidation and damage development is the controlling failure mechanism for short-lived specimens and uniform oxidation and damage development is the controlling failure mechanism for long-lived specimens.     

Investigation on non-uniform strains of a 2.5D woven ceramic matrix composite under in-plane tensile stress

In this study, the digital image correlation technique and strain gauges were applied to the in-plane tensile tests of a 2.5D woven ceramic matrix composite for the purpose of characterizing deformation and strain distribution features. The test results indicate a strong influence of the weave architecture on the strain distribution of the material. Mesoscale finite element models were established based on the weave architecture of the material. The experimental and simulation results show that: the strain distributions are periodic and fluctuating; the maximum strain fluctuation on the surface is about 30%, but the deviation of average strain decreases rapidly with the calculated area increase. The strain measurement methods were proposed to obtain the accurate average strain: the size of measurement/calculated area should be twice larger than the unit cell size; much more accurate result could be obtained when the area size is an integer multiple of the unit cell size.     

Theoretical prediction on structure evolution and optimal properties of silicon modified hexagonal boron nitride as interphase in SiCf/SiC composite

To predict the effects of Si doping on hexagonal boron nitride (h-BN) and to achieve a balance between mechanical and oxidation properties for the interphase modification in SiC_f/SiC composites, we herein calculate and analyze the crystal structures and mechanical properties of (BN)₆₄Si_x (x = 4, 8, 16, 32) models by means of density functional theory (DFT) calculations and ab initio molecular dynamics (aiMD) simulations. The possible trends of crack deflection and self-healing ability are discussed. The modeling shows an obvious transition of (BN)₆₄Si_x from the layered crystal structure and anisotropic mechanical property to amorphous structure and isotropic mechanical property as the Si doping content up to 36.1 wt%. Regarding to the application of interphase in SiC_f/SiC composites, (BN)₆₄Si₁₆ model structure possess the highest debonding potential according to Cook and Gordon’s criteria and illustrates the higher self-healing capacity at elevated temperature.     

Fatigue behavior and damage analysis of PIP C/SiC composite

In this work, we study the fatigue behavior of a C/SiC composite produced by several cycles of polymer infiltration and pyrolysis (PIP). Fatigue tests were performed with maximum stresses corresponding to 60–90% of the tensile strength of the composite. During the fatigue tests, acoustic emission (AE) monitoring was performed and the measured AE energy was utilized to quantify the damage and distinguish possible damage mechanisms. Most of the fatigue damage in the form of matrix cracking, interface damage and fiber breakage occurs in the first cycle. As loading cycles proceeded, damage in form of matrix crack re-opening and interfacial friction constantly accumulates. Nevertheless, all samples survived the run-out of 1,000,000 cycles. After the fatigue tests, an increase of the tensile strength is observed. This phenomenon is associated with the relief of process-induced internal thermal stresses and the weakening of the fiber-matrix interface. In general, the studied material shows very high relative fatigue limit of 90% of its tensile strength.     

On the tensile behaviors of 2D twill woven SiO2f/SiO2 composites at ambient and elevated temperatures: Mesoscale analysis and in situ experimental investigation

SiO_2f/SiO₂ composites are among the most ideal high-temperature wave-transparent materials used in hypersonic vehicles. The purpose of the study is the thorough experimental investigation of the tensile behavior of a 2D twill woven SiO_2f/SiO₂ composite, and the establishment of an accurate and efficient simulation method for such composites. The digital image correlation (DIC) method was utilized to capture local deformation data during tensile tests. Meanwhile, a progressive failure analysis (PFA) model employing the exponential damage evolution law was subsequently developed with UMAT in ABAQUS. Simulations of the mechanical properties and strain distributions show good consistency with experimental results. The results at room temperature and 900 °C demonstrate that the strain distributions exhibit obvious periodic patterns related to the woven structure. In addition, band-shaped strain concentrations can be observed at the intersection zones between adjacent longitudinal and transverse fiber bundles. These zones are regarded as critical regions. This was validated by the damage evolution observed in the simulations. Owing to the grain coarsening of quartz fibers and the embrittlement of different constituents at 900 °C, notable degradation of the mechanical properties and brittle fracture characteristics were observed.     

Analysis of SiC/SiC composites for energy applications at ambient conditions

Plain weave planar and biaxially braided tubular SiC/SiC CMCs are evaluated in tension and four-point bending, respectively, at ambient conditions. Custom-designed fixtures for CMC testing are developed for each loading mode and are coupled with three-dimensional digital image correlation. Stereoscopic image correlation analysis reveals crack initiation and failure sites to provide insight into stress redistribution mechanisms. Scanning electron microscopy is performed postmortem to determine the influence of microstructural features on crack initiation and failure. Crack spacing is measured in situ by stereoscopic image correlation and confirmed by SEM measurements to relate to underlying tow-tow crossing points. Triangulated surface heights of plain weave tow architecture are used to determine that subtle differences in neighboring transverse tow angle, which vary within a range of ±4° from horizontal, have no significant effect on final fracture location. The results presented reaffirm the state of current SiC/SiC CMCs developed for energy applications and will help to further improve SiC/SiC and other CMCs.     

Crack initiation and propagation in braided SiC/SiC composite tubes: Effect of braiding angle

Crack initiation and propagation in three braided SiC/SiC composite tubes with different braiding angles are investigated by in situ tensile tests with synchrotron micro-computed tomography. Crack networks are precisely detected after an image subtraction procedure based on Digital Volume Correlation. FFT based simulations are performed on the full-resolution 3D images to assess elastic stress/strain fields. Quantitative measurements of the crack geometries are performed using a novel method based on grey levels. The results show that braiding angle has no obvious effect on the location of crack onsets (initiation always occurs at tow interfaces), whereas it significantly affects the paths of crack propagation. This work provides an explicit demonstration of the crack propagation scenarios with respect to the mesoscopic fibre architectures.     

The oxidation behavior of MoSi2-CrSi2-Si/SiC coating for C/C composites in H2O-O2-Ar atmosphere: Experiment and first-principle investigation

The MoSi₂-CrSi₂-Si/SiC multi-component coating was prepared by two-step pack cementation on carbon/carbon composites. To investigate the effect of water vapor on the anti-oxidation of the coated samples, two kinds of atmosphere (50%H₂O-50%O₂, 50%O₂-50%Ar) were designed for comparison with a total pressure of 1 atm at 1773 K. The results showed that, after being tested for 10 h, the weight loss of the coated samples in O₂+Ar and H₂O+O₂ were 0.243% and 0.436% respectively. The reasons for different weight losses can be attributed to the water vapor, which could degrade the protective ability of the glass layer formed by SiO₂ and Cr₂O₃ and thereby accelerate the oxidation of MoSi₂ and CrSi₂. Based on the Mulliken analysis calculated by the first principle, the corresponding water vapor corrosion resistance of the prepared coating was in the following order: SiC>MoSi₂>CrSi₂, which was consistent with the experimental phenomenon.     

Effect of CVI SiC content on ablation and mechanism of C/C-SiC-ZrC-Cu composites

C/C-SiC-ZrC-Cu composites were fabricated by chemical vapor infiltration, precursor infiltration-pyrolysis and vacuum-pressure infiltration methods. During Cu infiltration, the Cu_6·69Si and Cu₃Si new phases are generated through reaction between SiC and molten Cu. The formed Cu_6·69Si, Cu₃Si, ZrC and SiC phases can improve the wettability and interface combination between Cu and the doped carbon matrix. The ablation tests demonstrate that the CVI SiC content significantly affects the structure of protective oxide layer, and induces inverse effects in ablation center at 2500 ^°C and 3000 ^°C. The relatively high CVI SiC content enhances the ablation resistance of composites at 2500 ^°C, but increases the linear ablation rate at 3000 ^°C due to the excessive evaporation and mechanical denudation. During ablation, the formed Si-Zr-C-O layer underneath ablation center and the Si-Cu-C-O layer on transition or marginal areas can prevent carbon matrix from serious oxidation. After ablation for 20 s, the C/C-SiC-ZrC-Cu composites with high CVI SiC content display the best anti-ablation property at 2500 ^°C, and the ablation rates are 3.5 ± 0.1 μm/s and 3.4 ± 0.1 mg/s.