Strength Of Ceramic Matrix Composites Research Articles

The oxidation life analysis method for SiC/SiC composite components based on an equivalent diffusion coefficient model

AbstractA method for the oxidation life analysis of SiC/SiC composite structures was proposed to predict the oxidation life of ceramic matrix composite (CMC) components. In this paper, multiscale diffusion–oxidation simulation of the CMCs’ structure can be carried out by establishing the equivalent diffusion coefficient model, to get the microscopic oxidation morphology parameters at each point. Based on the results, the residual stiffness is calculated with the homogenization method. Then, the residual stiffness is taken as the material parameter of element to calculate the stress distribution after oxidation in the CMC structure. And the residual strength of CMCs was calculated by progressive damage analysis. The oxidation life of CMC component is analyzed by judging the failure condition of each element. Finally, the oxidation life prediction of the CMC adjustment sheet under specific loading environment is realized, which is about 1 200 h. The results show that this method is feasible for the calculation of hot end parts under actual complex loads.

The design of special woven-preformed structures for the high-performance film cooling with undamaged fibers based on 2.5D ceramic matrix composites

A design-manufacturing integration method for the film cooling of 2.5D woven ceramic matrix composites (CMC) is proposed which can concurrently achieve film holes with undamaged fibers and improve cooling performance. Meanwhile, this study investigates the effect of different woven-preformed structures on gas-thermal-mechanical characteristics. The design-manufacturing integration method implements the collaborative design of the micro woven structure and macroscopical shaping of film holes. Firstly, the unique micro woven method can manufacture the film holes without damaged fibers which effectively guarantees the strength of CMC. Secondly, the macroscopically shaped special woven-preformed holes (SWPHs) own the special structural features related to woven structure. These special structures change the flow field and induce the counter-rotating/anticounter-rotating Vortex Pair (CVP/Anti-CVP) which effectively improves the cooling performance and controls the thermal stress. Compared with traditional drilled cylindrical hole DCH, the SWPHs’ maximum increase of averaged overall film cooling effectiveness reaches 110%. Besides, SWPHs avoid the large temperature difference between the yarn and the matrix around the drilled DCH which eventually reduces the thermal stress in the corresponding area.

Research on Predicting the Bending Strength of Ceramic Matrix Composites with Process of Incomplete Data

With the rapid development of machine learning, it is possible to use neural networks to build models to predict performance of Ceramic Matrix Composites (CMCs) with raw materials and environments. In the traditional material science engineering, it always took a long time to develop a new CMC. Furthermore, there is still no theoretical basis providing references to design experiments to develop CMCs with ideal performances. This work proposed a model to predict the bending strength of CMCs with a Convolution Neural Network (CNN) using 8 factors considered to affect the bending strength of CMCs mainly. For the data were all collected from papers published on journals and conferences, and there is no standard to describe an experiment, the incompleteness of data influences the performance of our model seriously. Then we tried several methods to fill the data, finally the regression imputation with a dual-hidden-layer neural network performed a significant improvement of the CNN bending strength prediction model.

Use of interphase in geopolymer matrix composites for improved toughness

Geopolymers are a lightweight material with excellent fire and thermal resistance. When combined with continuous fiber reinforcement, geopolymers demonstrate potential to replace heavier, structural components, especially those that are exposed to elevated temperature regimes (> 400 °C) in air. Oxidation resistance is essential to continuous operations in these environments. While oxide fibers can drastically improve the mechanical properties of geopolymers, their incorporation has been met with challenges. These geopolymer matrix composites (GMC) lack the necessary toughness to find much use as a structural component. In this study, concepts and materials used to reduce interfacial strength in ceramic matrix composites for the purpose of improving toughness are employed in GMCs. To investigate the potential to improve toughness, composites containing an alumina-based fiber and geopolymer were prepared with three different interfacial conditions: heat cleaned, carbon coated, and monazite coated. From the fabricated composites, specimens for four-point flexural testing and single fiber pushout were prepared. Additionally, composite samples were heat treated to 650 °C in oxidizing and non-oxidizing environments. To varying degrees, composites that utilized a fiber coating demonstrated improvements in toughness and relative damage tolerance as compared to those without any modification to the interface. In some cases, the improvement was over 160% as compared to those without a coating. Pushout testing showed specimens with coated fiber to have weaker interfacial conditions. The combined results support the use of tailored interface concepts in GMCs for improving toughness.

Fatigue Lifetime of Ceramic Matrix Composites at Intermediate Temperature by Acoustic Emission.

The fatigue behavior of a Ceramic Matrix Composite (CMC) at intermediate temperature under air is investigated. Because of the low density and the high tensile strength of CMC, they offer a good technical solution to design aeronautical structural components. The aim of the present study is to compare the behavior of this composite under static and cyclic loading. Comparison between incremental static and cyclic tests shows that cyclic loading with an amplitude higher than 30% of the ultimate tensile strength has significant effects on damage and material lifetimes. In order to evaluate the remaining lifetime, several damage indicators, mainly based on the investigation of the liberated energy, are introduced. These indicators highlight critical times or characteristic times, allowing an evaluation of the remaining lifetime. A link is established with the characteristic time around 25% of the total test duration and the beginning of the matrix cracking during cyclic fatigue.

In-situ damage monitoring of a SiC/SiC ceramic matrix composite using acoustic emission and digital image correlation

Continuous ceramic fiber reinforced ceramic matrix composites (CMCs) offer an innovative damage tolerant structural material for temperature regimes inaccessible to high temperature metals. However, when subjected to application environments, these materials exhibit damage and degrade over time depending on the severity of those conditions. The ultimate strength of CMCs is dictated by the properties of the load bearing fibers, but matrix cracking weakens the composite and can expose the load bearing fibers to harsh environments. The objective of the work performed was to develop a methodology for linking in situ detection of localized damage to final failure in continuous fiber reinforced CMCs. Initiation and growth of matrix cracking are measured and located linearly along the gage length via acoustic emission (AE) detection. High amplitude events at relatively low static loads can be associated with initiation of large matrix cracks. When there is a localization of high amplitude events in a given area, a measurable effect on the strain field can be observed. Full field surface strain measurements were obtained using digital image correlation (DIC). An estimation of the matrix cracking stress as well as localized areas of initiation were measured in real time.

Damage-based failure theory and its application to 2D-C/SiC composites

Damage evolution is of great importance to determine the final failure of ceramic matrix composites (CMCs). In order to characterize the damage coupling behavior and its effect on failure strength of CMCs, a new methodology for damage coupling analysis was formulated. And, new kind of damage-based failure criteria were established under plane stress state, including maximum damage criterion and quadratic damage criterion. Both the criteria require investigation upon the damage evolution laws. The proposed failure theory was applied to predict the coupled damage behavior and the off-axis strength of a 2D C/SiC composite. The theoretical results agree well with the experimental data.

Effects of Interface Properties on Tensile Strength of Ceramic Matrix Composites

A definition of central crack damaged zone(CCDZ) is introduced based on tension experiments of Ceramic Matrix Composites(CMCs).The width of the CCDZ is defined to equal the debond length and the effect of interface toughness is neglected.When the CMCs rupture,the stress carried by the fibers can be estimated by the mean fiber stress within the CCDZ.Furthermore,the rule of mixtures is modified.Considering that the interface properties control the fracture mode,two kinds of models are presented respectively for brittle and ductile fracture strength.The stress concentration factor and interface debond energy release rate are introduced and discussed.It indicates that both the above factors result in decline of the tensile strength as they increase.The predictions of the given formulae are in good agreement with experimental data.

Load-rate dependency of ultimate tensile strength in ceramic matrix composites at elevated temperatures

Ultimate tensile strength of three continuous fiber-reinforced ceramic matrix composites, including SiC f/CAS-II (1D), SiC f/MAS-5 (2D) and SiC f/SiC (2D), was determined as a function of load (test) rate in air at 1100–1200 °C. All three composites exhibited a significant dependency of ultimate tensile strength on test rate. The application of the preload technique as well as the prediction of life from one loading configuration (constant stress-rate) to another (constant stress) suggested that the overall phenomenological macroscopic failure mechanism of the composites would be a power-law type of slow crack growth or damage evolution/accumulation. It was further found that constant stress-rate testing could be used as an alternative to life prediction test methodology for ceramic matrix composites, at least for a short range of lifetimes and when ultimate tensile strength is used as a failure criterion.

Markov Process Analysis for the Strength of Ceramic Matrix Composites Reinforced with Continuous Fibers

A stochastic model for predicting the strength and reliability of unidirectional fiber-reinforced ceramic matrix composites is proposed, in order to find theoretically statistical properties in strength of the composites, composed of constituents with large variations in strength. In the proposed model, mechanical behaviors of the composites follows the Curtin's assumptions, of which validity was examined by a FEM analysis. The proposed model is based on a Markov process, in which it is assumed that a damage state in the composite is developed with each fiber breakage. When the Weibull distribution is used as a strength distribution of the fiber, the probability of being in each state is analytically solved as a function of stress. The expected value and variance in the composite stress were then estimated from the probabilities of being in states. Furthermore, the maximum stress of the expected value, i.e. the strength, is predicted together with the coefficient of variation. The results showed that, even if broken fibers are imperfectly recovered in stress along the fiber-axis from the breakage points, the composite exhibits a higher strength and reliability than that of a dry bundle. Finally, it is concluded that stress recovery in broken fibers is a significant mechanism to determine the strength and reliability of the composites.

Influence of Temperature on Fatigue and Creep Strength of Ceramic Matrix Composites.

For studying the mechanical properties of composite material at high temperature, fatigue and creep tests were carried out for two types of ceramics matrix composites, that is, the reaction bonded and the precursor SiC/SiC composites. And then analysis of data regression was conducted for evaluating damage accumulation. As a result, following conclusions were obtained. 1) At room temperature, fatigue strength of precursor CMC is above twice higher than that of reaction bonded CMC, but at 1 300°C, fatigue strength of both composites is reversed in the range of longer life. 2) Fatigue and creep strength of reaction bonded CMC correspond each other if it is explained that fractures are caused by the crack extension process of time dependence except for flexure strength. 3) In precursor CMC, fatigue and creep strength for a short term coincide with flexure strength, which can be also explained by the crack extension process of time dependence. But high temperature strength for a long term rapidly decline because of internal oxidation.

Effect of quartz inclusions on plastic flow in marble

To investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks, we produced synthetic marbles with a grainsize of 5–25 µm, containing quartz of similar size, with volume fractions of 0, 5.1, and 20.4 vol.%, and between 3 to 9% porosity. These rocks were deformed in triaxial compression at a strain‐rate of 3 × 10−5 s−1 confining pressure of 200 MPa, and 600°C. Marbles containing 20.4 vol.% quartz were almost 5 times stronger than those containing no quartz. The work‐hardening rate at given strain increased with quartz content. Marbles containing 20.4 vol.% quartz work‐hardened up to 10% strain whereas in single‐phase samples work hardening was negligible beyond 3% strain. The dislocation density in the calcite increased with increasing quartz content. The data are consistent with models for the strength of ceramic matrix composites, or with the hypothesis that strength is increased by decreasing porosity.

Strength versus Gauge Length in Ceramic‐Matrix Composites

The strength of ceramic‐matrix composites as a function of sample gauge length is derived as a function of the composite constituent material properties. Within the context of a global load‐sharing assumption for how load is transferred from broken to unbroken fibers, the analysis shows that, for samples shorter than 0.8δc (where δc is the characteristic‐slip length determining composite pullout), the composite ultimate strength increases with decreasing gauge length. For samples longer than 0.8δc, the strength is independent of gauge length. Implications of these results on the performance of composites with small‐scale stress concentrators is briefly discussed.

Characterization of Interlaminar Shear Strength of Ceramic Matrix Composites

The interlaminar shear strengths of three ceramic matrix composites have been characterized using a double‐notch shear (DNS) test. The material systems investigated are plain woven C/SiC, plain woven SiC/SiC, and cross‐plied SiC/calcium aluminosilicate‐II. The use of the double‐notch shear test for measuring the interlaminar shear strength of ceramic matrix composites is evaluated first. Numerical stress analyses are performed to investigate the effect of DNS specimen length, notch distance, and specimen supporting jig on the stress distribution in the expected fracture plane and the interlaminar shear strength. The numerical findings are then compared with an analytical model proposed elsewhere and correlated with the experimental results. The validity of this test technique has been established.

Interface and strength in ceramic matrix composites

The interfacial behaviour of SiC fibre-feinforced glass matrix composites was analysed by single-fibre pull-out testing. By using inverse analysis based on acoustic emission, the discontinuous debonding between fibre and matrix was evaluated quantitatively. Interfacial mechanical parameters such as interfacial shear strength and partial debonding stress were successfully estimated and the increase in fracture resistance of the ceramic matrix composite with crack extension due to fibre bridging in a unidirectionally compact tension specimen was calculated. The results were in good agreement with experimental results.