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Uniaxial Macro-Mechanical Property and Failure Analysis of a 2D-Woven SiC/SiC Composite
Abstract
The macro-mechanical behaviors of 2D-woven SiC/SiC composites produced with CVI technique were investigated by uniaxial tensile loading and unloading tests and compressive tests, and the changes in elastic modulus and residual strain were studied. Damage evolution and failure mode were analyzed by AE(Acoustic Emission) parameters and optical macroscope photos. Tensile test shows that the tensile stress-strain curve is obvious bilinear. Significant tensile damage begins at a specific tensile stress level of about 175 MPa. After the occurrence of significant damage, the unloading modulus decrease linearly as tensile stress increases, while the residual strains increase. The compressive stress-strain curve is almost linear until fracture, and compressive modulus slightly increase due to initial defects (pores and cracks) closure. Fracture surface shows that cracks in matrix can grow through fibers easily, and fibers suffer high extent of stress concentration during tensile test. The failure mode of compressive specimen is similar to crush which is correspond to high compressive strength. The main compressive damage mechanisms include cracks between bundles, within 90° bundles, and finally shear fracture occurs w'ithin 0° bundles.
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This article summarizes the current understanding of relationships between microstructure and mechanical properties in ceramics reinforced with aligned fibers. Emphasis is placed on definition of the micromechanical properties of the interface that govern the composite toughness. Issues such as the debond and sliding resistance of the interface are discussed based on micromechanics calculations and experiments conducted on both model composites and actual composites.RésuméCet article résume la compréhension courante des relations qui existent entre microstructure et propriétés mécaniques dans les céramiques renforcées par des fibres alignées. L'accent est mis sur la définition des propriétés mécaniques de l'interface qui régissent la résistance du composite. On discute les conséquences telles que la résistance à l'arrachement et au glissement à l'interface, en se basant sur des calculs de micromécanique et sur des expériences réalisées à la fois sur des composites modèles et sur des composites réels.ZusammenfassungDieser Artikel stellt den gegenwärtigen Stand im Verständnis des Zusammehanges zwischen Mikrostruktur und mechanischen Eigenschaften von Keramiken, die mit ausgerichteten Fasern verstärkt sind, zusammen. Besonderer Wert wird auf die Definition der mikromechanischen Eigenschaften der Grenzfläche gelegt, welche die Zähigkeit des Werkstoffes bestimmen. Bestimmte Fragenkreise, wie die Ablösung oder der Gleitwiderstand an der Grenzfläche, werden auf der Grundlage von mikromechanischen Rechnungen und von Experimenten, die an Modell- und echten Werkstoffen durchgeführt wurden, diskutiert.
Significantly improved fracture resistance (in terms of fracture toughness and fracture energy) can be imparted to monolithic ceramics by adopting composite design methodology based on fibre reinforcement technology. The present paper describes the fracture behaviour of one such fibre-reinforced material, namely the silica–silica based continuous fibre-reinforced, ceramic–matrix composite (CFCC) in two orthogonal notch orientations of crack divider and crack arrester orientations. Different fracture resistance parameters have been evaluated to provide a quantitative treatment of the observed fracture behaviour. From this study, it has been concluded that the overall fracture resistance of the CFCC is best reflected by total fracture energy release rate (Jc), which parameter encompasses most of the fracture events/processes. The Jc values of the composite are found to be more than an order of magnitude higher than the energy values corresponding to the plane strain fracture toughness (JKQ, derived from KIc, the plane strain fracture toughness) and >200% higher than elastic–plastic fracture toughness (JIc). Apart from this, the composite is found to exhibit high degree of anisotropy in the fracture resistance and also, a significant variation in the relative degree of shear component with crack extension.
composites have been elaborated with BN interphases of different thicknesses which were deposited on treated 1 Nicalon™ fibers by isothermal-isobaric chemical vapor infiltration from BCl3NH3H2 mixtures. Their mechanical behavior was investigated at 600 °C in air and compared to the results obtained on similar SiC/C/SiC composites. Although these materials exhibit similar stress and strain values at rupture when loaded at room temperature, whatever the interphase, their thermomechanical resistance depends on stress type, i.e. static or dynamic. Under static fatigue the BN interphases are more efficient than the pyrocarbon (PyC) ones. Thin BN interphases tend to maintain the interfacial properties. This result could be explained by the larger microcrack distances in the tows supporting the main part of the load, according to a lower interfacial sliding resistance. In contrast the materials with a PyC interphase, which have a much higher interfacial shear resistance at room temperature, exhibit better thermomechanical behavior under dynamic fatigue at 600 ° C. The mechanical characteristics are related to the evolution of the fibermatrix interfacial zone which has been studied by SEM, TEM and EELS.
2D woven Hi-Nicalon and Sylramic-iBN SiC fiber reinforced chemical vapor-infiltrated (CVI) SiC matrix composites were tested at room temperature with modal acoustic emission monitoring in order to determine relationships for stress-dependent matrix cracking. The Hi-Nicalon composites varied in the number of plies (1–36), specimen thickness, and constituent content. The Sylramic-iBN composites were fabricated with balanced and unbalanced 2D weaves in order to vary the fiber volume fraction in the orthogonal directions. Not surprisingly, matrix cracking stresses tended to be, but were not always, higher for composites with higher fiber volume fractions in the loading direction. It was demonstrated that simple relationships for stress-dependent matrix cracking could be related to the stress in the load-bearing CVI SiC matrix. For low-density composites, the 90° minicomposites do not share significant loads and matrix cracking was very similar to single tow minicomposites. For higher-density composites, where significant load is carried by the 0° minicomposites, matrix cracking was dependent on the unbridged “flaw” size, i.e., the 90° tow size or unbridged transverse crack size.
SiC-based ceramic matrix composites, consisting of carbon or SiC fibers embedded in a SiC-matrix, are tough ceramics when the fiber/matrix bonding is properly optimized through the use of a thin interphase. They are fabricated according to different processing routes (chemical vapor infiltration, polymer impregnation/pyrolysis, liquid silicon infiltration or slurry impregnation/hot pressing) each of them displaying advantages and drawbacks which are briefly discussed. SiC-matrix composites are highly tailorable materials in terms of fiber-type (carbon fibers of SiC-based fibers such as Si–C–O, SiC+C or quasi-stoichiometric SiC reinforcements), interphase (pyrocarbon or hexagonal BN, as well as (PyC–SiC)n or (BN–SiC)n multilayered interphases), matrix (simple SiC or matrices with improved oxidation resistance, such as self-healing matrices) and coatings (SiC or engineered multilayered coatings). The potential of SiC-matrix composites for application in advanced aerojet engines (after-burner hot section), gas turbine of electrical power/steam cogeneration (combustion chamber) and inner wall of the plasma chamber of nuclear fusion reaction, all of them corresponding to very severe conditions is discussed.
This work examines the mechanical behavior of a 2D woven, 0–90 SiC fiber-reinforced SiC matrix composite. Tensile experiments show that the short-term behavior is largely independent of test temperature below 1000 °C. Microscopic examination reveals that the extent of fiber pull-out and the integrity of the remaining material are also independent of temperature in this range. Conversely, at 1200 °C, the material retains much of its low-temperature stiffness and proportional limit, while the strength increases substantially. Micrographs of these specimens reveal little individual fiber pull-out and a higher density of matrix microcracks. Room-temperature tensile data show that the mechanical behavior is rate-dependent; higher strain rates lead to a lower Young's Modulus, higher proportional limit and higher ultimate strength. In-plane shear experiments demonstrate that the unreinforced matrix strength is approximately 10% of the composite tensile strength.
The use of fiber, interphase, CVI SiC minicomposites as structural elements for 2D-woven SiC fiber-reinforced chemically vapor infiltrated (CVI) SiC matrix composites is demonstrated to be a viable approach to model the elastic modulus of these composite systems when tensile loaded in an orthogonal direction. The 0° (loading direction) and 90° (perpendicular to loading direction) oriented minicomposites as well as the open porosity and excess SiC associated with CVI SiC composites were all modeled as parallel elements using simple Rule of Mixtures techniques. Excellent agreement for a variety of 2D woven Hi-Nicalon™ fiber-reinforced and Sylramic-iBN reinforced CVI SiC matrix composites that differed in numbers of plies, constituent content, thickness, density, and number of woven tows in either direction (i.e, balanced weaves versus unbalanced weaves) was achieved. It was found that elastic modulus was not only dependent on constituent content, but also the degree to which 90° minicomposites carried load. This depended on the degree of interaction between 90° and 0° minicomposites which was quantified to some extent by composite density. The relationships developed here for elastic modulus only necessitated the knowledge of the fractional contents of fiber, interphase and CVI SiC as well as the tow size and shape. It was concluded that such relationships are fairly robust for orthogonally loaded 2D woven CVI SiC composite system and can be implemented by ceramic matrix composite component modelers and designers for modeling the local stiffness in simple or complex parts fabricated with variable constituent contents.
Structural investigation of SiCf/SiC composites[J]
- Tatlisu H.