Standard Test Method for Flexural Strength of Continuous Fiber-Reinforced Advanced Ceramic Tubular Test Specimens at Ambient Temperature

Importancia y uso:

5.1 This test method may be used for material development, material comparison, quality assurance, characterization, and design data generation.

5.2 Continuous fiber-reinforced ceramic composites (CFCCs) may be composed of continuous ceramic-fiber directional (1D, 2D, and 3D) reinforcements which are often contained in a fine-grain-sized (<50 µm) ceramic matrix with controlled porosity. Usually these composites have an engineered thin (0.1 to 10 µm) interface coating on the fibers to produce crack deflection and fiber pull-out.

5.3 CFCC components have distinctive and synergistic combinations of material properties, interface coatings, porosity control, composite architecture (1D, 2D, and 3D), and geometric shape that are generally inseparable. Prediction of the mechanical performance of CFCC tubes (particularly with braid and 3D weave architectures) may not be possible by applying measured properties from flat CFCC plates to the design of tubes. This is because fabrication/processing methods may be unique to tubes and not replicable to flat plates, thereby producing compositionally similar but structurally and morphologically different CFCC materials. In particular, tubular components comprised of CFCC material form a unique synergistic combination of material, geometric shape, and reinforcement architecture that is generally inseparable. In other words, prediction of mechanical performance of CFCC tubes generally cannot be made by using properties measured from flat plates. Strength tests of transversely loaded CFCC tubes provide information on mechanical behavior and strength for a material subjected to a uniaxial, nonuniform stress.

5.4 Unlike monolithic advanced ceramics that fracture catastrophically from a single dominant flaw, CMCs generally experience “graceful” fracture from a cumulative damage process. Therefore, while the volume of material subjected to a nonuniform, uniaxial flexural stress for transversely loaded tube test may be a significant factor for determining matrix cracking stress, this same volume may not be as significant a factor in determining the ultimate strength of a CMC. However, the probabilistic nature of the strength distributions of the brittle matrices of CMCs requires a statistically significant number of test specimens for statistical analysis and design. Studies to determine the exact influence of test specimen volume on strength distributions for CMCs have not been completed. It should be noted that tensile flexural strengths obtained using different recommended test specimens with different volumes of material in the gage sections may be different due to these volume effects. Practice C1683 provides guidance on the scaling of statistical parameters for strength to account for differences in effective volume, effective area, or both.

5.5 Flexural strength tests provide information on the strength and deformation of materials under stresses induced from transverse loading of tubes. Nonuniform but uniaxial stress states are inherent in these types of tests, and subsequent evaluation of any nonlinear stress-strain behavior must take into account the asymmetric and anisotropic behavior of the CMC under multiaxial stressing. This nonlinear behavior may develop as the result of cumulative damage processes (for example, matrix cracking, matrix/fiber debonding, fiber fracture, delamination, etc.) which may be influenced by testing mode, testing rate, processing effects, or environmental effects. Some of these effects may be consequences of stress corrosion or subcritical (slow) crack growth that can be minimized by testing at sufficiently rapid rates as outlined in this test method.

5.6 The results of flexural strength tests of test specimens fabricated to standardized dimensions from a particular material or selected portions of a part, or both, may not totally represent the strength and deformation properties of the entire, full-size end product or its in-service behavior in different environments.

5.7 For quality control purposes, results derived from standardized flexural strength test specimens may be considered indicative of the response of the material from which they were taken for, given primary processing conditions and post-processing heat treatments.

5.8 The flexural behavior and flexural strength of a CMC are dependent on its inherent resistance to fracture, the presence of flaws, damage accumulation processes, or combinations thereof. Analyses of fracture surfaces and fractography, though beyond the scope of this test method, are highly recommended.

Subcomité:

C28.07

Volúmen:

15.01

Número ICS:

81.060.30 (Advanced ceramics)

Palabras clave:

ceramic matrix composite; CMC; continuous fiber composite; flexural strength; tubes;