Significance and Use
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.
Scope
1.1 This test method covers the determination of flexural strength, including stress-strain response, under monotonic loading of continuous fiber-reinforced advanced ceramic tubes at ambient temperature. This test method addresses tubular test specimen geometries, test specimen/grip fabrication methods, testing modes (force, displacement, or strain-control), testing rates (force rate, stress rate, displacement rate, or strain rate), and data collection and reporting procedures.
1.2 In this test method, an advanced ceramic composite tube/cylinder with a defined gage section and a known wall thickness is subjected to four-point flexure while supported in a four-point loading system utilizing two force-application points spaced an inner span distance that are centered between two support points located an outer span distance apart. The applied transverse force produces a constant moment in the gage section of the tube and results in uniaxial flexural stress-strain response of the composite tube that is recorded until failure of the tube. The flexural strength and the flexural fracture strength are determined from the resulting maximum force and the force at fracture, respectively. The flexural strains, the flexural proportional limit stress, and the flexural modulus of elasticity in the longitudinal direction are determined from the stress-strain data. Note that flexural strength as used in this test method refers to the maximum tensile stress produced in the longitudinal direction of the tube by the introduction of a monotonically applied transverse force, where ‘monotonic’ refers to a continuous, nonstop test rate without reversals from test initiation to final fracture. The flexural strength is sometimes used to estimate the tensile strength of the material.
1.3 This test method is intended for advanced ceramic matrix composite tubes with continuous fiber reinforcement: unidirectional (1D, filament wound and tape lay-up), bidirectional (2D, fabric/tape lay-up and weave), and tridirectional (3D, braid and weave). These types of ceramic matrix composites can be composed of a wide range of ceramic fibers (oxide, graphite, carbide, nitride, and other compositions) in a wide range of crystalline and amorphous ceramic matrix compositions (oxide, carbide, nitride, carbon, graphite, and other compositions). This test method may also be applicable to some types of functionally graded tubes such as ceramic fiber-wound tubes comprised of monolithic advanced ceramics. It is not the intent of this test method to dictate or normalize material fabrication including fiber layup or number of plies comprising the composite, but to instead provide an appropriate and consistent methodology for discerning the effects of different fabrication or fiber layup methods on flexural behavior of resulting tubular geometries.
1.4 This test method does not directly address discontinuous fiber-reinforced, whisker-reinforced, or particulate-reinforced ceramics, although the test methods detailed here may be equally applicable to these composites if it can be shown that these materials display the damage-tolerant behavior of continuous fiber-reinforced ceramics.
1.5 The test method is applicable to a range of test specimen tube geometries based on the intended application that includes composite material property and tube radius. Therefore, there is no “standard” test specimen geometry for a typical test setup. Lengths of the composite tube, lengths of the inner span, and lengths of the outer span are determined so as to provide a gage length with uniform bending moment. A wide range of combinations of material properties, tube radii, wall thicknesses, tube lengths, and lengths of inner and outer spans section are possible.
1.5.1 This test method is specific to ambient temperature testing. Elevated temperature testing requires high-temperature furnaces and heating devices with temperature control and measurement systems and temperature-capable testing methods that are not addressed in this test method.
1.6 This test method addresses tubular test specimen geometries, test specimen preparation methods, testing rates (that is, induced applied moment rate), and data collection and reporting procedures in the following sections:
| Scope | Section 1 |
| Referenced Documents | Section 2 |
| Terminology | Section 3 |
| Summary of Test Method | Section 4 |
| Significance and Use | Section 5 |
| Interferences | Section 6 |
| Apparatus | Section 7 |
| Hazards | Section 8 |
| Test Specimens | Section 9 |
| Test Procedure | Section 10 |
| Calculation of Results | Section 11 |
| Report | Section 12 |
| Precision and Bias | Section 13 |
| Keywords | Section 14 |
| Appendixes |
|
| Overview of Flexural Test Configurations | Appendix X1 |
| Fixtures with Cradles | Appendix X2 |
1.7 Values expressed in this test method are in accordance with the International System of Units (SI) and IEEE/ASTM SI 10.
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. Specific hazard statements are given in Section 8.
1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
