ASTM C1899-21 - Standard Test Method for Flexural Strength of Continuous Fiber-Reinforced Advanced Ceramic Tubular Test Specimens at Ambient Temperature

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.

1.6 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:

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.