Significance and Use
4.1 This test method may be used for material development, quality control, characterization, and design data generation purposes. This test method is intended to be used with ceramics
whose strength is 50 MPa (~7 ksi) or greater.
4.2 The flexure stress is computed based on simple beam theory with assumptions that the material is isotropic and homogeneous, the moduli of elasticity in tension and compression are
identical, and the material is linearly elastic. The average grain size should be no greater than one fiftieth of the beam thickness. The homogeneity and isotropy assumption in the standard
rule out the use of this test for continuous fiber-reinforced ceramics.
4.3 Flexural strength of a group of test specimens is influenced by several parameters associated with the test procedure. Such factors include the loading rate, test environment, specimen
size, specimen preparation, and test fixtures. Specimen sizes and fixtures were chosen to provide a balance between practical configurations and resulting errors, as discussed in
MIL-STD?1942?(MR) and Refs (1) and (2) .
Specific fixture and specimen configurations were designated in order to permit ready comparison of data without the need for Weibull-size scaling.
4.4 The flexural strength of a ceramic material is dependent on both its inherent resistance to fracture and the size and severity of flaws. Variations in these cause a natural scatter in
test results for a sample of test specimens. Fractographic analysis of fracture surfaces, although beyond the scope of this standard, is highly recommended for all purposes, especially if
the data will be used for design as discussed in MIL-STD-1942 (MR) and Refs (25) and Practices C1322 and C1239.
4.5 The three-point test configuration exposes only a very small portion of the specimen to the maximum stress. Therefore, three-point flexural strengths are likely to be much greater than
four-point flexural strengths. Three-point flexure has some advantages. It uses simpler test fixtures, it is easier to adapt to high temperature and fracture toughness testing, and it is
sometimes helpful in Weibull statistical studies. However, four-point flexure is preferred and recommended for most characterization purposes.
4.6 This method determines the flexural strength at ambient temperature and environmental conditions. The flexural strength under ambient conditions may or may not necessarily be the inert
flexural strength.
Note 7 time dependent effects may be minimized through the use of inert testing atmosphere such as dry nitrogen
gas, oil, or vacuum. Alternatively, testing rates faster than specified in this standard may be used. Oxide ceramics, glasses, and ceramics containing boundary phase glass are susceptible
to slow crack growth even at room temperature. Water, either in the form of liquid or as humidity in air, can have a significant effect, even at the rates specified in this standard. On the
other hand, many ceramics such as boron carbide, silicon carbide, aluminum nitride and many silicon nitrides have no sensitivity to slow crack growth at room temperature and the flexural
strength in laboratory ambient conditions is the inert flexural strength.
1. Scope
1.1 This test method covers the determination of flexural strength of advanced ceramic materials at ambient temperature. Four-point 1 / 4 point and three-point
loadings with prescribed spans are the standard as shown in Fig. 1 . Rectangular specimens of prescribed cross-section sizes are used with specified
features in prescribed specimen-fixture combinations. Test specimens may be 3 by 4 by 45 to 50 mm in size that are tested on 40 mm outer span four-point or three-point fixtures.
Alternatively, test specimens and fixture spans half or twice these sizes may be used. The method permits testing of machined or as-fired test specimens. Several options for machining
preparation are included: application matched machining, customary procedure, or a specified standard procedure. This method describes the apparatus, specimen requirements, test procedure,
calculations, and reporting requirements. The test method is applicable to monolithic or particulate- or whisker-reinforced ceramics. It may also be used for glasses. It is not applicable
to continuous fiber-reinforced ceramic composites.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 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 and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
ASTM Standards
C1239 Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
C1322 Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
C1368 Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Strength Testing at Ambient Temperature
E4 Practices for Force Verification of Testing Machines
E337 Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
Military Standard
MIL-STD-1942 (MR) Flexural Strength of High Performance Ceramics at Ambient Temperature Available from Standardization Documents Order Desk, DODSSP, Bldg. 4, Section D, 700 Robbins Ave.,
Philadelphia, PA 19111-5098, http://www.dodssp.daps.mil.
Keywords
advanced ceramics; flexural strength; four-point flexure; three-point flexure ;
ICS Code
ICS Number Code 81.060.20 (Ceramic products)
DOI: 10.1520/C1161
ASTM International is a member of CrossRef.
ASTM C1161