ASTM C1161-18(2023)

Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature

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, 2).4 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 (2-5) 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 sim...
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 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.

General Information

• Status: Published
• Publication Date: 31-Dec-2022
• ICS: 81.060.20 - Ceramic products
• Technical Committee: C28 - Advanced Ceramics
• Drafting Committee: C28.01 - Mechanical Properties and Performance

Relations

• Refers: ASTM C1322-15(2019) - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jul-2019
• Refers: ASTM C1239-13(2018) - Standard Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
• Effective Date: 01-Jul-2018
• Refers: ASTM C1368-18 - Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress Rate Strength Testing at Ambient Temperature
• Effective Date: 01-Jan-2018
• Refers: ASTM C1368-10(2017) - Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Strength Testing at Ambient Temperature
• Effective Date: 01-Feb-2017
• Refers: ASTM C1322-15 - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 01-Jul-2015
• Refers: ASTM E4-14 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Jun-2014
• Refers: ASTM C1239-13 - Standard Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
• Effective Date: 01-Aug-2013
• Refers: ASTM C1368-10 - Standard Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress-Rate Strength Testing at Ambient Temperature
• Effective Date: 01-Dec-2010
• Refers: ASTM C1322-05b(2010) - Standard Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• Effective Date: 15-Jul-2010
• Refers: ASTM E4-10 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Jun-2010
• Refers: ASTM E4-09a - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Nov-2009
• Refers: ASTM E4-09 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Apr-2009
• Refers: ASTM E4-08 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Dec-2008
• Refers: ASTM E337-02(2007) - Standard Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
• Effective Date: 01-Oct-2007
• Refers: ASTM C1239-07 - Standard Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
• Effective Date: 01-Feb-2007

Overview

ASTM C1161-18(2023) is the authoritative Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature. Published by ASTM International, this standard specifies procedures for determining the flexural strength of monolithic and reinforced advanced ceramics under ambient (room) conditions. The methodology addresses applications requiring high-strength materials, with particular focus on ceramics exhibiting strengths of 50 MPa (approximately 7 ksi) or greater.

Using prescribed three-point and four-point (1/4-point) loading techniques, the test provides vital data for material development, quality control, and mechanical performance characterization. Its robust approach considers specimen preparation, fixture configuration, and environmental influences, making it critical for laboratories and manufacturing sectors working with advanced ceramics and similar brittle materials.

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Key Topics

• Applicability: Designed for monolithic, particulate- or whisker-reinforced ceramics and glasses. Excludes continuous fiber-reinforced composites.
• Load Configurations: Details both three-point and four-point flexural test setups using rectangular specimens of specific cross-sections and span lengths.
• Material Properties: Emphasizes use with ceramics that have substantial inherent strength; assumes isotropy, homogeneity, and linear elasticity of test materials.
• Test Parameters: Specifies influences such as loading rate, environmental factors, specimen size, preparation methods, and fixture precision. These variables can affect the accuracy and comparability of results.
• Specimen Machining: Offers several machine preparation options-application-matched, customary, or standardized. Highlights the impact of surface condition on measured strength.
• Statistical Considerations: Requires appropriate sample sizes (minimum 10, preferably 30+) to support valid statistical analysis, especially for Weibull strength distribution estimation.
• Data Reporting: Outlines comprehensive documentation requirements including test configuration, material source, specimen preparation, environment, and complete strength data.

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Applications

The ASTM C1161-18(2023) flexural strength test has critical applications in industries such as:

• Quality Assurance: Enables consistent verification of batch-to-batch material properties for manufacturing control.
• Material Development: Supplies essential comparative data during new ceramic formulation and composite engineering.
• Product Design: Supports generation of robust mechanical property datasets for engineering design, modeling, and simulation of ceramic components.
• Failure Analysis: Fracture origins, though analysis is outside the direct scope, are often traced using this test in conjunction with fractographic examination.
• R&D: Forms a baseline evaluation tool in research environments exploring advanced ceramics, including whisker and particulate-reinforced matrices.
• Certification and Specification Compliance: Used for compliance with customer requirements and regulatory standards where mechanical strength of ceramics must be demonstrated.

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Related Standards

Key documents related to or referenced by ASTM C1161-18(2023) include:

• ASTM C1239 - Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
• ASTM C1322 - Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics
• ASTM C1368 - Test Method for Determination of Slow Crack Growth Parameters of Advanced Ceramics by Constant Stress Rate Strength Testing
• MIL-STD-1942(MR) - Flexural Strength of High Performance Ceramics at Ambient Temperature
• ASTM E4 - Practices for Force Calibration and Verification of Testing Machines
• ASTM E337 - Test Method for Measuring Humidity with a Psychrometer

Laboratories and engineers working with high-performance ceramic materials should ensure they are familiar with both ASTM C1161 and these related standards for accurate material properties assessment and reporting.

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Keywords: ASTM C1161-18(2023), flexural strength, advanced ceramics, standard test method, quality control, mechanical properties, three-point flexure, four-point flexure, ceramic testing, material development, Weibull statistics, specimen preparation.

Standards Content (Sample)

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.
Designation: C1161 − 18 (Reapproved 2023)
Standard Test Method for
Flexural Strength of Advanced Ceramics at Ambient
Temperature
This standard is issued under the fixed designation C1161; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope 2. Referenced Documents
1.1 This test method covers the determination of flexural 2.1 ASTM Standards:
strength of advanced ceramic materials at ambient temperature. C1239 Practice for Reporting Uniaxial Strength Data and
Four-point- ⁄4-point and three-point loadings with prescribed Estimating Weibull Distribution Parameters for Advanced
spans are the standard as shown in Fig. 1. Rectangular Ceramics
specimens of prescribed cross-section sizes are used with C1322 Practice for Fractography and Characterization of
specified features in prescribed specimen-fixture combinations. Fracture Origins in Advanced Ceramics
Test specimens may be 3 by 4 by 45 to 50 mm in size that are C1368 Test Method for Determination of Slow Crack
tested on 40-mm outer span four-point or three-point fixtures. Growth Parameters of Advanced Ceramics by Constant
Alternatively, test specimens and fixture spans half or twice Stress Rate Strength Testing at Ambient Temperature
these sizes may be used. The method permits testing of E4 Practices for Force Calibration and Verification of Test-
machined or as-fired test specimens. Several options for ing Machines
machining preparation are included: application matched E337 Test Method for Measuring Humidity with a Psy-
machining, customary procedure, or a specified standard pro- chrometer (the Measurement of Wet- and Dry-Bulb Tem-
cedure. This method describes the apparatus, specimen peratures)
requirements, test procedure, calculations, and reporting re- 2.2 Military Standard:
quirements. The test method is applicable to monolithic or MIL-STD-1942(MR) Flexural Strength of High Perfor-
particulate- or whisker-reinforced ceramics. It may also be mance Ceramics at Ambient Temperature
used for glasses. It is not applicable to continuous fiber-
3. Terminology
reinforced ceramic composites.
3.1 Definitions:
1.2 The values stated in SI units are to be regarded as the
3.1.1 complete gage section, n—the portion of the specimen
standard. The values given in parentheses are for information
between the two outer bearings in four-point flexure and
only.
three-point flexure fixtures.
1.3 This standard does not purport to address all of the
NOTE 1—In this standard, the complete four-point flexure gage section
safety concerns, if any, associated with its use. It is the
is twice the size of the inner gage section. Weibull statistical analysis only
responsibility of the user of this standard to establish appro-
includes portions of the specimen volume or surface which experience
priate safety, health, and environmental practices and deter-
tensile stresses.
mine the applicability of regulatory limitations prior to use.
–2
3.1.2 flexural strength, [FL ], n—a measure of the ultimate
1.4 This international standard was developed in accor-
strength of a specified beam in bending.
dance with internationally recognized principles on standard-
3.1.3 four-point- ⁄4-point flexure, n—configuration of flex-
ization established in the Decision on Principles for the
ural strength testing where a specimen is symmetrically loaded
Development of International Standards, Guides and Recom-
at two locations that are situated one-quarter of the overall span
mendations issued by the World Trade Organization Technical
away from the outer two support bearings (see Fig. 1).
Barriers to Trade (TBT) Committee.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
This test method is under the jurisdiction of ASTM Committee C28 on contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on Standards volume information, refer to the standard’s Document Summary page on
Mechanical Properties and Performance. the ASTM website.
Current edition approved Jan. 1, 2023. Published February 2023. Originally Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,
approved in 1990. Last previous edition approved in 2018 as C1161 – 18. DOI: Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http://
10.1520/C1161-18R23. www.dodssp.daps.mil.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1161 − 18 (2023)
surfaces. In addition, the upper or lower pairs are free to pivot
to distribute force evenly to the bearing cylinders on either
side.
NOTE 5—See Annex A1 for schematic illustrations of the required
pivoting movements.
NOTE 6—A three-point fixture has the inner pair of bearing cylinders
replaced by a single bearing cylinder.
3.1.9 slow crack growth (SCG), n—subcritical crack growth
(extension) which may result from, but is not restricted to, such
mechanisms as environmentally assisted stress corrosion or
diffusive crack growth.
3.1.10 three-point flexure, n—configuration of flexural
strength testing where a specimen is loaded at a location
midway between two support bearings (see Fig. 1).
4. 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.
NOTE 1—Configuration:
4.2 The flexure stress is computed based on simple beam
A: L = 20 mm
theory with assumptions that the material is isotropic and
B: L = 40 mm
homogeneous, the moduli of elasticity in tension and compres-
C: L = 80 mm
sion are identical, and the material is linearly elastic. The
FIG. 1 The Four-Point- ⁄4-Point and Three-Point Fixture Configura-
tion
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
3.1.4 fully articulating fixture, n—a flexure fixture designed
fiber-reinforced ceramics.
to be used either with flat and parallel specimens or with
4.3 Flexural strength of a group of test specimens is
uneven or nonparallel specimens. The fixture allows full
influenced by several parameters associated with the test
independent articulation, or pivoting, of all rollers about the
procedure. Such factors include the loading rate, test
specimen long axis to match the specimen surface. In addition,
environment, specimen size, specimen preparation, and test
the upper or lower pairs are free to pivot to distribute force
fixtures. Specimen sizes and fixtures were chosen to provide a
evenly to the bearing cylinders on either side.
balance between practical configurations and resulting errors,
NOTE 2—See Annex A1 for schematic illustrations of the required
as discussed in MIL-STD-1942(MR) and Refs (1, 2). Specific
pivoting movements.
fixture and specimen configurations were designated in order to
NOTE 3—A three-point fixture has the inner pair of bearing cylinders
permit ready comparison of data without the need for Weibull-
replaced by a single bearing cylinder.
size scaling.
–2
3.1.5 inert flexural strength, [FL ], n—a measure of the
4.4 The flexural strength of a ceramic material is dependent
strength of specified beam in bending as determined in an
on both its inherent resistance to fracture and the size and
appropriate inert condition whereby no slow crack growth
severity of flaws. Variations in these cause a natural scatter in
occurs.
test results for a sample of test specimens. Fractographic
NOTE 4—An inert condition may be obtained by using vacuum, low
analysis of fracture surfaces, although beyond the scope of this
temperatures, very fast test rates, or any inert media.
standard, is highly recommended for all purposes, especially if
–2
3.1.6 inherent flexural strength, [FL ], n—the flexural
the data will be used for design as discussed in MIL-STD-
strength of a material in the absence of any effect of surface
1942(MR) and Refs (2-5) and Practices C1322 and C1239.
grinding or other surface finishing process, or of extraneous
4.5 The three-point test configuration exposes only a very
damage that may be present. The measured inherent strength is
small portion of the specimen to the maximum stress.
in general a function of the flexure test method, test conditions,
Therefore, three-point flexural strengths are likely to be much
and test specimen size.
greater than four-point flexural strengths. Three-point flexure
3.1.7 inner gage section, n—the portion of the specimen
has some advantages. It uses simpler test fixtures, it is easier to
between the inner two bearings in a four-point flexure fixture.
adapt to high temperature and fracture toughness testing, and it
3.1.8 semi-articulating fixture, n—a flexure fixture designed
is sometimes helpful in Weibull statistical studies. However,
to be used with flat and parallel specimens. The fixture allows
some articulation, or pivoting, to ensure the top pair (or bottom
pair) of bearing cylinders pivot together about an axis parallel
The boldface numbers in parentheses refer to the references at the end of this
to the specimen long axis, in order to match the specimen test method.
C1161 − 18 (2023)
four-point flexure is preferred and recommended for most from biaxial disk or plate strength tests, wherein machining
characterization purposes. direction cannot be aligned.
4.6 This method determines the flexural strength at ambient
6. Apparatus
temperature and environmental conditions. The flexural
6.1 Loading—Specimens may be loaded in any suitable
strength under ambient conditions may or may not necessarily
testing machine provided that uniform rates of direct loading
be the inert flexural strength.
can be maintained. The force measuring system shall be free of
NOTE 7—time dependent effects may be minimized through the use of
initial lag at the loading rates used and shall be equipped with
inert testing atmosphere such as dry nitrogen gas, oil, or vacuum.
a means for retaining read-out of the maximum force applied to
Alternatively, testing rates faster than specified in this standard may be
the specimen. The accuracy of the testing machine shall be in
used. Oxide ceramics, glasses, and ceramics containing boundary phase
glass are susceptible to slow crack growth even at room temperature. accordance with Practices E4 but within 0.5 %.
Water, either in the form of liquid or as humidity in air, can have a
6.2 Four-Point Flexure—Four-point- ⁄4-point fixtures (Fig.
significant effect, even at the rates specified in this standard. On the other
1) shall have support and loading spans as shown in Table 1.
hand, many ceramics such as boron carbide, silicon carbide, aluminum
nitride, and many silicon nitrides have no sensitivity to slow crack growth
6.3 Three-Point Flexure—Three-point fixtures (Fig. 1) shall
at room temperature and the flexural strength in laboratory ambient
have a support span as shown in Table 1.
conditions is the inert flexural strength.
6.4 Bearings—Three- and four-point flexure:
5. Interferences
6.4.1 Cylindrical bearing edges shall be used for the support
of the test specimen and for the application of load. The
5.1 The effects of time-dependent phenomena, such as stress
corrosion or slow crack growth on strength tests conducted at cylinders shall be made of hardened steel which has a hardness
no less than HRC 40 or which has a yield strength no less than
ambient temperature, can be meaningful even for the relatively
short times involved during testing. Such influences must be 1240 MPa (;180 ksi). Alternatively, the cylinders may be
considered if flexure tests are to be used to generate design made of a ceramic with an elastic modulus between 2.0 and 4.0
5 6
data. Slow crack growth can lead a rate dependency of flexural × 10 MPa (30 to 60 × 10 psi) and a flexural strength no less
strength. The testing rate specified in this standard may or may than 275 MPa (;40 ksi). The portions of the test fixture that
not produce the inert flexural strength whereby negligible slow support the bearings may need to be hardened to prevent
crack growth occurs. See Test Method C1368. permanent deformation. The cylindrical bearing length shall be
at least three times the specimen width. The above require-
5.2 Surface preparation of test specimens can introduce
ments are intended to ensure that ceramics with strengths up to
machining microcracks which may have a pronounced effect
1400 MPa (;200 ksi) and elastic moduli as high as 4.8 ×
on flexural strength. Machining damage imposed during speci-
5 6
10 MPa (70 × 10 psi) can be tested without fixture damage.
men preparation can be either a random interfering factor, or an
Higher strength and stiffer ceramic specimens may require
inherent part of the strength characteristic to be measured. With
harder bearings.
proper care and good machining practice, it is possible to
6.4.2 The bearing cylinder diameter shall be approximately
obtain fractures from the material’s natural flaws. Surface
1.5 times the beam depth of the test specimen size employed.
preparation can also lead to residual stresses. Universal or
See Table 2.
standardized test methods of surface preparation do not exist. It
6.4.3 The bearing cylinders shall be carefully positioned
should be understood that final machining steps may or may
such that the spans are accurate within 60.10 mm. The load
not negate machining damage introduced during the early
application bearing for the three-point configurations shall be
course or intermediate machining.
positioned midway between the support bearing within
5.3 This test method allows several options for the machin-
60.10 mm. The load application (inner) bearings for the
ing of specimens, and includes a general procedure (“Stan-
four-point configurations shall be centered with respect to the
dard” procedure, 7.2.4), which is satisfactory for many (but
support (outer) bearings within 60.10 mm.
certainly not all) ceramics. The general procedure used pro-
6.4.4 The bearing cylinders shall be free to rotate in order to
gressively finer longitudinal grinding steps that are designed to
relieve frictional constraints (with the exception of the middle-
minimize subsurface microcracking. Longitudinal grinding
load bearing in three-point flexure which need not rotate). This
aligns the most severe subsurface microcracks parallel to the
can be accomplished by mounting the cylinders in needle
specimen tension stress axis. This allows a greater opportunity
bearing assemblies, or more simply by mounting the cylinders
to measure the inherent flexural strength or “potential strength”
as shown in Figs. 2 and 3. The cylinders are held in place by
of the material as controlled by the material’s natural flaws. In
low-stiffness springs, rubber bands, or magnets. (If these
contrast, transverse grinding aligns the severest subsurface
fixtures are used for some fracture toughness tests, whereby
machining microcracks perpendicular to the tension stress axis
very low forces are used to break pre-cracked test specimens,
and the specimen is more likely to fracture from the machining
microcracks. Transverse-ground specimens in many instances
TABLE 1 Fixture Spans
may provide a more “practical strength” that is relevant to
Configuration Support Span (L), mm Loading Span, mm
machined ceramic components whereby it may not be possible
A 20 10
to favorably align the machining direction. Transverse-ground
B 40 20
specimens may be tested in accordance with 7.2.2. Data from
C 80 40
transverse-ground specimens may correlate better with data
C1161 − 18 (2023)
TABLE 2 Nominal Bearing Diameters
6.10 Micrometer—A micrometer with a resolution of
Configuration Diameter, mm 0.002 mm (or 0.0001 in.) or smaller should be used to measure
A 2.0 to 2.5 the test specimen dimensions. The micrometer shall have flat
B 4.5
anvil faces. The micrometer shall not have a ball tip or sharp tip
C 9.0
since these might damage the test specimen if the specimen
dimensions are measured prior to fracture. Alternative dimen-
sion measuring instruments may be used provided that they
have a resolution of 0.002 mm (or 0.0001 in.) or finer and do
then very low-stiffness rubber bands or springs should be
no harm to the specimen.
used.) Annex A1 illustrates the action required of the bearing
cylinders. Note that the outer support bearings roll outward and
7. Specimen
the inner loading bearings roll inward.
7.1 Specimen Size—Dimensions are given in Table 3 and
6.5 Semi-Articulating Four-Point Fixture—Specimens pre-
shown in Fig. 4. Cross-sectional dimensional tolerances are
pared in accordance with the parallelism requirements of 7.1
60.13 mm for B and C specimens, and 60.05 mm for A. The
may be tested in a semi-articulating fixture as illustrated in Fig.
parallelism tolerances on the four longitudinal faces are
2 and in Fig. A1.1a. All four bearings shall be free to roll. The
0.015 mm for A and B and 0.03 mm for C. The two end faces
two inner bearings shall be parallel to each other to within
need not be precision machined.
0.015 mm over their length and they shall articulate together as
a pair. The two outer bearings shall be parallel to each other to
7.2 Specimen Preparation—Depending upon the intended
within 0.015 mm over their length and they shall articulate
application of the flexural strength data, use one of the
together as a pair. The inner bearings shall be supported
following four specimen preparation procedures:
independently of the outer bearings. All four bearings shall rest
NOTE 8—This test method does not specify a test specimen surface
uniformly and evenly across the specimen surfaces. The fixture
finish. Surface finish may be misleading since a ground, lapped, or even
shall be designed to apply equal load to all four bearings.
polished surface may conceal hidden, beneath-the-surface cracking dam-
age from rough or intermediate grinding.
6.6 Fully Articulating Four-Point Fixture—Specimens that
are as-fired, heat treated, or oxidized often have slight twists or 7.2.1 As-Fabricated—The flexural specimen shall simulate
unevenness. Specimens which do not meet the parallelism
the surface condition of an application where no machining is
requirements of 7.1 shall be tested in a fully articulating fixture to be used; for example, as-cast, sintered, or injection-molded
as illustrated in Fig. 3 and in Fig. A1.1b. Well-machined
parts. No additional machining specifications are relevant. An
specimens may also be tested in fully articulating fixtures. All edge chamfer is not necessary in this instance. As-fired
four bearings shall be free to roll. One bearing need not
specimens are especially prone to twist or warpage and might
articulate. The other three bearings shall articulate to match the not meet the parallelism requirements. In this instance, a fully
specimen’s surface. All four bearings shall rest uniformly and
articulating fixture (6.6 and Fig. 3) shall be used in testing.
evenly across the specimen surfaces. The fixture shall apply 7.2.2 Application-Matched Machining—The specimen shall
equal load to all four bearings.
have the same surface preparation as that given to a compo-
nent. Unless the process is proprietary, the report shall be
6.7 Semi-Articulated Three-Point Fixture—Specimens pre-
specific about the stages of material removal, wheel grits,
pared in accordance with the parallelism requirements of 7.1
wheel bonding, and the amount of material removed per pass.
may be tested in a semi-articulating fixture. The middle bearing
7.2.3 Customary Procedures—In instances where a custom-
shall be fixed and not free to roll. The two outer bearings shall
ary machining procedure has been developed that is completely
be parallel to each other to within 0.015 mm over their length.
satisfactory for a class of materials (that is, it induces no
The two outer bearings shall articulate together as a pair to
unwanted surface damage or residual stresses), this procedure
match the specimen surface, or the middle bearing shall
shall be used.
articulate to match the specimen surface. All three bearings
7.2.4 Standard Procedures—In the instances where 7.2.1 –
shall rest uniformly and evenly across the specimen surface.
7.2.3 are not appropriate, then 7.2.4 shall apply. This procedure
The fixture shall be designed to apply equal load to the two
shall serve as minimum requirements and a more stringent
outer bearings.
procedure may be necessary.
6.8 Fully Articulated Three-Point Flexure—Specimens that
7.2.4.1 All grinding shall be done with an ample supply of
do not meet the parallelism requirements of 7.1 shall be tested
appropriate filtered coolant to keep workpiece and wheel
in a fully articulating fixture. Well-machined specimens may
constantly flooded and particles flushed. Grinding shall be in
also be tested in a fully articulating fixture. The two support
two or three stages, ranging from coarse to fine rates of
(outer) bearings shall be free to roll outwards. The middle
material removal. All machining shall be in the surface
bearing shall not roll. Any two of the bearings shall be capable
grinding mode, and shall be parallel to the specimen long axis
of articulating to match the specimen surface. All three
shown in Fig. 5. No Blanchard or rotary grinding shall be used.
bearings shall rest uniformly and evenly across the specimen
Machine the four long faces in accordance with the following
surface. The fixture shall be designed to apply equal load to the
paragraphs. The two end faces do not require special machin-
two outer bearings.
ing.
6.9 The fixture shall be stiffer than the specimen, so that 7.2.4.2 Coarse grinding, if necessary, shall be with a dia-
most of the crosshead travel is imposed onto the specimen. mond wheel no coarser than 150 grit. The stock removal rate
C1161 − 18 (2023)
NOTE 1—Configuration:
A: L = 20 mm
B: L = 40 mm
C: L = 80 mm
NOTE 2—Load is applied through a ball which permits the loading member to tilt as necessary to ensure uniform loading.
NOTE 3—Bearing cylinders are held in place by low-stiffness springs, rubber bands, or magnets.
FIG. 2 Schematics of Two Semi-Articulating Four-Point Fixtures Suitable for Flat and Parallel Specimens
(wheel depth of cut) shall not exceed 0.03 mm (0.001 in.) per 7.2.4.5 Wheel speed should not be less than 25 m/sec
pass to the last 0.060 mm (0.002 in.) per face. Remove (~1000 in./sec). Table speeds should not be greater than
approximately equal stock from opposite faces. 0.25 m ⁄sec (45 ft/min).
7.2.4.3 Intermediate grinding, if utilized, should be done 7.2.4.6 The procedures in 7.2.4 address diamond grit size
with a diamond wheel that is between 240 and 320 grit. The for coarse, intermediate, and finish grinding but leaves the
stock removal rate (wheel depth of cut) shall not exceed choice of bond system (resin, vitrified), diamond type (natural
0.006 mm (0.00025 in.) per pass to the last 0.020 mm or synthetic, coated or uncoated, friability, shape, etc.) and
(0.0008 in.) per face. Remove approximately equal stock from concentration (percent of diamond in the wheel) to the discre-
opposite faces. tion of the user.
7.2.4.4 Finish grinding shall be with a diamond wheel that is
NOTE 9—The sound of the grinding wheel during the grinding process
between 400 and 600 grit. The stock removal rate (wheel depth
may be a useful indicator of whether the grinding wheel condition and
of cut) shall not exceed 0.006 mm (0.00025 in.) per pass. Final material removal conditions are appropriate. It is beyond the scope of this
standard to specify the auditory responses, however.
grinding shall remove no less than 0.020 mm (0.0008 in.) per
face. The combined intermediate and final grinding stages shall 7.2.4.7 Materials with low fracture toughness and a greater
remove no less than 0.060 mm (0.0025 in.) per face. Remove susceptibility to grinding damage may require finer grinding
approximately equal stock from opposite faces. wheels at very low removal rates.
C1161 − 18 (2023)
NOTE 1—Configuration:
A: L = 20 mm
B: L = 40 mm
C: L = 80 mm
NOTE 2—Bearing A is fixed so that it will not pivot about the x axis. The other three bearings are free to pivot about the x axis.
NOTE 3—Bearing cylinders are held in place by low-stiffness springs, rubber bands, or magnets.
FIG. 3 Schematics of Two Fully Articulating Four-Point Fixtures Suitable Either for Twisted or Uneven Specimens, or for Flat and Paral-
lel Specimens
TABLE 3 Specimen Size
bars. Larger chamfers or rounded edges may be used with
Configuration Width (b), mm Depth (d), mm Length (L ), min, mm C-test specimens. Consult Annex A2 for guidance and whether
T
A 2.0 1.5 25
corrections for flexural strength are necessary. No chipping is
B 4.0 3.0 45
allowed. Up to 50× magnification may be used to verify this.
C 8.0 6.0 90
Alternatively, if a test specimen can be prepared with an edge
that is free of machining damage, then a chamfer is not
required.
7.2.4.8 The four long edges of each B-sized test specimen
7.2.4.9 Very deep skip marks or very deep single striations
shall be uniformly chamfered at 45°, a distance of 0.12 6
(which may occur due to a poor quality grinding wheel or due
0.03 mm as shown in Fig. 4. They can alternatively be rounded
to a failure to true, dress, or balance a wheel) are not
with a radius of 0.15 6 0.05 mm. Edge finishing must be
acceptable.
comparable to that applied to the test specimen surfaces. In
7.2.5 Handling Precautions and Scratch Inspection—
particular, the direction of machining shall be parallel to the
Exercise care in storing and handling of specimens to avoid the
test specimen long axis. If chamfers or rounds are larger than
the tolerance allows, then corrections shall be made to the introduction of random and severe flaws, such as might occur
if specimens were allowed to impact or scratch each other. If
stress calculation in accordance with Annex A2. Smaller
chamfer or rounded edge sizes are re
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