ASTM C1161-18(2023)

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 service@astm.org. 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 m
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