ASTM C1161-94(1996)
(Test Method)Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
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. Rectangular specimens of prescribed cross-section sizes are used with specified features in prescribed specimen-fixture combinations.
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.
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NOTICE: This standard has either been superseded and replaced by a new version or
withdrawn. Contact ASTM International (www.astm.org) for the latest information.
Designation: C 1161 – 94 (Reapproved 1996)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Flexural Strength of Advanced Ceramics at Ambient
Temperature
This standard is issued under the fixed designation C 1161; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the determination of flexural
strength of advanced ceramic materials at ambient temperature.
Four-point– ⁄4 point and three-point loadings with prescribed
spans are the standard. Rectangular specimens of prescribed
cross-section sizes are used with specified features in pre-
scribed specimen-fixture combinations.
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 appro-
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E 4 Practices for Force Verification of Testing Machines
E 337 Test Method for Measured Humidity with a Psy-
chrometer (The Measurement of Wet- and Dry-Bulb Tem-
peratures) NOTE 1—Configuration:
A: L = 20 mm
2.2 Military Standard:
B: L = 40 mm
MIL-STD-1942 (MR) Flexural Strength of High Perfor-
4 C: L = 80 mm
mance Ceramics at Ambient Temperature
FIG. 1 1 The Four-Point– ⁄4 Point and Three-Point Fixture
3. Terminology Configuration
3.1 Definitions:
between two support bearings (see Fig. 1).
3.1.1 flexural strength—a measure of the ultimate strength
4. Significance and Use
of a specified beam in bending.
3.1.2 four-point– ⁄4 point flexure—configuration of flexural 4.1 This test method may be used for material development,
quality control, characterization, and design data generation
strength testing where a specimen is symmetrically loaded at
two locations that are situated one quarter of the overall span, purposes.
4.2 The flexure stress is computed based on simple beam
away from the outer two support bearings (see Fig. 1).
3.1.3 three-point flexure—configuration of flexural strength theory with assumptions that the material is isotropic and
homogeneous, the moduli of elasticity in tension and compres-
testing where a specimen is loaded at a location midway
sion are identical, and the material is linearly elastic. The
average grain size should be no greater than one fiftieth of the
This test method is under the jurisdiction of ASTM Committee C-28 on
beam thickness. The homogeneity and isotropy assumption in
Advanced Ceramics and is the direct responsibility of Subcommittee C28.01 on
the standard rule out the use of this test for continuous
Properties and Performance.
fiber-reinforced ceramics.
Current edition approved July 25, 1994. Published February 1995. Originally
published as C 1161 – 90. Last previous edition C 1161 – 90.
4.3 Flexural strength of a group of test specimens is
Annual Book of ASTM Standards, Vol 03.01.
influenced by several parameters associated with the test
Annual Book of ASTM Standards, Vol 11.03.
procedure. Such factors include the loading rate, test environ-
Available from Standardization Documents, Order Desk, Bldg. 4, Section D,
700 Robbins Ave., Philadelphia, PA 19111-5094. ment, specimen size, specimen preparation, and test fixtures.
C 1161
Specimen sizes and fixtures were chosen to provide a balance 1240 MPa (z180 ksi). Alternatively, the cylinders may be
between practical configurations and resulting errors, as dis- made of a ceramic with an elastic modulus between 2.0 and 4.0
5 6
cussed in MIL-STD 1942 (MR) and Refs (1) and (2). Specific 3 10 MPa (30–60 3 10 psi) and a flexural strength no less
fixture and specimen configurations were designated in order to than 275 MPa (z40 ksi). The portions of the test fixture that
permit ready comparison of data without the need for Weibull- support the bearings may need to be hardened to prevent
size scaling. permanent deformation. The cylindrical bearing length shall be
4.4 The flexural strength of a ceramic material is dependent at least three times the specimen width. The above require-
on both its inherent resistance to fracture and the presence of ments are intended to ensure that ceramics with strengths up to
defects. Analysis of a fracture surface, fractography, though 1400 MPa (z200 ksi) and elastic moduli as high as 4.8 3 10
beyond the scope of this test method, is highly recommended MPa (70 3 10 psi) can be tested without fixture damage.
for all purposes, especially for design data as discussed in Higher strength and stiffer ceramic specimens may require
MIL-STD-1942 (MR) and Refs (2–5). harder bearings.
6.4.2 The bearing cylinder diameter shall be approximately
5. Interferences
1.5 times the beam depth of the test specimen size employed.
See Table 2.
5.1 The effects of time-dependent phenomena, such as stress
corrosion or slow crack growth on strength tests conducted at 6.4.3 The bearing cylinders shall be carefully positioned
such that the spans are accurate within 60.10 mm. The load
ambient temperature, can be meaningful even for the relatively
short times involved during testing. Such influences must be application bearing for the three-point configurations shall be
positioned midway between the support bearing within 60.10
considered if flexure tests are to be used to generate design
mm. The load application (inner) bearings for the four-point
data.
configurations shall be centered with respect to the support
5.2 Surface preparation of test specimens can introduce
(outer) bearings within 60.10 mm.
machining flaws which may have a pronounced effect on
6.4.4 The bearing cylinders shall be free to rotate in order to
flexural strength. Machining damage imposed during specimen
relieve frictional constraints (with the exception of the middle-
preparation can be either a random interfering factor, or an
load bearing in three-point flexure which need not rotate). This
inherent part of the strength characteristic to be measured.
can be accomplished by mounting the cylinders in needle
Surface preparation can also lead to residual stresses. Universal
bearing assemblies, or more simply by mounting the cylinders
or standardized test methods of surface preparation do not
as shown in Fig. 2 and Fig. 3. Note that the outer-support
exist. It should be understood that final machining steps may or
bearings roll outward and the inner-loading bearings roll
may not negate machining damage introduced during the early
inward.
course or intermediate machining.
6.5 Semiarticulating–Four-Point Fixture—Specimens pre-
6. Apparatus
pared in accordance with the parallelism requirements of 7.1
may be tested in a semiarticulating fixture as illustrated in Fig.
6.1 Loading—Specimens may be loaded in any suitable
2. The bearing cylinders themselves must be parallel to each
testing machine provided that uniform rates of direct loading
other to within 0.015 mm (over their length).
can be maintained. The load-measuring system shall be free of
6.6 Fully Articulating–Four-Point Fixture—Specimens that
initial lag at the loading rates used and shall be equipped with
are as-fired, heat treated, or oxidized often have slight twists or
a means for retaining read-out of the maximum load applied to
unevenness. Specimens which do not meet the parallelism
the specimen. The accuracy of the testing machine shall be in
requirements of 7.1 shall be tested in a fully articulating fixture
accordance with Practices E 4 but within 0.5 %.
as illustrated in Fig. 3.
6.2 Four-Point Flexure—Four-point– ⁄4 point fixtures (Fig.
6.7 The fixture shall be stiffer than the specimen, so that
1) shall have support and loading spans as shown in Table 1.
most of the crosshead travel is imposed onto the specimen.
6.3 Three-Point Flexure—Three-point fixtures (Fig. 1) shall
have a support span as shown in Table 1.
7. Specimen
6.4 Bearings—Three- and four-point flexure:
7.1 Specimen Size—Dimensions are given in Table 3 and
6.4.1 Cylindrical bearing edges shall be used for the support
shown in Fig. 4. Cross-sectional dimensional tolerances are
of the test specimen and for the application of load. The
60.13 mm for B and C specimens, and 60.05 mm for A. The
cylinders shall be made of hardened steel which has a hardness
parallelism tolerances on the four longitudinal faces are 0.015
no less than HRC 40 or which has a yield strength no less than
mm for A and B and 0.03 mm for C. The two end faces need
not be precision machined.
7.2 Specimen Preparation—Depending upon the intended
The boldface numbers in parentheses refer to the references at the end of this
application of the flexural strength data, use one of the
test method.
following four specimen preparation procedures:
TABLE 1 Fixture Spans TABLE 2 Nominal Bearing Diameters
Configuration Support Span (L), mm Loading Span, mm Configuration Diameter, mm
A20 10 A 2.0 to 2.5
B40 20 B 4.5
C80 40 C 9.0
C 1161
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
FIG. 2 Schematic of a Semiarticulated Four-Point Fixture Suitable for Flat and Parallel Specimens
7.2.1 As-Fabricated—The flexural specimen shall simulate pass. Remove approximately equal stock from opposite faces.
the surface condition of an application where no machining is 7.2.4.3 Materials with low fracture toughness and a greater
to be used; for example, as-cast, sintered, or injection-molded susceptibility to grinding damage may require finer grinding
parts. No additional machining specifications are relevant. An wheels at very low removal rates.
edge chamfer is not necessary in this instance. As-fired 7.2.4.4 The four long edges of each specimen shall be
specimens are especially prone to twist or warpage and might uniformly chamfered at 45°, a distance of 0.12 6 0.03 mm as
not meet the parallelism requirements. In this instance, a fully shown in Fig. 4. They can alternatively be rounded with a
articulating fixture (6.6 and Fig. 3) shall be used in testing. radius of 0.156 0.05 mm. Edge finishing must be comparable
7.2.2 Application-Matched Machining—The specimen shall to that applied to the specimen surfaces. In particular, the
have the same surface preparation as that given to a compo- direction of machining shall be parallel to the specimen long
nent. Unless the process is proprietary, the report shall be axis. If chamfers are larger than the tolerance allows, then
specific about the stages of material removal, wheel grits, corrections shall be made to the stress calculation (1). Alter-
wheel bonding, and the amount of material removed per pass. natively, if a specimen can be prepared with an edge that is free
7.2.3 Customary Procedures—In instances where a custom- of machining damage, then a chamfer is not required.
ary machining procedure has been developed that is completely 7.2.5 Handling Precautions—Care should be exercised in
satisfactory for a class of materials (that is, it induces no storing and handling of specimens to avoid the introduction of
unwanted surface damage or residual stresses), this procedure random and severe flaws, such as might occur if specimens
shall be used. were allowed to impact or scratch each other.
7.2.4 Standard Procedures—In the instances where 7.2.1 7.3 Number of Specimens—A minimum of 10 specimens
through 7.2.3 are not appropriate, then 7.2.4 shall apply. This shall be required for the purpose of estimating the mean. A
procedure shall serve as minimum requirements and a more minimum of 30 shall be necessary if estimates regarding the
stringent procedure may be necessary. form of the strength distribution are to be reported (for
7.2.4.1 All grinding shall be done with an ample supply of example, a Weibull modulus). The number of specimens
appropriate filtered coolant to keep workpiece and wheel required by this test method has been established with the
constantly flooded and particles flushed. Grinding shall be in at intent of determining not only reasonable confidence limits on
least two stages, ranging from coarse to fine rates of material strength distribution parameters, but also to help discern
removal. All machining shall be in the surface grinding mode, multiple-flaw population distributions. More than 30 speci-
and shall be parallel to the specimen long axis shown in Fig. 5. mens are recommended if multiple-flaw populations are
No Blanchard or rotary grinding shall be used. present.
7.2.4.2 The stock-removal rate shall not exceed 0.03 mm
8. Procedure
(0.001 in.) per pass to the last 0.06 mm (0.002 in.) per face.
Final (and intermediate) finishing shall be performed with a 8.1 Test specimens on their appropriate fixtures in specific
diamond wheel that is between 320 and 500 grit. No less than testing configurations. Test specimens Size A on either the
0.06 mm per face shall be removed during the final finishing four-point A fixture or the three-point A fixture. Similarly, test
phase, and at a rate of not more than 0.002 mm (0.0001 in.) per B specimens on B fixtures, and C specimens on C fixtures. A
C 1161
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.
FIG. 3 Schematic of a Fully Articulating Four-Point Fixture Suitable for Twisted or Uneven Specimens
TABLE 3 Specimen Size
of contact between the bearings and the specimen to ensure
even line loading and that no dirt or contamination is present.
Configuration Width (b), mm Depth (d), mm Length (L ), min,
T
If uneven line loading of the specimen occurs, use fully
mm
articulating fixtures.
A 2.0 1.5 25
8.4 Mark the specimen to identify the points of load
B 4.0 3.0 45
C 8.0 6.0 90 application and also so that the tensile and compression faces
can be distinguished. Carefully drawn pencil marks will
suffice.
fully articulating fixture is required if the specimen parallelism
8.5 Put cotton, crumbled tissues, or other appropriate
requirements cannot be met. An alternative procedure with a D
material around specimen to prevent pieces from flying out of
specimen is given in the Appendix.
the fixtures upon fracture. This s
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