ISO 14704:2016
(Main)Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method for flexural strength of monolithic ceramics at room temperature
Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method for flexural strength of monolithic ceramics at room temperature
ISO 14704:2016 specifies a test method for determining the flexural strength of monolithic fine ceramics, and whisker- or particulate-reinforced ceramic composites, at room temperature and applies to materials with grain size less than 200 µm. This test method may be used for materials development, quality control, characterization and design data-generation purposes. NOTE Since fracture is due to tensile stress, flexural strength data can be used to calculate a uniaxial tensile strength considering the effect of the tested volume and Weibull-statistics. So, flexural strength is often used in substitute for uniaxial tensile strength.
Céramiques techniques — Méthode d'essai de résistance en flexion des céramiques monolithiques à température ambiante
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 14704
Third edition
2016-04-15
Fine ceramics (advanced ceramics,
advanced technical ceramics) —
Test method for flexural strength
of monolithic ceramics at room
temperature
Céramiques techniques — Méthode d’essai de résistance en flexion des
céramiques monolithiques à température ambiante
Reference number
ISO 14704:2016(E)
©
ISO 2016
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ISO 14704:2016(E)
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ISO 14704:2016(E)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 3
5 Apparatus . 3
5.1 Testing machine . 3
5.2 Test fixture . 3
5.2.1 General. 3
5.2.2 Bearings . 4
5.2.3 Four-point fixture: semi-articulating . 4
5.2.4 Four-point fixture: fully articulating . 4
5.2.5 Three-point fixture: semi-articulating . 4
5.2.6 Three-point fixture: fully articulating . 5
5.2.7 Positioning of bearings . 5
5.2.8 Fixture material . 6
5.3 Micrometer . 6
6 Test pieces . 6
6.1 Test piece size . 6
6.1.1 Machined test pieces . 6
6.1.2 As-fired or heat-treated test pieces . 6
6.2 Test piece preparation . 7
6.2.1 General. 7
6.2.2 As-fired . 7
6.2.3 Customary machining procedure . 7
6.2.4 Component-matched procedure . 7
6.2.5 Basic machining procedure . 7
6.2.6 Parallelism, orthogonality and chamfer sizes . 8
6.2.7 Handling of test pieces . 9
6.2.8 Number of test pieces . 9
7 Procedure. 9
8 Calculation .11
9 Test report .12
10 Strength scaling factors .13
Annex A (informative) General information.14
Annex B (normative) Test fixtures .15
Annex C (informative) Typical fracture patterns in ceramic test pieces .21
Annex D (informative) Chamfer correction factors .23
Annex E (informative) Weibull scaling factors .26
Bibliography .28
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ISO 14704:2016(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 206, Fine ceramics.
This third edition cancels and replaces the second edition (ISO 14704:2008), which has been technically
revised.
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INTERNATIONAL STANDARD ISO 14704:2016(E)
Fine ceramics (advanced ceramics, advanced technical
ceramics) — Test method for flexural strength of
monolithic ceramics at room temperature
1 Scope
This International Standard specifies a test method for determining the flexural strength of monolithic
fine ceramics, and whisker- or particulate-reinforced ceramic composites, at room temperature and
applies to materials with grain size less than 200 µm. This test method may be used for materials
development, quality control, characterization and design data-generation purposes.
NOTE Since fracture is due to tensile stress, flexural strength data can be used to calculate a uniaxial tensile
strength considering the effect of the tested volume and Weibull-statistics. So, flexural strength is often used in
substitute for uniaxial tensile strength.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 3611, Geometrical product specifications (GPS) — Dimensional measuring equipment: Micrometers for
external measurements — Design and metrological characteristics
ISO 7500-1, Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1:
Tension/compression testing machines — Calibration and verification of the force-measuring system
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
flexural strength
maximum nominal stress at fracture of a specified elastic beam loaded in bending
3.2
four-point flexure
configuration of flexural strength testing where a test piece is loaded equally by two bearings
symmetrically located between two support bearings
Note 1 to entry: See Figure 1, a) and b).
Note 2 to entry: The bearings may be cylindrical rollers or cylindrical bearings.
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ISO 14704:2016(E)
L = 40 mm ± 0,1 mm
a) Four-point-1/4 point flexure
L = 30 mm ± 0,1 mm
b) Four-point-1/3 point flexure
L = 30 mm ± 0,1 mm or 40 mm ± 0,1 mm
c) Three-point flexure
Key
1 loading bearings
2 support bearing
3 test piece
Figure 1 — Flexural test configurations
3.3
four-point-1/4 point flexure
specific configuration of four-point flexural strength testing where the inner bearings are situated one-
quarter of the support span away from the two outer bearings
Note 1 to entry: See Figure 1 a).
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ISO 14704:2016(E)
3.4
four-point-1/3 point flexure
specific configuration of four-point flexural strength testing where the inner bearings are situated one-
third of the support span away from the two outer bearings
Note 1 to entry: See Figure 1 b).
3.5
semi-articulating fixture
test fixture designed to apply uniform and even loading to test pieces that have flat and parallel surfaces
3.6
fully articulating fixture
test fixture designed to apply uniform and even loading to test pieces that may have uneven, non-
parallel or twisted surfaces
3.7
three-point flexure
configuration of flexural strength testing where a test piece is loaded at a location midway between
two support bearings
Note 1 to entry: See Figure 1 c).
Note 2 to entry: Four-point flexure (3.2) is usually preferred, since a large amount of material is exposed to the
maximum stress (see Annex A for more information).
4 Principle
A beam test piece with a rectangular cross-section is loaded in flexure until fracture. The load at
fracture, the test fixture and test piece dimensions are used to compute the flexural strength which
is often used in substitute for uniaxial tensile strength of a ceramic. The material is assumed to be
isotropic and linearly elastic.
5 Apparatus
5.1 Testing machine
The testing machine shall be capable of applying a force to the loading roller (three-point flexure) or
equally to the loading rollers (four-point flexure) in order to stress the test piece. The machine shall
be capable of applying the force at a constant loading or displacement rate. The test machine shall be
equipped for recording the peak load applied to the test piece. The accuracy of the test machine shall be
in accordance with ISO 7500-1, Class 1, with an accuracy of 1 % of indicated load at fracture.
5.2 Test fixture
5.2.1 General
Three- or four-point flexure configurations shall be used, as illustrated in Figure 1. The four-point-1/4
point configuration is recommended. The fixture shall have bearings that are free to roll, as described in
5.2.2, in order to eliminate frictional constraints when the test piece surfaces expand or contract during
loading. In addition, the fixture shall be designed so that parts “articulate” or tilt to ensure uniform
loading to the test piece. The articulation is designed so that parts of the fixture can rotate, as shown in
Figure B.1, to ensure even loading on the left and right bearings. An articulation is also needed to ensure
that all the bearings evenly contact the test piece surfaces and apply uniform load. Semi-articulated
fixtures have some articulating or tilting capabilities and may be used with test pieces that have flat and
parallel surfaces, such as on as-machined test pieces. A semi-articulating fixture has pairs of upper and
lower bearings that articulate to match the test piece surfaces, as shown in Figures B.2 and B.3. Fully
articulated fixtures have more moving parts and are necessary for test pieces that do not have flat and
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ISO 14704:2016(E)
parallel surfaces. They allow independent articulation of the bearings. Fully articulated fixtures often
are necessary for as-fired, heat-treated or oxidized test pieces, since uneven loading can cause twisting
and severe errors. A fully articulating fixture may also be used with machined test pieces.
5.2.2 Bearings
Test pieces shall be loaded and supported by bearings. The bearings may be cylindrical rollers or
cylindrical bearings. The bearings shall be made of a steel which has a hardness of no less than HRC 40
for test piece strengths up to 1 400 MPa, or no less than HRC 46 for test piece strengths up to 2 000 MPa.
Alternatively, the bearing may be made of a ceramic or hardmetal with an elastic modulus between
200 GPa and 500 GPa and a flexural strength greater than 275 MPa. The bearing length shall be greater
than or equal to 12 mm. The bearing diameter shall be approximately 1,5 times the test piece thickness
(d). Diameters between 4,5 mm and 5 mm are recommended. The bearings shall have a smooth surface
and shall have a diameter that is uniform to ± 0,015 mm. The bearings shall be free to roll in order to
eliminate friction. In four-point flexure, the two inner bearings shall be free to roll inwards, and the
two outer bearings shall be free to roll outwards. In three-point flexure, the two outer bearings shall be
free to roll outwards, and the inner (middle) bearing shall not roll.
NOTE 1 Friction can cause errors in the stress calculations. The rolling can be accomplished by several
designs. The bearing can be mounted in roller bearing or cylindrical bearing assemblies. It is also acceptable, and
simpler, for the bearings to be free to roll on the fixture surface, as shown in Figure 2.
The bearing diameter is specified on the basis of competing requirements. The bearings should not
be so large as to cause excessive change in the moment arm as a test piece deflects, as this can create
errors from contact-point tangency shift. On the other hand, the bearings should not be so small as to
create excessive wedging stresses in the test piece or create contact stresses that damage the fixture.
NOTE 2 The bearing hardness and stiffness requirements and guidelines are intended to ensure that test
pieces with strengths up to 1 400 MPa (or 2 000 MPa), and elastic moduli as high as 500 GPa, can be tested
without damaging the fixture. Higher-strength or stiffer ceramic test pieces can require harder bearings. For
example, if the bearing elastic modulus is greater than 500 GPa, then it is advisable to lengthen the bearings and
the fixture support width to more than 12 mm to distribute the forces over a longer bearing length.
5.2.3 Four-point fixture: semi-articulating
Figure B.2 a) shows the actions of the bearings in this fixture. The two inner bearings shall be parallel
to each other to within 0,015 mm over their length (≥ 12 mm in accordance with 5.2.2). The two outer
bearings shall be parallel to each other to within 0,015 mm over their length. Either the two inner or
the two outer bearings shall be capable of articulating (tilting) together as a pair to match the test piece
surface. All four bearings shall rest uniformly and evenly across the test piece surface. The fixture shall
apply equal load to all four bearings. All four bearings shall be free to roll.
5.2.4 Four-point fixture: fully articulating
Figure B.2 b) shows the actions of the bearings in this fixture. One bearing need not articulate (tilt).
The other three bearings shall articulate (tilt) independently to follow the test piece surface. All four
bearings shall rest uniformly and evenly across the test piece surface. The fixture shall apply equal load
to all four bearings. All four bearings shall be free to roll.
5.2.5 Three-point fixture: semi-articulating
Figure B.3 a) shows the actions of the bearings in this fixture. The two outer bearings shall be parallel
to each other to within 0,015 mm over their length (≥ 12 mm in accordance with 5.2.2). The two outer
bearings shall articulate together to follow the test piece surface, or the middle bearing shall articulate
to follow the test piece surface. All three bearings shall rest uniformly and evenly across the test piece
surface. The fixture shall be designed to apply equal load to the two outer bearings. The two support
(outer) bearings shall be free to roll outwards. The middle bearing shall be fixed and not free to roll.
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ISO 14704:2016(E)
5.2.6 Three-point fixture: fully articulating
Figure B.3 b) and c) show the actions of the bearings in this fixture. Any two of the bearings shall be
capable of articulating (tilting) independently to rest uniformly and evenly across the test piece surface.
The fixture shall be designed to apply equal load to the two outer bearings. The two support (outer)
bearings shall be free to roll outwards. The middle bearing shall not roll.
5.2.7 Positioning of bearings
The bearings shall be positioned so that the spans are accurate to within ±0,1 mm. The middle bearing
for the three-point fixture shall be positioned midway between the outer bearings to within ±0,1 mm.
The inner bearings for the four-point fixture shall be centered over the outer bearings to within
±0,1 mm.
NOTE The positions of the bearings can be defined either by the use of captive bearings, or by appropriate
stops against which the bearings are held at the commencement of a test. The spans can be measured to the
nearest 0,1 mm using a traveling microscope or other suitable device. The spans can also be verified by
measurement of the distances between bearing stops and adding (outer span) or subtracting (inner span) the
radii of the bearing cylinders.
2
1
a) Four-point flexure
3
1
b) Three-point flexure
Key
1 test piece
2 alternative rolling bearings
3 alternative loading bearing arrangements
NOTE 1 For a), the four bearings shall be free to roll.
NOTE 2 For b), the two outer bearings are free to roll outwards, but the middle bearing shall be non-rolling.
Figure 2 — Schematic representation of fixtures showing the rolling action of the bearing
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ISO 14704:2016(E)
5.2.8 Fixture material
The fixture which supports and aligns the bearings shall be sufficiently hard, so that the bearings do
not permanently deform the fixture.
NOTE Line-contact loadings can deform the fixture. The hardness of the fixture will depend upon the design
of the fixture. If the bearings are at least 12 mm wide and the fixture is 12 mm wide or more, then a fixture made
of steel with an HRC of 25 or greater will be adequate.
5.3 Micrometer
A micrometer, such as that described in ISO 3611, but with a resolution of 0,002 mm, shall be used to
measure the test piece dimensions. The micrometer shall have flat anvil faces such as those shown in
ISO 3611. The micrometer shall not have a ball tip or sharp tip since these might damage the test piece.
Alternative dimension-measuring instruments may be used, provided that they have a resolution of
0,002 mm or finer.
6 Test pieces
6.1 Test piece size
6.1.1 Machined test pieces
Test piece dimensions are shown in Figure 3. Cross-sectional tolerances shall be ±0,2 mm. The
parallelism tolerance on opposite longitudinal faces is 0,015 mm.
6.1.2 As-fired or heat-treated test pieces
Test piece dimensions may be altered, as required, but deviations from the specifications in 6.1.1 and
Figure 3 shall be stated in the test report.
Dimensions in millimetres
or
X
X
X
a
L
T
Key
a
L ≥ 35 mm for 30 mm test fixtures and L ≥ 45 mm for 40 mm test fixtures.
T T
Figure 3 — Standard test piece
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ISO 14704:2016(E)
6.2 Test piece preparation
6.2.1 General
This International Standard allows several options for test piece preparation. In all cases, the end faces
of the test piece do not need special preparation or finishing. A minimum of two long edges on one
4 mm wide face shall be chamfered or rounded, as shown in Figure 3. It is highly recommended that all
four long edges be chamfered or rounded. The test piece surface condition (as-fired, ground, polished
etc.) shall be reported.
Although a surface finish specification is not part of this International Standard, it is highly
recommended that the surface roughness be measured and reported.
NOTE Surface preparation of test pieces can introduce machining flaws (especially microcracks beneath the
test piece surface) which can have a pronounced effect on flexural strength. Machining damage can either be a
random interfering factor, or an inherent part of the strength characteristics to be measured. Surface preparation
can also create residual stresses. Final machining steps (including polishing) can or cannot negate machining
damage introduced from prior, coarser machining steps.
6.2.2 As-fired
The flexure test piece is fabricated by sintering or some other process, such that no machining is
required. In this case, the purpose is to measure the strength of the test piece with an as-fired surface.
An edge chamfer or rounding is recommended and can be made before sintering.
As-fired test pieces are especially prone to twist or warpage. They may not meet the parallelism
requirements given in 6.1.1, in which case a fully articulating fixture should be used in testing.
One surface of an as-fired part may be machined to help minimize twisting or warpage effects. The
machined surface should be placed in contact with the inner bearings (test piece compression side)
during testing.
6.2.3 Customary machining procedure
In instances where a customary machining procedure has been developed that is completely satisfactory
for a class of materials (i.e. it introduces minimal or no unwanted surface damage or residual stress),
then this customary procedure is permitted. The test report shall include details of the procedure,
especially the wheel grits, wheel bonding (resin, metal, vitreous glass, other) and the material removed
per pass. The long edges of the test piece shall be rounded or chamfered, as shown in Figure 3.
6.2.4 Component-matched procedure
The test piece shall have the same surface preparation as that given to a component. The test report
shall include details of the procedure, especially the wheel bonding (resin, metal, vitreous, other) and
the material removed per pass. The long edges of the test piece shall be rounded or chamfered, as shown
in Figure 3.
6.2.5 Basic machining procedure
If the procedures in 6.2.2 to 6.2.4 are not applicable, then the following procedure may be used.
NOTE The procedure specified below is a general-duty, conservative practice. It is intended to minimize
machining damage or residual stresses in a broad range of ceramics. Faster or more aggressive removal rates
can be suitable for some materials. Alternatively, some very brittle ceramics can require a more conservative
preparation.
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ISO 14704:2016(E)
6.2.5.1 Test pieces shall be ground in the longitudinal direction, as shown in Figure 4.
Figure 4 — Surface grinding parallel to the test piece longitudinal axis
6.2.5.2 All grinding shall be done with an ample supply of filtered coolant, in order to keep the work
piece and wheel flooded and particles flushed. Grinding shall be in at least two stages, ranging from
coarse to fine rates of material removal.
6.2.5.3 Coarse grinding shall be carried out using a diamond wheel rounded to within 0,03 mm and
of grit size not exceeding 120 mesh (D 126), using a depth of cut not exceeding 0,03 mm per pass.
Alternatively, a creep-feed grinding process may be used for the coarse grinding step.
6.2.5.4 Finishing machining shall be carried out using a diamond wheel of grit size between 320 mesh
and 800 mesh (e.g. D 46 or finer), using a depth of cut not exceeding 0,002 mm per pass. Final finishing
shall remove no less than 0,06 mm of material per face. Approximately, equal stock shall be removed
from opposite faces.
6.2.5.5 The long edges shall be uniformly chamfered at 45° to a size of 0,12 mm ± 0,03 mm, as shown
in Figure 3. Alternatively, they can be rounded to a radius of 0,15 mm ± 0,05 mm. Edge chamfering or
rounding shall be comparable to that applied to the test piece surfaces in the fine-finishing step. The
direction of machining shall be parallel to the test piece’s long axis.
If, for some reason, the chamfers are larger than the specified size range (e.g. for the removal of very
large chips), then the stresses should be corrected for the reduced second moment of inertia of the test
piece cross-section. Annex D may be consulted for this correction.
6.2.5.6 The final dimensions of the test piece shall be in accordance with
...
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