SIST EN ISO 18756:2005

SLOVENSKI STANDARD
01-november-2005
Fine ceramics (advanced ceramics, advanced technical ceramics) - Determination
of fracture toughness of monolithic ceramics at room temperature by the surface
crack in flexure (SCF) method (ISO 18756:2003)
Fine ceramics (advanced ceramics, advanced technical ceramics) - Determination of
fracture toughness of monolithic ceramics at room temperature by the surface crack in
flexure (SCF) method (ISO 18756:2003)
Hochleistungskeramik - Bestimmung der Bruchzähigkeit monolithischer Keramik bei
Raumtemperatur für Biegeproben mit Oberflächenriss (Knoop-Riss) (SCF-Verfahren)
(ISO 18756:2003)
Céramiques techniques - Détermination de la ténacité a la rupture des céramiques
monolithiques a température ambiante par fissuration superficielle en flexion (ISO
18756:2003)
Ta slovenski standard je istoveten z: EN ISO 18756:2005
ICS:
81.060.30 Sodobna keramika Advanced ceramics
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 18756
NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2005
ICS 81.060.30
English Version
Fine ceramics (advanced ceramics, advanced technical
ceramics) - Determination of fracture toughness of monolithic
ceramics at room temperature by the surface crack in flexure
(SCF) method (ISO 18756:2003)
Céramiques techniques - Détermination de la ténacité à la Hochleistungskeramik - Bestimmung der Bruchzähigkeit
rupture des céramiques monolithiques à température monolithischer Keramik bei Raumtemperatur für
ambiante par fissuration superficielle en flexion (ISO Biegeproben mit Oberflächenriss (Knoop-Riss) (SCF-
18756:2003) Verfahren) (ISO 18756:2003)
This European Standard was approved by CEN on 2 June 2005.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Central Secretariat or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18756:2005: E
worldwide for CEN national Members.

Foreword
The text of ISO 18756:2003 has been prepared by Technical Committee ISO/TC 206 "Fine
ceramics" of the International Organization for Standardization (ISO) and has been taken over as
secretariat of which is held by BSI.

This European Standard shall be given the status of a national standard, either by publication of
an identical text or by endorsement, at the latest by January 2006, and conflicting national
standards shall be withdrawn at the latest by January 2006.

This document is part of a series:

CEN/TS 14425-1 Advanced technical ceramics — Test methods for determination of fracture
toughness of monolithic ceramics — Part 1: Guide to test method selection

CEN/TS 14425-3 Advanced technical ceramics — Test methods for determination of fracture
toughness of monolithic ceramics — Part 3: Chevron notched beam (CNB) method

CEN/TS 14425-5 Advanced technical ceramics — Test methods for determination of fracture
toughness of monolithic ceramics — Part 5: Single-edge V-notch beam (SEVNB) method

EN ISO 15732 Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method
for fracture toughness of monolithic ceramics at room temperature by single edge precracked
beam (SEPB) method
EN ISO 18756 Fine ceramics (advanced ceramics, advanced technical ceramics) —
Determination of fracture toughness of monolithic ceramics at room temperature by the surface
crack in flexure (SCF) method
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Cyprus,
Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

Endorsement notice
The text of ISO 18756:2003 has been approved by CEN as EN ISO 18756:2005 without any
modifications.
INTERNATIONAL ISO
STANDARD 18756
First edition
2003-12-01
Fine ceramics (advanced ceramics,
advanced technical ceramics) —
Determination of fracture toughness of
monolithic ceramics at room temperature
by the surface crack in flexure (SCF)
method
Céramiques techniques — Détermination de la ténacité à la rupture des
céramiques monolithiques à température ambiante par fissuration
superficielle en flexion
Reference number
ISO 18756:2003(E)
©
ISO 2003
ISO 18756:2003(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2003
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2003 — All rights reserved

ISO 18756:2003(E)
Contents Page
Foreword. iv
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols . 3
5 Principle . 4
6 Apparatus. 5
7 Test specimens . 6
7.1 Specimen size, preparation and edge chamfering . 6
7.2 Number of specimens. 6
8 Procedure. 7
8.1 Introduction of the precrack by Knoop indentation . 7
8.2 Specimen fracture. 11
8.3 Crack size measurement. 12
8.4 Environmental effects. 13
8.5 Optional: Estimate of R-curve behaviour .14
8.6 Optional: Reference materials . 14
9 Calculation. 14
10 Test report. 15
Annex A (informative) Environmental effects. 17
Annex B (normative) Precrack characterization . 18
Annex C (informative) R-curve estimation by the SCF method. 25
Annex D (normative) Chamfer correction factors. 27
Bibliography . 29

ISO 18756:2003(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 18756 was prepared by Technical Committee ISO/TC 206, Fine ceramics.

iv © ISO 2003 — All rights reserved

INTERNATIONAL STANDARD ISO 18756:2003(E)

Fine ceramics (advanced ceramics, advanced technical
ceramics) — Determination of fracture toughness of monolithic
ceramics at room temperature by the surface crack in flexure
(SCF) method
1 Scope
This International Standard describes a test method that covers the determination of fracture toughness of
monolithic ceramic materials at room temperature by the surface crack in flexure (SCF) method.
This International Standard is intended for use with monolithic ceramics and whisker- or particulate-reinforced
ceramics that are regarded as macroscopically homogeneous. It does not include continuous-fibre reinforced
ceramic composites.
The test method is applicable to materials with either flat or rising crack growth resistance curves. This
method is similar to ISO 15732 except that precracks are smaller and are made by a different procedure. The
methods should produce similar or identical results for materials with a flat R-curve.
1/2
NOTE This test method is usually applicable to ceramic materials with a fracture toughness less than ≈ 10 MPa m . It
may be difficult to form precracks with a Knoop indenter for materials with greater fracture toughness or those materials
which are soft (low hardness) such as some zirconias, or for porous ceramics.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 3611:1978, Micrometer callipers for external measurement
1)
ISO 7500-1:— , Metallic materials — Verification of static uniaxial testing machines — Part 1: Tension/
compression testing machines — Verification and calibration of the force-measuring system
ISO 14704:2000, Fine ceramics (advanced ceramics, advanced technical ceramics) — Test method for flexural
strength of monolithic ceramics at room temperature
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

1) To be published. (Revision of ISO 7500-1:1999)
ISO 18756:2003(E)
3.1
stress intensity factor
K
I
magnitude of the elastic stress field singularity at the tip of a crack subjected to opening mode displacement
NOTE It is a function of applied force and test specimen size, geometry and crack length, and has dimensions of
force times length to the power of three over two.
3.2
fracture toughness
generic term for measures of resistance of extension of a crack
3.3
fracture toughness value
K
Isc
fracture toughness value measured by the SCF method
NOTE This is the measured stress intensity factor corresponding to the crack extension resistance of a semi-elliptical
small crack formed underneath a Knoop indentation. The measurement is performed to the operational procedure herein
and satisfies all the validity requirements.
3.4
precrack
crack introduced into the test specimen artificially prior to testing the specimen to fracture
3.5
crack front line
line to indicate the position of the tip of the crack
3.6
critical stress intensity factor
K
Ic
critical value of K at which fast fracture occurs
I
3.7
critical crack
crack at fracture at maximum load and whose stress intensity factor just reaches the critical stress intensity
factor
3.8
critical crack size
size of the critical crack at fracture
NOTE The critical crack will be larger than the precrack if stable crack extension occurs due to environmentally-
assisted slow crack growth or rising R-curve behaviour.
3.9
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
3.10
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
3.11
flexural strength
maximum nominal stress at fracture of a specified elastic beam loaded in bending
2 © ISO 2003 — All rights reserved

ISO 18756:2003(E)
4 Symbols
a Crack depth
A Flexure fixture moment arm
B Specimen width, the cross section dimension perpendicular to the direction of loading in bending
c Crack half width
C Chamfer size
d Length of Knoop indentation long diagonal
h Depth of Knoop indentation
F Knoop indentation load
F Chamfer correction factor
c
H (a/c, a/W) A polynomial in the stress intensity factor coefficient, for the point on the crack periphery where it
intersects the specimen surface
H (a/c, a/W) A polynomial in the stress intensity factor coefficient, for the deepest part of the surface crack
K Stress intensity factor, Mode I
I
K Critical stress intensity factor, Mode I
Ic
K Fracture toughness value, surface crack in flexure method
Isc
L Flexure fixture support span
L Specimen length
T
M (a/c, a/W) A polynomial in the stress intensity factor coefficient
P Load at fracture
Q (a/c) A polynomial function of the surface crack ellipticity
S (a/c, a/W) Factor in the stress intensity factor coefficient
W Specimen depth, the cross section dimension parallel to the direction of loading in bending
Y Stress intensity factor coefficient
Y Stress intensity factor coefficient at the deepest part of the surface crack
d
Y The maximum stress intensity factor coefficient along the boundary of the surface crack
max
Y Stress intensity factor coefficient at the intersection of the surface crack with the specimen
s
surface
ISO 18756:2003(E)
5 Principle
This International Standard is for material development, material comparison, quality assurance,
characterization, reliability, and design data generation. The method determines the fracture toughness value,
K by fracturing a common flexure specimen which has a small surface precrack (see Figure 1). The
Isc
specimen is indented with a Knoop indenter in order to make a small, semi-elliptical surface crack. The
specimen is polished or ground carefully until the indentation and associated residual stress field are removed.
The specimen is fractured in four-point flexure. The fracture toughness, K , is calculated from the fracture
Isc
load and the measured critical crack size. Fractography is required to measure the precrack size and to
determine whether the crack has grown in size. Fracture toughness as a function of crack size may be
evaluated by varying the Knoop indentation load that is used to make the precrack. Background information
concerning this test method may be found in References [1] and [2]. An international interlaboratory
comparison study (round robin) project on this method is described in References [3], [4] and [5].
If the ceramic is too soft (low hardness) or has too great a fracture toughness, it may be difficult to create a
precrack by the SCF method. In addition, for some materials (particularly those with coarse grain or
heterogeneous microstructures), it may be difficult to detect the crack on the fracture surface. If the user is not
sure of the applicability of this method, then a single trial specimen may be tested with an abbreviated
procedure. Indent the specimen and fracture it without removal of the indentation and residual stresses.
Inspect the fracture surface to confirm that the specimen fractured from the precrack (and not from a material
flaw) and that the precrack can be detected on the fracture surface.
Precracking is by Knoop indentation only in this International Standard. Residual stresses underneath the
indentation are removed in this test method. There is some limited experience with SCF precracking by Vickers
indentation [3, 4, 5, 6, 7]
Key
1 indentation and precrack
2 polished or lapped surface
Figure 1 — Indentation and precrack in a flexural specimen
4 © ISO 2003 — All rights reserved

ISO 18756:2003(E)
6 Apparatus
6.1 Testing machine, capable of applying a uniform cross-head speed. The testing machine shall be in
accordance with ISO 7500-1:— Class 1 with an accuracy of 1 % of indicated load at fracture.
6.2 Flexural fixtures, four-point as shown in Figure 2. Flexural fixtures shall meet the requirements of
ISO 14704.
The fixtures shall either be semi-articulating or fully-articulating depending upon the condition of the specimens.
If the specimens meet the parallelism requirements of 7.1, then semi-articulating fixtures may be used. Semi-
articulating fixtures are usually completely satisfactory for machined specimens. If the specimens do not meet
the parallelism requirements of 7.1 (due to hand grinding unevenness, problems with machining, or other
causes), then fully-articulating fixtures shall be used. Fully-articulating fixtures also may be used with machined
specimens. Specimens shall be loaded and supported by bearings. The bearings shall be free to roll in order to
eliminate friction. For four-point flexure, the two inner bearings shall be free to roll inwards, and the two outer
bearings shall be free to roll outwards.
Four-point 1/4-point fixtures with 20 mm support span and 10 mm inner span are also permitted. Such fixtures
shall meet the requirements of ISO 14704.
Dimensions in millimetres
L = 40,0 ± 0,10 or L = 20,0 ± 0,10
a) Four-point 1/4-point flexure

L = 30,0 ± 0,10
b) Four-point 1/3-point flexure
Key
1 loading bearings
2 support bearings
Figure 2 — Four-point flexure
ISO 18756:2003(E)
6.3 Micrometer, such as described in ISO 3611 but with a resolution of 0,002 mm shall be used to
measure the specimen dimensions. The micrometer shall have flat anvil faces such as shown in ISO 3611.
The micrometer shall not have a ball tip or sharp tip since these might damage the specimen. Alternative
dimension measuring instruments may be used provided that they have a resolution of 0,002 mm or finer.
6.4 Hardness testing machine, conventional type, to create the Knoop indentation. The machine shall be
able to apply loads of 20 N to 50 N or greater. If a hardness machine with this load range is not available, then
a strength testing machine (6.1) may be used although some loss of accuracy or control of indentation and
crack size may result.
6.5 Microscopes, optical and/or scanning electron, shall be used to detect the precrack (or critical crack)
and measure its size on the specimen fracture surface after the test. Magnifications of 100× to 500× are
usually required. The microscope shall be capable of making photographic or digital records of the cracks.
6.6 Dye penetrants, to highlight the crack. Dye penetrants that do not promote slow crack growth nor bleed
(spread on the fracture surface after fracture) are preferred.
6.7 Temperature measuring device, a thermometer or other device to measure ambient temperature
during the fracture of the specimen.
6.8 Humidity measuring device, such as a hygrometer, sling psychrometer, or other device to measure
ambient humidity during the fracture of the specimen.
7 Test specimens
7.1 Specimen size, preparation and edge chamfering
7.1.1 Rectangular beam specimens with dimensions as shown in Figure 3 shall be used. The cross-
sectional tolerances are ± 0,2 mm. The parallelism tolerance on opposite longitudinal faces is 0,015 mm.
7.1.2 Specimens shall be prepared in accordance with ISO 14704 with the exceptions noted below. The
indentation may be placed in either a 3 mm or 4 mm wide face. A diamond-grit wheel 320 or finer shall be
used to remove the last 0,04 mm on the surface that is used for the indentation. This surface shall be polished,
lapped or fine ground to provide a flat, smooth surface for the surface crack.
NOTE The surface does not require a polished, high-quality finish such as required for a hardness measurement. The
surface need only be flat so that the Knoop indentation is not affected by machining striations or marks, or specimen
unevenness.
7.1.3 Chamfers or edge rounds are optional. If premature fracture occurs from edge damage, then the
edges shall be chamfered or rounded as specified in ISO 14704. The chamfer size should be 0,15 mm or less.
See Figure 3.
7.2 Number of specimens
The number of specimens shall be not less than five. It is recommended that at least ten specimens be
prepared. This will provide specimens for practice tests to determine the best indentation loads and provide
specimens to make up for unsuccessful or invalid tests. More specimens are needed if environment, testing
rate, or precrack sizes are to be varied.
6 © ISO 2003 — All rights reserved

ISO 18756:2003(E)
Dimensions in millimetres
L W 25 for 20 mm test fixtures or
T
L W 35 for 30 mm test fixtures or
T
L W 45 for 40 mm test fixtures
T
a
Edge chamfers or rounding.
Figure 3 — Test specimen dimensions
8 Procedure
8.1 Introduction of the precrack by Knoop indentation
8.1.1 Use a Knoop indenter to indent the middle of the polished, lapped or fine ground surface of the
o
specimen. The indentation shall be perpendicular to the specimen long axis to within 2 as shown in Figure 1.
o o
One end of the specimen shall be tilted approximately 0,25 to 0,5 as shown in Figure 4. A full load dwell time
of 15 s or more during the indentation cycle shall be used. The indentation may be placed in either a 3 mm or
4 mm wide face as shown in Figure 5. It is recommended that the indentation be placed near to the exact
centre of the surface, both along the width dimension B and along the specimen length, in order to make it
easy to confirm that fracture occurs from the precrack.
NOTE 1 The 0,25° to 0,5° tilt makes the precrack easier to detect on the fracture surface. The specimen tilt causes
precrack tilts from 0° to 5°.
NOTE 2 In some instances such as with zirconia, indentation times longer than 15 s may be helpful.
NOTE 3 A trial specimen may be tested to help determine the best indentation load. Indent then fracture the specimen
in the flexure fixtures without removal of the indentation and residual stress damage zone. Examine the fracture surface to
confirm that the specimen has fractured from the precrack, that the precrack is discernible, and within the prescribed size
limits.
NOTE 4 The Knoop indentation procedure to create a surface crack will not be successful on very soft or porous
ceramics since a precrack will not form under the indentation. The process may not work on very tough ceramics which
are resistant to the formation of cracks, or where the cracks are very small and likely to be removed during the subsequent
polishing step to remove the residual stress and damage zone.
ISO 18756:2003(E)
a) Specimen side view b) Specimen end view
Key
1 Knoop indenter
2 precrack
3 indent with precrack
4 platform tilts specimen
Figure 4 — Indentation of the precrack by Knoop indentation

NOTE 1 The indentation may be placed on either the wide 4 mm face or the narrow 3 mm face.
NOTE 2 The crack size has been exaggerated for illustrative purposes and is usually much smaller.
Figure 5 — Specimen cross section
8.1.2 The optimum indentation load used may have to be determined for each different class of material by
the use of a few trial specimens. The load shall be sufficient to create a crack that is larger than the naturally-
occurring flaws in the material, but not too large relative to the specimen cross section size (2c < 0,5B and
a < 0,5W) nor so large that the indentation is badly spalled or shattered. Indentation loads of approximately
20 N are suitable for very brittle ceramics, 25 N to 50 N for medium tough ceramics, and 49  to 98 N for very
tough ceramics or ceramics with some porosity. Indentation loads of 98 N to 147 N may be necessary for
materials with medium- to coarse-grain sizes. In such materials, it is necessary to make large precracks that
will stand out against the normal microstructural roughness on the fracture surface.
8.1.3 Measure the length of the long diagonal, d, of the Knoop impression to within 0,005 mm (5 µm).
8 © ISO 2003 — All rights reserved

ISO 18756:2003(E)
NOTE A conventional microhardness machine (6.4) may be used for this measurement. The measurement does not
require the precision needed for hardness measurements. If the Knoop hardness is reported, greater care is
recommended in making the diagonal size measurement and in the preparation of the initial specimen surface.
8.1.4 Compute the depth, h, of the Knoop impression as follows:
hd= /30 (1)
8.1.5 Measure the specimen depth, W, in the middle of the specimen at the indent location to within
0,002 mm. A hand micrometer (6.3) with vernier graduations marked in 0,002 mm increments is suitable.
8.1.6 Mark the side of the specimen with a pencil or other marker with an arrow to indicate which surface
has the precrack.
8.1.7 Remove the indentation and the residual stress damage zone.
8.1.7.1 Remove from the indented surface an amount of material that is approximately equal to 4,5h to
5,0h as shown in Figure 6. The material removal process shall not induce residual stresses or excessive
machining damage in the specimen surface. Be careful to remove material from the correct face. Mark with
pencil or permanent marker the faces that will not be ground or polished. Material may be removed by any
one of the three procedures described in 8.1.7.2, 8.1.7.3 and 8.1.7.4.
NOTE The removal of 4,5h to 5,0h eliminates the residual stress damage zone under the impression, and usually will
leave a precrack shape that has the greatest stress intensity factor at the deepest part of the precrack periphery. The
location of the maximum stress intensity can be controlled by the amount of material removed. The initial precrack under
the Knoop indent is roughly semicircular and the maximum Y, stress intensity factor coefficient, Y is at the surface. As
max
material is removed, the precrack becomes more semi-elliptical in shape (or like a section of a circle) and Y will shift to
max
the deepest part of the precrack. If too much material is removed, the remaining precrack will be too small and fracture will
not occur from the precrack. In such cases it is preferable to remove smaller amounts, provided that no less than 3h is
removed. If this step is not adequate to ensure fracture from the precrack, then a greater indentation load may be needed.

Key
1 material to be removed after indentation
NOTE The precrack extends below the Knoop hardness indentation, which has a depth h.
Figure 6 — The indentation and the residual stress damage zone
8.1.7.2 Material may be removed by hand grinding, hand lapping or hand polishing with abrasive papers
under wet or dry conditions. Hand polishing the specimen with 180 to 220 grit silicon carbide paper can
remove the required amount in 5 min to 10 min per specimen for many ceramics. Check the specimen height
frequently during this process. Remove the last 0,005 mm with a finer grit (220 to 280 grit) paper with less
pressure, so as to minimize polishing damage. Monitor the specimen depth, W, frequently during the material
removal step, with special emphasis on monitoring the evenness of the material removal.
ISO 18756:2003(E)
Hand grinding, hand lapping or hand polishing may not be effective with very hard ceramics. For very hard
ceramics, material may be removed by machine polishing or lapping (8.1.7.3), or by machine surface grinding
(8.1.7.4).
Regularly change the orientation of the surface being hand polished or ground during material removal in
order to minimize unevenness. Unevenness may cause misalignments during subsequent flexure testing or
cause errors in the cross section size measurements.
NOTE 1 Dry hand grinding may be faster than wet grinding. Diamond impregnated polishing discs (30 µm) are also
effective in removing material by hand grinding.
NOTE 2 Hand lapping or grinding may make the precracked surface uneven or not parallel to the opposite specimen
surface. A slight rounding of the specimen edges is usually inconsequential.
NOTE 3 Finer grit (320 to 500 grit) papers are recommended for glasses for both rough removal and fine finishing steps
CAUTION — Fine ceramic powders or fragments may be created if the lapping or hand sanding is
done dry. Masks should be used or the removal done wet if there is an inhalation hazard, especially if
the ceramic contains silica or fine whiskers.
8.1.7.3 Material may be removed by machine polishing or lapping with diamond slurry or paste containing
about 0,3 µm particles. This requires about 10 min to 15 min per specimen for many ceramics. A trial
specimen may be polished to obtain an appropriate removal rate by adjusting applied masses, rotating speed
of the disc and polishing times in order to obtain the correct amount of material removal.
8.1.7.4 Material may be removed by surface grinding with diamond wheels on a grinding machine for
very hard materials. Take care to ensure that the correct amount of material has been removed from each
specimen. Avoid aggressive grinding conditions that may introduce residual stresses. If machine surface
grinding is used, fine wheel grits (320 to 600 grit) and small removal rates are recommended. Grinding may be
carried out under wet conditions.
8.1.7.5 After the prescribed amount of material has been removed, examine the ground-indented surface
for evidence of remnant lateral cracks. Figure 7 provides guidance. A low power reflected light metallurgical
optical microscope (6.5) with magnifications from 100× to 500× may be used to examine the ground-indented
tensile surface. If there is evidence of remnants of lateral cracks, then additional material should be removed
(6h to 10h) to ensure that the lateral cracks remnants are removed.
1/2
NOTE Deeper than normal lateral cracks may occur in materials with very low fracture toughness (< 3,0 MPa m ) or
if larger indentation loads (W 98 N) are used.

a) shows the ground surface after material removal. The Knoop indentation (dashed lines) has been removed and the
median crack is very tight and not visible. There are no traces of lateral cracks.
b) to e) show examples of remnants of lateral cracks which should be removed in accordance with 8.1.7.4 and 8.1.7.5.
Figure 7 — A ground surface and remnants of lateral cracks existing after removal of the damage zone
in some brittle materials
10 © ISO 2003 — All rights reserved

ISO 18756:2003(E)
8.1.7.6 Annealing or heat treating to remove the residual stresses under the indentation is not permitted
by this International Standard due to the risk of crack tip blunting or crack healing.
8.1.8 Dry the specimen prior to testing if the material removal is done wet.
NOTE There is no consensus on the best conditions for drying specimens. Heating in an air or vacuum oven at

100 °C to 150 °C for times up to 1 h and then storage in a desiccator prior to testing may be sufficient.
8.1.9 If necessary, a dye penetrant (6.6) may be applied to aid crack detection. If a dye penetrant is used,
the specimen should be dried thoroughly prior to fracture.
8.1.10 Measure and record the specimen dimensions, B and W, in the vicinity of the precrack to within
0,002 mm.
8.2 Specimen fracture
8.2.1 Ensure that the specimen is dry.
8.2.2 Test the precracked specimens to fracture in four-point flexure in laboratory ambient conditions. If the
material is susceptible to slow crack growth, it is recommended that the testing conditions of 8.4 be applied.
NOTE Many oxides, glasses and ceramics having glassy boundary phases may be susceptible to slow crack growth.
Measured fracture toughness may be sensitive to displacement rate and moisture in the atmosphere. See Annex A for
additional background information.
8.2.3 Insert the specimen into the flexure fixture as shown in Figure 8, with the surface crack on the tension
face, approximately in the middle (within 1 mm) of the two inner loading rollers. The specimen may be
preloaded to no more than 25 % of the expected fracture load. Place cotton, crumpled tissue or other
appropriate material under the specimen to prevent the pieces from impacting the fixture upon fracture and to
prevent the fracture surface from being damaged by impact after the specimen breaks. Place a simple shield
around the fixture to ensure operator safety and to preserve the primary fracture pieces for subsequent
fracture analysis. If the specimen precracked face is not parallel to the opposite face to within 0,015 mm, then
fully-articulating fixtures shall be used.
8.2.4 Use a standard displacement rate of 0,5 mm/min for specimens tested with 30 mm or 40 mm support
span fixtures. Use a displacement rate of 0,10 mm/min to 0,13 mm/min for specimens tested with 20 mm
support span fixtures.
8.2.5 Apply a compressive load to the fixture until the specimen fractures. Measure the fracture load to an
accuracy of ± 1 %. The time to fracture should not exceed 20 s in order to minimize environmental effects. If
the time to fracture is greater than 20 s, use a faster displacement rate than those given in 8.2.4.
8.2.6 Measure the ambient temperature during the test series.
8.2.7 Measure the ambient relative humidity during the test series if tests are performed in laboratory
ambient conditions.
ISO 18756:2003(E)
The flexure specimen may be tested with either the wide or narrow face on the loading bearings.
The precrack shall be on the tension surface which is the bottom surface in this figure.
Figure 8 — The flexural specimens on the loading bearings
8.3 Crack size measurement
8.3.1 Examine the fracture surface of the specimens and measure the critical crack dimensions a and 2c as
shown in Figure 4 or 5. Fractographic techniques and fractographic skill are needed for this step. 8.3.2 to
8.3.10 give more detail on the procedures that may be used. Annex B provides guidance on crack detection
and characterization. If stable crack extension is not detected, the critical crack size should be the same as
the precrack size. Measure the crack depth, a, to within 0,005 mm (5 µm) or less if possible and the crack
width, 2c, to within 0,010 mm (10 µm) or less if possible.
NOTE The achievable precision of the crack size measurement depends upon the material and its microstructure, the
clarity of the crack and the mode of viewing. For some materials, it is possible to measure the crack size with greater
precision than suggested in 8.3.1, but in other materials the achievable precision may be less than suggested in 8.3.1. In
many instances, the computed fracture toughness is not very sensitive to the precision of the crack size measurement as
discussed in References [3] and [5]. Depending upon the crack sizes and specimen geometries, satisfactory estimates of
fracture toughness may be obtained even with crack size measurements that are less precise than suggested in 8.3.1.
8.3.2 The optimum procedure will vary from material to material. Either an optical microscope or a scanning
electron microscope, or both may be used. Low magnifications (50× to 100×) may be used to locate the crack,
and higher magnifications (100× to 500×) to directly measure or photograph the crack for measurement.
8.3.3 If an optical microscope is used, then variation of the lighting source direction can be used to highlight
the crack. Stereo binocular optical microscopes are preferred to metallographic microscopes. Crack sizes may
be measured from photos taken of the fracture surface, by direct measurement while viewing the specimen if
the microscope has a precision transversing stage for the specimen, or by an eyepiece filar measurement
device. If photos are taken, the fracture surface plane should be normal to the camera axis and a stage
micrometer should be used to confirm the magnifications.
12 © ISO 2003 — All rights reserved

ISO 18756:2003(E)
8.3.4 If a scanning electron microscope (SEM) is used, then an SEM magnification calibration standard
shall be used to confirm the magnification.
NOTE Additional details on techniques to find and characterize the cracks for both optical and SEM microscopy are
given in Annex B.
8.3.5 The crack shape may be approximated by a semi-ellipse. This approximation is most accurate for
instances where the greatest stress intensity factor coefficient is at the deepest part of the crack (Y = Y ;
max d
see 9.1). If the maximum stress intensity factor coefficient is at the surface (Y = Y ), then re-examine the
max s
crack shape to confirm that the crack is semi-elliptical. If it is not, then reject the datum.
8.3.6 If the crack form is severely distorted in the third dimension (i.e. is not flat), or the crack front line is
incomplete over more than 33 % of its periphery, reject the datum. See Figures B.7 c), B.7 e) and B.7 f).
8.3.7 If hand grinding or machining damage [see Figure B.7 a)] interfere with the determination of the crack
shape and Y > Y , then reject the datum.
s d
8.3.8 If the precrack shows evidence of excessive extension (corner pop-in) at the intersection of the
surface [see Figure B.7 b)], then reject the datum.
8.3.9 If the precrack shows evidence of stable crack extension prior to fracture, then measure both the
initial precrack size and the critical crack size. [See Figures B.4 and B.7 d)].
8.3.10 If the crack width is such that 2c > 0,5B or the depth is such that a > 0,5W, then reject the datum. A
smaller indentation load may be used.
8.4 Environmental effects
8.4.1 If susceptibility to environmental degradation, such as slow crack growth, is a concern, then tests
should be performed in accordance with either 8.4.2, 8.4.3 or 8.4.4.
8.4.2 Perform tests at two different displacement rates. The two test rates should differ by at least two or
three orders of magnitude. One rate should be very slow, so that the crack has a chance to react to the
environment. Susceptibility may be evaluated by comparing the mean fracture toughness values at the two
rates. Environmental susceptibility may also be determined by examining the fracture surfaces for evidence of
crack extension such as halos at the slower testing rate [Figures B.4 and B.7 d)]. If the material is susceptible
to environmental effects, then determination of the critical crack size is required. Annex A provides some
examples of the use of the critical crack sizes and also variations of displacement rate. If the precrack size or
incorrect critical crack size is used for the calculation, the fracture toughness values may strongly depend on
the displacement rate.
8.4.3 Perform tests in an inert environment such as dry nitrogen gas. Select an atmosphere which is
considered not to adversely affec
...