Bsi bs en 00820 1 2002 (2003)

BS EN
820-1:2002

BRITISH STANDARD

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Advanced technical
ceramics — Methods of
testing monolithic
ceramics —
Thermomechanical
properties —
Part 1: Determination of flexural
strength at elevated temperatures

The European Standard EN 820-1:2002 has the status of a
British Standard

ICS 81.060.30

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BS EN 820-1:2002

National foreword
This British Standard is the official English language version of
EN 820-1:2002. It supersedes DD ENV 820-1:1994 which is withdrawn.
The UK participation in its preparation was entrusted to Technical Committee
RPI/13, Advanced technical ceramics, which has the responsibility to:
—

aid enquirers to understand the text;

—

present to the responsible international/European committee any
enquiries on the interpretation, or proposals for change, and keep the
UK interests informed;

—

monitor related international and European developments and
promulgate them in the UK.

A list of organizations represented on this committee can be obtained on
request to its secretary.
Cross-references
The British Standards which implement international or European
publications referred to in this document may be found in the BSI Catalogue
under the section entitled “International Standards Correspondence Index”, or
by using the “Search” facility of the BSI Electronic Catalogue or of British
Standards Online.

This publication does not purport to include all the necessary provisions of a
contract. Users are responsible for its correct application.
Compliance with a British Standard does not of itself confer immunity
from legal obligations.

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This British Standard was
published under the authority
of the Standards Policy and
Strategy Committee on
6 March 2003

Summary of pages
This document comprises a front cover, an inside front cover, the EN title page,
pages 2 to 10, an inside back cover and a back cover.
The BSI copyright date displayed in this document indicates when the
document was last issued.

Amendments issued since publication
Amd. No.

Date

Comments

© BSI 6 March 2003

ISBN 0 580 41375 6

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EUROPEAN STANDARD

EN 820-1

NORME EUROPÉENNE
EUROPÄISCHE NORM

October 2002

ICS 81.060.30

Supersedes ENV 820-1:1993

Advanced technical ceramics - Methods of testing monolithic
ceramics - Thermomechanical properties - Part 1: Determination
of flexural strength at elevated temperatures
Céramiques techniques avancées - Méthode d'essai des
céramiques monolithiques - Propriétés thermodynamiques
- Détermination de la résistance à la flexion à températures
élevées

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English version

Hochleistungskeramik - Monolithische Keramik Thermomechanische Eigenschaften - Teil 1: Bestimmung
der Biegefestigkeit bei erhöhten Temperaturen

This European Standard was approved by CEN on 8 August 2002.
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 Management Centre 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 Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: rue de Stassart, 36

© 2002 CEN

All rights of exploitation in any form and by any means reserved
worldwide for CEN national Members.

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Ref. No. EN 820-1:2002 E

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EN 820-1:2002 (E)

Contents
page
Foreword......................................................................................................................................................................3
Scope ..............................................................................................................................................................4

2

Normative references ....................................................................................................................................4

3

Terms and definitions....................................................................................................................................4

4
4.1
4.2
4.3
4.4
4.5

4.6

Apparatus .......................................................................................................................................................5
General............................................................................................................................................................5
Test jig.............................................................................................................................................................5
Heating device................................................................................................................................................6
Test machine ..................................................................................................................................................6
Linear measuring devices.............................................................................................................................6
Drying oven ....................................................................................................................................................7

5
6
7
8
9
10

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1

Test pieces......................................................................................................................................................7
Test temperatures and number of tests ......................................................................................................7
Test procedure ...............................................................................................................................................7
Calculations....................................................................................................................................................8
Accuracy and references ..............................................................................................................................9
Test report ....................................................................................................................................................10

2
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EN 820-1:2002 (E)

Foreword
This document EN 820-1:2002 has been prepared by Technical Committee CEN/TC 184 "Advanced technical
ceramics", the 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 April 2003, and conflicting national standards shall be withdrawn at the latest by
April 2003.
This document supersedes ENV 820-1:1993.
This document has been prepared under a mandate given to CEN by the European Commission and the European
Free Trade Association.
EN 820 'Advanced technical ceramics – Methods of testing monolithic ceramics – Thermomechanical properties'
consists of three Parts:


Part 1 : Determination of flexural strength at elevated temperatures



Part 2 : Determination of self-loaded deformation



Part 3 : Determination resistance to thermal shock by water quenching

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According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Czech Republic, Denmark, Finland,
France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Spain,
Sweden, Switzerland and the United Kingdom.

3
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EN 820-1:2002 (E)

1

Scope

This Part of this European Standard specifies a method of determining the three-point or four-point flexural strength of
advanced monolithic technical ceramics at elevated temperatures as agreed between parties to the test. The test can
be performed in any appropriate atmosphere.

2

Normative references

This European Standard incorporates by dated or undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text, and in the publications listed hereafter. For dated
references, subsequent amendments to or revisions of any of these publications apply to this European Standard only
when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred
to applies (including amendments).
EN 843-1, Advanced technical ceramics – Monolithic ceramics – Mechanical tests at room temperature – Part 1:
Determination of flexural strength.
ENV 843-5, Advanced technical ceramics – Monolithic ceramics – Mechanical tests at room temperature – Part 5:
Statistical analysis of fracture data.
EN 10002-2, Metallic materials – Tensile testing – Part 2: Verification of the force measuring system of the tensile
testing machine.

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EN ISO 17025, General requirements for the competence of testing and calibration laboratories.
IEC 60584, Thermocouples - Part 1: Reference tables.
IEC 60584, Thermocouples - Part 2: Tolerances.
ISO 3611, Micrometer callipers for external measurement.

3

Terms and definitions

For the purposes of this European Standard, the following terms and definitions apply.
3.1
nominal flexural strength

maximum nominal stress at the instant of failure supported by the material when loaded in elastic bending
NOTE
It is recognised that flexural strength tests on ceramics at elevated temperature may reveal inelastic behaviour in
the material. Under such conditions, the nominal flexural strength calculated in accordance with this standard is not strictly a
valid result, since it tends to overestimate the true surface stress in the test piece. This method requires that the
load/displacement relationship for each test piece at each temperature is inspected, and the validity of the result determined.

3.2
three-point flexure
means of bending a beam test piece whereby the test piece is supported on bearings near its ends, and a central load
is applied

4
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EN 820-1:2002 (E)

3.3
four-point flexure
means of bending a beam test piece whereby the test piece is supported on bearings near its ends, and is loaded
equally at two points symmetrically disposed about the centre of the supported span
3.4

inelastic deformation
for the purposes of the test, the deformation of a test specimen under load which is not entirely elastic and reversible on
removal of the load
3.5
subcritical crack growth
extension of existing cracks or flaws under a stress which does not produce instant failure
NOTE

4
4.1

This effect can be due to the intrinsic crack propagation of the material, additionally influenced by the environment.

Apparatus
General

The test is carried out in a similar manner to that prescribed in EN 843-1 for testing at room temperature, with the
exceptions that the test jig shall be constructed from materials which are capable of remaining linearly elastic at the
temperature of testing, and that a means of heating the test jig and test piece to the test temperature is required.

4.2

Test jig

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The test jig shall be of a design that allows full articulation for alignment of the support and loading rollers on the
surfaces of the test piece, and in which the rollers are capable of rolling to minimise friction between the rollers and the
test piece.
NOTE 1

It is recognised that –practical limitations can restrict the design of the test jig, and oxidation effects can restrict its
function. In such cases, alternatives may be employed, but deviations from the function specified above should be reported in
the report (see clause 10, d)).

The outer span of the test jig shall be either 20,0 mm ± 0,5 mm (span A) or 40,0 mm ± 0,5 mm (span B). For three-point
flexure the loading roller shall be centralised relative to the span to within 0,2 mm. For four-point flexure, the inner span
shall be 10,0 ± 0,2 mm (span A) or 20,0 mm ± 0,2 mm (span B). The inner rollers shall be symmetrically positioned
relative to the outer rollers to better than 0,2 mm.
NOTE 2
This represents a relaxation of centralisation requirements compared with room temperature testing (requirement
0,1 mm, see EN 843-1) because it can become relatively more difficult to set up test jigs inside furnaces.

Measurements of the positions of the rollers shall be made with a travelling microscope (4.5.2).
The rollers shall be made from a material which is capable of remaining linearly elastic up to the maximum temperature
of the series of tests. For tests to 400 °C, hardened steel rollers as in EN 843-1 shall suffice. For higher temperatures it
is recommended that the test jig is constructed from fine-grained strong refractory ceramic materials.
NOTE 3
For temperatures up to 1 400 °C, high-purity high-alumina ceramic materials can be used. For temperatures up to 1
°
500 °C in air or up to 1700 C in neutral atmospheres, sintered silicon carbide is recommended.

Consideration should be given to any potential reaction between the test material and the rollers at the maximum test
temperature, and the choice of roller material shall be made such that reactions are minimised. In particular, non-oxide
ceramic materials may tend to oxidise, and the oxide then may react with the loading rollers. Rollers shall be
maintained with a clean smooth burr-free surface for all tests. They shall be inspected before any test to ensure they
are undamaged, and are capable of rolling freely in the jig at the start of the test. If necessary, rollers should be
replaced to maintain correct jig function. Similarly, reactions between the main ceramic components of the test jig shall
be minimized by appropriate choice of material.
5
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EN 820-1:2002 (E)

NOTE 4
At very high temperatures, the progressive oxidation of non-oxide jig components can lead to the development of
frictional effects due to inability of the rollers to roll freely. Such effects can lead to uncertainties in nominal flexural strength of
up to 5%. It is not possible to quantify this effect, which is likely to be time, temperature and material dependent. Jig components
can be found to have welded together on cooling to room temperature. They should be gently separated and cleaned before reuse.

The load shall be applied to the test jig through solid or hollow columns which are loaded by the test machine. These
columns shall be of a material which remains linearly elastic to the maximum testing temperature.
NOTE 5
To ascertain this, the columns of the test jig can be brought into contact at the appropriate test temperature with or
without the jig in place. A load equivalent to that estimated to be needed to fracture test pieces is applied, and the apparent load
displacement relationship is examined for evidence of non-linearity with increasing load (see clause 8).

4.3

Heating device

The test jig shall be contained within a furnace or suitable heating device of design appropriate to the maximum
temperature for the tests, and capable of reaching the maximum testing temperature within 3 h.

NOTE 1
The use of very fast heating rates is not recommended because of risk of fracture of ceramic components in the test
jig or columns.

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The temperature control system used with the furnace shall be capable of controlling to within ± 5 °C using a suitable
thermocouple. The temperature of the test piece shall be recorded using a second thermocouple manufactured in
accordance with the manufacturing tolerances stated in IEC 60584-2, allowing the use of reference tables in IEC
60584-1 or, alternatively, calibrated in a manner traceable to the International temperature scale ITS-90. The tip of the
thermocouple shall be close to but not touching the test piece. It shall previously have been determined that the
temperature of the test piece shall not vary by more than ± 5 °C over its length after a 15 minute hold time at the
required testing temperature.

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NOTE 2
For use in an air atmosphere, the furnace can most conveniently be of a split design allowing access to the test jig
in the position for testing. For a controlled atmosphere, a similar arrangement may be used with the test jig surrounded by a
gas-tight work-tube, or alternatively, a sealed atmosphere furnace may be employed. The flow conditions for gas should also be
considered. It should preferably be static or of limited flow rate to minimise problems of inhomogeneous test piece temperature.
In all cases it is desirable to have ready access to the test jig during set-up. A multiple test jig or hot specimen insertion system
can have some advantages in terms of total time required for testing, but care should be taken that the jig and test piece
alignment requirements are maintained for all jigs and/or test pieces, and the 15 min temperature equilibration time is used for
each test piece.
NOTE 3
It is advisable to employ water cooling for parts of the furnace and loading column system to minimise the
temperature variations of the test machine and to protect load measuring devices.
NOTE 4
It is permissible to employ a system for rapid cooling of the test piece fragments after failure to allow inspection of

fracture surfaces without subjecting them to further heating or to oxidation. This can most readily be done by allowing the
fragments to drop out of the furnace down the support tube, preferably onto ceramic fibre blanket to avoid further damage.

4.4

Test machine

The test machine shall be capable of applying a force to the test jig at constant loading or cross-head displacement
rate. The test machine shall be equipped for recording the load and displacement output for inspection, and for
determining the peak load at fracture. The accuracy of the test machine shall be in accordance with EN 10002-2,
Grade 1 (error limit of 1% of indicated load).
Ensure that the force calibration of the test machine has recently been checked in accordance with EN 10002-2.

4.5

Linear measuring devices

4.5.1 A micrometer in accordance with ISO 3611, capable of recording to 0,01 mm and accurate to this level.
Flat anvils shall be used to minimise risks of producing indentation damage in test pieces during measurement.

6
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EN 820-1:2002 (E)

4.5.2 Travelling microscope or other suitable device reading to the nearest 0,05 mm (used for measurement of
the distance between loading rollers).

4.6

Drying oven
°

A drying oven capable of maintaining (120 ± 10) C.

Test pieces

Test pieces shall be prepared in accordance with the procedures defined in EN 843-1, according to agreement
between parties, and in the surface finish categories:
i

As-fired;

ii

Machined (agreed procedure);

iii.1 Standard finish by grinding;

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5

iii.2 Standard finish by lapping and/or polishing.

°

After machining they shall be washed free from cutting and grinding fluids, and dried in an oven at (120 ± 10) C for 2 h,
followed by equilibration for at least 2 h in the laboratory atmosphere.
NOTE

6

If the test temperature is greater than about 120 °C, equilibration is generally not necessary.

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Test temperatures and number of tests

The test temperatures and the number of test pieces to be tested at each temperature shall be agreed between parties.
At least 5 test pieces shall be tested at each temperature. For statistical evaluation of strength data (e.g. Weibull
parameters, etc.) the minimum number shall be 30 (see ENV 843-5).
NOTE
Weibull parameters estimated from the test data can be seriously in error if the number of nominally identical testspieces is less than 30. The uncertainty in the parameters is sufficient as to render comparisons between materials meaningless.

7

Test procedure

Assemble the furnace and the test jig in the test machine.
Measure the width and thickness of each test piece at a minimum of three positions approximately equidistant along its
length, using the micrometer (4.5.1). Do not apply any marks to the test piece, e.g. pencil marking, except to the ends
°
outside the test span, if the test temperature is to exceed 400 C.
NOTE 1

Marking materials that withstand high temperatures can contain a clay or other inorganic components which can
react with the test piece and influence the measured strength properties.

Place each test piece in turn in the test jig orientated so that the appropriate face is subjected to tensile stress on
loading. Ensure that both test piece and jig are centralised with respect to the loading axis, and that the test jig rollers
are in their correct starting positions as defined in 4.2.
NOTE 2
If the available furnace arrangements do not allow convenient access to the test jig for aligning the test piece, an
alignment jig should be used. Alternatively, the alignment can be performed out of the furnace, and then the whole arrangement
placed between the loading columns. Care should be taken to ensure that the test piece does not move during this process. If
there is a risk of movement, a temporary adhesive, such as polystyrene cement, can be used to fix the test piece and rollers in
position. This burns out at 200 °C to 300 °C on heating in the furnace, leaving all components correctly positioned.

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EN 820-1:2002 (E)

If necessary apply a small load not more than 5% of the expected fracture load to keep the test piece in position. Raise
the furnace temperature to the required test temperature, and maintain this temperature for a period of 15 minutes to
ensure thermal equilibrium. During heating ensure that any load applied to the test piece does not increase beyond 5%
of the expected failure load as a consequence of thermal expansion of the columns and the test jig.
After stabilisation of the test temperature, select a rate of load application or cross-head displacement rate such that
fracture is obtained in a period of 5 - 15 s, this rate to be kept the same for all tests in the series. Choose a recording

range on the test machine such that the expected average force at fracture is near the centre of the range. If the
recordings of load and displacement are to be made using a chart recorder, select a speed range on the chart recorder
o
such that the load/displacement trace is drawn at about 45 to the chart axis.
NOTE 3
If the test machine does not have constant loading rate control, selection of a displacement rate may have to be
determined by experiment, depending on the elastic compliance of the test machine, the stiffness of the test jig and the loading
columns, and the elastic properties of the test piece. A machine displacement rate of 0,5 mm/min is a convenient starting point
for most machines in cases where the expected strength of the material is 200 MPa to 400 MPa. For materials which are much
weaker or much stronger than this, the displacement rate can have to be respectively decreased or increased by an appropriate
factor.
NOTE 4
The strength of advanced monolithic technical ceramics can be markedly affected by the test rate as a consequence
of the phenomenon of sub-critical crack growth, especially at temperatures at which inelastic deformations become significant.
The short time required by this test is intended to match the testing rate prescribed in EN 843-1, and to compromise between
the elimination of significant testing rate effects and the ability of test machines and load recording equipment to provide an
accurate measure of peak fracture load.

Apply the test force at the chosen rate and record the load/displacement behaviour. Record the peak load supported by
the test piece at the instant of fracture and the time to failure.
Unless a multiple test piece system or a hot test piece insertion and rapid quench arrangement has been employed,
cool the furnace to ambient temperature and retrieve the broken fragments of test piece for later examination.

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Even if the test piece has failed away from the central loading roller in three-point bending or outside the uniformly
stressed zone between the two loading rollers in four-point bending, the result shall not be ignored, and shall be
included in the report of the test series and in the calculation of nominal mean strength.
If the test pieces were in the as-fired condition before testing, remeasure the thickness and width of the test piece at the
fracture position.

Inspect the test piece fragments for signs of oxidation and/or reaction with the loading rollers. Inspect the loading rollers
for signs of reaction with the specimen or oxidation or flattening which would impair the performance of the jig.
Renovate or replace the loading rollers if they do not conform to the requirements for free rolling movement.
Repeat the procedure for each test piece.

8

Calculations

For three-point flexure:

σf =

3 Fm l
2 b h2

(1)

For four-point flexure:

σf =

3 Fm d
b h2

(2)

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Calculate the nominal flexural strength of each test piece from its dimensions and peak load at failure in accordance
with EN 843-1.


EN 820-1:2002 (E)

where
is the nominal fracture stress expressed in Mega Pascal (numerically equivalent to N mm );

Fm

is the peak force at fracture expressed in Newtons;

b

is the test piece width, expressed in millimetres, the mean of three determinations, or at the fracture
position for the as-fired condition (I);

h

is the test piece thickness in the direction of bending, expressed in millimetres, the mean of three
determinations, or at the fracture position for the as-fired condition (I);

l

is the distance between centres of the outer support rollers (three-point flexure), expressed in
millimetres;

d

is the mean of the distances between the centres of the inner loading and outer support rollers (fourpoint bending), expressed in millimetres.

If load/displacement data have been digitally recorded by the test machine, prepare a graph of load versus nominal
displacement. Inspect each load/displacement graph or chart recorder trace in the following way. Ignoring the first part
of the curve where the slack in the load train is being taken up, place a ruler against the linear part of the trace. If the
trace shows any obvious signs of curvature indicating loss of stiffness as the load increases, it shall be taken that the
material is displaying inelastic behaviour. This fact shall be identified in the test report (see clause 10, k)).
NOTE 1
If inelastic deformation is apparent from the load/displacement trace with a test piece in place, but absent when no
test piece is employed (see 4.1, NOTE 3), this could be due to two causes: plastic deformation of the test piece and sub-critical
crack growth in the test piece. Assuming the effect originates from the test piece, under conditions where the effect is small,
distinguishing between these possible causes is difficult unless further testing and examination of fracture surfaces is
performed. In the case of plastic deformation, the nominal stresses that are calculated according to the formulae above become
seriously in error; and there is no convenient simple method of correcting apparent nominal strength to true nominal strength. In
the case of sub-critical crack growth, the test piece compliance increases a little, but behaviour on unloading remains essentially
elastic, and the formulae remain correct. Because of the difficulty in distinguishing these causes, any sign of inelasticity should
be noted in the test report.
NOTE 2
A progressive increase in apparent stiffness with load may be noted. This is a result of progressive stiffening of the
load train. If this occurs, it is more difficult to distinguish the onset of inelastic deformation in the test piece.

Calculate the mean fracture strength at each temperature.
NOTE 3

If the number of valid test results at a given temperature is sufficiently high, e.g. 30 or more, it is strongly
recommended that statistical analysis is performed according to the procedures described in ENV 843-5.

9

Accuracy and references

The accuracy of the test procedure depends on the ability of the test jig to conform to the requirements laid down in
EN 843-1 concerning room temperature tests. Provided that the jig is free to align itself, and the rollers are free to roll,
errors in the actual stress introduced in the test piece are small, probably less than 1 %. Errors may also be introduced
by inaccuracies in measurement of fracture load and test piece dimensions, giving an overall error limit of ± 2% if all
aspects of specification in this test are adhered to.
The principal interferences in this test are changes to the above conditions due to degradation of the contact surfaces
of the rollers on the test piece and on their bases. For example, in the case of sintered silicon carbide rollers, oxidation
at high temperatures may lead to some frictional component being introduced because of adhesion or reaction
between the contact surfaces. The effects are presently non-quantified at high temperatures, and thus frictional errors,
which can be as high as a 5 % overestimate of nominal flexural strength, may be introduced.
At elevated temperatures, the thermal expansion of the test jig and of the test piece causes possible errors in the true
nominal flexural strength because test piece and test jig dimensions are measured only at ambient temperature, and
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-2

f


EN 820-1:2002 (E)

will have all increased. Errors of up to about 3 % may arise, increasing with increasing temperature and with increasing
expansion coefficient of the test jig or test material.
NOTE
Unless otherwise agreed between parties, it is not normal to correct the results for thermal expansion effects.
However, if the expansion coefficients of the test jig and test piece materials are known, the true dimensions at each test
temperature can be calculated and used in Equations 1 and 2 in place of the room temperature figures. Care should be taken to
allow for rolling effects of the rollers in the calculation of true span length. The procedure adopted should be given in the test
report.

10 Test report
The test report shall be in accordance with EN ISO 17025, and shall contain the following information:
a)

the name and address of the testing establishment;

b)

the date of the test, a unique identification of the report and of each page, the name and address of the
customer, and the signatory to the report;

c)

a reference to this method, i.e. determined in accordance with ENV 820-1, and whether the test was in three or
four-point bending;

d)

a description of the test apparatus used, including jig design and construction material, furnace and loading
columns, and the testing machine;

e)

details of the test material type, manufacturing code, batch number;

f)

the surface finish category appropriate to the preparation method, i.e. I, ii, iii.1 or iii.2 (see EN 843-1);

g)

the exact method of test piece surface preparation, including details of the grinding machine, the grinding and
finishing operations, grit sizes, directions of grinding and depths of cut, chamfering procedure, and any
subsequent thermal treatment prior to testing;

h)

the average test temperatures during the tests, and the soak periods;

i)

the test machine displacement rate or stressing rate employed;

j)

the average time to failure at each temperature of testing;

k)

the individual nominal strength result for each test piece tested, expressed in MPa to three significant figures,
including those failing substantially away from the loading rollers (three-point bend) or outside the central
loading rollers (four-point bend), and for each test piece the result of inspection of the load/displacement trace
for inelastic effects; if a correction has been made for thermal expansion effects, this should be stated and the
method of correction described;

l)

unless otherwise agreed, the mean strength and standard deviation at each test temperature;

m) any relevant comments concerning deterioration or sticking of the test jig, and concerning the external surface
or fracture surface condition of the test pieces after testing;
n)

deviations from this method and comments on the test or test results.

10
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