SIST ENV 820-4:2002

SLOVENSKI STANDARD
SIST ENV 820-4:2002
01-januar-2002
Advanced technical ceramics - Monolithic ceramics - Thermomechanical
properties - Part 4: Determination of flexural creep deformation at elevated
temperatures
Advanced technical ceramics - Monolithic ceramics - Thermomechanical properties -
Part 4: Determination of flexural creep deformation at elevated temperatures
Hochleistungskeramik - Monolithische Keramik - Thermomechanische Eigenschaften -
Teil 4: Bestimmung der Kriechverformung unter Biegebeanspruchung bei erhöhten
Temperaturen
Ta slovenski standard je istoveten z: ENV 820-4:2001
ICS:
81.060.30 Sodobna keramika Advanced ceramics
SIST ENV 820-4:2002 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ENV 820-4:2002

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SIST ENV 820-4:2002
EUROPEAN STANDARD prENV 820-4
NORME EUROPÉNNE
EUROPÄISCHE NORM
UDC 666.3 : 620.179.13 : 536.4
Descriptors: Ceramics, thermal tests, thermal shock tests, determination, thermodynamic properties, thermal shock resistance
English version
Advanced technical ceramics - Monolithic Ceramics -
Thermomechanical properties - Part 4: Determination
of flexural creep deformation at elevated temperatures
Céramiques techniques avancées - deHochleistungskeramik - Monolitische
Céramiques monolithiques - Propriétés Keramik - Thermomechanische
thermomécaniques - Partie 3: Détermination Eigenschaften - Teil 3: Bestimmung der
This draft European Prestandard is submitted to CEN members for formal vote. It
has been drawn up by Technical Committee CEN/TC184.
CEN Members are the national standards bodies of Austria, Belgium, Denmark, Finland,
France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway,
Portugal, Spain, Sweden, Switzerland and United Kingdom.
CEN
European Committee for Standardization
Comité Européen de Normalisation
Europäisches Komitee für Normung
Central Secretariat: rue de Stassart 36, B-1050 Brussels
© 1999 Copyright reserved to CEN members
Ref. No. EN 820-4 : 1999 E
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SIST ENV 820-4:2002
ENV 820-4:2001 (E)
Contents Page
Foreword 3
1 Scope 4
2 Normative references 4
3 Terms and definitions 5
4 Significance and use 5
5 Principle 6
6 Apparatus 6
7 Test pieces 10
8 Procedure 11
9 Accuracy and interferences 16
10 Test report 17
Bibliography for detailed analysis of flexural creep 18
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SIST ENV 820-4:2002
ENV 820-4:2001 (E)
Foreword
This European Prestandard has been prepared by Technical Committee CEN/TC 184 "Advanced
technical ceramics", the secretariat of which is held by BSI.
EN 820 consists of four parts:
Part 1: Determination of flexural strength at elevated temperatures;
Part 2: Determination of self-loaded deformation;
Part 3: Determination of resistance to thermal shock by water quenching;
Part 4: Determination of flexural creep deformation at elevated temperatures.
This standard includes a bibliography.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to announce this European Prestandard: Austria, Belgium, Czech
Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg,
Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United Kingdom.
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SIST ENV 820-4:2002
ENV 820-4:2001 (E)
1 Scope
This Part of ENV 820 describes a procedure for undertaking flexural creep tests at elevated
temperatures on advanced technical ceramics, mainly for the purposes of comparison of
deformation behaviour of materials under stressed conditions and under any appropriate
atmospheric condition.
NOTE This prestandard does not provide a method of acquiring engineering performance data since
the stress distribution under flexural loading is indeterminate.
2 Normative references
This European Prestandard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Prestandard only when incorporated in it by amendment
or revision. For undated references the latest edition of the publication referred to applies (including
amendments).
ENV 820-1 Advanced technical ceramics - Monolithic ceramics - Thermo-mechanical properties
- Part 1: Determination of flexural strength at elevated temperatures
EN 843-1 Advanced technical ceramics - Monolithic ceramics - Mechanical properties at room
temperature - Part 1: Determination of flexural strength
ENV 1006 Advanced technical ceramics - Methods of testing monolithic ceramics - Guidance
on the sampling and selection of test pieces
EN 10002-2 Metallic materials - Tensile testing - Part 2: Verification of the force measuring
system of the tensile testing machines.
EN 60584-1 Thermocouples - Part 1: Reference tables (IEC 60584-1: 1995)
EN 60584-2 Thermocouples - Part 2: Tolerances (IEC 60584-2: 1989 + A1: 1989)
EN ISO/IEC 17025 General requirements for the competence of testing and calibration
laboratories (ISO/IEC 17025:1999)
ISO 3611 Micrometer callipers for external measurement
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SIST ENV 820-4:2002
ENV 820-4:2001 (E)
3 Terms and definitions
For the purposes of this Part of ENV 820 the following terms and definitions apply.
3.1
creep
the time-dependent non-elastic deformation of a material under an applied stress
3.2
creep rupture
the failure of a test piece under nominally constant loading conditions resulting from an
accumulation of microstructural damage
3.3
stress rupture
the catastrophic extension of a flaw having previously grown subcritically under constant nominal
stress leading to failure of the test piece
3.4
subcritical crack growth
the extension of existing cracks or flaws under stress which does not produce instant failure
4 Significance and use
The test is intended to evaluate the deformation of a test piece under nominally constant force as a
function of time at elevated temperatures. In particular it can be used for materials comparison, or
for determining the temperature at which creep deformation becomes significant for a prospective
engineering use.
During the course of such a test, the test piece can fracture. This can be due either to a subcritical
crack growth process unrelated to the mechanism of creep (stress rupture), or to the accumulation of
creep damage leading to acceleration of creep rate and the linking of damage to form cracks (creep
rupture). In some circumstances it is not possible to distinguish the mechanism of failure. In either
case, the test piece lifetime under the imposed temperature and stress conditions can be an important
aspect of a material's performance.
The analysis given in this Prestandard (see 8.6) produces purely nominal data, assuming that the
actual maximum nominal stress in the test piece is linearly proportional to the test force applied and
is constant during the test. Moreover, an additional assumption of linear dependence of strain on
stress is made for some deflection measurement methods. Furthermore, it does not give engineering
creep data equivalent to separate pure tensile or compressive conditions. In many cases, the creep
rate dependence is to the maximum stress, and can differ in tension and compression. Typically, the
true maximum stress in the test piece is less than that calculated using equation 1 because of faster
relaxation at higher stress levels, and the true surface strain rate can be greater than a linear
prediction in certain geometrical arrangements for determining the deformation, particularly if this
is done using the relative displacement of the loading system. The Bibliography contains references
to more detailed theoretical analyses of flexural creep accounting for such non-linearities.
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SIST ENV 820-4:2002
ENV 820-4:2001 (E)
5 Principle
The method involves supporting a bar test piece on two supports near its ends, heating it to the
required elevated temperature which is maintained constant, applying a force to two loading points
spaced symmetrically between the support points, and recording the deflection of the test bar with
time.
The deflection of the test piece is measured indirectly and continuously or at appropriate time
intervals during the test using the displacement of the loading system (see e.g. Figure 1(a)), or by
using contacting extensometer rods at given positions on the test piece (see e.g. Figures 1(b) to
1(e)). The indirect measurement of deflection (Figure 1(a)) is converted into a nominal maximum
surface strain in the test piece assuming a linear relationship between stress and accumulated strain.
Similar assumptions are involved in analysing deflections between support points and the span
centre (Figure 1(b) and 1(c)). When employing displacement measurement between the loading
points and the span centre (Figures 1(d) and 1(e)), the analysis assumes uniform curvature of the test
piece, a linear relationship between strain and distance from the neutral axis, and equal behaviour in
tension and compression. The slope of the strain/time curve can be converted to a creep strain rate.
6 Apparatus
6.1 Creep test loading jig
The test jig is essentially a four-point bend flexural test jig similar to that described in ENV 820-1
for flexural strength testing at elevated temperatures. It comprises a pair of parallel 5 mm diameter
support rods positioned 40 mm apart on a refractory supporting structure. These rods shall be free to
roll to eliminate friction effects. In contrast to the articulating requirement in ENV 820-1,
articulation is not required provided that the rods are accurately parallel in the horizontal plane to
within 0,001 mm per mm length of rod.
The loading assembly comprises a similar pair of freely rolling rods positioned on a loading block.
The spacing between these rods shall be between 30 % and 50 % of the spacing of the support rods.
The loading block shall be free to articulate relative to the loading column in order to permit
alignment of the loading rods on the test piece upper surface.
NOTE 1 Subject to agreement between parties, other test piece support and loading spans can be
employed. This can be particularly advantageous for creep-resistant materials. In addition, in some
conditions it is recognized that freely rolling rollers, although preferred, may not be feasible. Such
deviations from this method should be reported. The effect of restricted roller rotation may or may
not be significant depending on the test material and the testing conditions. There is some evidence
to suggest that the surface of glass-phase containing materials, or materials which oxidize to give a
viscous glassy surface layer, can have a low coefficient of friction against the roller material at the
test temperature, such that over the period of the test any friction becomes negligible. However, this
situation cannot always be guaranteed.
The loading block shall be guided appropriately such that the loading rods are positioned mid-way
between the support rods, thus centrally loading a test piece when placed on the support rods.
The parts of the loading jig shall be constructed from a ceramic material which is anticipated to be
more resistant to deformation than the materials under test. In addition the support and loading rods
shall be of a material which does not chemically react with the test piece.
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SIST ENV 820-4:2002
ENV 820-4:2001 (E)
NOTE 2 Suitable materials include high-purity alumina for use with most oxide-based test pieces,
or sintered silicon carbide for most non-oxide ceramics.
NOTE 3 Test jig parts manufactured from sintered silicon carbide or other silicon-based non-oxide
ceramics develop oxidation films in a short period of time when exposed to temperatures typically
o
above 1300 C in an oxygen-containing atmosphere. This can cause prevention of rolling of rollers
and impairment of jig function.
6.2 Heating device
A heating device surrounds the loading jig in such a manner as permit access to the jig for the
purposes of mounting and demounting test pieces. The heating device shall be capable of
o
maintaining a constant test piece temperature to ± 3 C over the duration of the test.
The temperature of the test piece shall be recorded using a thermocouple manufactured in
accordance with EN 60584-2 allowing the use of reference tables in EN 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
o
determined that the temperature of the test piece does not vary by more than ± 3 C over its length
when temperature has stabilized for more than 30 min.
The heating device can incorporate or be incorporated within a vacuum or other appropriate
chamber for control of gas atmosphere if appropriate to the determination.
6.3 Loading device
The load shall be applied through a loading column to which the upper part of the jig is connected.
The load can be generated by any appropriate means provided that a force constant to within ± 1 %
can be generated at the test piece.
The force applied to the test piece shall be calibrated to an accuracy of 1% in accordance with
EN 10002-2.
NOTE The mass of the loading system between the load cell and the test piece can be significant
and should be taken into account, including any spring force due to the use of bellows systems for
protective atmospheres, and any pressure differential between the external atmosphere and inside
the test chamber.
6.4 Deflection measuring device
The flexural deflection of the test piece is monitored most effectively by determining the relative
vertical displacement of two defined positions on the free under-surface of the deforming test piece
(see Figures 1 (d) and 1(e)).
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SIST ENV 820-4:2002
ENV 820-4:2001 (E)
The displacement is detected by use of extensometer rods passing into the loading jig and freely
contacting the test piece surface, using a linear displacement transducer or similar device. The
output of the displacement transducer shall have a sensitivity of better than 0,5 μm, shall have linear
output with displacement to better than 1 % of the range over which it is used, and shall be
calibrated to an accuracy of better than 1 % of range. The contacting force between the test piece
and the extensometer rods shall be less than 1 % of the force applied to the test piece.
NOTE 1 The above method is preferred. Alternatively, the test piece displacement can be
determined by recording the relative displacement of the support and loading parts of the load train
provided that the deflection of the load train at the temperature of operation is less than 5 % of that
of the test piece. Typical methods are shown in Figure 1 (a) to (1c). The load train deflection can be
determined by using a thick non-deforming test piece of a material such as sintered silicon carbide.
The disadvantages of this method are that it does not account for time-dependent flattening of
rollers and that linearity of stress and strain is assumed.
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SIST ENV 820-4:2002
ENV 820-4:2001 (
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