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IS0
I NT ERN AT1 ON A L ORGANIZATION FOR STA N DARDlZATl ON
IS0 RECOMMENDATION
R 899
DETERMINATION OF TENSILE CREEP
OF PLASTICS
1 st EDITION
Decem ber 1968
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The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
documents, in any country, may be authorized therefore only
by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
Also issued in French and Russian. Copies to be obtained through the national standards organizations.
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BRIEF HISTORY
The IS0 Recommendation R 899, Determination of tensile creep of plastics, was drawn up by
Technical Committee ISO/TC 61, Plastics, the Secretariat of which is held by the United States of
America Standards Institute (USASI).
Work on this question by the Technical Committee began in 1961 and led, in 1963, to the
adoption of a Draft IS0 Recommendation.
4
‘4 first Draft IS0 Recommendation (No. 748) was circulated in 1964 to all the IS0 Member
Bodies for enquiry. The voting summary having been reviewed, it was decided to prepare a second
Draft IS0 Recommendation which was circulated to ail the IS0 Member Bodies in February 1967.
This second Draft was approved,subject to a few modifications of an editorial nature, by the following
Member Bodies :
Australia India South Africa, Rep. of
Austria Iran Spain
Belgium Israel Sweden
Canada Italy Switzerland
Chile Japan Thailand
Czechoslovakia Netherlands U.A.R.
U.S.A.
Finland New Zealand
U.S.S.R.
France Poland
Portugal Yugoslavia
Germany
Hungary Romania
One Member Body opposed the approval of the Draft :
United Kingdom
IS0 Recommendation was then submitted by correspondence to the IS0
The second Draft
Council, which decided, in December 1968, to accept it as an IS0 RECOMMENDATION.
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E
ISO/R 899-1968 (E
D ece m be r I 968
IS0 Recommendation R 899
DETERMINATION OF TENSILE CREEP
OF PLASTICS
1. SCOPE
1.1 This 1SO Recommendation describes the testing of plastics specimens in tension for long
periods of time under conditions of constant temperature and humidity.
1.2 The test consists in applying a tensile load to the test specimen under exactly defined con-
ditions :
measuring the elongation of the specimen in the course of time during which these
(U)
conditions are acting on it, andior
determining the time elapsed during the action of these conditions from the start of
(b)
the test to the rupture of the specimen and/or to attaining the specified value of
strain.
1.3
Since the creep test is a time-consuming test whose results are sensitive to variations in material
composition and in the effects of the environment, it is at present recommended for use
where the variations in the environment can be kept under close control.
1.4 The mechanical behaviour of plastics in tensile creep tests is of importance in determining the
resistance and stability of these materials under conditions of loads applied for long periods of
time. The tensile creep test results are useful in predicting the probable dimensional changes
(or occurrence of fracture) of plastics parts under these service conditions.
2. DEFINITIONS
2.1 Creep. The timedependent strain resulting from force.
For the purposes of obtaining significant measurements by this method, tensile creep of
plastics is determined at constant load, constant temperature and constant humidity, under the
conditions defined in section 3.
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ISO/R 899-1968 (E)
2.2 Strain. For the purpose of this IS0 Recommendation, the relative elongation
A!.?
e =-
where PO
AII = II - II,
is the gauge length at the respective moment of the test,
II
is the initial gauge length of the unstressed specimen (prior to the application of
IIo
load under environmental conditions).
The strain may be reported as a percentage:
AII
e (O/,J =- X 100
II,
2.3
Recovery. The decrease of strain at any given moment after full unloading of the specimen,
defined as a percentage decrease of the strain at the instant of removal of stress.
2.4 Stress (nominal). The force acting per unit area of original specimen cross-section. It is
denoted by the following formula :
F
u =-(kgf/cm2)
S
where
Y
F is the load (kgf),
S is the original cross-sectional area of the specimen (cm’).
2.5 Rate of creep (k,). Increase in strain per unit of time. The usual practice is to report the
average rate of creep in a time interval defined by the relation
where e, and e, are total strains observed at the moments t, and t, respectively.
NOTES
1. The instantaneous rate of creep at a time instant t, is defined as the slope of the creepcurve at the
point corresponding to the time t,.
2. When reporting the rate of creep, the conditions (temperature, humidity, initial stress), under which it
has been determined, should be stated in each case.
2.6 Index of creep (k,’). Increase of strain e2 - e, in a specified interval of time from t, to r2
expressed as a percentage of the strain e,. it is defined conventionally by the following
formula :
Appurent modulus. The ratio stress/strain at a specified moment of the creep experiment. It is
2.7
denoted by the symbol E, calculated by the following formula :
U
E =-
Cif e,
where U is the nominal stress and e, is the relative elongation at the time t.
NOTES
1. Owing to the viscoelastic noniinearity the apparent modulus depends on the applied stress. In report-
ing the apparent modulus, the nominal stress of the particular creep experiment should be stated.
2. Instead of the creep curve (depending on the purpose of testing) the curve E, versus i may be used for the
interpretation of the results of creep experiments.
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ISO/R 899-1968 (1
2.8 isochronous stress-strain curve. From the series of creep curves measured at several stress levels,
the strain values should be taken at the same time of loading. Corresponding values of the
nominal stress (x axis) and strain (y axis) give the isochronous stress - strain curve.
2.9 Time to rupture. The time elapsing between the moment of full loading of the specimen and the
moment of rupture.
2.10 Creep strength limit. The nominal stress that leads to rupture (uB t) or to a specified strain
(U,,*) for a specified time t at a given temperature and relative humidity.
NOTE. - It may be useful (depending on the purpose of testing) to differentiate between the so-called
instantaneous strain and creep strain (instantaneous strain + creep strain = total strain) and/or instantaneous
recovery and total recovery. It is recommended that the instantaneous strain be denoted by the symbol e, as a
value of strain occurring after short measurable time interval of loading, for example, at 1 minute.
3. TEST CONDITIONS
3.1 A constant temperature should be maintained during the test over the whole length of the
working portion of the specimen (or along the gauge length). Unless otherwise specified, the
I 1 degC or less.
temperature should be constant to
3.2 of the test specimen should operate on
The device for maintaining the constant temperature
the airbath principle. It is recommended that no liquid be used. The device should be equipped
with a reliable thermostat. Automatic recording of the temperature during the whole time of
the test is recommended.
3.3
For creep tests of high moisture absorptivity plastics, it is recommended that a constant
relative humidity be maintained in the space of the test specimen. Relative humidity should
not fluctuate during the test more than I 5 relative humidity. Control may be obtained
either by carrying out the test in a controlled atmosphere laboratory, or in a small controlled
enclosure around the specimen.
3.4 Unless otherwise required, one of the recommended standard atmospheres in IS0 Recommen-
datio
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