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Ref. No. : ISO/R 373 - 1964 (E)
IS0
O RG A N IZATl O N
I NTERN AT1 O N A L FOR STANDARD IZATl O N
IS0 RECOMMENDATION
R 373
bl
GENERAL PRINCIPLES FOR FATIGUE TESTING OF METALS
1st E D IT I ON
August 1964
COPYRIGHT RESERVED
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 373, General Principles for Fatigue Testing of Metals, was
drawn up by Technical Committee ISO/TC 17, Steel, the Secretariat of which is held by the
British Standards Institution (BSI).
Work on this question by the Technical Committee began in 1958 and led, in 1961, to the
adoption of a Draft IS0 Recommendation.
In October 1962, this Draft IS0 Recommendation (No. 516) was circulated to all the
IS0 Member Bodies for enquiry. It was approved subject to a few modifications of an editorial
nature, by the following Member Bodies:
Australia France
Norway
Austria Germany Poland
Belgium Greece Portugal
Bulgaria Hung a r y Romania
Burma India
Spain
Canada Ireland Sweden
Chile Italy Switzerland
Czechoslovakia Japan Turkey
Denmark Morocco United Kingdom
Netherlands U.S.S.R.
Egypt
Finland New Zealand Yugoslavia
One Member Body opposed the approval of the Draft: U.S.A.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council,
which decided, in August 1964, to accept it as an IS0 RECOMMENDATION.
c
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ISO/R 373 - 1964 (E)
IS0 Recommendation R 373 August 1964
GENERAL PRINCIPLES FOR FATIGUE TESTING OF METALS
1. SCOPE
This IS0 Recommendation consists mainly of general recommendations for the definitions of the
terms used, for the preparation of fatigue test pieces, their subsequent testing procedure and the
presentation of results. The recommendations are intended to apply mainly to fatigue tests
under tension-compression (direct stress), bending or torsion of plain or notched test pieces of
simple forms. It does not cover, for example, fatigue under repeated impact or thermal fatigue.
In this IS0 Recommendation, the term ‘‘fatigue’’ applies to changes in properties which can
occur in a metallic material due to the repeated application of stresses or strains, although usually
this term applies specially to those changes which lead to cracking or failure.
2. OBJECT
The object of fatigue testing is to provide data relating to the behaviour of materials or struc-
tural components, when subjected to stresses or strains which vary repeatedly with time.
3. DEFINITIONS AND SYMBOLS
3.1 General. Stresses in service may be of simple form, for example, tension-compression,
bending or torsion, or they may occur in combination. According to the information
required, the stresses applied in fatigue tests may similarly be one of those modes or a
combination of two or more of them. Whatever the mode of stress, whether applied singly
or in combination, the direct and/or shear stress to which the test specimen is subjected
will usually vary approximately sinusoidally with time, as illustrated in Figure 1.
1 Stress cycle
t- 1
Maximum stress
Mean stress
Minimum stress
min.
I Time
Fig. 1. - Fatigue stress cycle
NOTE. - Range of stress = 2 (Stress amplitude).
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ISO/R 373 - 1964 (E)
The statements in the following clauses are thus applicable whatever the mode of stressing,
although, for reasons of simplicity, reference will be made generally to simple stress systems.
3.2 Stress. In general, the stress will be a nominal stress calculated by reference to the net area
under consideration. It will usually be calculated by the use of conventional elastic
formulae.
It should be noted that in some instances tests may be conducted and the results expressed
entirely in terms of strain; in particular, where strains beyond the elastic limit are employed,
it is undesirable to calculate stresses from them.
3.2.1 Applied stress cycle. It will be seen from Figure 1 that the smallest section of the stress-
time function which is repeated periodically is the stress-cycle. Any stress varying
periodically over a given range can be regarded as comprising a variable stress com-
ponent alternating between two values opposite in sign but equal in magnitude (the
stress amplitude) and a static stress component (the mean stress) superimposed. The
recommended notation is algebraic as in Figure 2.
Fluctuating Fluctuating
tension compression
+
Stress
1
a mox.
t
Time
- - T
0-
T
I
mox.
Stress
Fig. 2. - Stress cycle with algebraic notation
3.2.2 Symbols, designations and definitions
Symbol
Designation I Deiinition
Maximum stress The highest algebraic value of stress in the stress cycle; tensile stress
is considered positive and compressive stress negative.
Minimum stress The lowest algebraic value of stress in the stress cycle; tensile stress
is considered positive and compressive stress negative.
Mean stress Static component of the stress. It is one half of the algebraic sum
of the maximum and minimum stresses.
Stress amplitude Variable component of stress. It is one half of the algebraic dif-
ference between the maximum stress and the minimum stress.
Range of stress Algebraic difference between the maximum stress and the minimum
stress in the stress cycle.
NOTE. - For shear stress, the symbol T will be used instead of o.
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ISO/R 373 - I964 (E)
Definition
Symbol Designation
n Number of the stress cycles Number of cycles applied at any stage during the test.
Frequency of cycles Number of applied cycles per unit time (cycles per minute or cycles
f
per second).
Theoretical stress Geometric stress concentration factor based on net area and cal-
Kt
concentration factor culated in accordance with the elastic theory.
Algebraic ratio of the minimum stress to the maximum stress in
Stress ratio
RS
one cycle.
amin.
~
O'rnax.
3.2.3 Types of stress cycles. The stress cycle may take any of the forms shown in Figure 3.
U
0
0
LJ. n
A
Time
3 4 5 6 7
Fluctuating Reversed Fluctuat ing
compression symmetrical tension
Reversed Reversed
asymmetrical asymmetrical
Fig. 3. - Types of cyclic stress
3.3 Fatigue strength
.
3.3.1 Symbols, designations, and deBinitions
Symbol Designation Definition
N Endurance or fatigue life Number of stress cycles to failure, generally stated as decimal frac-
of 10".
tions or multiples
Fatigue strength at N Value of the stress condition under which the test-piece would have
ON
cycles or fatigue strength a life (which may be statistically determined) of N cycles.
for .finite life (sometimes
known as endurance limit)
Fatigue limit Value which may be statistically determined of the stress condition
OD
below which a material may endure an infinite number of stress
cycles.
If aA is the stress amplitude at fatigue limit, then, for a particular
a,,
value of the mean stress
aD =am &aA
NOTE. - Certain materials and environments preclude the attain-
ment of fatigue limits.
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IÇO/R 373 - 1964 (E)
Symbol Designation Definition
Cycle ratio Ratio of the applied stress cycles (see clause 3.2.2) to cycles to failure:
nlN
used in multi-level tests with reference to the Woehler curve.
Fatigue strength reduction
Ratio of fatigue limit for plain polished test pieces to the fatigue
Kf
factor (based on the net limit for test pieces with a stress-concentration. The term “strcss-
area) concentration” is here used in a general sense, to denote not only
a mechanical notch, but also, for example, the influence
the effect of
of corrosion or of an unmachined surface.
Values of fatigue strength at N cycles (GN) may be employed in the
calculation of this factor. The ratio will vary with the value of N
chosen.
4. PROCEDURE AND CRITERIA OF FAILURE
4.1
Design of specimen. The design of the specimen has to provide for all the variables that have
to be investigated, and should be undertaken with a view to satisfying the conditions of the
test programme.
For tests of materials, the test piece may be of simple shape, but for fatigue tests on struc-
tural components and assemblies, the design of the specimen should reproduce as closely
as possible the true condition and distribution of the loads.
4.2 Test piece preparation. The majority of fatigue failures originate at a free surface in the
metal, and, as a consequence, the fatigue strength of a test piece or of a structural or mechan-
ical component may be profoundly influenced by conditions at the surface. The method of
preparation of a series of test pieces for a fatigue determination should take account of this
in relation to the purpose for which the fatigue determination is being undertaken.
Thus, in preparing test pieces without any deliberately introduced stress-concentration, the
machining and polishing technique employed to form the test portion should be designed
to minimize surface imperfections and residual surface stresses and other effects, such as
overheating.
Where the test piece being prepared contains an intentional stress-concentration (for
example, a discontinuity such as a notch, a change in cross-section or a hole), the precautions
noted above will apply to the machining process employed in forming the stress-
concentration.
4.3 Mounting the test piece in the testing machine. Each test piece should be mounted in the
testing machine in such a manner that operation of the machine does not subject the test
piece to appreciable stresses additional to the required stress.
As examples, a rotating beam test piece should be clamped Co-axially with the rotating shaft
of the machine to avoid vibration; similarly, misalignment of an axial tension or compression
stress test piece will result in the superimposition of bending stress.
4.4 Application of the load. The general procedure adopted in arriving at full load-running
Conditions should be the same for each test piece tested in a fatigue determination, and will
be controlled by the type and conditions of the test being undertaken. The speed of a rotat-
ing test piece in a bending fatigue test should be increased to a value approximating to that
required for the test before any load is applied. The load is then raised incrementally or
continuously, but without shock, to the required value as quickly as is convenient. In
general, tests will then be run continuously until failure of the test piece occurs.
Taking into account the volume of the material being stressed, the frequency of the stress
cycles should be so chosen as to avoid overheating the test piece during testing. If, for any
reason, this is inconvenient, the test piece may be continuously cooled by any appropriate
method which does not influence the result (for example, by inducing corrosion).
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ISO/R 373 - 1964 (E)
4.5 Test procedure. The number of test pieces employed in a fatigue determination may vary
over a wide range, according to the nature of the information being sought. Where the
experiment is of a statistical nature, the number may be very large; if, on the other hand,
each “test piece” is a costly machine component, the number will necessarily be small.
If the number is less than 6, it may be desirable to adopt a special procedure.
Many fatigue determinations employ at least 10 similar test pieces, each of them being
subjected to a particular stress amplitude until either failure (for criteria of failure, see
clause 4.6) occurs, or until a predetermined number of cycles have been reached without
failure. The values of the stress amplitudes applied to the individual test pieces should be
so chosen as to result in one test piece * remaining unbroken at the predetermined endurance
and one test piece broken at a very slightly different value. The remaining test pieces
should be subjected to stress amplitudes which result in them falling over a range of endur-
ances sufficiently wide to be plotted against the stress amplitude so that a curve may be
The
drawn through the points. This curve is generally referred to as the Woehler curve.
range of endurances covered by the tests will depend upon the information being sought
from the determination.
In some cases, the object of a fatigue determination may not be to plot a Woehler curve;
for example, a fatigue limit may be obtained by a statistically designed procedure, such as
the “staircase” method (see Section 7).
4.6 Criterion of the failure. In the majority of fatigue determinations, the criterion of failure is
either the occurrence of a visible fatigu
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