ASTM D671-93
(Test Method)Standard Test Method for Flexural Fatigue of Plastics by Constant-Amplitude-of-Force (Withdrawn 2002)
Standard Test Method for Flexural Fatigue of Plastics by Constant-Amplitude-of-Force (Withdrawn 2002)
SCOPE
1.1 This test method covers the determination of the effect of repetitions of the same magnitude of flexural stress on plastics by fixed-cantilever type testing machines, designed to produce a constant-amplitude-of-force on the test specimen each cycle.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D 671 – 93
Standard Test Method for
Flexural Fatigue of Plastics by Constant-Amplitude-of-
Force
This standard is issued under the fixed designation D 671; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 4.2 This test method is useful to determine the effect of
variations in material, stress, and environmental conditions on
1.1 This test method covers the determination of the effect
the ability of a material to resist deterioration resulting from
of repetitions of the same magnitude of flexural stress on
repeated stress. It may also be used to provide data for use as
plastics by fixed-cantilever type testing machines, designed to
a guide to design and selection of materials for service under
produce a constant-amplitude-of-force on the test specimen
conditions of repeated stress.
each cycle.
4.3 The results are suitable for direct application in design
1.2 The values stated in SI units are to be regarded as the
only when all design factors including magnitude and mode of
standard. The values given in parentheses are for information
stress, size and shape of part, ambient and part temperature,
only.
heat transfer conditions, cyclic frequency, and environmental
1.3 This standard does not purport to address all of the
conditions are comparable to the test conditions.
safety problems, if any, associated with its use. It is the
4.4 The results obtained from testing machines other than
responsibility of the user of this standard to establish appro-
the type described here may not agree due to differences in
priate safety and health practices and determine the applica-
specimen size and geometry, testing machine speeds, heat
bility of regulatory limitations prior to use.
transfer, material fabrication, etc.
2. Referenced Documents 4.5 The type of machine covered in this test method is
suitable for determining the fatigue strength for a range of
2.1 ASTM Standards:
mean stress in flexure. However, for plastic materials, which
D 618 Practice for Conditioning Plastics and Electrical
creep and stress relax, the effect of a mean stress other than
Insulating Materials for Testing
zero is to cause relaxation so that the stress cycle tends to
D 4000 Classification System for Specifying Plastic Mate-
approach the condition of complete reversal of stress.
rials
4.6 Tests of thin sheet yield results which vary with the
3. Summary of Test Method
thickness of the sheet (Note 1). Because of this fact the
thickness of the sheet shall be specified when reporting results
3.1 This test method measures the ability of a material to
of tests of thin sheet; and all comparisons of different materials,
resist deterioration from cyclic stress. The test results provide
or selection of materials on the basis of fatigue strength, shall
data on the number of cycles of stress to produce specimen
be made from results of tests of standard specimens or tests in
failure by fracture, softening, or reduction in stiffness by
which the same thickness of sheet is used for all materials.
heating as a result of internal friction (damping).
NOTE 1—For the purposes of this test a thin sheet shall be defined as a
4. Significance and Use
sheet less than 7.6 mm (0.3 in.) in thickness or a material for which the
4.1 The flexural fatigue test provides information on the ratio of the modulus of elasticity to the fatigue limit is less than 100. The
reason for these restrictions is that thin sheets and materials having a low
ability of rigid plastics to resist the development of cracks or
modulus of elasticity are bent so much under the required loads that the
general mechanical deterioration of the material as a result of
fatigue specimen cannot (in the deflected position) be considered a straight
a relatively large number of cycles of constant amplitude of
beam and hence the following equation is not accurate:
force.
S 5 Mc/I
This test method is under the jurisdiction of ASTM Committee D-20 on Plastics
where:
and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
S = stress in outer fiber,
Current edition approved Oct. 15, 1993. Published December 1993. Originally
M = bending moment (PL),
published as D 671 – 42 T. Last previous edition D 671 – 90.
c = distance from neutral axis to outer fiber, and
Annual Book of ASTM Standards, Vol 08.01.
I = moment of inertia.
Annual Book of ASTM Standards, Vol 08.03.
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NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D671–93
4.7 In any plastic part fatigue may be frequency dependent.
Data should not be extrapolated to other frequencies unless the
frequency response is known.
4.8 In any plastic having appreciable damping, fatigue is
dependent on the heat transfer of the specimen or part to the
surroundings. Changes in testing temperature, test frequency,
rate of removal of heat (as by current of air from a fan) will
affect test results. It may be desirable to measure the effect of
these variables or combinations thereof to more closely simu-
late end-use conditions for some specific application.
4.9 The nominal stress or strain resulting from the applied
load does not always represent the actual magnitude of the
applied stress or strain at the test section of the specimen. The
elementary beam formula will not yield precise results for
materials whose: (a) stress-strain relationship is not linear, (b)
stress-strain curve in tension is not identical to that in com-
pression, or (c) internal damping is large. Most plastics have
one or more of these characteristics. No generally satisfactory
method of taking these factors into account is yet available.
4.10 For many materials, there may be a specification that
requires the use of this test method, but with some procedural
modifications that take precedence when adhering to the
specification. Therefore, it is advisable to refer to that material
specification before using this test method. Table 1 of Classi-
fication System D 4000 lists the ASTM materials standards that
currently exist.
5. Apparatus (Fig. 1)
5.1 Testing Machine—A fatigue testing machine of the
fixed-cantilever, repeated-constant-force type (see Appendix
X1). In this machine the specimen, A, shall be held as a
cantilever beam in a vise, B, at one end, and bent by a
concentrated load applied through a yoke, C, fastened to the
opposite end. The alternating force shall be produced by an
unbalanced, variable eccentric, D, mounted on a shaft. The
FIG. 1 Fixed-Cantilever, Repeated-Constant-Load Type Fatigue
Testing Machine
shaft shall be rotated at constant speed by a motor.
5.2 Counter—A counter, E, to record the number of cycles.
the stress range over which the measurements are to be made.
5.3 Cut-off Switch—A suitable mechanically or electrically
operated cut-off switch, F, shall be provided to stop the The triangular form of these specimen types provides for
uniform stress distribution over their respective test spans.
machine when the specimens fail.
5.4 Thermometer—A suitable means of measuring the 6.2 Machining of each specimen shall be accomplished with
a very sharp cutting tool, using such combination of speed and
specimen temperature during the fatigue test such that the
fatigue stress is not disturbed. One approach which has been feed as will give a good finish with a minimum of heating of
the specimen. The test specimen shall be polished with
successful utilized a radiation thermometer to measure the
specimen surface temperature. Other approaches such as ad- successively finer emery paper, finishing with No. 00 to
remove all scratches and tool marks. The final polishing shall
hering thermocouples to the specimen surface may be adequate
provided the stress distribution in the fatigue experiments is not be lengthwise of the specimen, since even small scratches
disturbed. transverse to the direction of tensile stress tend to lower the
fatigue strength. In order to avoid heating, all polishing shall be
6. Test Specimens
done either by hand or with light pressure on a slowly
6.1 The test specimens shall conform to one of the two revolving sanding drum. Care shall be taken to avoid rounding
geometries (Type A or Type B) shown in Fig. 2. Selection of a the edges and corners of the specimen.
particular specimen will depend upon specimen thickness and 6.3 Specimens may be molded to the dimensions specified
in Fig. 2, but care should be taken to stress relieve internal
stress unless the effect of molded-in stress is to be measured.
Model SF-2U Constant-Amplitude-of-Force Fatigue Machine, available from
Satec Systems, Inc., Liberty St. Extension, Grove City, PA 16127, has been found
7. Conditioning
satisfactory for this purpose.
7.1 Conditioning—Condition the test specimens at 23 6
Ircon Radiation Thermometer Series 700, available from Ircon Inc., 207
Lawrence Rd., Niles, IL, has been found satisfactory for this purpose. 2°C (73.4 6 3.6°F) and 50 6 5 % relative humidity for not less
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D671–93
FIG. 2 Dimensions of Constant Force Fatigue Specimens
than 40 h prior to test in accordance with Procedure A of 8. Procedure
Practice D 618, for those tests where conditioning is required.
8.1 Tuning Machine—Tune machine by using a thin metal
In cases of disagreement, the tolerances shall be6 1°C
specimen having a natural frequency of 1800 6 4 cpm when
(61.8°F) and 62 % relative humidity.
vibrating as a free cantilever beam. (Tuning specimens and
7.2 Test Conditions—Conduct tests in the Standard Labora-
weights are furnished by the manufacturer of the test appara-
tory Atmosphere of 23 6 2°C (73.4 6 3.6°F) and 50 6 5%
tus.) Tune by the following procedure:
relative humidity, unless otherwise specified in the test method.
8.1.1 Set the eccentric, D, Fig. 1, very near zero load, that is,
In cases of disagreement, the tolerances shall be 61°C
at about 0.1 to 0.3 units. Do not change this setting during
(61.8°F) and 62 % relative humidity.
subsequent tuning runs (tests).
7.3 The mechanical properties of many plastics change
8.1.2 Make several runs of a minute or two each, using
rapidly with small changes in temperature. Since heat is
different total tuning weights, G, Fig. 1, and plot the total
generated as a result of the flexing of the specimen, tests shall
tuning mass versus the amplitude of vibration to the nearest 0.5
be conducted without forced cooling to ensure uniformity of
mm (0.02 in.). At low values of mass the amplitude will be
test conditions. The temperature at the region of the highest
very small, increasing to higher values as proper tuning is
stress in the specimen shall be measured and recorded.
reached by the addition of more weights. Further addition of
7.3.1 When the effect of heat transfer is being measured, it
mass will then result in a decrease of amplitude.
is acceptable to cool the test specimen to simulate end use
8.1.3 Select the proper tuning mass (to the nearest 0.02 % of
conditions. However, it should be realized that heat transfer is
the dynamic machine capacity) from the peak of the mass-
undoubtedly the most difficult variable to simulate or control.
versus-amplitude curve. This value is a constant for the
To develop fatigue data under refrigeration or at isothermal
machine and is called the “complementary mass.”
conditions is only of value if the material will be used under
similar conditions. Artificial cooling shall not be used to force 8.2 Measurements—Test three or more specimens at each of
the material to fail mechanically when thermal failure is the at least four different stress amplitudes. Choose stress ampli-
controlling mechanism in the application. tudes to yield a mean log of cycles to failure (log N) of about
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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D671–93
4, 5, 6, and 7. Measure the minimum thickness of the triangular mass system so that tuning (see 8.1) can be achieved. When
portion of each specimen to the nearest 0.03 mm (0.001 in.). frequency is not specified, tests shall be conducted at 30 Hz 6
Clamp the specimen snugly in the vise, B, Fig. 1, with its 5%.
smaller end screwed securely to the vibrating yoke, C. Measure 8.7 Readings—Set the revolution counter at zero before
the test span, L, to the nearest 0.5 mm (0.02 in.) from the starting a test. Upon failure of the test specimen, read the
leading edge of the vise, B, along the principal axis of the counter to determine the number of cycles to failure.
specimen to the line-of-centers of the mounting screws in the 8.8 Temperature Measurement—Measure the steady-state
yoke, C. Measure the width, b, of the specimen defined by the temperature. If steady-state conditions do not occur, measure
intersection of the leading edge of the vise with the sides (or the temperature throughout the fatigue test. In all fatigue tests,
projections thereof) of the triangular portion of the specimen, measure the temperature at failure unless it can be shown that
to the nearest 0.3 mm (0.01 in.). the heat rise is insignificant for the specific material and test
8.3 Calculation of Effective Mass of Test Specimen— condition. Focus the radiation thermometer on the expected
Calculate the effective mass (Note 2) of the standard specimen failure area of specimen. Temperature measurements should be
Types A and B (Fig. 2) as follows: measured over small areas, such as over a 1-mm (0.040-in.)
For Type A Specimen: diameter spot, and the specimen should be periodically
scanned to ensure that the maximum localized heating is being
W 5 kDd
measured.
For Type B Specimen:
9. Plotting and Interpreting Results
W 5 k8Dd
9.1 Plotting Results—Plot an S-N (stress versus cycles-to-
where: failure) diagram with the alternating stress amplitude as the
W = effective mass, g (or lb),
ordinate against the common logarithm of the number of cycles
3 3
D = density of the specimen (material), Gg/m (or lb/in. ),
required for failure (see X3.21) as the abscissa. Plot all test data
d = average specimen thickness, mm (or in.),
and define the S-N diag
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