ASTM D7791-12

Standard Test Method for Uniaxial Fatigue Properties of Plastics

SIGNIFICANCE AND USE
These fatigue tests are used to determine the effect of processing, surface condition, stress, and so forth, on the fatigue resistance of plastic material subjected to uniaxial stress for relatively large numbers of cycles. The results can also be used as a guide for the selection of plastic materials for service under conditions of repeated flexural stress.
Properties can vary with specimen depth and test frequency. Test frequency can be 1-25 Hz but it is recommended that a frequency of 5 Hz or less be used.
Material response in fatigue is not identical for all plastics. If a plastic does not exhibit an elastic region, where strain is reversible, plastic deformation will occur during fatigue testing, causing the amplitude of the programmed load or deformation to change during testing. In this situation, caution shall be taken when using the results for design as they are generally not indicative of the true fatigue properties of the material.
The results of these fatigue tests are suitable for application in design only when the specimen test conditions realistically simulate service conditions or some methodology of accounting for service conditions is available and clearly defined.
This procedure accommodates various specimen preparation techniques. Comparison of results obtained from specimens prepared in different manners shall not be considered comparable unless equivalency has been demonstrated.
SCOPE
1.1 This test method covers the determination of dynamic fatigue properties of plastics in uniaxial loading. This method is applicable to rigid and semi-rigid plastics. Uniaxial loading systems with tension and compression capabilities are used to determine these properties. Stress and strain levels are below the proportional limits of the material where the strains and stresses are relatively elastic.
1.2 This test method can be used with two procedures:
1.2.1 Procedure A, fatigue testing in tension.
1.2.2 Procedure B, fatigue testing in compression, only for rigid plastics.
1.3 Comparative tests can be run in accordance with either procedure, provided that the procedure is found satisfactory for the material being tested.
1.4 The values stated in SI units are to be regarded as the standard. The values provided in parentheses are for information only.
1.5 This standard does not purport to address all of the safety concerns, 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.
Note 1—There is no known ISO equivalent to this standard.

General Information

• Status: Historical
• Publication Date: 31-Mar-2012
• ICS: 83.080.01 - Plastics in general
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.10 - Mechanical Properties

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D7791 − 12
Standard Test Method for
Uniaxial Fatigue Properties of Plastics
This standard is issued under the fixed designation D7791; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope D1505 Test Method for Density of Plastics by the Density-
Gradient Technique
1.1 This test method covers the determination of dynamic
D2839 Practice for Use of a Melt Index Strand for Deter-
fatigue properties of plastics in uniaxial loading. This method
mining Density of Polyethylene
is applicable to rigid and semi-rigid plastics. Uniaxial loading
D3479/D3479M Test Method for Tension-Tension Fatigue
systems with tension and compression capabilities are used to
of Polymer Matrix Composite Materials
determine these properties. Stress and strain levels are below
D4883 Test Method for Density of Polyethylene by the
the proportional limits of the material where the strains and
Ultrasound Technique
stresses are relatively elastic.
D5947 Test Methods for Physical Dimensions of Solid
1.2 This test method can be used with two procedures:
Plastics Specimens
1.2.1 Procedure A, fatigue testing in tension.
E4 Practices for Force Verification of Testing Machines
1.2.2 Procedure B, fatigue testing in compression, only for
E83 Practice for Verification and Classification of Exten-
rigid plastics.
someter Systems
E466 Practice for Conducting Force Controlled Constant
1.3 Comparative tests can be run in accordance with either
procedure,providedthattheprocedureisfoundsatisfactoryfor Amplitude Axial Fatigue Tests of Metallic Materials
E1942 Guide for Evaluating DataAcquisition Systems Used
the material being tested.
in Cyclic Fatigue and Fracture Mechanics Testing
1.4 The values stated in SI units are to be regarded as the
standard. The values provided in parentheses are for informa-
3. Terminology
tion only.
3.1 Definitions—Definitions of terms applying to this test
1.5 This standard does not purport to address all of the
method appear in Terminology D883.
safety concerns, if any, associated with its use. It is the
3.2 Definitions:
responsibility of the user of this standard to establish appro-
3.2.1 compressive proportional limit—maximum elastic
priate safety and health practices and determine the applica-
stress or strain exhibited by a material in compression as
bility of regulatory limitations prior to use.
observed in Test Method D695.
NOTE 1—There is no known ISO equivalent to this standard.
3.2.2 mean strain—algebraic average of the maximum and
minimum strains in one cycle.
2. Referenced Documents
3.2.3 mean stress—algebraic average of the maximum and
2.1 ASTM Standards:
minimum stresses in one cycle.
D618 Practice for Conditioning Plastics for Testing
3.2.4 R ratio—ratio of the minimum stress or strain to the
D638 Test Method for Tensile Properties of Plastics
maximum stress or strain that the specimen is loaded.
D695 Test Method for Compressive Properties of Rigid
Plastics
3.2.5 tensile proportional limit—maximum elastic stress or
D792 Test Methods for Density and Specific Gravity (Rela-
strain exhibited by a material in tension as observed in Test
tive Density) of Plastics by Displacement
Method D638.
D883 Terminology Relating to Plastics
4. Summary of Test Method
4.1 Procedure A—A specimen of rectangular or circular
This test method is under the jurisdiction ofASTM Committee D20 on Plastics
cross section is gripped by the ends of the specimen, pulled
and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
equally in opposite directions, and released back to its original
Current edition approved April 1, 2012. Published May 2012. DOI:10.1520/
D7791-12.
position or load. The specimen is cyclically loaded in tension
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
to a specific stress or strain level at a uniform frequency until
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
the specimen ruptures or yields. From these tests, fatigue
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. strengths can be determined at specified numbers of cycles.At
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7791 − 12
least, four different stress or strain levels are tested to construct 6.2 Recording Equipment—Calibrated equipment must be
a stress versus number of cycles to failure (S-N) curve or a used to record the following information during testing at a
data acquisition rate and filter in accordance with Guide
strain versus number of cycles to failure (r-N) to determine the
E1942:
uniaxial endurance limit of the material in tension.
6.2.1 Load versus time
4.2 Procedure B—A specimen of rectangular or circular
6.2.2 Change in length versus time
cross section is compressed equally from opposite directions
6.2.3 Number of cycles
and released back to its original position or load.The specimen
6.3 Micrometers—Applicable apparatus in accordance with
is cyclically loaded in compression to a specific stress or strain
Test Methods D5947 shall be used to measure the width and
level at a uniform frequency until the specimen ruptures or
thickness or diameter of the test specimen.
yields. From these tests, fatigue strengths can be determined at
6.4 Extensometers/Compressometer—A suitable instrument
specified numbers of cycles. At least, four different stress or
shall be used for determining the distance between two
strain levels are tested to construct a stress versus number of
designatedpointswithinthegagelengthofthetestspecimenas
cycles to failure (S-N) curve or a strain versus number of
the specimen is stretched or compressed. This instrument shall
cyclestofailure(r-N)todeterminetheuniaxialendurancelimit
be essentially free of inertia at the specified speed of testing.
of the material in compression.
Extensometers shall be classified and their calibration periodi-
cally verified in accordance with Practice E83.An
5. Significance and Use
extensometer/compressometer with a maximum strain error of
5.1 These fatigue tests are used to determine the effect of
0.0002 mm/mm (in./in.) that automatically and continuously
processing, surface condition, stress, and so forth, on the
records shall be used. Crosshead stroke of the testing machine
fatigueresistanceofplasticmaterialsubjectedtouniaxialstress
can also be used to record deflection as long as it meets the
for relatively large numbers of cycles. The results can also be
same error requirements listed above.
used as a guide for the selection of plastic materials for service
6.5 Supporting Jig (Procedure B Only)—A supporting jig
under conditions of repeated flexural stress.
can be used for thin specimens or high compressive loads and
shall be in accordance with Test Method D695.
5.2 Properties can vary with specimen depth and test fre-
quency. Test frequency can be 1-25 Hz but it is recommended
7. Sampling, Test Specimens, and Test Units
that a frequency of 5 Hz or less be used.
7.1 Specimens shall be taken from samples that accurately
5.3 Material response in fatigue is not identical for all
represent the material or design that is being tested.
plastics. If a plastic does not exhibit an elastic region, where
7.2 The specimens can be cut from sheets, plates, or molded
strain is reversible, plastic deformation will occur during
shapes, or can be molded to the desired finished dimensions.
fatigue testing, causing the amplitude of the programmed load
The actual dimensions used for calculations shall be measured
or deformation to change during testing. In this situation,
in accordance with Test Methods D5947.
caution shall be taken when using the results for design as they
7.3 Procedure A—Specimen dimensions, shape, surface
are generally not indicative of the true fatigue properties of the
conditions, and limitations shall be in compliance with Test
material.
Method D638.
5.4 The results of these fatigue tests are suitable for appli-
7.4 Procedure B—Specimen dimensions, shape, surface
cation in design only when the specimen test conditions
conditions, and limitations shall be in compliance with Test
realistically simulate service conditions or some methodology
Method D695.
of accounting for service conditions is available and clearly
7.5 Specimens cut from non-uniform thick molded part
defined.
sectionsshallbemachinedequallyandminimallyonbothsides
5.5 This procedure accommodates various specimen prepa-
to create a uniform thickness in the gage. It must be noted that
ration techniques. Comparison of results obtained from speci-
machining the thickness of plastic can change the mechanical
mens prepared in different manners shall not be considered
properties and caution shall be taken when applying the results
comparable unless equivalency has been demonstrated.
to design.
7.6 It is recommended that density measurements be taken
6. Apparatus
from each sample in the gage in accordance withTest Methods
6.1 Testing Machine—The testing machine shall essentially
D792, Test Method D1505, Practice D2839, or Test Method
meet the specifications set forth by Test Method D638 for
D4883 to ensure that the process used to fabricate the speci-
ProcedureAand Test Method D695 for Procedure B except as
mens creates consistent and uniform material.
described below. The error in the deflection measuring system
8. Number of Test Specimens
shall not exceed
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