ASTM D7791-22

Standard Test Method for Uniaxial Fatigue Properties of Plastics

SIGNIFICANCE AND USE
5.1 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.  
5.2 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.  
5.3 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.  
5.4 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.  
5.5 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
Note 1: There is no known ISO equivalent to this standard.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Jun-2022
• ICS: 83.080.01 - Plastics in general
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.10 - Mechanical Properties

Relations

• Refers: ASTM D883-24 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2024
• Refers: ASTM D883-23 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2023
• Refers: ASTM D883-20 - Standard Terminology Relating to Plastics
• Effective Date: 01-Jan-2020
• Refers: ASTM D883-19c - Standard Terminology Relating to Plastics
• Effective Date: 01-Aug-2019
• Refers: ASTM D883-19a - Standard Terminology Relating to Plastics
• Effective Date: 15-Apr-2019
• Refers: ASTM D883-19 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2019
• Refers: ASTM D883-18a - Standard Terminology Relating to Plastics
• Effective Date: 01-Dec-2018
• Refers: ASTM D883-18 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2018
• Refers: ASTM D4883-18 - Standard Test Method for Density of Polyethylene by the Ultrasound Technique
• Effective Date: 01-Nov-2018
• Refers: ASTM E1942-98(2018)e1 - Standard Guide for Evaluating Data Acquisition Systems Used in Cyclic Fatigue and Fracture Mechanics Testing
• Effective Date: 01-Jun-2018
• Refers: ASTM D883-17 - Standard Terminology Relating to Plastics
• Effective Date: 15-Aug-2017
• Refers: ASTM E4-14 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Jun-2014
• Refers: ASTM E177-14 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2014
• Refers: ASTM E177-13 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2013
• Refers: ASTM E691-13 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-May-2013

Overview

ASTM D7791-22 is the Standard Test Method for Uniaxial Fatigue Properties of Plastics. Developed by ASTM International, this method is essential for evaluating the dynamic fatigue characteristics of rigid and semi-rigid plastics when subjected to uniaxial (single axis) loading in either tension or compression. This standard provides guidelines for determining how plastics perform under repeated or cyclic stress, such as flexural loads, which is crucial for materials selection and engineering design in applications where durability and long-term reliability are critical.

Key Topics

• Scope and Applicability

  • Covers both rigid and semi-rigid plastics.
  • Uniaxial fatigue testing in tension (Procedure A) and compression (Procedure B).
  • Suitable for comparative and characterization testing under controlled laboratory conditions.

• Test Conditions

  • Fatigue testing is typically performed at frequencies from 1 to 25 Hz, but 5 Hz or less is recommended to avoid excessive heating or non-representative material behavior.
  • Stress and strain levels should remain below the material's proportional limit to maintain primarily elastic responses.
  • Specimens must be properly conditioned by standard procedures (e.g., ASTM D618).

• Fatigue Test Procedure

  • Specimens are cyclically loaded using sinusoidal, square, or trapezoidal waveforms until failure or a specified number of cycles.
  • Results are used to construct S-N (stress vs. number of cycles) or ε-N (strain vs. number of cycles) curves to determine endurance or fatigue limits.

• Significance and Use

  • Enables assessment of the impact of material processing, surface condition, and other variables on fatigue life.
  • Results inform material selection for repeated or cyclic loading environments.

• Limitations

  • Fatigue behavior can differ among plastics; not all materials exhibit a clear elastic region, influencing the interpretation of fatigue results.
  • Results are reliable for design purposes only when test conditions closely simulate actual service conditions.
  • Procedures accommodate various specimen preparation techniques, and results should only be compared if specimen equivalency is established.

Applications

The ASTM D7791-22 test method is widely used across industries where plastics are subject to repeated, cyclic loading, including:

• Automotive Components
  • Testing durability of plastic parts exposed to long-term vibrations and cyclic loads, such as bushings and mounts.
• Consumer Products
  • Evaluating the fatigue resistance of plastic housings, enclosures, and wear-prone parts.
• Aerospace Engineering
  • Selecting plastics for components exposed to flexural and vibrational stresses in aircraft interiors or ancillary systems.
• Construction and Infrastructure
  • Assessing the longevity of plastic materials used in building elements and civil infrastructure, where cyclic loads are common.

Using the results from this method, engineers and designers can optimize material selection and product designs to ensure safety, performance, and regulatory compliance under repeated loading conditions.

Related Standards

ASTM D7791-22 works in conjunction with several other ASTM standards to ensure comprehensive material testing:

• ASTM D618 - Practice for Conditioning Plastics for Testing.
• ASTM D638 - Test Method for Tensile Properties of Plastics.
• ASTM D695 - Test Method for Compressive Properties of Rigid Plastics.
• ASTM D792 - Test Methods for Density and Specific Gravity of Plastics.
• ASTM D883 - Terminology Relating to Plastics.
• ASTM D3479/D3479M - Test Method for Tension-Tension Fatigue of Polymer Matrix Composite Materials.
• ASTM E4, E83, E466, E691 - Practices and guidelines for calibration, extensometer systems, and statistical analysis in testing.

Note: There is no known ISO equivalent for this ASTM standard.

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By adhering to ASTM D7791-22, organizations can generate reliable, repeatable data on the uniaxial fatigue properties of plastics, supporting informed decisions in product development, quality assurance, and regulatory compliance. For full compliance, always refer to the official ASTM document.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D7791 − 22
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* D638 Test Method for Tensile Properties of Plastics
D695 Test Method for Compressive Properties of Rigid
1.1 This test method covers the determination of dynamic
Plastics
fatigue properties of plastics in uniaxial loading. This method
D792 Test Methods for Density and Specific Gravity (Rela-
is applicable to rigid and semi-rigid plastics. Uniaxial loading
tive Density) of Plastics by Displacement
systems with tension and compression capabilities are used to
D883 Terminology Relating to Plastics
determine these properties. Stress and strain levels are below
D1505 Test Method for Density of Plastics by the Density-
the proportional limits of the material where the strains and
Gradient Technique
stresses are relatively elastic.
D2839 Practice for Use of a Melt Index Strand for Deter-
1.2 This test method can be used with two procedures:
mining Density of Polyethylene
1.2.1 Procedure A, fatigue testing in tension.
D3479/D3479M Test Method for Tension-Tension Fatigue
1.2.2 Procedure B, fatigue testing in compression, only for
of Polymer Matrix Composite Materials
rigid plastics.
D4883 Test Method for Density of Polyethylene by the
Ultrasound Technique
1.3 Comparative tests can be run in accordance with either
procedure,providedthattheprocedureisfoundsatisfactoryfor D5947 Test Methods for Physical Dimensions of Solid
the material being tested. Plastics Specimens
E4 Practices for Force Calibration and Verification of Test-
1.4 The values stated in SI units are to be regarded as the
ing Machines
standard. The values provided in parentheses are for informa-
E83 Practice for Verification and Classification of Exten-
tion only.
someter Systems
1.5 This standard does not purport to address all of the
E177 Practice for Use of the Terms Precision and Bias in
safety concerns, if any, associated with its use. It is the
ASTM Test Methods
responsibility of the user of this standard to establish appro-
E466 Practice for Conducting Force Controlled Constant
priate safety, health, and environmental practices and deter-
Amplitude Axial Fatigue Tests of Metallic Materials
mine the applicability of regulatory limitations prior to use.
E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
NOTE 1—There is no known ISO equivalent to this standard.
E1942 Guide for Evaluating DataAcquisition Systems Used
1.6 This international standard was developed in accor-
in Cyclic Fatigue and Fracture Mechanics Testing
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
3. Terminology
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
3.1 Definitions—Definitions of terms applying to this test
Barriers to Trade (TBT) Committee.
method appear in Terminology D883.
2. Referenced Documents
3.2 Definitions:
2.1 ASTM Standards: 3.2.1 compressive proportional limit—maximum elastic
D618 Practice for Conditioning Plastics for Testing stress or strain exhibited by a material in compression as
observed in Test Method D695.
This test method is under the jurisdiction ofASTM Committee D20 on Plastics
3.2.2 mean strain, n—algebraic average of the maximum
and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
and minimum strains in one cycle.
Current edition approved July 1, 2022. Published July 2022. Originally approved
in 2012. Last previous edition approved in 2017 as D7791 - 17. DOI:10.1520/
3.2.3 mean stress, n—algebraic average of the maximum
D7791-22.
and minimum stresses in one cycle.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
3.2.4 R ratio, n—ratio of the minimum stress or strain to the
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. maximum stress or strain that the specimen is loaded.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7791 − 22
3.2.5 tensile proportional limit, n—maximum elastic stress 6. Apparatus
or strain exhibited by a material in tension as observed in Test
6.1 Testing Machine—The testing machine shall essentially
Method D638.
meet the specifications set forth by Test Method D638 for
ProcedureAand Test Method D695 for Procedure B except as
4. Summary of Test Method
described below. The error in the deflection measuring system
shall not exceed 60.5 % of the maximum deflection. The
4.1 Procedure A—A specimen of rectangular or circular
machine shall be able to execute sinusoidal or square/
cross section is gripped by the ends of the specimen, pulled
trapezoidal load or deflection programs at the specified test
equally in opposite directions, and released back to its original
frequency and maintain an error of 61 % or less of the
position or load. The specimen is cyclically loaded in tension
maximum programmed load or deflection.
to a specific stress or strain level at a uniform frequency until
the specimen ruptures or yields. From these tests, fatigue
6.2 Recording Equipment—Calibrated equipment must be
strengths can be determined at specified numbers of cycles.At
used to record the following information during testing at a
least, four different stress or strain levels are tested to construct data acquisition rate and filter in accordance with Guide
a stress versus number of cycles to failure (S-N) curve or a
E1942:
strain versus number of cycles to failure (r-N) to determine the
6.2.1 Load versus time
uniaxial endurance limit of the material in tension.
6.2.2 Change in length versus time
6.2.3 Number of cycles
4.2 Procedure B—A specimen of rectangular or circular
cross section is compressed equally from opposite directions 6.3 Micrometers—Applicable apparatus in accordance with
Test Methods D5947 shall be used to measure the width and
and released back to its original position or load.The specimen
is cyclically loaded in compression to a specific stress or strain thickness or diameter of the test specimen.
level at a uniform frequency until the specimen ruptures or
6.4 Extensometers/Compressometer—A suitable instrument
yields. From these tests, fatigue strengths can be determined at
shall be used for determining the distance between two
specified numbers of cycles. At least, four different stress or
designated points within the gauge length of the test specimen
strain levels are tested to construct a stress versus number of
as the specimen is stretched or compressed. This instrument
cycles to failure (S-N) curve or a strain versus number of
shall be essentially free of inertia at the specified speed of
cyclestofailure(r-N)todeterminetheuniaxialendurancelimit
testing. Extensometers shall be classified and their calibration
of the material in compression.
periodically verified in accordance with Practice E83.An
extensometer/compressometer with a maximum strain error of
5. Significance and Use
0.0002 mm/mm (in./in.) that automatically and continuously
records shall be used. Crosshead stroke of the testing machine
5.1 These fatigue tests are used to determine the effect of
can also be used to record deflection as long as it meets the
processing, surface condition, stress, and so forth, on the
same error requirements listed above.
fatigueresistanceofplasticmaterialsubjectedtouniaxialstress
for relatively large numbers of cycles. The results can also be
6.5 Supporting Jig (Procedure B Only)—A supporting jig
used as a guide for the selection of plastic materials for service
can be used for thin specimens or high compressive loads and
under conditions of repeated flexural stress.
shall be in accordance with Test Method D695.
5.2 Properties can vary with specimen depth and test fre-
7. Sampling, Test Specimens, and Test Units
quency. Test frequency can be 1-25 Hz but it is recommended
that a frequency of 5 Hz or less be used.
7.1 Specimens shall be taken from samples that accurately
represent the material or design that is being tested.
5.3 Material response in fatigue is not identical for all
7.2 The specimens can be cut from sheets, plates, or molded
plastics. If a plastic does not exhibit an elastic region, where
strain is reversible, plastic deformation will occur during shapes, or can be molded to the desired finished dimensions.
The actual dimensions used for calculations shall be measured
fatigue testing, causing the amplitude of the programmed load
in accordance with Test Methods D5947.
or deformation to change during testing. In this situation,
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- tocreateauniformthicknessinthegauge.Itmustbenotedthat
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.
D7791 − 22
7.6 It is recommended that density measurements be taken 11.3 Test Program Setup:
from each sample in the gauge in accordance with Test 11.3.1 Program the testing machine to follow a dynamic
Methods D792, Test Method D1505, Practice D2839, or Test load(sinusoidal,square,ortrapezoidal)ordeflectionwaveform
Method D4883 to ensure that the process used to fabricate the with the amplitude set to the selected load or deflection level.
specimens creates consistent and uniform material. Unless specified, the default waveform shall be sinusoidal.The
selected level shall be below the proportional limit of the
8. Number of Test Specimens materialasdeterminedbyTestMethodsD638(ProcedureA)or
Test Method D695 (Procedure B). Set the test frequency
8.1 At least three test specimens shall be tested at each of
(maximum 25 Hz).
the four stress or strain levels (minimum of twelve specimens
11.3.2 Stress Level Selection—All stress levels shall not
for each test). For additional sample sizes consult Table 1 of
exceed the proportional limit of the material. The associated
Test Method D3479/D3479M.
load of a selected stress level can be determined by the
8.2 In the case of anisotropic materials, a single direction
following equation:
shall be chosen and maintained for all stress or strain levels.
P 5σA (1)
9. Calibration and Standardization
where:
P = load in N,
9.1 All equipment shall be calibrated in accordance with the
σ = stress level in MPa, and
manufacturer’s specifications and Practice E4 or Practice E83,
A = gauge cross sectional area (mm ).
Class B-2.
11.3.3 Strain Level Selection—All strain levels shall not
9.2 Specimens f
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7791 − 17 D7791 − 22
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*
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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
NOTE 1—There is no known ISO equivalent to this standard.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D618 Practice for Conditioning Plastics for Testing
D638 Test Method for Tensile Properties of Plastics
D695 Test Method for Compressive Properties of Rigid Plastics
D792 Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
Current edition approved May 1, 2017July 1, 2022. Published May 2017July 2022. Originally approved in 2012. Last previous edition approved in 20122017 as
D7791 - 12.D7791 - 17. DOI:10.1520DOI:10.1520/D7791-22.⁄D7791-17.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7791 − 22
D883 Terminology Relating to Plastics
D1505 Test Method for Density of Plastics by the Density-Gradient Technique
D2839 Practice for Use of a Melt Index Strand for Determining Density of Polyethylene
D3479/D3479M Test Method for Tension-Tension Fatigue of Polymer Matrix Composite Materials
D4883 Test Method for Density of Polyethylene by the Ultrasound Technique
D5947 Test Methods for Physical Dimensions of Solid Plastics Specimens
E4 Practices for Force Calibration and Verification of Testing Machines
E83 Practice for Verification and Classification of Extensometer Systems
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E466 Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E1942 Guide for Evaluating Data Acquisition Systems Used in Cyclic Fatigue and Fracture Mechanics Testing
3. Terminology
3.1 Definitions—Definitions of terms applying to this test method appear in Terminology D883.
3.2 Definitions:
3.2.1 compressive proportional limit—maximum elastic stress or strain exhibited by a material in compression as observed in Test
Method D695.
3.2.2 mean strain—strain, n—algebraic average of the maximum and minimum strains in one cycle.
3.2.3 mean stress—stress, n—algebraic average of the maximum and minimum stresses in one cycle.
3.2.4 R ratio—ratio, n—ratio of the minimum stress or strain to the maximum stress or strain that the specimen is loaded.
3.2.5 tensile proportional limit—limit, n—maximum elastic stress or strain exhibited by a material in tension as observed in Test
Method D638.
4. Summary of Test Method
4.1 Procedure A—A specimen of rectangular or circular cross section is gripped by the ends of the specimen, pulled equally in
opposite directions, and released back to its original position or load. The specimen is cyclically loaded in tension to a specific
stress or strain level at a uniform frequency until the specimen ruptures or yields. From these tests, fatigue strengths can be
determined at specified numbers of cycles. At least, four different stress or strain levels are tested to construct a stress versus
number of cycles to failure (S-N) curve or a strain versus number of cycles to failure (r-N) to determine the uniaxial endurance
limit of the material in tension.
4.2 Procedure B—A specimen of rectangular or circular cross section is compressed equally from opposite directions and released
back to its original position or load. The specimen is cyclically loaded in compression to a specific stress or strain level at a uniform
frequency until the specimen ruptures or yields. From these tests, fatigue strengths can be determined at specified numbers of
cycles. At least, four different stress or strain levels are tested to construct a stress versus number of cycles to failure (S-N) curve
or a strain versus number of cycles to failure (r-N) to determine the uniaxial endurance limit of the material in compression.
5. Significance and Use
5.1 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.
5.2 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.
5.3 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
D7791 − 22
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.
5.4 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.
5.5 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.
6. Apparatus
6.1 Testing Machine—The testing machine shall essentially meet the specifications set forth by Test Method D638 for Procedure
A and Test Method D695 for Procedure B except as described below. The error in the deflection measuring system shall not exceed
60.5 % of the maximum deflection. The machine shall be able to execute sinusoidal or square/trapezoidal load or deflection
programs at the specified test frequency and maintain an error of 61 % or less of the maximum programmed load or deflection.
6.2 Recording Equipment—Calibrated equipment must be used to record the following information during testing at a data
acquisition rate and filter in accordance with Guide E1942:
6.2.1 Load versus time
6.2.2 Change in length versus time
6.2.3 Number of cycles
6.3 Micrometers—Applicable apparatus in accordance with Test Methods D5947 shall be used to measure the width and thickness
or diameter of the test specimen.
6.4 Extensometers/Compressometer—A suitable instrument shall be used for determining the distance between two designated
points within the gauge length of the test specimen as the specimen is stretched or compressed. This instrument shall be essentially
free of inertia at the specified speed of testing. Extensometers shall be classified and their calibration periodically verified in
accordance with Practice E83. An extensometer/compressometer with a maximum strain error of 0.0002 mm/mm (in./in.) that
automatically and continuously records shall be used. Crosshead stroke of the testing machine can also be used to record deflection
as long as it meets the same error requirements listed above.
6.5 Supporting Jig (Procedure B Only)—A supporting jig can be used for thin specimens or high compressive loads and shall be
in accordance with Test Method D695.
7. Sampling, Test Specimens, and Test Units
7.1 Specimens shall be taken from samples that accurately represent the material or design that is being tested.
7.2 The specimens can be cut from sheets, plates, or molded shapes, or can be molded to the desired finished dimensions. The
actual dimensions used for calculations shall be measured in accordance with Test Methods D5947.
7.3 Procedure A—Specimen dimensions, shape, surface conditions, and limitations shall be in compliance with Test Method D638.
7.4 Procedure B—Specimen dimensions, shape, surface conditions, and limitations shall be in compliance with Test Method
D695.
7.5 Specimens cut from non-uniform thick molded part sections shall be machined equally and minimally on both sides to create
a uniform thickness in the gauge. It must be noted that machining the thickness of plastic can change the mechanical properties
and caution shall be taken when applying the results to design.
D7791 − 22
7.6 It is recommended that density measurements be taken from each sample in the gauge in accordance with Test Methods D792,
Test Method D1505, Practice D2839, or Test Method D4883 to ensure that the process used to fabricate the specimens creates
consistent and uniform material.
8. Number of Test Specimens
8.1 At least three test specimens shall be tested at each of the four stress or strain levels (minimum of twelve specimens for each
test). For additional sample sizes consult Table 1 of Test Method D3479/D3479M.
8.2 In the case of anisotropic materials, a single direction shall be chosen and maintained for all stress or strain levels.
9. Calibration and Standardization
9.1 All equipment shall be calibrated in accordance with the manufacturer’s specifications and Practice E4 or Practice E83, Class
B-2.
9.2 Specimens for each test shall be identically processed and prepared for accurate results.
10. Conditioning
10.1 Conditioning—Condition the test specimens in accordance with Procedure A of Practice D618 unless otherwise specified by
contract or the relevant ASTM material specification. Condition time is specified as a minimum. Temperature and humidity
tolerances shall be in accordance with Practice D618 unless specified differently by contract or material specification.
10.2 Test Conditions—Conduct the tests at the same temperature and humidity used for conditioning with tolerances in accordance
with Practice D618 unless otherwise specified by contract or the relevant ASTM material specification.
11. Procedure
11.1 Specimen Measurements:
11.1.1 Procedure A—Take measurements of all specimens. Measure and record the depth and width, or diameter of the specimen
to the nearest 0.025 mm (0.001 in) at several points along its length. The minimum measurement shall be used for calculations.
These measurements shall be made in accordance with Test Methods D5947 and Test Method D638.
11.1.2 Procedure B—Take measurements of all specimens. Measure and record the depth and width, or diameter of the specimen
to the n
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