ASTM D7774-22

Standard Test Method for Flexural 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 flexural 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 flexure. This method is applicable to rigid and semi-rigid plastics. Stress and strain levels are below the proportional limit of the material where the strains and stresses are relatively elastic. Three-point or four-point bending systems are used to determine these properties.  
1.2 This test method can be used with two procedures:  
1.2.1 Procedure A, designed for materials that use three-point loading systems to determine flexural strength. Three-point loading system is used for this procedure.  
1.2.2 Procedure B, designed for materials that use four-point loading systems to determine flexural strength. Four-point loading system is used for this procedure.  
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: This standard and ISO 13003 address the same subject matter, but differ in technical content and results cannot be directly compared between the two test methods.  
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 D4883-18 - Standard Test Method for Density of Polyethylene by the Ultrasound Technique
• Effective Date: 01-Nov-2018
• Refers: ASTM D883-18 - Standard Terminology Relating to Plastics
• 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 D790-17 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
• Effective Date: 01-Jul-2017
• Refers: ASTM D6272-17 - Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending
• Effective Date: 01-Mar-2017
• Refers: ASTM D6272-17e1 - Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending
• Effective Date: 01-Mar-2017
• Refers: ASTM D790-15e1 - Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
• Effective Date: 01-Dec-2015

Overview

ASTM D7774-22 is the Standard Test Method for Flexural Fatigue Properties of Plastics, established by ASTM International. This standard outlines systematic procedures for evaluating the dynamic fatigue properties of rigid and semi-rigid plastics under flexural (bending) conditions. By applying repeated flexural stress across numerous cycles at controlled frequencies, ASTM D7774-22 enables assessment of fatigue resistance, the flexural endurance limit, and aids in the comparative evaluation of different plastic materials. It is a critical resource for material selection and mechanical design, particularly where plastics experience cyclic loading in service.

Key Topics

• Flexural Fatigue Testing: The standard describes two primary procedures:
  • Procedure A (Three-point bending): For materials tested on a single mid-span loading point.
  • Procedure B (Four-point bending): For materials loaded at two points, distributing the stress more evenly.
• Testing Parameters: Specifies test frequencies ranging from 1 to 25 Hz, with a recommendation of 5 Hz or less for optimal results.
• Material Variability: Highlights that fatigue performance varies based on factors such as processing, surface condition, and differences in specimen preparation.
• Elastic vs. Plastic Deformation: Notes that not all plastics exhibit a purely elastic region under cyclic stress, and plastic deformation may lead to changes in applied load or strain during testing.
• Test Conditions and Equivalency: Emphasizes the importance of simulating service conditions during testing for design purposes, and cautions against comparing results from specimens prepared by different methods unless equivalence is established.

Applications

ASTM D7774-22 is widely used in quality assurance, research and development, and engineering design involving plastics. Its applications include:

• Material Selection: Helps manufacturers and engineers compare the flexural fatigue resistance of various plastics, directly informing material choices for components subjected to repeated bending.
• Product Validation: Supports the evaluation of finished parts, molded shapes, or samples cut from larger components, ensuring that products will withstand expected service conditions over long periods.
• Design Optimization: Provides data for mechanical and structural design, ensuring plastic components are robust against cyclic flexural loads, particularly in automotive, aerospace, electronics, and construction industries.
• Failure Analysis: Allows developers to plot S-N (stress vs. number of cycles) or r-N (strain vs. number of cycles) curves, identifying endurance or fatigue limits critical for safe, long-term product deployment.

Related Standards

ASTM D7774-22 references and aligns with several other established materials standards, ensuring a comprehensive approach to testing:

• ASTM D618: Conditioning Plastics for Testing
• ASTM D790: Flexural Properties of Plastics
• ASTM D6272: Four-Point Bending Flexural Properties
• ASTM D638: Tensile Properties of Plastics
• ASTM D883: Fundamental Terminology Relating to Plastics
• ASTM D792, D1505, D2839, D4883: Density and Specific Gravity of Plastics
• ASTM E4, E83, E177, E691, E1942: Precision, calibration, and data acquisition procedures for mechanical testing
• ISO 13003: Fatigue Properties of Fibre-Reinforced Plastics (note: technical differences exist, so results from ASTM D7774-22 and ISO 13003 are not directly comparable)

Practical Value

Implementing ASTM D7774-22 enables organizations to:

• Ensure reliable, comparable assessment of plastic fatigue performance
• Improve safety and durability in plastic product design
• Meet international expectations for standardization and product quality
• Support compliance with regulations and customer requirements relating to mechanical properties

By providing a robust and internationally recognized method, ASTM D7774-22 is a key standard for any laboratory or manufacturer involved in advanced plastics testing under cyclic flexural loads.

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: D7774 − 22
Standard Test Method for
Flexural Fatigue Properties of Plastics
This standard is issued under the fixed designation D7774; 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* 2. Referenced Documents
1.1 This test method covers the determination of dynamic 2.1 ASTM Standards:
D618 Practice for Conditioning Plastics for Testing
fatigue properties of plastics in flexure. This method is appli-
D638 Test Method for Tensile Properties of Plastics
cable to rigid and semi-rigid plastics. Stress and strain levels
D790 Test Methods for Flexural Properties of Unreinforced
are below the proportional limit of the material where the
and Reinforced Plastics and Electrical Insulating Materi-
strains and stresses are relatively elastic. Three-point or four-
als
point bending systems are used to determine these properties.
D792 Test Methods for Density and Specific Gravity (Rela-
1.2 This test method can be used with two procedures:
tive Density) of Plastics by Displacement
1.2.1 Procedure A, designed for materials that use three- D883 Terminology Relating to Plastics
point loading systems to determine flexural strength. Three- D1505 Test Method for Density of Plastics by the Density-
Gradient Technique
point loading system is used for this procedure.
D2839 Practice for Use of a Melt Index Strand for Deter-
1.2.2 Procedure B, designed for materials that use four-
mining Density of Polyethylene
point loading systems to determine flexural strength. Four-
D3479/D3479M Test Method for Tension-Tension Fatigue
point loading system is used for this procedure.
of Polymer Matrix Composite Materials
1.3 Comparative tests can be run in accordance with either
D4883 Test Method for Density of Polyethylene by the
procedure, provided that the procedure is found satisfactory for
Ultrasound Technique
the material being tested.
D5947 Test Methods for Physical Dimensions of Solid
Plastics Specimens
1.4 The values stated in SI units are to be regarded as the
D6272 Test Method for Flexural Properties of Unreinforced
standard. The values provided in parentheses are for informa-
and Reinforced Plastics and Electrical Insulating Materi-
tion only.
als by Four-Point Bending
1.5 This standard does not purport to address all of the E4 Practices for Force Calibration and Verification of Test-
ing Machines
safety concerns, if any, associated with its use. It is the
E83 Practice for Verification and Classification of Exten-
responsibility of the user of this standard to establish appro-
someter Systems
priate safety, health, and environmental practices and deter-
E177 Practice for Use of the Terms Precision and Bias in
mine the applicability of regulatory limitations prior to use.
ASTM Test Methods
NOTE 1—This standard and ISO 13003 address the same subject matter,
E691 Practice for Conducting an Interlaboratory Study to
but differ in technical content and results cannot be directly compared
Determine the Precision of a Test Method
between the two test methods.
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-
2.2 ISO Standard:
ization established in the Decision on Principles for the
ISO 13003 Fibre-Reinforced Plastics—Determination of
Development of International Standards, Guides and Recom-
Fatigue Properties Under Cyclic Loading Conditions
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
This test method is under the jurisdiction ofASTM Committee D20 on Plastics Standards volume information, refer to the standard’s Document Summary page on
and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties. the ASTM website.
Current edition approved July 1, 2022. Published July 2022. Last previous Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
edition approved in 2017 as D7774 - 17. DOI:10.1520/D7774-22. 4th Floor, New York, NY 10036, http://www.ansi.org.
*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
D7774 − 22
3. Terminology caution shall be taken when using the results for design as they
are generally not indicative of the true fatigue properties of the
3.1 Definitions—Definitions applying to this test method
material.
appear in Terminology D883.
5.4 The results of these fatigue tests are suitable for appli-
3.2 mean strain, n—the algebraic average of the maximum
cation in design only when the specimen test conditions
and minimum strains in one cycle.
realistically simulate service conditions or some methodology
3.3 mean stress, n—the algebraic average of the maximum
of accounting for service conditions is available and clearly
and minimum stresses in one cycle.
defined.
3.4 R Ratio, n—the ratio of the minimum stress or strain to
5.5 This procedure accommodates various specimen prepa-
the maximum stress or strain that the specimen is loaded.
ration techniques. Comparison of results obtained from speci-
3.5 proportional limit, n—the maximum elastic stress or mens prepared in different manners shall not be considered
strain exhibited by the material as observed during Test comparable unless equivalency has been demonstrated.
Methods D790 (for Procedure A) or Test Method D6272 (for
6. Apparatus
Procedure B).
6.1 Testing Machine—The testing machine shall essentially
4. Summary of Test Method
meet the specifications of Test Method D790 except as de-
scribed below. The error in the deflection measuring system
4.1 ProcedureA—Aspecimenofrectangularcrosssectionis
shall not exceed 60.5 % of the maximum deflection. The
bracedbytwodouble-sidedsupportsandisloadedbymeansof
machine shall be able to execute sinusoidal or square/
a double-sided loading nose midway between the supports. A
trapezoidal load or deflection programs at the specified test
support span-to-depth ratio of 16:1 is used. The specimen is
frequency and maintain an error of 61 % or less of the
cyclically loaded to predetermined stress or strain range at a
maximum programmed load or deflection.
uniform frequency and R ratio until the specimen ruptures or
yields. From these tests, fatigue strengths can be determined at 6.2 Recording Equipment—Calibrated equipment must be
specified numbers of cycles. At least four different stress or used to record the following information during testing at a
strain levels are tested to construct a stress versus number of data acquisition rate and filter in accordance with Guide
cycles to failure (S-N) curve or a strain versus number of E1942:
cycles to failure (r-N) to determine the flexural endurance limit 6.2.1 Load versus time,
of the material at a specific R ratio. 6.2.2 Deflection versus time, and
6.2.3 Number of cycles.
4.2 Procedure B—A specimen of rectangular cross section
6.3 Micrometers—Applicable apparatus in accordance with
is braced by two double-sided supports and is loaded by means
oftwodouble-sidedloadingnoses,eachanequaldistancefrom Test Methods D5947 shall be used to measure the width and
thickness of the test specimen.
the adjacent support. Load span-to-support span ratios can be
1:2 or 1:3. The specimen is cyclically loaded to a predeter-
6.4 Bending Fixture
mined stress or strain range at a uniform frequency and R ratio
6.4.1 Procedure A—A three-point bending fixture shall be
until the specimen ruptures or yields. From these tests, fatigue
used. A double-sided loading nose and two double-sided
strengths can be determined at specified numbers of cycles.At
supports are required for this procedure. One side loads or
least four different stress or strain levels are tested to construct
supportsthetopofthespecimenandtheotherloadsorsupports
a stress versus number of cycles to failure (S-N) curve or a
the bottom of the specimen.The dimensions of both sides shall
strain versus number of cycles to failure (r-N) to determine the
be identical. The radii of the noses and supports will be in
flexural endurance limit of the material at a specific R ratio.
accordance with Test Method D790. An example of a fixture
configuration for Procedure A is shown in Fig. 1. This fixture
5. Significance and Use
hasasupportspanrangeof50.8to254mm(2to10in.)at50.8
mm (2 in.) intervals.
5.1 These fatigue tests are used to determine the effect of
6.4.1.1 The supports are set to simply support the specimen
processing, surface condition, stress, and so forth, on the
using minimal pressure to hold the specimen in place. The
fatigue resistance of plastic material subjected to flexural stress
specimenshallbelongenoughsothattheendsdonotslipfrom
for relatively large numbers of cycles. The results can also be
the supports during testing.
used as a guide for the selection of plastic materials for service
6.4.2 Procedure B—Four-point bending fixture shall be
under conditions of repeated flexural stress.
used. Two double-sided loading noses and two double-sided
5.2 Properties can vary with specimen depth and test fre-
supports are required for this procedure. One side loads or
quency. Test frequency can be 1-25 Hz but it is recommended
supportsthetopofthespecimenandtheotherloadsorsupports
that a frequency of 5 Hz or less be used.
the bottom of the specimen.The dimensions of both sides shall
5.3 Material response in fatigue is not identical for all be identical. The radii of the noses and supports will be in
plastics. If a plastic does not exhibit an elastic region, where accordance with Test Method D6272. Load-span-to-support-
strain is reversible, plastic deformation will occur during span ratios shall be set at 1:2 or 1:3. An example of a 1:3
fatigue testing, causing the amplitude of the programmed load load-span-to-support-span ratio fixture configuration for Pro-
or deformation to change during testing. In this situation, cedure B is shown in Fig. 2. This fixture has a load span range
D7774 − 22
FIG. 1 Three-Point Fixture for Flexural Fatigue Tests
FIG. 2 Four-Point Fixture for Flexural Fatigue Tests
of 50.8 to 101.6 mm (2 to 4 in.) and a support span range of 7.4 Procedure B—Specimen dimensions, shape, surfaces,
101.6 to 203.2 mm (4 to 8 in.), both at 50.8 mm (2 in.) and limitations shall be in compliance with Test Method
intervals. D6272.Thistestmethodonlycoversspecimenswith1:2or1:3
6.4.2.1 The supports are set to simply support the specimen load span-to-support span ratios. The specimen shall be long
using minimal pressure to hold the specimen in place. The enough to allow for overhanging on each end of at least 10 %
specimenshallbelongenoughsothattheendsdonotslipfrom of the support span, but in no case less than 12.8 mm ( ⁄2 in.).
the supports during testing. Overhang shall be sufficient to prevent the specimen from
slipping through the supports.
7. Sampling, Test Specimens, and Test Units
7.5 Specimens that do not naturally fail in the load span
7.1 Specimens shall be taken from samples that accurately
duringtestingcanbemadethinnerinthegaugeareatopromote
represent the material or design that is being tested. failure in the load span. The reduced width of the specimen
shall be used to calculate the test stress or strain.
7.2 The specimens can be cut from sheets, plates, or molded
shapes, or can be molded to the desired finished dimensions. 7.6 Samples cut from non-uniform thick molded part sec-
The actual dimensions used for calculations shall be measured
tions shall be machined equally and minimally on both sides to
in accordance with Test Methods D5947. create a uniform thickness in the support span. It must be noted
that machining the thickness of plastic can change the me-
7.3 Procedure A—Specimen dimensions, shape, surfaces,
chanical properties and caution shall be taken when applying
and limitations shall be in compliance withTest Method D790.
the results to design.
The specimen shall be long enough to allow for overhanging
on each end of at least 10 % of the support span, but in no case
7.7 It is recommended that density measurements be taken
less than 12.8 mm 164 ( ⁄2 in.). Overhang shall be sufficient to from each sample in the support span in accordance with Test
prevent the specimen from slipping through the supports.
Methods D792, Test Method D1505, Practice D2839 or Test
D7774 − 22
Method D4883 to ensure that the process used to fabricate the 11.2.2.1 Determine and set the support span and the load
specimens creates consistent and uniform material. span. Measure the spans accurately to the nearest 0.1 mm
(0.004 in.) for spans less than 63 mm (2.5 in.). For spans
8. Number of Test Specimens
greater than or equal to 63 mm (2.5 in.), measure to the nearest
0.3 mm (0.012 in.). Use the actual measured spans for all
8.1 At least three test specimens shall be tested at each of
calculations.
the four stress or strain levels (minimum of twelve specimens
for each test). For additional sample sizes consult Table 1 of 11.2.2.2 Position the four-point bend fixture so that the
centerline between the load noses and the load axis of the
Test Method D3479/D3479M.
testing mach
...

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: D7774 − 17 D7774 − 22
Standard Test Method for
Flexural Fatigue Properties of Plastics
This standard is issued under the fixed designation D7774; 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 full reversed fatigue properties of plastics in flexure. This method is
applicable to rigid and semi-rigid plastics. Stress and strain levels are below the proportional limit of the material where the strains
and stresses are relatively elastic. Three-point or four-point bending systems are used to determine these properties.
1.2 This test method can be used with two procedures:
1.2.1 Procedure A, designed for materials that use three-point loading systems to determine flexural strength. Three-point loading
system is used for this procedure.
1.2.2 Procedure B, designed for materials that use four-point loading systems to determine flexural strength. Four-point loading
system is used for this procedure.
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—This standard and ISO 13003 address the same subject matter, but differ in technical content and results cannot be directly compared between
the two test methods.
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
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. Last previous edition approved in 20122017 as D7774 - 12.D7774 - 17.
DOI:10.1520/D7774-17.DOI:10.1520/D7774-22.
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
D7774 − 22
D790 Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
D792 Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
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
D6272 Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by
Four-Point Bending
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
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
2.2 ISO Standard:
ISO 13003 Fibre-Reinforced Plastics—Determination of Fatigue Properties Under Cyclic Loading Conditions
3. Terminology
3.1 Definitions—Definitions applying to this test method appear in Terminology D883.
3.2 mean strain—strain, n—the algebraic average of the maximum and minimum strains in one cycle.
3.3 mean stress—stress, n—the algebraic average of the maximum and minimum stresses in one cycle.
3.4 R Ratio—Ratio, n—the ratio of the minimum stress or strain to the maximum stress or strain that the specimen is loaded.
3.5 proportional limit—limit, n—the maximum elastic stress or strain exhibited by the material as observed during Test Methods
D790 (for Procedure A) or Test Method D6272 (for Procedure B).
4. Summary of Test Method
4.1 Procedure A—A specimen of rectangular cross section is braced by two double-sided supports and is loaded by means of a
double-sided loading nose midway between the supports. A support span-to-depth ratio of 16:1 is used. The specimen is cyclically
loaded equally in the positive and negative directions to a specific to predetermined stress or strain levelrange at a uniform
frequency and R ratio 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 flexural endurance limit of the material.material
at a specific R ratio.
4.2 Procedure B—A specimen of rectangular cross section is braced by two double-sided supports and is loaded by means of two
double-sided loading noses, each an equal distance from the adjacent support. Load span-to-support span ratios can be 1:2 or 1:3.
The specimen is cyclically loaded equally in the positive and negative directions to a specificto a predetermined stress or strain
levelrange at a uniform frequency and R ratio 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 flexural endurance limit of
the material.material at a specific R ratio.
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 flexural 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.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
D7774 − 22
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.
6. Apparatus
6.1 Testing Machine—The testing machine shall essentially meet the specifications of Test Method D790 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 Deflection versus time, and
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
of the test specimen.
6.4 Bending Fixture
6.4.1 Procedure A—A three-point bending fixture shall be used. A double-sided loading nose and two double-sided supports are
required for this procedure. One side loads or supports the top of the specimen and the other loads or supports the bottom of the
specimen. The dimensions of both sides shall be identical. The radii of the noses and supports will be in accordance with Test
Method D790. An example of a fixture configuration for Procedure A is shown in Fig. 1. This fixture has a support span range of
50.8 to 254 mm (2 to 10 in.) at 50.8 mm (2 in.) intervals.
6.4.1.1 The supports are set to simply support the specimen using minimal pressure to hold the specimen in place. The specimen
shall be long enough so that the ends do not slip from the supports during testing.
6.4.2 Procedure B—Four-point bending fixture shall be used. Two double-sided loading noses and two double-sided supports are
required for this procedure. One side loads or supports the top of the specimen and the other loads or supports the bottom of the
specimen. The dimensions of both sides shall be identical. The radii of the noses and supports will be in accordance with Test
Method D6272. Load-span-to-support-span ratios shall be set at 1:2 or 1:3. An example of a 1:3 load-span-to-support-span ratio
fixture configuration for Procedure B is shown in Fig. 2. This fixture has a load span range of 50.8 to 101.6 mm (2 to 4 in.) and
a support span range of 101.6 to 203.2 mm (4 to 8 in.), both at 50.8 mm (2 in.) intervals.
6.4.2.1 The supports are set to simply support the specimen using minimal pressure to hold the specimen in place. The specimen
shall be long enough so that the ends do not slip from the supports during testing.
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FIG. 1 Three-Point Fixture for Flexural Fatigue Tests
FIG. 2 Four-Point Fixture for Flexural Fatigue Tests
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, surfaces, and limitations shall be in compliance with Test Method D790. The
specimen shall be long enough to allow for overhanging on each end of at least 10 % of the support span, but in no case less than
12.8 mm 164 ( ⁄2 in.). Overhang shall be sufficient to prevent the specimen from slipping through the supports.
7.4 Procedure B—Specimen dimensions, shape, surfaces, and limitations shall be in compliance with Test Method D6272. This
test method only covers specimens with 1:2 or 1:3 load span-to-support span ratios. The specimen shall be long enough to allow
for overhanging on each end of at least 10 % of the support span, but in no case less than 12.8 mm ( ⁄2 in.). Overhang shall be
sufficient to prevent the specimen from slipping through the supports.
7.5 Specimens that do not naturally fail in the load span during testing can be made thinner in the gauge area to promote failure
in the load span. The reduced width of the specimen shall be used to calculate the test stress or strain.
7.6 Samples cut from non-uniform thick molded part sections shall be machined equally and minimally on both sides to create
a uniform thickness in the support span. 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.
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7.7 It is recommended that density measurements be taken from each sample in the support span 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.
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 Section 7 of Practice D618 unless specified differently by contr
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