ASTM D7774-12

Standard Test Method for Flexural 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 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.
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 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 and health 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.

General Information

• Status: Historical
• Publication Date: 04-Jul-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: D7774 − 12
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.
Note—Balloted equations and text were included editorially and the year date changed on July 5, 2012.
1. Scope D638 Test Method for Tensile Properties of Plastics
D790 Test Methods for Flexural Properties of Unreinforced
1.1 This test method covers the determination of dynamic
and Reinforced Plastics and Electrical Insulating Materi-
full reversed fatigue properties of plastics in flexure. This
als
methodisapplicabletorigidandsemi-rigidplastics.Stressand
D792 Test Methods for Density and Specific Gravity (Rela-
strain levels are below the proportional limit of the material
tive Density) of Plastics by Displacement
where the strains and stresses are relatively elastic.Three-point
D883 Terminology Relating to Plastics
or four-point bending systems are used to determine these
D1505 Test Method for Density of Plastics by the Density-
properties.
Gradient Technique
1.2 This test method can be used with two procedures:
D2839 Practice for Use of a Melt Index Strand for Deter-
1.2.1 Procedure A, designed for materials that use three-
mining Density of Polyethylene
point loading systems to determine flexural strength. Three-
D3479/D3479M Test Method for Tension-Tension Fatigue
point loading system is used for this procedure.
of Polymer Matrix Composite Materials
1.2.2 Procedure B, designed for materials that use four-
D4883 Test Method for Density of Polyethylene by the
point loading systems to determine flexural strength. Four-
Ultrasound Technique
point loading system is used for this procedure.
D5947 Test Methods for Physical Dimensions of Solid
Plastics Specimens
1.3 Comparative tests can be run in accordance with either
procedure, provided that the procedure is found satisfactory for D6272 Test Method for Flexural Properties of Unreinforced
and Reinforced Plastics and Electrical Insulating Materi-
the material being tested.
als by Four-Point Bending
1.4 The values stated in SI units are to be regarded as the
E4 Practices for Force Verification of Testing 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
E1942 Guide for Evaluating DataAcquisition Systems Used
safety concerns, if any, associated with its use. It is the
in Cyclic Fatigue and Fracture Mechanics Testing
responsibility of the user of this standard to establish appro-
2.2 ISO Standard:
priate safety and health practices and determine the applica-
ISO 13003 Fibre-Reinforced Plastics—Determination of
bility of regulatory limitations prior to use.
Fatigue Properties Under Cyclic Loading Conditions
NOTE 1—This standard and ISO 13003 address the same subject matter,
but differ in technical content and results cannot be directly compared
3. Terminology
between the two test methods.
3.1 Definitions—Definitions applying to this test method
2. Referenced Documents
appear in Terminology D883.
2.1 ASTM Standards:
3.2 mean strain—thealgebraicaverageofthemaximumand
D618 Practice for Conditioning Plastics for Testing
minimum strains in one cycle.
3.3 mean stress—thealgebraicaverageofthemaximumand
This test method is under the jurisdiction ofASTM Committee D20 on Plastics
minimum stresses in one cycle.
and is the direct responsibility of Subcommittee D20.10.24 on Engineering and
Design Properties.
3.4 R Ratio—the ratio of the minimum stress or strain to the
Current edition approved July 5, 2012. Published July 2012. Last previous
maximum stress or strain that the specimen is loaded.
edition approved in 2011 as D7774 - 11. DOI:10.1520/D7774-12.
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 Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036, http://www.ansi.org.
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D7774 − 12
3.5 proportional limit—the maximum elastic stress or strain mens prepared in different manners shall not be considered
exhibited by the material as observed during Test Methods comparable unless equivalency has been demonstrated.
D790 (for ProcedureA) or Test Method D6272 (for Procedure
6. Apparatus
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
cyclicallyloadedequallyinthepositiveandnegativedirections
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,
flexural endurance limit of the material.
6.2.2 Deflection versus time, and
6.2.3 Number of cycles.
4.2 Procedure B—A specimen of rectangular cross section
is braced by two double-sided supports and is loaded by means
6.3 Micrometers—Applicable apparatus in accordance with
oftwodouble-sidedloadingnoses,eachanequaldistancefrom
Test Methods D5947 shall be used to measure the width and
the adjacent support. Load span-to-support span ratios can be
thickness of the test specimen.
1:2 or 1:3. The specimen is cyclically loaded equally in the
6.4 Bending Fixture
positive and negative directions to a specific stress or strain
6.4.1 Procedure A—A three-point bending fixture shall be
level at a uniform frequency until the specimen ruptures or
used. A double-sided loading nose and two double-sided
yields. From these tests, fatigue strengths can be determined at
supports are required for this procedure. One side loads or
specified numbers of cycles. At least four different stress or
supportsthetopofthespecimenandtheotherloadsorsupports
strain levels are tested to construct a stress versus number of
the bottom of the specimen.The dimensions of both sides shall
cycles to failure (S-N) curve or a strain versus number of
be identical. The radii of the noses and supports will be in
cycles to failure (r-N) to determine the flexural endurance limit
accordance with Test Method D790. An example of a fixture
of the material.
configuration for Procedure A is shown in Fig. 1. This fixture
hasasupportspanrangeof50.8to254mm(2to10in.)at50.8
5. Significance and Use
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
be identical. The radii of the noses and supports will be in
5.3 Material response in fatigue is not identical for all
accordance with Test Method D6272. Load-span-to-support-
plastics. If a plastic does not exhibit an elastic region, where
span ratios shall be set at 1:2 or 1:3. An example of a 1:3
strain is reversible, plastic deformation will occur during
load-span-to-support-span ratio fixture configuration for Pro-
fatigue testing, causing the amplitude of the programmed load
cedure B is shown in Fig. 2. This fixture has a load span range
or deformation to change during testing. In this situation,
of 50.8 to 101.6 mm (2 to 4 in.) and a support span range of
caution shall be taken when using the results for design as they
101.6 to 203.2 mm (4 to 8 in.), both at 50.8 mm (2 in.)
are generally not indicative of the true fatigue properties of the
intervals.
material.
6.4.2.1 The supports are set to simply support the specimen
5.4 The results of these fatigue tests are suitable for appli-
using minimal pressure to hold the specimen in place. The
cation in design only when the specimen test conditions
specimenshallbelongenoughsothattheendsdonotslipfrom
realistically simulate service conditions or some methodology
the supports during testing.
of accounting for service conditions is available and clearly
defined. 7. Sampling, Test Specimens, and Test Units
5.5 This procedure accommodates various specimen prepa- 7.1 Specimens shall be taken from samples that accurately
ration techniques. Comparison of results obtained from speci- represent the material or design that is being tested.
D7774 − 12
FIG. 1 Three-Point Fixture for Flexural Fatigue Tests
FIG. 2 Four-Point Fixture for Flexural Fatigue Tests
7.2 The specimens can be cut from sheets, plates, or molded failure in the load span. The reduced width of the specimen
shapes, or can be molded to the desired finished dimensions. shall be used to calculate the test stress or strain.
The actual dimensions used for calculations shall be measured
7.6 Samples cut from non-uniform thick molded part sec-
in accordance with Test Methods D5947.
tions shall be machined equally and minimally on both sides to
7.3 Procedure A—Specimen dimensions, shape, surfaces,
create a uniform thickness in the support span. It must be noted
and limitations shall be in compliance withTest Method D790.
that machining the thickness of plastic can change the me-
The specimen shall be long enough to allow for overhanging
chanical properties and caution shall be taken when applying
on each end of at least 10 % of the support span, but in no case
the results to design.
less than 12.8 mm 164 ( ⁄2 in.). Overhang shall be sufficient to
7.7 It is recommended that density measurements be taken
prevent the specimen from slipping through the supports.
from each sample in the support span in accordance with Test
7.4 Procedure B—Specimen dimensions, shape, surfaces,
Methods D792, Test Method D1505, Practice D2839 or Test
and limitations shall be in compliance with Test Method
Method D4883 to ensure that the process used to fabricate the
D6272.Thistestmethodonlycoversspecimenswith1:2or1:3
specimens creates consistent and uniform material.
load span-to-support span ratios. The specimen shall be long
enough to allow for overhanging on each end of at least 10 %
8. Number of Test Specimens
of the support span, but in no case less than 12.8 mm ( ⁄2 in.).
Overhang shall be sufficient to prevent the specimen from
8.1 At least three test specimens shall be tested at each of
slipping through the supports.
the four stress or strain levels (minimum of twelve specimens
7.5 Specimens that do not naturally fail in the load span for each test). For additional sample sizes consult Table 1 of
duringtestingcanbemadethinnerinthegaugeareatopromote Test Method D3479/D3479M.
D7774 − 12
8.2 In the case of anisotropic materials, a single direction testing machine is co-axial. This insures that no moments are
shall be chosen and maintained for all stress or strain levels. created on the apparatus and that the load is being applied
directly at the midpoint of the fixture.
9. Calibration and Standardization
11.2.2.3 Position the specimen on the supports so that the
centerline between the load noses and the middle of the
9.1 All equipment shall be calibrated in accordance with the
specimen are aligned and the bottom face of the specimen is
manufacturer’s specifications and Practice E4.
perpendicular to the loading axis.
9.2 Specimens for each test shall be identically processed
11.3 Test Program Setup:
and prepared for accurate results.
11.3.1 Program the testing machine to follow a dynamic
10. Conditioning load(sinusoidal,square,ortrapezoidal)ordeflectionwaveform
with the amplitude set to the selected load or deflection level.
10.1 Conditioning—Condition the test specimens in accor-
Unless specified, the default waveform shall be sinusoidal.The
dance with Procedure A of Practice D618 unless otherwise
selected level shall be below the proportional limit of the
specified by contract or the relevantASTM material specifica-
material as determined by Test Methods D790 (for Procedure
tion. Condition time is specified as a minimum. Temperature
A) or Test Method D6272 (for Procedure B). The R ratio is
and humidity tolerances shall be in accordance with Section 7
equalto-1andthemeanstress/strainisequaltozerosinceboth
of Practice D618 unless specified differently by contract or
Procedure A and Procedure B use fully reversed loading. Set
material specification.
the test frequency (maximum 25 Hz).
10.2 Test Conditions—Conduct the tests at the same tem-
11.3.2 Stress Level Selection—Stress levels shall not exceed
perature and humidity used for conditioning with tolerances in
the proportional limit of the material. The associated load of a
accordance with Section 7 of Practice D618 unless otherwise
selected stress level
...