ASTM D6112-23

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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: D6112 − 18 D6112 − 23
Standard Test Methods for
Compressive and Flexural Creep and Creep-Rupture of
Plastic Lumber and Shapes
This standard is issued under the fixed designation D6112; 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 These test methods cover the determination of the creep and creep-rupture properties of plastic lumber and shapes, when
loaded in compression or flexure under specified environmental conditions. Test specimens in the “as-manufactured” form are
employed. As such, these are test methods for evaluating the properties of plastic lumber or shapes as a product and not material
property test methods.
1.2 Plastic lumber and plastic shapes are currently made predominantly with recycled plastics. However, this test method would
also be applicable to similar manufactured plastic products made from virgin resins where the product is non-homogenous in the
cross-section.
1.3 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are for information only.
1.4 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.
1.5 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:
D543 Practices for Evaluating the Resistance of Plastics to Chemical Reagents
D883 Terminology Relating to Plastics
D2990 Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics
D4000 Classification System for Specifying Plastic Materials
D5033 Guide for Development of ASTM Standards Relating to Recycling and Use of Recycled Plastics (Withdrawn 2007)
D5947 Test Methods for Physical Dimensions of Solid Plastics Specimens
These test methods are under the jurisdiction of ASTM Committee D20 on Plastics and are the direct responsibility of Subcommittee D20.20 on Plastic Lumber (Section
D20.20.01).
Current edition approved June 1, 2018Nov. 1, 2023. Published June 2018November 2023. Originally approved in 1997. Last previous edition approved in 20132018 as
D6112 - 13.D6112 - 18. DOI: 10.1520/D6112-18.10.1520/D6112-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
*A Summary of Changes section appears at the end of this standard
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E4 Practices for Force Calibration and Verification of Testing Machines
E176 Terminology of Fire Standards
E456 Terminology Relating to Quality and Statistics
3. Terminology
3.1 Definitions of Terms—For definitions of terms used in these test methods and associated with plastics issues refer to the
terminology contained in Terminology D883 or in Guide D5033. For definitions of terms used in this specification and associated
with fire issues refer to the terminology contained in Terminology E176. For terms relating to precision and bias and associated
issues, the terms used in this standard are defined in accordance with Terminology E456.
3.1.1 plastic lumber, n—a manufactured product made primarily from plastic materials (filled or unfilled), typically used as a
building material for purposes similar to those of traditional lumber, which is usually rectangular in cross-section.
3.1.1.1 Discussion—
Plastic lumber is typically supplied in sizes similar to those of traditional lumber board, timber and dimension lumber; however
the tolerances for plastic lumber and for traditional lumber are not necessarily the same. D883
3.1.2 resin, n—a solid or pseudo-solid organic material often of high molecular weight, that exhibits a tendency to flow when
subjected to stress, usually has a softening or melting range, and usually fractures conchoidally.
3.1.2.1 Discussion—
In a broad sense, the term is used to designate any polymer that is a basic material for plastics. D883
3.2 Definitions:Definitions of Terms Specific to This Standard:
3.2.1 compression—in a compressive creep test, the decrease in length produced in the gauge length or the total length of a test
specimen.
3.2.2 creep modulus—the ratio of initial applied stress to creep strain.
3.2.3 creep strain—the total strain, at any given time, produced by the applied stress during a creep test.
3.2.3.1 Discussion—
The term creep, as used in this test method, reflects current plastics engineering usage. In scientific practice, creep is often defined
to be the nonelastic portion of strain. However, this definition is not applicable to existing engineering formulas. Plastics have a
wide spectrum of retardation times, and elastic portions of strain cannot be separated in practice from nonelastic. Therefore,
wherever “strain” is mentioned in these test methods, it refers to the sum of elastic strain plus the additional strain with time.
3.1.3.1 Discussion—The term creep, as used in this test method, reflects current plastics engineering usage. In scientific practice,
creep is often defined to be the nonelastic portion of strain. However, this definition is not applicable to existing engineering
formulas. Plastics have a wide spectrum of retardation times, and elastic portions of strain cannot be separated in practice from
nonelastic. Therefore, wherever “strain” is mentioned in these test methods, it refers to the sum of elastic strain plus the additional
strain with time.
3.2.4 deflection—in a flexural creep test, the change in mid-span position of a test specimen.
3.2.5 deformation—a change in shape, size or position of a test specimen as a result of compression, deflection, or extension:
extension.
3.1.6 plastic lumber, n—a manufactured product made primarily from plastic materials (filled or unfilled), typically used as a
building material for purposes similar to those of traditional lumber, which is usually rectangular in cross-section. (Terminology
D883)
3.1.6.1 Discussion—
Plastic lumber is typically supplied in sizes similar to those of traditional lumber board, timber and dimension lumber; however
the tolerances for plastic lumber and for traditional lumber are not necessarily the same. (Terminology D883)
3.2.6 plastic shape, n—a manufactured product made primarily from plastic materials (filled or unfilled), which is not necessarily
rectangular in cross section.
3.1.8 resin, n—a solid or pseudo-solid organic material often of high molecular weight, that exhibits a tendency to flow when
subjected to stress, usually has a softening or melting range, and usually fractures conchoidally. (Terminology D883)
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3.1.8.1 Discussion—In a broad sense, the term is used to designate any polymer that is a basic material for plastics.
3.2.7 stress—for compressive creep, the ratio of the applied load to the initial cross-sectional area. For flexural creep, maximum
fiber stress is calculated according to Eq 1.
3.2.7.1 Discussion—
Maximum fiber stress for flexible creep is calculated based on the load at a given point on the load-deflection curve.
3.1.10 Additional definition of terms applying to this test method appear in Terminology D883 and Guide D5033.
4. Summary of Test Method
4.1 These test methods consist of measuring the deflection or compression as a function of time and time-to-rupture, or failure
of a specimen subject to constant flexural or compressive load under specified environmental conditions.
4.2 The four-point loading a outlined in this testing standard shall be used for the flexural creep tests.
4.3 Compressive loading as outlined in this testing standard shall be used for the compressive creep tests.
4.4 These test methods represent modifications of the compressive and flexural creep and creep rupture test methods specified in
Test Methods D2990.
5. Significance and Use
5.1 Data from creep and creep-rupture tests are necessary to predict the creep modulus and strength of materials under long-term
loads and to predict dimensional changes that have the potential to occur as a result of such loads.
5.2 Data from these test methods can be used to characterize plastic lumber: for comparison purposes, for the design of fabricated
parts, to determine long-term performance under constant load, and under certain conditions, for specification purposes.
5.3 For many products, it is possible that there will be a specification that requires the use of this test method, but with some
procedural modifications that take precedence when adhering to the specification. Therefore, it is advisable to refer to that product
specification before using this test method. Table 1 in Classification D4000 lists the ASTM materials standards that currently exist.
6. Apparatus
6.1 General:
6.1.1 Loading System:
6.1.1.1 The loading system must be so designed that the load applied and maintained on the specimen is within 61 % of the
desired load. The loading mechanism must allow reproductively rapid and smooth loading as specified in 11.1.3. In creep-rupture
tests, provision must be made to ensure that shock loading, caused by a specimen failure, is not transferred to other specimens
undergoing testing. The accuracy of the loading system shall be verified at least once each year in accordance with Practices E4.
6.1.1.2 Loading systems that provide a mechanical advantage require careful design to maintain constant load throughout the test.
For example, lever systems must be designed so that the load does not change as the lever arm moves during the test.
6.1.2 Compression and Deflection Measurements:
6.1.2.1 The accuracy of the deformation measuring device shall be within 61 % of the deformation to be measured.
6.1.2.2 Deformation measuring devices shall be calibrated against a precision micrometer screw or other suitable standard under
conditions are nearly identical as possible with those encountered in the test. Caution is necessary when using deformation
measuring devices whose calibration is subject to drifting with time and is dependent on temperature and humidity.
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6.1.2.3 Deformation measuring devices shall be firmly attached to or seated on the specimen so that no slippage occurs. Electrical
resistance gauges are suitable only if the material tested will permit perfect adhesion to the specimen and if they are consistent with
6.2.1
6.1.3 Time Measurement—The accuracy of the time measuring device shall be 61 % of the time-to-rupture or failure or the
elapsed time of each creep measurement, or both.
6.1.4 Temperature Control and Measurement:
6.1.4.1 The temperature of the test space, especially close to the gauge length of the specimen, shall be maintained within 62°C
by a suitable automatic device and shall be stated in reporting the results.
NOTE 2—The thermal contraction and expansion associated with small temperature changes during the test has the potential to produce changes in the
apparent creep rate, especially near transition temperatures.
6.1.4.2 Care must be taken to ensure accurate temperature measurements over the gauge length of the specimen throughout the
test. The temperature measuring devices shall be checked regularly against temperature standards and shall indicate the
temperature of the specimen gauge area.
6.1.4.3 Temperature measurements shall be made at frequent intervals, or continuously recorded to ensure an accurate
determination of the average test temperature and compliance with 6.1.5.
6.1.5 Environmental Control and Measurements:
6.1.5.1 When the test environment is air, the relative humidity shall be controlled to 50 6 5 % during the test unless otherwise
specified, or unless the creep behavior of the material under testing has been shown to be unaffected by humidity. The controlling
and measuring instruments shall be stable for long time intervals and accurate to within 61 %. (The control of relative humidity
is known to be difficult at temperatures much outside the range from 50 to 104°F (10 to 40°C).)
6.1.5.2 If, for any reason, the specified relative humidity cannot be achieved or the test is conducted to determine the sensitivity
of the product to high humidity, report the actual average value and fluctuation of relative humidity used.
6.1.5.3 The composition of the test environment shall be maintained constant throughout the test. (Warning—Take special
precautions to avoid personal contact, to eliminate toxic vapors, and to guard against explosion hazards in accordance with any
possible hazardous nature of the particular environment being used.)
6.1.6 Vibration Control—Creep tests are quite sensitive to shock and vibration. The location of the apparatus, the test equipment,
and mounting shall be so designated that the specimen is isolated from vibration. Multiple-station test equipment must be of
sufficient rigidity so that no significant deflection occurs in the test equipment during creep or creep-rupture testing. During
time-to-rupture or failure, means to prevent jarring of other test specimens by the falling load from a failed test specimen shall be
provided by a suitable net or cushion.
6.2 Compressive Creep:
6.2.1 Platens—Parallel platens shall be used to apply the load to the unconfined-type specimen (see 8.2). One of the platens of
the machine shall preferably be self-aligning and shall, so that it is possible to apply the load evenly over the face of the specimen,
be arranged so that the specimen is accurately centered and the resultant of the load is through its center.
6.2.2 The compression of specimen gauge length under load shall be measured by means of any device that will not influence the
specimen behavior by mechanical (undesirable deformation, notches, etc.) physical (heating of specimen, etc.), or chemical effects.
Alternatively, the compression of the specimen can be measured using platen displacement with the entire length of the specimen
serving as the gauge length.
6.3 Flexural Creep:
6.3.1 Test Rack—A rigid test rack shall be used to provide support of the test specimen at both ends with a span equal to 16
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(tolerant +4 and −2) times the depth of the specimen. In order to avoid excessive indentation of the specimen, the radius of the
support shall be a minimum of 0.5 in. (12.7 mm) and up to 1.5 times the depth of the specimen. Sufficient space must be allowed
below the specimen for dead-weight loading.
6.3.2 Loading Beam—The loading beam shall be configured with loading noses with cylindrical surfaces (see Fig. 1). The radius
of noses shall be at least 0.5 in. (12.7 mm) or all specimens. For large specimens it is possible that the radius of the supports will
be up to 1.5 times the specimen depth.
6.3.3 A four point loading arrangement shall be used as shown in Fig. 1.
6.3.4 For flexural testing the deflection of the specimen shall be measured at the midpoint of the load span at the bottom face of
the specimen.
7. Reagents
7.1 Purity of Reagents—Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where
such specification are available . It is acceptable to use other grades, provided it is first ascertained that the reagent is of sufficiently
high purity to permit its use without lessening the accuracy of the determination.
7.2 Purity of Water—Unless otherwise indicated, references to water shall be understood to mean distilled water or water of equal
purity.
7.3 Specified Reagents—If this test method is referenced in a material specification, the specific reagent to be used shall be as
stipulated in the specification.
7.4 Standard Reagents—A list of standard reagents is also available in Test Method D543.
8. Test Specimen
8.1 General:
8.1.1 It is acceptable to make test specimens by any of the techniques normally employed to produce plastic lumber. When the
testing objective is to obtain design data, the method of sample fabrication shall be the same as that used in the application.
8.1.2 In the case of materials whose dimensions are known to change significantly due to the specified environment alone (for
example, the shrinkage of some thermosetting plastics due to post-curing at elevated temperatures), provision shall be made to test
unloaded control specimens alongside the test specimen so as to provide compensation for changes other than creep. A minimum
of three control specimens shall be tested at each test temperature.
NOTE 1—Minimum radius = 0.5 in. (12.7 mm); maximum radius = 1.5 times the specimen depth.
FIG. 1 Four Point Loading and Support Noses at Maximum Radius
Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by
the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Derner, U.K., and the United States Pharmacopeia and National
Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
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8.1.3 In creep testing at a single temperature, the minimum number of test specimens at each stress shall be two if four or more
levels of stress are used or three if fewer than four levels are used.
8.1.4 In creep-rupture testing, a minimum of two specimens shall be tested at each of the stress levels specified in 10.2.1 at each
temperature.
NOTE 3—The scatter of creep-rupture data is considerable, with one half to a full decade of variation in time-to-rupture being typical. Therefore, it is some
times necessary to test more than two specimens at each stress level to obtain satisfactory results.
8.2 Compressive Creep:
8.2.1 The standard test specimen shall be in the form of a right prism. With the exception that specimen cross sections are the full
sections of any manufactured plastic lumber or shape. Surfaces of the test specimens shall be plane and parallel.
8.2.2 Test specimens for determining compressive properties of plastic lumber and shapes shall be cut from the “as manufactured”
profile. Great care shall be taken in cutting and machining the ends so that smooth, flat parallel surfaces and sharp, clean edges
to within ⁄300 (0.0033) of the specimen length perpendicular to the long axis of the specimen results. Plastic lumber is generally
nonuniform through the cross-section; no machining operations other than those required to provide flat, parallel ends shall be
carried out.
8.2.2.1 The standard test specimen, except as indicated in 8.2.2.2 to 8.2.2.3, shall be in the form of a right cylinder or prism whose
height is twice its minimum width or diameter.
8.2.2.2 For rod material, the test specimen shall have a diameter equal to the diameter of the rod and whose height is twice its
diameter.
8.2.2.3 When testing hollow profiles, the test specimen shall have a minimum length equal to twice its minimum cross sectional
dimension.
8.3 Flexural Creep:
8.3.1 The specimens shall be full size as manufactured. The original surfaces shall be unaltered.
8.3.2 For flatwise (plank) tests, the depth of the specimen shall be the thickness, or smaller dimension, of the material. For
edgewise (joist) tests the width becomes the smaller dimension and depth the larger. For all tests, the support span shall be 16
(tolerance +4 and −2) times the depth of the beam. 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 0.25 in. (6.4 mm) on each end. Overhand shall be sufficient to prevent
the specimen from slipping through the supports.
9. Conditioning
9.1 The specimen shall be preconditioned in the test environment for at least 48 h prior to being tested or for a longer period if
needed to establish an equilibrium condition. Those materials whose creep properties are suspected to be affected by moisture
content shall be brought to moisture equilibrium appropriate to the test conditions prior to testing.
9.2 If warranted, based on projected usage, submerge the test specimen in water for at least 24 h or until it achieves an equilibrium
moisture content prior to conditioning. Tape the ends of the test specimen prior to water immersion.
10. Selection of Test Conditions
10.1 Test Temperatures—Selection of temperatures for creep and creep-rupture testing depends on the intended use of the test
results and shall be made as follows:
10.1.1 To characterize a material, select two or more test temperatures to cover the useful temperature range usually at elevated
temperatures, in suitable increments that reflect the variation of the creep of the material with temperature and transitions of the
material. Unless actual conditions warrant otherwise, test temperatures of 50, 73.4, and 104°F (10, 23, and 40°C) are
recommended.
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10.1.2 To obtain design data, the test temperatures and environment shall be the same as those of the intended end-use application.
10.1.3 To obtain the stress for 1 % strain at 1000 h (see 10.3.2) or for other simple material comparisons such as data sheets, use
the recommended test temperatures cited in 10.1.1.
10.2 Creep-Rupture:
10.2.1 At each test temperature, make creep-rupture tests at a minimum of seven stress levels selected so as to produce rupture
at approximately the following times: 1, 10, 30, 100, 300, 1000, and 3000 h.
10.2.1.1 The objective of these tests is to produce at each test temperature, a curve of stress-at-rupture versus time-to-rupture, often
called a “creep-rupture envelope,” which indicates a limit of a material’s load-bearing capability at the test temperature. For the
prediction of long-term performance, for example, in the design of parts that will bear constant loads six months or longer, test
times longer than 3000 h are usually necessary, particularly at elevated temperatures where it is possible that heat aging of the
material will be occurring, and in aggressive environments, both of which can greatly affect creep-rupture.
10.2.2 For materials that fail catastrophically (that is, with negligible yielding, drawing, or flowing) measure and report the
time-to-rupture. For materials that yield, draw, or flow significantly prior to rupture, measure and report the time at the onset of
tertiary creep (onset of yielding, flowing, or drawing), which shall be considered the time-to-failure and shall be measured and
reported. For materials that yield, draw, or flow, it is possible that creep strain will have to be measured with a recorder or some
other method.
10.3 Creep:
10.3.1 To obtain design data or to characterize a material, select stress levels as follows:
10.3.1.1 For materials that show linear viscoelasticity, that is, successive creep modulus versus time for different stresses that
superimpose upon each other (Boltzman superposition principle, select a minimum of three stress levels for each temperature of
interest.
10.3.1.2 For materials that are significantly affected by stress, select at least five stresses (and preferably more) for each
temperature of interest.
10.3.1.3 Select stress levels in approximately even increments up to the 1000–h creep-rupture stress: stress levels above 1000 psi
(6.9 MPa) to the nearest 500 psi (3.4 MPa); stress levels below 1000 psi (6.9 MPa) to the nearest 100 psi (0.7 MPa).
10.3.1.4 Do not use stress levels that produce failure in less than 1000 h in creep testing.
10.3.2 For simple material comparisons, as for data sheets and the like, determine the stress to produce 1 % strain in 1000 h. Do
this by selecting several loads to produce strains in the approximate range of 1 % (both somewhat greater and less than 1 % in 1000
h) and plotting a 1000-h isochronous stress-strain curve from which it will be possible to determine the stress to produce 1 % strain
by interpolation.
NOTE 4—Isochronous stress-strain curves are cartesian plots of the applied stress used in the creep test versus the creep strain at a specific time, in this
case 1000 h. Since only one point of an isochronous plot is obtained from each creep test, it is usually necessary to run creep tests at least three stress
levels (and preferably more) to obtain an isochronous plot (See Fig. 2).
11. Procedure
11.1 General:
11.1.1 Mount a properly conditioned and measured specimen in the compressive creep fixture of flexural creep rack. If necessary,
mount a properly conditioned and measured control specimen alongside the test specimen in the same manner.
Nielsen, L.E., Mechanical Properties of Polymers, Reinhold Publishing Corp., New York, NY, 1962.
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FIG. 2 Cartesian Isochronous Stress Strain Curves at Various Times
11.1.2 Attach the deformation measuring devices to the specimen (and control specimen) or, if these are optical devices, install
ready for measurements. Make the initial or reference measurement for compression or deflection.
11.1.2.1 If the test environment would be disturbed during the attachment of the deformation measuring device, mount the device
prior to mounting the specimen.
11.1.3 Apply the full load rapidly and smoothly to the specimen, preferably in 1 to 5 s. In no case shall the loading time exceed
5 s. Start the timing at the onset of loading.
11.1.4 If an environmental agent is used, apply it to the entire gauge length of the specimen immediately after loading.
11.1.4.1 If the environmental agent is volatile, cover the specimen to retard evaporation without affecting the applied load.
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