ASTM F791-96(2002)e1
(Test Method)Standard Test Method for Stress Crazing of Transparent Plastics
Standard Test Method for Stress Crazing of Transparent Plastics
SIGNIFICANCE AND USE
This test method provides a guide for evaluating a specific solvent, chemical, or compound that may be detrimental to a transparent plastic as a result of a manufacturing process, a fabrication operation, or the operational environment. All transparent plastics are susceptible to crazing, though in widely varying degree and from a variety of causes. This test method is intended to allow establishment of the crazing stress when the simultaneous action of both load and a material that would cause crazing is applied producing non-reversible damage that might limit the usage of that transparent plastic in a specific application.
SCOPE
1.1 This practice covers the determination of the critical crazing stress for a transparent-plastic material when exposed to a specific solvent, chemical, or compound at a specific temperature.
1.2 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 .
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Designation:F791–96 (Reapproved 2002)
Standard Test Method for
Stress Crazing of Transparent Plastics
This standard is issued under the fixed designation F 791; 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 (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
e NOTE—Eq 1 was corrected editorially in December 2006.
1. Scope index of refraction, which causes light to be reflected off of the
crazes. This light reflection causes the crazes to sparkle when
1.1 This test method covers the determination of the critical
viewed from certain angles. The crazes are sometimes random
crazing stress for a transparent plastic material when exposed
and scattered with varied lengths and depths but usually are
to a specific solvent, chemical, or compound at a specific
oriented perpendicular to a tensile stress. Crazing may be
temperature.
difficult to detect. It becomes more pronounced when viewed
1.2 This standard does not purport to address all of the
with a light source that is at an oblique angle.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Significance and Use
priate safety and health practices and determine the applica-
4.1 This test method provides a guide for evaluating a
bility of regulatory limitations prior to use.
specific solvent, chemical, or compound that may be detrimen-
2. Referenced Documents tal to a transparent plastic as a result of a manufacturing
process, a fabrication operation, or the operational environ-
2.1 ASTM Standards:
ment.Alltransparentplasticsaresusceptibletocrazing,though
D 618 Practice for Conditioning Plastics for Testing
inwidelyvaryingdegreeandfromavarietyofcauses.Thistest
E 691 Practice for Conducting an Interlaboratory Study to
method is intended to allow establishment of the crazing stress
Determine the Precision of a Test Method
when the simultaneous action of both load and a material that
2.2 Other Method:
would cause crazing is applied producing non-reversible dam-
ARTC (Aircraft Research and Testing Committee of the
age that might limit the usage of that transparent plastic in a
Aircraft Industries Association of America, Inc.) Condi-
3 specific application.
tioning Method
5. Apparatus
3. Terminology
5.1 Test Fixture, with fluorescent light source illustrated and
3.1 Definitions of Terms Specific to This Standard:
constructed as shown in Figs. 1 and 2.
3.1.1 crazing—a group of surface fissures that appear to be
5.2 Drill Fixture constructed as shown in Fig. 3.
small cracks in the material, although they are not.
5.3 Marking Fixture, constructed as shown in Fig. 3.
3.1.1.1 Discussion—Crazing is a form of yielding in poly-
5.4 Portable Specimen Rack, constructed in the manner as
mers characterized by a spongy void filled fibrillar structure.
shown in Fig. 4 for handling and conditioning test specimens.
The density in the craze changes resulting in a change in the
5.5 Weights—A container and shot for the application of
weight on the rack as shown in Fig. 1.
This test method is under the jurisdiction of ASTM Committee F07 on 5.6 Filter Paper, quantity of 0.50 by 1.0-in. (12.7 by
Aerospace and Aircraft and is the direct responsibility of Subcommittee F07.08 on
25.4-mm) pieces of filter, medium-retention filter paper.
Transparent Enclosures and Materials.
Current edition approved Oct. 10, 1996. Published December 1996. Originally
6. Test Specimens
published as F 791 – 82. Last previous edition F 791 – 82 (1988).
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
6.1 The test specimen shall be machined from the transpar-
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ent plastic material to be evaluated. A minimum of six
Standards volume information, refer to the standard’s Document Summary page on
specimens for each solvent, chemical, or compound is re-
the ASTM website.
quired. It is preferred that the transparent plastic sheet material
Available from Aircraft Industries Association, 1725 DeSales St. NW, Wash-
ington, DC 20034.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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F791–96 (2002)
FIG. 1 Accelerated Crazing Test Fixture
FIG. 3 Fixtures for Specimen Preparation
FIG. 2 Application of Test Liquid to Piece of Filter Paper on Top
Surface of Test Specimen
rack, rod, lead weights, container, shot, and the radiused nut.
For convenience of assembly, the weight rack pan may be
thickness be 6.35 6 0.64 mm (0.250 6 0.025 in.), but any
stamped with the total weight of the pan, rod, and nuts as a
thickness material may be used. Orientation of each test
unit. A container, such as a ⁄2-pt (0.24-L) paint can with a
specimen within the test sheet or part should be recorded.
6.4-mm ( ⁄4-in.) hole drilled in the center of the bottom and
6.2 Thetestspecimensshallbe25.4 60.8mm(1.00 60.03
installed so it slides up and down on the rod, can serve as a
in.) wide by 177.8 6 1.27 mm (7.00 6 0.05 in.) long by
receptacle for the lead shot to attain required weight.
thickness.
6.3 The edges shall be smooth machined surfaces without
8. Conditioning
cracks, and the test specimen surface shall be free of defects or
irregularities. If the test specimen has been machined to 8.1 Precondition the test specimens in accordance with one
of the following procedures:
thickness, the nonmachined surface shall be the test surface.
8.1.1 Practice D 618 Procedure B—Forty-eight hours at
7. Preparation of Apparatus
50°C (122°F) followed by cooling to room temperature in
7.1 Once the load for a particular stress is calculated, that desiccator over anhydrous calcium chloride for at least 5 h.
load will be the sum of the individual weights of the weight Designate as Condition 1 and test within 15 min.
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F791–96 (2002)
10. Procedure A—Craze Stress Iteration
10.1 Place each specimen in the drill fixture and drill a 7.94
mm (0.3125 in.) diameter hole at a distance of 12.7 6 1.27 mm
(0.50 6 0.050 in.) from one end and on the longitudinal
centerline of the specimen.
10.2 Place each specimen in the marking fixture and draw a
pencil line on the edge of the specimen 101.6 mm (4.0 in.)
from the center of the 7.94-mm (0.3125-in.) diameter hole and
perpendicular to the length of the specimen (see mark in Fig.
2).
10.3 Measure the width and thickness of each specimen to
the nearest 0.03 mm (0.001 in.) at the pencil line. Handle each
specimen only by its edges. Do not clean test specimens in any
manner during the time period between conditioning and
testing.
FIG. 4 Specimen Rack
10.4 Inserttheconditionedspecimenimmediatelyinthetest
fixture with the pencil mark on the specimen in line with the
center of the fulcrum. Raise the weights and insert the end of
8.1.2 Two hours at 90°C (194°F), ambient cooled, and
the rod through the 7.94-mm (0.3125-in.) hole in the test
followed by 7 days at 23 6 1.1°C (73.5 6 2°F) and 50 65%
specimen. Carefully center the load supporting nut (with a
relative humidity. Designate as Condition 2 and test within 1 h.
7.94-mm (0.3125-in.) machined radius on the surface contact-
8.1.3 ARTC Method—Sixteen hours at 14°C (25°F) below
ing the plastic) in the hole and tighten the nut. Align the
the average heat deflection temperature, cool at a rate not
specimen edges so they are exactly perpendicular to the
exceeding 28°C (50°F)/h and follow by 96 h at 23 6 1.1°C
fulcrum and slowly lower the weights until the specimen
(73.5 6 2°F) and 50 6 5 % relative humidity. Designate as
accepts the load.
Condition 3 and test within 1 h.
10.5 Test the first specimen at 27.58 MPa (4000 psi) outer
8.1.4 As received, no preconditioning. Designate as Condi-
fiber stress.Apply the load for 10 6 0.5 min and observe to be
tion 4.
sure no crazing has occurred. Place the 12.5 by 25-mm ( ⁄2 by
1 in.) filter paper directly over the fulcrum in the middle of the
NOTE 1—The conditions listed above may not result in uniform
specimen so there is a clear area along each edge to avoid
moisture content for certain plastics. Moisture content reportedly may
have a strong effect on craze results for certain plastics. If other inducing edge crazing. Apply the test chemical to the filter
preconditionings are required to ensure uniform or desired moisture
paper only. Use care so that the test material does not extend
content, the use of these should be reported in the test report.
beyond this area and defeat the purpose of the test. Keep the
filter paper moist with test chemical for the duration of the test,
9. Calculation of Loads
15 min, 30 min, or any duration desired. Remove the filter
9.1 The width and thickness of each specimen shall be
paper after the test period and inspect for craze. Turn on the
measured to the nearest 0.03 mm (0.001 in.). Enter this data
fluorescent lamps for inspection only to avoid undesired
along with the identification of the specimen in the required
heating of the test specimen. Terminate the testing of that
records.
specimen.
9.2 Calculate the load to be used with each specimen in
NOTE 3—It is recommended that a control test be run with each set of
accordance with the following equation:
craze tests. This control test is conducted exactly the same as the other
craze tests, except that no chemicals should be applied to this control
P 5 ~S 3 B 3 D !/~6L! (1)
specimen during the craze test. This provides a baseline and allows a
where: determination of whether the crazing observed in the tests with the
chemical applied is due to the chemical/stress combination, or is a
P = load, N (lb.),
function of stress alone.
S = maximum outer fiber stress, MPa (psi), determined by
test sequence in 10.5-10.8,or 11.5-11.8,
10.6 Ifthefirstspecimeniscrazed,testthesecondspecimen
L = length of specimen from fulcrum to center of applied
at13.79MPa(2000psi).Ifthefirstspecimendidnotcraze,test
load, mm (in.),
the second specimen at 20.68 MPa (6000 psi).
B = width of specimen, mm (in.), and
10.7 If the second specimen does not craze at 2000 psi, test
D = thickness of specimen, mm (in.).
the third specimen at 20.68 MPa (3000 psi). Test the fourth at
NOTE 2—This equation is valid only for relatively small deflections. a lower or higher stress depending on whether the third
For large deflections, the dimension L should be replaced by the actual
specimen did or did not craze. Continue this procedure in
horizontal distance from the point of load application to the fulcrum in the
suitable increments until the critical crazing stress for specific
displaced condition. A deflection of 25.4 mm (1 in.) at the point of load
solvent, chemical, or compound is determined to the desired
application will result in an actual stress at the fulcrum which is
accuracy.
approximately 5 % less than the expected stress, and a 38.1 mm (1.5 in.)
10.8 Report the critical crazing stress as the stress midway
deflection will result in an actual stress at the fulcrum which is approxi-
mately 10 % less than the expected stress. between the stress at which crazing was and was not observed
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F791–96 (2002)
on duplicate specimens. For example: Crazing was observed at between the butyl dams placed at each top surface edge of the
27.58 MPa (4000 psi) and none at 24.13 MPa (3500 psi). The specimens as described in 10.4. Apply chemical as needed
criticalcrazingstresswouldbe25.86 61.72MPa(3750 6250 throughout the test to maintain a wetted condition on the entire
psi). test surface of the specimen, from the fulcrum down to the
10.9 In the examination of the crazing, note all cracks at the point of load application. Place a pan under the point of load
edge of the specimen as “edge crazing.” Disregard this application to catch the chemical which drips off of the
condition when ascertaining the end point unless the edge of specimen. Continue the test for 15 minutes, 30 minutes, or any
crazing grows and extends across the entire width of the duration desired. During the test, monitor the specimen con-
specimen. tinuously for craze development. Record the time and location
10.10 Testing shall be at the specified temperature 63°C of the craze front as crazing progresses down the beam.
(65°F).
11.6 Calculate the craze stress for each recorded time with
the following equation:
11. Procedure B—Craze Stress Tracking
S 5 ~6 3 P 3 Z!/~B 3 D ! (2)
11.1 Place each specimen in the drill fixture and drill a
7.94-mm (0.3125-in.) diameter hole at a distance of 12.7 6
where:
1.27 mm (0.50 6 0.050 in.) from one end and on the
S = maximum outer fiber stress, MPa (psi),
longitudinal centerline of the specimen.
P = load, N (lb.),
11.2 Place each specimen in the marking fixture and draw a
Z = the distance from the point of load application to the
pencil line on the edge of the specimen 101.6 mm (4.0 in.)
craze front, mm (in.),
from the center of the 7.94-mm (0.3125-in.) diameter hole and B = width of specimen, mm (in.), and
D = thickness of specimen, mm (in.)
perpendicular to the length of the specimen (see the mark in
Fig. 2). On the side opposite the test surface, using an ink
11.7 If the first specimen does not craze, test the second
suitable for marking plastic, mark lines across the specimen
specimen at 41.37 MPa (6000 psi). If no crazing is observed at
surface perpendicular to the edge, from edge to edge, at 6.4
41.37 MPa (6000 psi), discontinue testing.
mm (0.25 in.) intervals, starting at the hole and progressing
11.8 If the first specimen breaks before the completion of
101.6 mm (4.0 in.) to the point at which the beam will rest on
thetest,testtheremainingspecimensatastresslevelbelowthe
the fulcrum.
stress at which the specimen fractured.
11.3 Measure the width and thickness of each specimen to
11.9 If the first specimen crazes below 6.895 MPa (1000
the nearest 0.03 mm (0.001 in.) at the pencil line. Handle each
psi) (more than ⁄4 of the way down the beam) conduct the
specimen only by its edges. Do not clean test specimens in any
remaining tests with 13.79 MPa (2000 psi) at the fulcrum.
manner during the time period between conditioning and
11.10 Calculate the minimum stress at which crazing oc-
testing.
curs. This stress may be calculated from the equation in 11.6.
11.4 Firmly press a 3.18 mm (0.125 in.) wide ribbon of
11.11 In the examination of the crazing, note all cracks at
non-compressible black butyl glazing tape sealant on the test
the edge of the specimen as edge crazing. Disregard this
surface of the
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