ASTM D6991-05
(Test Method)Standard Test Method for Measurements of Internal Stresses in Organic Coatings by Cantilever (Beam) Method
Standard Test Method for Measurements of Internal Stresses in Organic Coatings by Cantilever (Beam) Method
SIGNIFICANCE AND USE
Stresses in coatings arise as a result of their shrinkage or expansion if expected movements are prevented by coating adhesion to its substrate.
There are several causes leading to arrival of stresses in the coatings: film formation (cross-linking, solvent evaporation, etc.); differences in thermal expansion coefficients between coating and substrate; humidity and water absorption; environmental effects (ultraviolet radiation, temperature and humidity), and others.
Knowledge of the internal stresses in coatings is very important because they may effect coating performance and service life. If the internal stress exceeds the tensile strength of the film, cracks are formed. If stress exceeds adhesion between coating and substrate, it will reduce adhesion and can lead to delamination of coatings. Quantitative information about stresses in coatings can be useful in coating formulation and recommendations for their application and use.
This method has been found useful for air-dry industrial organic coatings but the applicability has not yet been assessed for thin coatings (thickness 0.0254 mm (.001 in.), for powder and thermally-cured coatings.
SCOPE
1.1 This test method covers the procedure for measurements of internal stresses in organic coatings by using the cantilever (beam) method.
1.2 This method is appropriate for the coatings for which the modulus of elasticity of substrate (Es) is significantly greater than the modulus of elasticity of coating (Ec) and for which the thickness of substrate is significantly greater than thickness of coating (see and Note 0).
1.3 The stress values are limited by the adhesion values of coating to the substrate and by the tensile strength of the coating, or both.
1.4 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
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 to determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D6991 – 05
Standard Test Method for
Measurements of Internal Stresses in Organic Coatings by
Cantilever (Beam) Method
This standard is issued under the fixed designation D6991; 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 3. Terminology
1.1 Thistestmethodcoverstheprocedureformeasurements 3.1 Definitions of Terms Specific to This Standard:
of internal stresses in organic coatings by using the cantilever 3.1.1 cantilever, n—abeamormembersecurelyfixedatone
(beam) method. end and hanging free at the other end.
1.2 This method is appropriate for the coatings for which 3.1.2 deflection, n—the displacement of a beam from its
the modulus of elasticity of substrate (Es) is significantly original position by an applied force.
greater than the modulus of elasticity of coating (Ec) and for 3.1.2.1 Discussion—The deflection of the beam is used to
which the thickness of substrate is significantly greater than measure that force acting on the tip.
thickness of coating (see Note 4 and Note 5). 3.1.3 internal stress, n—astresssystemwithinasolidthatis
1.3 The stress values are limited by the adhesion values of not dependent on external forces.
coating to the substrate and by the tensile strength of the
4. Test Method
coating, or both.
4.1 Internal stresses in coatings are determined by the
1.4 The values stated in SI units are to be regarded as the
standard. The values given in parentheses are for information cantilever method (Fig. 1). Substrate A in the shape of a
rectangularcantileverbeamisclampedbyitsendBinaspecial
only.
1.5 This standard does not purport to address all of the fixture E. Coating (F) is applied to one side of the beam.
Internal stresses occur in the film when it is being cured
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- (drying, cross-linking, etc.). When there is sufficient adhesion
between the coating and the substrate, the stresses bend the
priate safety and health practices and to determine the
cantilever beam, forcing its free end D to be deflected from its
applicability of regulatory limitations prior to use.
original position by a distance of h. The deflection of the beam
2. Referenced Documents
is measured under an optical microscope and internal stress is
2.1 ASTM Standards: calculated using the equation for the cantilever method. See Eq
D823 Practices for Producing Films of Uniform Thickness 1 in Section 9, (Formula 1).
of Paint, Varnish, and Related Products on Test Panels
5. Significance and Use
D1186 Test Methods for Nondestructive Measurement of
5.1 Stressesincoatingsariseasaresultoftheirshrinkageor
Dry Film Thickness of Nonmagnetic Coatings Applied to
a Ferrous Base expansion if expected movements are prevented by coating
adhesion to its substrate.
D1400 Test Method for Nondestructive Measurement of
DryFilmThicknessofNonconductiveCoatingsAppliedto 5.2 There are several causes leading to arrival of stresses in
the coatings: film formation (cross-linking, solvent evapora-
a Nonferrous Metal Base
tion, etc.); differences in thermal expansion coefficients be-
tween coating and substrate; humidity and water absorption;
This test method is under the jurisdiction of ASTM Committee D01 on Paint
environmental effects (ultraviolet radiation, temperature and
and Related Coatings, Materials, andApplications and is the direct responsibility of
humidity), and others.
Subcommittee D01.23 on Physical Properties of Applied Paint Films.
Current edition approved Feb. 1, 2005. Published February 2005. DOI: 10.1520/
5.3 Knowledge of the internal stresses in coatings is very
D6991-05.
important because they may effect coating performance and
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
service life. If the internal stress exceeds the tensile strength of
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
the film, cracks are formed. If stress exceeds adhesion between
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
coating and substrate, it will reduce adhesion and can lead to
Withdrawn. The last approved version of this historical standard is referenced
delamination of coatings. Quantitative information about
on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6991 – 05
A- Cantilever beam (substrate)
B- Beam end clamped in Fixture E
c- Coating thickness
D- Free end deflected under stress
E- Fixture
F- Coating
G- Width of beam
h- Deflection
L- Distance between the deflecting point and the clamping point.
t- Substrate thickness
FIG. 1 Diagram of the Cantilever Method for Measurements of Internal Stresses in Organic Coatings
a – Original position b – Free end deflected from its original position as a result of stress
stresses in coatings can be useful in coating formulation and 7.2 Cantilever substrates are selected with a slight cylindri-
recommendations for their application and use. cal curvature with a “concave” side to be coated. If the strips
5.4 This method has been found useful for air-dry industrial
are flat the “slight curvature” can be made by gently bending
organic coatings but the applicability has not yet been assessed them with hand to achieve 2-3 mm deflection.
for thin coatings (thickness <0.0254 mm (.001 in.), for powder
7.3 Install the cantilever in the fixture and measure using
and thermally-cured coatings.
microscope the deflection at fixed point before coating appli-
cation.
6. Apparatus
7.4 Substrate should be degreased or solvent-cleaned; in
6.1 Measurement Fixture (Fig. 2)—The fixture consists of
some cases, surface can be slightly and uniformly abraded
the support A and the stop B to which the cantilever substrate
using abrasive paper.
C is clamped with the screw D and shim E. On the side of the
7.5 Theclampedareaandtheuncoatedsideofthecantilever
support there is an engraved mark called the fixed point at an
substrate are masked with tape during the application of
exact known distance (L) from the edge clamping point. By
coating.
moving the fixture under an optical microscope, the deflection
7.6 Apply uniform coatings of the material to be tested to
of the cantilever is always measured at the fixed point.
the “concave” side of the cantilever strip at specified thickness
6.2 Optical Microscope—Capable of measuring deflection
with resolution 0.0254 mm (0.001 in.). in accordance with Practices D823. The thickness should not
be greater than half the thickness of the cantilever panel (see
7. Test Specimen
Note 4). For example, if substrate thickness is 0.254 mm (0.01
in.) the recommended coating thickness should be not greater
7.1 Use stainless strips (stainless steel 304SS is acceptable)
than 0.127 mm (0.005 in.). Due to the slower process of curing
as a cantilever substrate with the following dimensions: width,
in very thick coatings it is recommended to limit the coating
12 mm (0.5 in.); length, 102 mm (4 in.); and thickness, 0.254
thickness to 0.254 – 0.381 mm (0.01 – 0.015 in.).
mm (0.01 in.).
7.7 Removeanypaintfromtheuncoatedsidebysharprazor
NOTE 1—Other dimensions could be used. However, to reduce effect of
blade if necessary. Prepare a minimum of three coated panels
clamping, the length of cantilever strip between the edge point at which it
is clamped and the point at which deflection is measured (see Fig. 1) for the material.
4,5
should be greater than 80 mm.
7.8 Cure the coated panels under humidity and temperature
Stainlesssteelwasselectedtoavoidcorrosionofthestrips.However,in
conditions as agreed upon between the producer and the user.
cases where the coating can not adhere to the stainless steel, the other
7.9 The thickness of the dry coatings should be measured in
materials can be used (carbon steel, aluminum, etc.).
accordance with Test Methods D1186, Test Method D1400 or
anyothertestmethodasagreeduponbetweentheproducerand
the user.
Perera,D.Y.,Eynde,D.V.,“ConsiderationsonaCantilever(Beam)Methodfor
Measuring the Internal Stress in Organic Coatings.” Journal of Coatings Technol-
7.10 Take any precautions in handling of the cantilever
ogy, Vol. 53, No. 677, June 1981.
5 beam during preparation for application, masking, application,
Korobov, Y., Salem, L., “Stress Analysis as a Tool in Coatings Research,”
Materials Performance, Vol. 29, No. 4, April 1990. mask removal, etc., to avoid any deformation or damage.
D6991 – 05
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