ASTM D6991-05(2010)
(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 Note 4 and Note 5).
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.
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 to determine the applicability of regulatory limitations prior to use.
General Information
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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: D6991 − 05 (Reapproved 2010)
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 Nonferrous Metal Base (Withdrawn 2006)
1.1 Thistestmethodcoverstheprocedureformeasurements
3. Terminology
of internal stresses in organic coatings by using the cantilever
3.1 Definitions of Terms Specific to This Standard:
(beam) method.
3.1.1 cantilever, n—a beam or member securely fixed at one
1.2 This method is appropriate for the coatings for which
end and hanging free at the other end.
the modulus of elasticity of substrate (Es) is significantly
3.1.2 deflection, n—the displacement of a beam from its
greater than the modulus of elasticity of coating (Ec) and for
original position by an applied force.
which the thickness of substrate is significantly greater than
3.1.2.1 Discussion—The deflection of the beam is used to
thickness of coating (see Note 4 and Note 5).
measure that force acting on the tip.
1.3 The stress values are limited by the adhesion values of
3.1.3 internal stress, n—astresssystemwithinasolidthatis
coating to the substrate and by the tensile strength of the
not dependent on external forces.
coating, or both.
1.4 The values stated in SI units are to be regarded as the
4. Test Method
standard. The values given in parentheses are for information
4.1 Internal stresses in coatings are determined by the
only.
cantilever method (Fig. 1). Substrate A in the shape of a
1.5 This standard does not purport to address all of the
rectangularcantileverbeamisclampedbyitsendBinaspecial
safety concerns, if any, associated with its use. It is the
fixture E. Coating (F) is applied to one side of the beam.
responsibility of the user of this standard to establish appro-
Internal stresses occur in the film when it is being cured
priate safety and health practices and to determine the
(drying, cross-linking, etc.). When there is sufficient adhesion
applicability of regulatory limitations prior to use.
between the coating and the substrate, the stresses bend the
cantilever beam, forcing its free end D to be deflected from its
2. Referenced Documents
original position by a distance of h. The deflection of the beam
2.1 ASTM Standards:
is measured under an optical microscope and internal stress is
D823 Practices for Producing Films of Uniform Thickness
calculated using the equation for the cantilever method. See Eq
of Paint, Varnish, and Related Products on Test Panels
1 in Section 9, (Formula 1).
D1186 Test Methods for Nondestructive Measurement of
Dry Film Thickness of Nonmagnetic Coatings Applied to
5. Significance and Use
a Ferrous Base (Withdrawn 2006)
5.1 Stressesincoatingsariseasaresultoftheirshrinkageor
D1400 TestMethodforNondestructiveMeasurementofDry
expansion if expected movements are prevented by coating
Film Thickness of Nonconductive Coatings Applied to a
adhesion to its substrate.
5.2 There are several causes leading to arrival of stresses in
This test method is under the jurisdiction of ASTM Committee D01 on Paint
the coatings: film formation (cross-linking, solvent
and Related Coatings, Materials, andApplications and is the direct responsibility of
evaporation,etc.);differencesinthermalexpansioncoefficients
Subcommittee D01.23 on Physical Properties of Applied Paint Films.
between coating and substrate; humidity and water absorption;
Current edition approved Dec. 1, 2010. Published December 2010. Originally
approved in 2005. Last previous edition approved in 2005 as D6991 – 05. DOI:
environmental effects (ultraviolet radiation, temperature and
10.1520/D6991-05R10.
humidity), and others.
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
5.3 Knowledge of the internal stresses in coatings is very
Standards volume information, refer to the standard’s Document Summary page on
important because they may effect coating performance and
the ASTM website.
service life. If the internal stress exceeds the tensile strength of
The last approved version of this historical standard is referenced on
www.astm.org. the film, cracks are formed. If stress exceeds adhesion between
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6991 − 05 (2010)
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
coating and substrate, it will reduce adhesion and can lead to exact known distance (L) from the edge clamping point. By
delamination of coatings. Quantitative information about moving the fixture under an optical microscope, the deflection
stresses in coatings can be useful in coating formulation and of the cantilever is always measured at the fixed point.
recommendations for their application and use.
6.2 Optical Microscope—Capable of measuring deflection
5.4 This method has been found useful for air-dry industrial with resolution 0.0254 mm (0.001 in.).
organic coatings but the applicability has not yet been assessed
7. Test Specimen
for thin coatings (thickness <0.0254 mm (.001 in.), for powder
and thermally-cured coatings. 7.1 Use stainless strips (stainless steel 304SS is acceptable)
as a cantilever substrate with the following dimensions: width,
6. Apparatus
12 mm (0.5 in.); length, 102 mm (4 in.); and thickness, 0.254
mm (0.01 in.).
6.1 Measurement Fixture (Fig. 2)—The fixture consists of
the support A and the stop B to which the cantilever substrate
NOTE 1—Other dimensions could be used. However, to reduce effect of
C is clamped with the screw D and shim E. On the side of the
clamping, the length of cantilever strip between the edge point at which it
support there is an engraved mark called the fixed point at an is clamped and the point at which deflection is measured (see Fig. 1)
A- Support
B- Stop
C- Coated cantilever beam
D- Screw clamp
E- Pressure shim
L- Distance between end of pressure shim to the engraved point where the deflection is measured
FIG. 2 Fixture with the Clamped Coated Cantilever Sample for the Measurements of Internal Stresses in Organic Coatings
D6991 − 05 (2010)
4,5
should be greater than 80 mm. position of the cantilever substrate cannot be correctly determined. For
Stainless steel was selected to avoid corrosion of the strips. However, such cases the deflection obtained with the uncoated substrate is taken as
in cases where the coating can not adhere to the stainless steel, the other the “zero” point.
materials can be used (carbon steel, aluminum, etc.).
8.3 The difference between current values taken at the
7.2 Cantilever substrates are selected with a slight cylindri-
specifiedtimeintervalsand“zero”valuesisthedeflectiontobe
cal curvature with a “concave” side to be coated. If the strips
used in stress calculations.
are flat the “slight curvature” can be made by gently bending
8.4 The stress (deflection) can be measured as a function of
them with hand to achieve 2-3 mm deflection.
time and of difference variables (temperature, humidity, coat-
7.3 Install the cantilever in the fixture and measure using
ing thickness, etc.). Coated samples can be clamped in the
microscope the deflection at fixed point before coating appli-
fixture for the period of testing, or can be removed for various
cation.
exposures and reinstalled again.
7.4 Substrate should be degreased or solvent-cleaned; in
NOTE 3—Sample reinstallation may increase the error of measurement.
some cases, surface can be slightly and uniformly abraded
using abrasive paper. 9. Calculation of Internal Stress
7.5 Theclampedareaandtheuncoatedsideofthecantilever
9.1 Internal Stress is calculated by using Eq 1 (Formula 1)
substrate are masked with tape during the application of
developed by Corcoran :
coating.
hE t
s
S 5 (1)
7.6 Apply uniform coatings of the material to be tested to
3L c t1c 1 2 γ
~ !~ !
s
the “concave” side of the cantilever strip at specified thickness
where:
in accordance with Practices D823. The thickness should not
S = internal stress, MPa (PSI),
be greater than half the thickness of the cantilever panel (see
h = deflection of the cantilever, mm (in.),
Note 4). For example, if substrate thickness is 0.254 mm (0.01
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