ASTM D6991-05  Measurement of Internal Stresses in Organic Coatings

ASTM D6991-05  Measurement of Internal Stresses in Organic Coatings

ASTM D6991-05 is the standard test method that uses the cantilever (beam) method for evaluating the internal stresses in organic coatings. Analysis of the internal stresses of a coating is essential to predict its performance and service life. This also helps in coating formulation and deciding application and use.

    Scope:

    ASTM D6991-05 test method defines the procedure for measuring internal stresses of organic coatings by the cantilever (beam) method. When the elasticity of substrate (Es) is much greater than the elasticity of coating (Ec), the cantilever method for determining internal stresses is found to be suitable. The method is also appropriate where the thickness of the substrate is significantly greater than that of the coating. Adhesion and tensile strength are parameters that limit the stress values.

    Test Procedure:

    In ASTM D6991-05 test method the substrate (rectangular cantilever beam) is clamped by its end in a fixture. On one side of the beam, the coating under test is applied and allowed to cure. This is when internal stresses occur in the film. These stresses, when sufficient in magnitude, deflect the free end of the cantilever beam from its original position. The distance of deflection is measured using an optical microscope and the internal stress is calculated by the cantilever method equation. The stress values are generally reported in MPa or PSI.

    Specimen size:

    Cantilever substrate- Stainless strips (stainless steel 304 SS is acceptable)
    Dimensions: width – 2 mm (0.5 inches); length – 102 mm (4 inches); and thickness – 0.254 mm (0.01 inches). Apply uniform coatings of the material to be tested to the “concave” side of the cantilever strip at a specified thickness.

    Data:

    The stresses are reported in SI units. The formula used is as follows,

    Where:
    S = internal stress, MPa (PSI),
    h = deflection of the cantilever, mm (inch),
    Es = modulus of elasticity of substrate, for stainless steel 304SS, typically 19.3 by 104 MPa (28 by 106 PSI),
    γs = Poisson’s ratio of the cantilever substrate (for stainless steel: 0.25),
    L = length of the substrate between the edge point at which it is clamped and point of which deflection is measured, mm (inch),
    t = thickness of the cantilever substrate, mm (inch), and
    c = thickness of the coating, mm (inch)
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