Deformation - Is Your Coating Tough Enough?
The performance of coatings when influenced by external stresses caused by stretching, bending or impact determines the suitability for their designed application. Coated steel objects are subject to such stresses either during the production stage or service life. Tests in the laboratory are essential to determine if a specific coating is able to withstand those stresses.
The steel deck of a ship, for example, expands and contracts with changes in temperature. The paint that is protecting the steel has to have enough flexibility to withstand the stress imposed on it. Additionally, a coating designed for use in the coil coating industry should have the ability to stretch as the substrate is formed into its desired shape without damage or discolourisation, for example.
Other stresses come from sources such as physical impact from the movement of people, equipment and machinery. In order to fully test a coating's performance we can use the following tests:
Mandrel Bend Test
A coated metal sheet is bent over a conical or cylindrical mandrel and flaws such as cracks, colour change or adhesion of the coating to the substrate or secondary coating are evaluated. Corresponding results, produced by decreasing mandrel sizes, indicate the degree of elasticity of the coating.
The Cupping Test
The cupping test helps to determine the ductile property of a coating (the extent it can be drawn out or deformed) before it fractures, by gradually increasing the depth of indentation. The test is used to determine the minimum depth at which the coating cracks and/or becomes detached from the substrate.
This test is used to ascertain a coating’s ability to withstand damage by a defined falling object, the test specimen is fixed into position by the quick release clamp.The defined weight, fitted with a known punch diameter, is lifted to a predetermined height, set using the adjustable collar device. The weight is then released and the resulting deformation is observed.
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