The effect of substrate material properties on the failure behaviour of coatings in the Erichsen cupping test

Pre-painted sheet metal produced by coil coating is subjected to large deformations during manufacture of white goods and architectural cladding. The thermosetting polyester coatings must resist failure by cracking, and their formability can be assessed qualitatively using the industry-standard Erichsen cupping test. However, this only provides strains much smaller than the coatings can withstand, and hence does not discriminate between coating behaviour. Finite element (FE) modelling has been used to show that the applied strain governs the failure of coil coatings during forming, and to demonstrate how increased surface strains can be achieved by altering key parameters to make the Erichsen cupping test discriminating and quantitative. The surface strains are increased by increasing the coefficient of friction between the indenter and the substrate, and by increasing the thickness of the substrate. A parametric study on substrate properties showed that a smaller strain hardening exponent (i.e. more plastic behaviour) gave higher surface strains. There was no variation in the surface strains over a temperature range of -60 degrees C to 60 degrees C. Understanding how the test conditions and substrate properties influence the surface strains improves the efficacy of the Erichsen cupping test. The surface strains applied to a coating can be varied by changing the substrate properties, which allows for greater differentiation between coatings and for the coating failure strains to be determined quantitatively. This provides a data-driven approach to develop and formulate better coatings using a single, efficient and easy test.