Fracture energy of automotive clearcoats—I. Experimental methods and mechanics

The fracture energy of five automotive clearcoats was measured using fracture mechanics methods originally developed for measuring the fracture energy of brittle inorganic thin films. The fracture energy of the clearcoats ranged from 12 J m⁻² to 140 J m⁻². A discrepancy between the actual fracture energy and the ‘apparent’ fracture energy was observed and attributed to differences in the tensile stress-strain behavior of the materials, in particular the yield strength. Correcting for differences in their tensile stress-strain behavior, the apparent fracture energy of the films ranged from 13 J m⁻² to 330 J m⁻². Contrary to expectations, the fracture energy of the clearcoats did not decrease with increasing cross-link density. This was ascribed to differences in chemical architecture and additives in these commercial materials. Measuring the toughness in this manner shows promise for evaluating the long-term cracking resistance of automotive clearcoats.