Anticipating Paint Cracking: The Application of Fracture Mechanics To the Study of Paint Weathering

By Mark E. Nichols

The relationships between the fracture energy (G_c), film thickness, stress level, and weathering time were investigated for a number of automotive clearcoats. As weathering progressed, most clear-coats embrittled due to the combined effects of photooxidation, hydrolysis, and other degradation mechanisms.

This embrittlement was measured and related to the chemical composition changes that take place in the clearcoat during weathering. When the fracture energy dropped below the driving energy for cracking, G, brittle cracking occurred. The driving force for cracking was shown to depend on stress level and film thickness.

The importance of fatigue loading was also qualitatively investigated. These techniques were successfully used to anticipate the long-term weather-ing behavior of automotive clearcoats.