Automotive exterior paint systems can significantly affect the impact performance of thermoplastic body panels. To utilize the benefits of predictive engineering as a tool to assist in the design and development of thermoplastic body panels, thermoplastic body panel materials have been characterized with typical automotive paint systems for use for finite element modeling and analysis.
Paint systems used for exterior body panels can vary from rigid to more flexible, depending on the vehicle manufacturer's specifications. Likewise, thermoplastics for body panels vary in mechanical properties, primarily depending on the heat performance requirements of the application. To understand the effects of paint systems on impact performance of thermoplastic body panels, two different paint systems, representing “rigid” and “more flexible,” were evaluated on two body panel grades of thermoplastics with different mechanical properties.
A two-step process was followed to characterize the mechanical properties of the thermoplastic-paint system. First, the deformation properties of the thermoplastic materials were determined, including the modulus, yield stress, and post-yield behavior. Next, the failure criteria of the painted systems were determined by correlating experimental failure displacements of painted disk specimens with maximum principal stress levels obtained through finite element simulation of the disk impact tests. Unpainted disks, disks impacted on their painted side and disks impacted on their unpainted side were tested. Tests were performed at two temperatures, 23°C and -30°C, over a range of strain rates. The results of the material characterization, deformation models, and failure criteria as a function of rate and temperature were incorporated into a user-defined material subroutine for use in finite element analyses.
To validate the material model described above, fender impact testing was performed on full-scale fenders and load-displacement traces were recorded along with failure information. Next, finite element analyses were performed simulating the fender impact tests. The material model was evaluated based on its ability to predict the failure behavior of the fender as well as its ability to match the experimental load-displacement trace.