Correlating Stressed Environmental Testing of Structural Composites to Service

A compact in-situ tensile stress fixture was designed for the study of the combined effects of stress and automotive environments on structural glass fiber-reinforced composite materials. With this fixture, a standardized 300 hour laboratory screening test was developed to compare the residual property loss of composite materials due to concurrent exposure to stress and environment. It is of great importance that the data gathered in the laboratory have correlation to on-vehicle (in-service) performance, and that both lab and real world data be taken with a test system (in-situ test fixtures) capable of providing accurate and consistent results under either test condition. For the correlation of the 300 hour screening test results of a P4 (programmable powdered preform process) chopped-glass-fiber material to actual vehicle service conditions, stressed (within fixture) and unstressed tensile samples of P4 chopped-glass-fiber automotive structural composite were placed on a semi-truck for one year. The stressed composite specimens were loaded to 25% of the ultimate strength of the material. During that year, the truck logged over 100,000 miles, experienced all four weather seasons, and traveled through every region of the contiguous United States. Further in-situ fixture durability testing of the chopped-glass composite was conducted on the roof of a laboratory located in Dearborn, Michigan for one year. The on-roof test was conducted to investigate the influences of environment, stress, and UV exposure. With one exception (rooftop modulus), the on-vehicle and rooftop durability tests showed no significant change in tensile properties in either the stressed or unstressed specimens when compared to material baseline tensile properties. Furthermore, the two one year durability tests indicated slightly less degradation than observed for the 300 hour screening test results of the chopped-glass-fiber composite.