Dynamic Property and Fatigue Crack Propagation Research on Tire Sidewall and Model Compounds

Research was conducted to define appropriate compound loading conditions and energy parameters required to properly control and analyze fatigue crack propagation experiments for tire sidewall applications. The effects of strain level, pulse frequency, overall cycle frequency, sample thickness, and oven temperature were screened, and strain level was shown to be the dominant variable in the region of interest. Designed experiments further confirmed that frequency (i.e., strain rate) effects upon strain energy are small at normal rates of tire deformation (equivalent to 40 Hz). However, at typical laboratory test frequencies (≤5 Hz), strain rate effects on strain energy are large, and the differences vs. results under tire conditions depend heavily on polymer type as well as test temperature. Thus, the use of strain level, strain rate, and temperature conditions which simulate the tire service environment are critical to give representative results in laboratory testing. A constitutive equation was defined which provides an excellent model for strain energy in pure (or simple) shear as a function of the principal extension ratio (i.e., strain level) at constant frequency. Therefore, computer modeling of such experiments appears straightforward using an on-line minicomputer. Fatigue crack propagation studies showed major effects of pure-shear sample thickness, processing prior to molding, different types of reference compounds, and different polymer types. Halobutyl compounds and halobutyl/EPDM/NR blends were shown to provide superior FCP resistance at a given strain or strain energy level. These results were consistent with earlier tire and laboratory data.