EFFECT OF CROSS-LINK STRUCTURES ON THE FATIGUE-CRACK GROWTH-BEHAVIOR OF NR VULCANIZATES WITH VARIOUS AGING CONDITIONS

The effect of crosslink structures on the fatigue crack growth (FCG) behavior of unfilled and HAF-filled vulcanizates with conventional (Conv), semi-efficient (Semi-EV) and efficient (EV) cure systems, all with similar levels of total crosslink densities, was investigated before and after aging at 100 degrees C in air and in nitrogen. The unfilled vulcanizates before aging did not show differences in the FCG rate through the whole range of tearing energy applied in this study. However, HAF-filled vulcanizates before aging showed some differences in the FCG rate with an order of EV > Semi-EV > Conv in the range of high tearing energy corresponding to the strain levels from 46 to 85%. Heat aging in air results in a higher FCG rate (compared to before aging) due mainly to the changes of the total crosslink density and the crosslink structures. The Conv cure system in unfilled rubber showed a higher FCG rate than the other cure systems since it had the poorest resistance to heat aging at all the aging periods. Also, heat aging of vulcanizates in air gave higher FCG rates than heat aging in nitrogen because of the dominant effect of oxidative degradation on the mechanical fatigue resistance of the rubber. The various vulcanizates after aging at 100 degrees C in air showed that the FCG rate and slopes (between the FCG rate and the tearing energy) were drastically changed at similar critical tearing energy. The slopes of all samples changed at the critical strain levels of around 40-50% over which the classical statistics theory for rubber elasticity disobeys.