Investigation of fatigue crack growth characteristics of NR/BR blend based tyre tread compounds

Abstract

Tyre tread directly comes in contact with various road surfaces and is prone to damage due to cuts from sharp objects during service. As tyres undergo millions of fatigue cycles, these cuts propagate continuously and lead to catastrophic failure. Therefore fatigue crack growth (FCG) characteristics should be an essential criterion for tread compound selection. The present study investigates FCG behavior of blends comprising of Natural Rubber (NR) and Polybutadiene Rubber (BR) over a wide range of tearing energy. Pure shear specimens with a notch on both edges were tested in a Tear Analyser. Rapid increase in FCG rate after a certain strain level was observed. This transition point appeared in a strain range of 20-35 %, depending on the blend composition. The higher BR containing compounds exhibited better FCG characteristics below the transition point but reversal of ranking was seen above this point. The influence of temperature, R ratio, waveforms and cure system on FCG characteristics was also investigated in NR and 60-40 NR/BR blend compounds. Higher FCG rate was achieved under pulse loading compared to the sine waveform. The relaxation time between pulse cycles played an important role. With an increase in relaxation time, FCG rate decreases significantly. The higher sensitivity towards R ratio was observed in NR compound. The 60-40 NR/BR blend showed higher FCG rate with increase in temperature compared to the NR compound. The NR compound with high Sulfur/Accelerator (S/Ac) ratio showed better FCG characteristics whereas for 60-40 NR/BR blend with low S/Ac ratio achieved superior FCG characteristics.