The objective of this research is to determine the deformation and fracture characteristics of natural rubber under tensile impact loading. Material tension and fracture experiments were performed on unfilled and 25 phr carbon black-filled natural rubber. The material tension tests were done in order to characterize material stiffness and strength as they vary with strain rate. The fracture tests were done in order to study the effect of high strain rate on the tear energy of natural rubbers.
Tensile stress-extension ratio curves up to material failure were generated from the test data at constant strain rates ranging from 0.1 to 229 s-1. Several transitions associated with stain-induced crystallization were observed in both materials. Transitions occurred between 0.1-110 s-1 and above 110 s-1 in the unfilled natural rubber, and for the 25 phr carbon black-filled natural rubber transitions occurred between 0.1-132 s-1 and beyond 132 s-1. The unfilled natural rubber became more compliant when the strain rate increased from 110 to 206 s-1, and it became stiffer when the strain rate increased from 206 to 229 s-1. The filled natural rubber became stiffer when the strain rate increased from 83 to132 s-1, and it became more compliant when the strain rate increased from 132 to 194 s-1.
It was found from the fracture experiments that the fracture energy of the unfilled natural rubber did not vary significantly over a range of sample strain rate from 0.01 to 56 s-1, but there was significant variation in the fracture energy of the 25 phr carbon black filled natural rubber from 0.01 to 71 s-1 sample strain rate. The fracture energy of the 25 phr carbon black filled natural rubber at a sample strain rate of 0.1 s-1 was 25 kJ/m2, which was three times greater than it was at 10 s-1 sample strain rate. Furthermore, the carbon black fillers increased the fracture energy of natural rubber at quasi-static sample strain rates (0.01 – 0.1 s-1) by about 200%, but the carbon black fillers did not improve the fracture energy of natural rubber at sample strain rates between 5-29 s-1. In this strain rate range, the fracture energy of 25 phr carbon black-filled natural rubber was almost the same as that in the unfilled natural rubber. Above a sample strain rate of 30 s-1, the fracture energy was greater than the unfilled natural rubber. Finally, the crack speed did not vary significantly with loading rates for both unfilled and filled natural rubber.