Effect of Sample Geometry On the Fracture of Natural Rubber Vulcanizates

Natural rubber (NR) has extraordinary green strength and cut-growth resistance due to its ability to undergo strain-induced crystallization. Our research group has studied the effect of  crosslink density, carbon black type, carbon black level and strain rate on the fracture of NR and its ability to strain-crystallize. Now, we have found that the width of the edge-cut tensile can affect strain-crystallization prior to fracture.

When an edge-cut is introduced in gum (no filler) natural rubber vulcanizates, strength initially decreases gradually as the cut size increases. But, at certain critical cut size, the strength drops discontinuously. This is attributed to bulk crystallization when a cut is shallow, and the lack of bulk crystallization when a cut is deep. With a small edge-cut, crystallization occurs not only at a cut-tip, but also in bulk. However, with a cut larger than the critical value, crystallization occurs only at the cut-tip.

Width, length, width to length ratio, shape and volume defines the geometry of a bulk specimen. In this study, we focus on the effects of this geometry on the strength of the edge-cut NR specimens and their ability to strain-crystallize prior to rupture.