Effect of Sample Geometry and Dimensions on the Fracture of Natural Rubber Vulcanizates

Natural rubber (NR) has extraordinary green strength and cut-growth resistance due to its ability to strain-crystallize. We have discovered that the width of an edge-cut tensile specimen of NR can affect strain-crystallization prior to fracture.

Width, length, width to length ratio, shape and volume define the geometry of bulk specimens. Therefore, we focused on the effects of geometry on the strength of edge-cut NR specimens and their ability to strain-crystallize.

Sample geometry, width and thickness in combination, affect strength that results from strain-crystallization. Because relatively thick specimens have significant stress in the thickness direction and in the remaining width (w_R), the width in front of the cut tip is the actual width sustaining stretch. Thus, samples with high thickness/w_R ratio (0.84 in this study) promote strain-crystallization when cut depth is near the critical value. When the thickness/w_R ratio < 0.84 and w_R > 4mm, the thickness effect is minute. However, when the thickness/w_R ratio < 0.84 and w_R < 1.3mm, some specimens promote strain-crystallization, but some do not.