We report on duality in stiffness values for both carbon black and silica-filled SBR-based (styrene butadiene rubber) tire rubber materials after cyclic loading (and not with no-cycle, neat samples). We believe, this behavior is due to morphological changes occurring due to cycling and not necessarily due to larger scale void/crack initiation. Causes may be chain breakage, reduced crosslinking in all samples, and agglomerate break-up/particle redistribution in silica systems, which represent early damage initiation and morphological changes in these systems. Therefore, we get a dual stiffness vs. strain behavior which is essentially superposition of two separate stiffness vs. strain curves, each being similar to the stiffness-strain curves for the neat (no fatigue) samples. We believe that the second superposed portion represents the sections deteriorated/rearranged due to cyclic loading (lower crosslinking/rearranged particle distributions) coming in-line during the straining process, and when the non-deteriorated/non-rearranged sections weaken.

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