The Effect of Carbon Black Parameters on the Fatigue Life of Filled Rubber Compounds

Fatigue is defined as decay caused by cyclic deformations at an amplitude less than necessary for fracture in one cycle. Such failures are initiated by flaws which act as stress concentrators. These flaws occur in the material either through mechanical or chemical action during service or through agglomeration of certain ingredients during mixing and fabrication. This paper deals with the latter process, where the nature and size of the flaws as well as the properties of the matrix are contingent on carbon black variables. Using the tearing energy concept of fatigue developed by Lake and Lindley, it was shown that the size of the flaw is primarily determined by carbon black particle size. On the other hand, the cut growth constant depends on carbon black structure. When translated to actual fatigue life using the Monsanto Fatigue-to-Failure Tester, these relationships mean that under constant strain conditions, compounds containing coarse carbons will have a significantly higher fatigue life than those with fine carbons. Under conditions of constant strain, higher structure carbons will impart a slight positive effect. However, under conditions of constant stress, the beneficial effects of structure become magnified. Other factors known to affect fatigue life were also considered. These are : set, stress relaxation, hysteretic energy dissipation, and flaw size distribution.