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.

A new type of wire adhesion test method has been developed and has been compared to the conventional ASTM (D-2229) method on the basis of both accuracy and precision. The extent to which the ASTM slot type method can bias adhesion measurements by concentrating stresses at the leading edge of the wire-rubber interface has been demonstrated and is in agreement with the findings of other investigators. No such limitation was found with the CSRC method. In terms of precision, the ASTM and CSRC methods were found to be comparable, but the CSRC method was found to be more sensitive to differences in adhesion. The new method was used to show that carbon blacks with higher levels of EM surface area, unit form factor, porosity, and volatile content increased wire adhesion in a NR-sulfur compound, unaged and tested at room temperature. The effects of EM surface area and unit form factor became more important when the compounds were aged severely and tested at 212° F. Increased porosity and volatile content improved adhesion unaged at room temperature, but when adhesion was tested hot and aged, the quality of the rubber deteriorated and adhesion was lowered as the levels of these variables increased. Carbon black loading and type are important in a natural rubber recipe in developing good levels of wire adhesion at various test conditions: first, by altering the rate of vulcanization in a direction that favorably affects the competing reactions of the rubber and brass for sulfur and second, by increasing the reinforcement of the rubber, particularly at high temperatures.