Laboratory Evaluation of Flex-Cracking Resistance

Abstract

Abstract A machine has been designed for laboratory evaluation of the flex-cracking resistance of any stock from which a dumb-bell strip may be cut. The conditions under which the stock is tested may be varied at the will of the operator. The results obtained on this machine on tread and sidewall stocks have always evaluated the flex-cracking resistance of stocks in the same order that actual road tests have. MANY factors are involved in the proper compounding of rubber stock. In tread compounding wear resistance is the major consideration, but other factors closely approach it in importance. Among these is the ability to withstand repeated flexing without cracking. Flex cracking produces an unsightly appearance which conveys the idea of inherent weakness in the tire, and also, if the flex cracking becomes very pronounced, the tire may fail from carcass break at that point. A satisfactory tire must not, flex-crack. Flex-cracking resistance is dependent on two things—the stock itself and the stress under which it is working. The design of the tire—that is, size and shape of the buttons, the size and position of the side ribs, and the amount of lettering on the side—regulates the working stress under which the sidewall and tread are operating. This accounts for the fact that two tires made by different companies may be different in flex-cracking resistance although of very similar or even identical compounding.