A Fatigue Cracking Test for Tire Tread Compounds. Some of the Laws of Fatigue
Abstract AN IMPORTANT factor in the service rendered by a tire tread is its ability to resist cracking. Cracking of tire treads in service may be classified into two kinds or phases—the initiation or start of cracks, and the growth of cracks or cuts once started—and the two phases must be differentiated. Nearly all tires in service soon develop small cracks or “checking” due to the action of ozone in the atmosphere (9), and may suffer cuts due to sharp objects on the road. In many cases such cuts and ozone cracks may be harmless, but if the cracks grow too rapidly they will extend to the carcass before the tire is worn out (4). We define the growth of cracks under the influence of mechanical action as fatigue cracking. The quality of rubber in respect to its resistance to fatigue cracking can be improved by addition of antioxidants (3, 7), and by such factors as the proper choice of accelerators (4), the proper balance of fillers (1, 3), and the proper cure. The quantitative evaluation of such chemicals and compounding ingredients in the laboratory depends upon the validity and precision of a test. It is the purpose of this paper to describe a laboratory test which has been found to correlate with shoulder cracking in tires, and to present data from which certain laws of fatigue cracking have been deduced.