Dunlop Fatigue Test for Rubber Compounds

This paper considers the effects produced inside a block of rubber subjected to repeated cycles of stress. Other types of fatigue, such as flex cracking or the phenomenon recently referred to by Cadwell, Merrill, Sloman and Yost as static fatigue, are not discussed. The fatigue test which will be described in the following pages has been used for a number of years in the Dunlop Rubber Company's laboratories. The test machine, like others which have been described by Martens, Vogt, Abbott, Depew and Snyder, Cooper, Havenhill and Macbride, Havenhill, Lessig, and Roelig, can be used for assessing heat development, the failure produced in the test-piece being then of the “blow-out” type, or it can be used to produce breakdown in specimens which have been stressed for prolonged periods at a moderately high, although steady temperature. The failure may then be considered to be one of fatigue and not one of thermal decomposition, although, as will be shown, the heating effect is one of the most important factors operating to produce failure. Among the test machines referred to above, the one described by Depew and Snyder is similar to the Dunlop machine, an early form of which was already in use for the purpose of evaluating heat generation in solid tire compounds when Depew and Snyder's paper was published in 1929. The present improved machine and technique are the result of several years of subsequent development work. The stressing conditions of the machine, which produce direct compression, are as simple as possible and give results which vary less and are more easily interpreted than those obtained when they are more complex, as, for example, when they introduce shear in addition to compression.