Dynamic Fatigue Life of Rubber S. M. CADWELL, R. A. RIERRILL, C. AI. SLOXAN, AND F. L. I-OST United States Rubber Company, Detroit, RIich. Dynamic fatigue is the gradual deterioration and rupture of a rubber member due to mechanical vi- brations imposed on it. The number of repeated mechanical vibrations required to rupture the rub- ber member is referred to here as the dynamic fatigue life of the member for that particular con- dition of vibration. Mention has been made in the literature (1-5) that the dynamic fatigue life of rubber in extension is less when the minimum of the oscillation cycle falls near zero strain; but heretofore no complete study has been published. The fatigue lives of rubber as a function of the os- cillation stroke are examined for minimum dis- ",rtions varying from high compressions through all possible elongations. F A rubber member is continuously vibrated it will sooner I or later crack and ultimately rupture, owing to the re- peated oscillations to which it has been subjected. The gradual deterioration of physical and chemical properties which accompanies such vibration is called dynamic fatigue. The number of such repeated vibrations required to rupture the rubber member is here defined as the dynamic fatigue life of the member for the particular condition of vibration imposed. The authors ivish to outline briefly the general nature of the results they have obtained in their studies on the dynamic fatigue life of rubber and, in particular, to show the critical dependence of the life on the imposed oscillation conditions. The dynamic fatigue life of rubber which is being vibrated linearly-that is, back and forth along its own length-be- FIGURE 1. RUBBER SAJIPLE IS IDEALIZED TEST MACHINE The general dynamic fatigue characteristics of rubber in linear vibration in a dark, dry enclosure are (hn. = minimum length during the vibra- tion; LO = free unstrained length): (1) For a given oscillation stroke the dynamic fatigue life is a minimum w-hen Lmi,. = LO; (2) for a constant value of Ld,. the dynamic fatigue life decreases as the oscillation stroke increases; (3) for given strain limits of oscillation the dynamic fatigue life is usually lower the harder the stock; (4) the dynamic fatigue life depends to a large degree on the rubber temperature. The dynamic fatigue life of rubber worked in shear can be related to the dynamic fatigue life of rubber vibrated through linear strains. tween constant strain limits mill be discussed first. Figures 1 to 3 facilitate a general statement of the problem and also serve to define certain oscillation limits which are of funda- mental importance with reference to the dynamic fatigue life. The sketches are conventionalized for the sake of simplicity. In Figure 1, a is a side view of a cylindrical body of rubber bonded between two circular metal end plates. In b this rubber member has been placed between two heads of a test machine; the position of the stationary head is adjustable; the other head oscillates back and forth in a direction parallel to the axis of the rubber sample. b, c, and d show various possible test conditions resulting from variation of the position of the adjustable head. Besides the alteration of the adjustable head (which does not change the oscillation stroke) the stroke itself can be changed by varying the position of the eccentric connection on the rotating wheel. Its free unstrained length is referred to as LO. b and c show the two ex- tremes in length assumed by the rubber member for a particular oscillation condition. The minimum length, which is the condi- tion in 6, is referred to as Ldn,. The maximum length of the sample in the oscillation, as illustrated in c, is referred to as Lax. The difference between Lmsx, and L,,,, is the oscillation stroke, In Figure 2, a again represents the rubber sample. FIGURE 2. RUBBER SAMPLE IX IDEALIZED TEST MACHINE (100) = % oscillation stroke La k+~2 1100) = % minimum strain { + - extension rompression I, Q 19
Dynamic Fatigue Life of Rubber
Apr 28, 2023
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