When a thin bonded rubber disk is subjected to repeated torsional strains by rotating one end-piece with respect to the other, an initial circumferential crack grows inwards. This is the basis of a new test method for determining rates of growth of a fatigue crack in rubber. Values of torsional stiffness K for a partially-cracked disk are computed here by FEA and corresponding fracture energies G are deduced in terms of the shear modulus μ, radius r of the still-uncracked portion, and rotation angle ϕ, assuming that the rubber compound is linearly elastic. Values obtained for G are shown to vary with r2 , and thus decrease rapidly as the crack grows and the radius of the uncracked portion decreases. They are approximately the same as values obtained when the cracked disk is assumed to be equivalent to a solid disk having a slightly larger radius than the actual uncracked portion. For thin disks the correction in radius for K is approximately 0.22h, and for G approximately 0.33h, where h is the disk thickness. Measurements of crack growth rates dc/dn for a wide range of values of fracture energy G are described in Part II, the next paper in this issue.
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Aboutorabi et al. (1998) studied this question.