ABSTRACT
When a load is carried over an extended period, a crack in a viscoelastic material might grow, even if the load is less than the static tearing strength Tc for unstable rupture, and even in the absence of dynamic cycles. The rate of crack growth under these conditions is governed by the instantaneous value of the energy release rate. In the oil and gas industry, a seal or packer may well be required to tolerate high pressures encountered for an extended time without developing a crack in the material, even in the absence of dynamic cyclic loading. In this case, understanding the development of time-dependent cracking is essential to the successful material selection and design. A finitely scoped, strain-controlled procedure was used in which static strain is slowly increased on a planar tension test piece over a prespecified total test duration while a camera records images of crack development. The procedure was applied on a carbon black-filled hydrogenated nitrile. The information obtained in the experiment can be used to evaluate creep-crack growth, or in combination with measurements of the fatigue crack growth rate law, to analyze cases where crack growth might occur under mixed conditions.
