The failure theory of Bueche and Halpin is generalized and expanded to obtain a prediction of the time dependence of the tensile strength and ultimate elongation. The model pictures rupture as the propagation of tears or cracks within the material. The growth of a tear or crack is viewed as an ideally simple process in which the molecular chains at the tear tip stretch viscoelastically under the influence of a high stress concentration until they rupture. As a consequence, the failure process is a nonequilibrium one, developing with time and involving the consecutive rupture of the molecular chains. Substantial support for the theory is found by comparing the theoretical prediction against experimental results obtained for SBR and EPR rubbers. In addition, it is experimentally demonstrated that delayed and forced rupture experiments can yield qualitatively identical data for viscoelastic bodies. Also discussed is the basis for the approach to the nonlinear response of a viscoelastic body required here in the development of a fracture theory.