A MATERIALS FAILURE RELATION OF ACCELERATING CREEP AS EMPIRICAL DESCRIPTION OF DAMAGE ACCUMULATION

A general materials failure relation, OMEGA = A OMEGA(alpha), describes accelerating creep of materials with rate coefficients alpha and A, by relating rates of deformation, OMEGA to changes in deformation rate, OMEGA (Voight, 1988). Time of failure can be extrapolated from inverse rate versus time, data, and alpha and A may be derived to permit one to calculate the failure time. The method is of value for quantitative hazard assessments. Mechanisms leading to damage accumulation during accelerating creep include creep fracture by stress corrosion and power law lattice deformation. These mechanisms are examined here as phenomenologically related to the materials failure relation. Apparently, both mechanisms favour alpha congruent-to 2, where alpha is the parameter of the materials failure relation controlling the sensitivity to accelerating activity. For pure shear governed by power law creep of power p, under constant load, alpha = 2.0 and A = p. Stress corrosion is widely described by Charles' equation, relating crack velocity to stress intensity during subcritical crack growth by the stress corrosion index n. The relationship between n and alpha is given by alpha = (2 n - 2)/n. The quantitative, predictive qualities of the general materials failure relationship are illustrated with examples from Mount Toc, Italy, and Mount St. Helens, Washington. Six chevron-notched short bar experiments under constant load serve as examples for accelerating creep fracture in the laboratory. The experiments were conducted on siltstones of the Ithaca formation, which show a mean stress corrosion index of n = 78 +/- 24. Analysis with regard to the materials failure relation results in a mean alpha = 2.0 +/- 0.3, which complies with the established relationship between n and alpha.