Dimensional transitions in creeping materials due to nonlinearity and microstructural disorder

The transition from extremely brittle to very ductile behaviours of creeping materials is discussed, where analogies with power-law hardening materials are pointed out. Considering Norton's Law as a viscous constitutive law, it is possible to define a generalized stress-intensity factor Kc ―characterizing the intermediate asymptotic behaviour under steady-state creep conditions― with physical dimensions depending upon the Norton stress exponent n. In the two limit cases of creep resistant materials (n≅1) and creep sensitive materials (n ≫ 1), Kc assumes respectively the dimensions of an elastic stress-intensity factor (FL−3/2) and of a stress (FL−2). Such a dimensional transition, with consequent stress-singularity attenuation, is completely analogous to that occurring through the introduction of a fractal stress-intensity factor (Kc)*, when the influence of microstructural disorder is considered.