The relation between stress-relaxation and creep for some zirconium alloys during neutron irradiation

In-reactor stress-relaxation tests on beam specimens of several zirconium alloys have been performed at 566 K in a fast neutron flux (E>1 MeV) of 2 × 10¹⁷ n/m² · s. The stress-relaxation behaviour is characterized by an initial rapid decrease in the unrelaxed stress ratio followed by a slower steady-state decrease which can be expressed by the relation,

$\text{ln}\text{(σ}\text{σ}{}_0\text{) = ? (AE)t +}\text{ln}\text{D}$

, where $\text{σ}\text{σ}{}_0$ is the unrelaxed stress at a given time t as a fraction of the initial stress σ₀, A is a flux-, material-,temperature-dependent constant, E is Young's modulus, and D is given by $\text{σ}{}_{\mathrm{p}}\text{σ}{}_0$ where σ_p approximates the initial stress decrease.The linear dependence of In $\text{σ}\text{σ}{}_0$ on time and the independence of the stress-relaxation behaviour on the initial stress implies that the creep rate in the steady-state period can be given by the expression, $\text{ε}\text{?}\text{=}\text{A}\text{σ}$. Good agreement was obtained between creep rates derived from stress-relaxation tests on experimental pressure tube materials and creep rates derived from diameter measurements of pressure tubes for identical temperature, flux and stress conditions.