A regression approach for Zircaloy-2 in-reactor creep constitutive equations

Abstract

Typical data analysis procedures used in developing current phenomenological in-reactor creep equations for Zircaloy cladding materials are examined. It is found that the data normalization assumptions and the curve fitting techniques generally adopted in these procedures can make the prediction of creep strain rate from the resulting equations highly questionable. Multiple regression analysis performed on a set of carefully selected Zr-2 in-reactor creep data on the basis of comparable as fabricated metallurgical conditions indicates that models of the form e = Aaynm exp (-Q/RT) can give significant account for the data. Both regression statistics and residual plots have provided strong evidences for the significance of the regression equations.--

ABSTRACT OF THE ADDENDUM

The addendum of this report contains the results and discussions of the analysis we made on our revised Zircaloy-2 in-reactor creep data. Although fewer creep rate data were used, much of the same general conclusions can be made as we did previously. The differences between the final recommended regression Zircaloy-2 in-reactor creep equations in the report and the addendum are primarily those of the estimated coefficients. The starting model equations continue to provide excellent correlations for the creep strain rate data. Our attempt to verify the additivity of thermal and irradiation creep components directly from the creep data set show that there is little merit in using a composite formula made up by the above two components for ranges covered by the test parameters. This statement should not be looked upon as a denial to such a treatment, however, especially in situations of combinations of high , a and T where changes in dominating deformation mechanism are possible. At present, we do not have reliable data in those regimes.