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Low Fatigue Design Basis for Zircaloy Components

“Fatigue Design Basis for Zircaloy Component” W. J. O’Donnell, B. F. Langer, Nuclear Science & Engineering, Vol. 20, 1964

Keywords: Low cycle fatigue; Zircaloy; cyclic stress-strain; residual stresses; fatigue tests; notch sensitivity

General methods have recently been developed for low cycle fatigue design. The required basic strain-controlled data for both non-irradiated and irradiated Zircaloy-2, -3, -4 were obtained for temperature between 70 F and 600 F. Data include both rolled and base-annealed material, and as-welded material tested in various directions. The cyclic stress-strain properties of these materials were also obtained and were found to differ quite significantly from the conventional properties.

Using the cyclic properties in a Modified Goodman Diagram, fatigue-failure curves were developed which included the deleterious effect of the maximum possible mean stress that can exist in the material as it is cycled. Limited available test data confirm the validity of this test method. Using the resulting curves, one need only consider the cyclic stress loads.

The worst possible effects of residual stresses due to welding and other fabrication methods, and mean stresses due to differential thermal expansion are included in the curves. The phenomenon of fuel growth introduces a monotonically increasing strain which accompanies the cyclic strain. The effects of such a gradually accumulating increment of strain were investigated and were found to be adequately covered by the adjustment for maximum mean stress.

Design curves were constructed from the mean failure curves by applying approximate factors to cover the effects of size, environment, surface finish, and scatter of data. The results of fatigue tests on notched irradiated Zircaloy indicate that this material is somewhat less notch sensitive than 100,000 psi tensile strength steel. Non-irradiated Zircaloy is even less notch sensitive. However, fatigue tests on notched weld metal indicate considerably greater notch sensitivity.