Mechanical and constitutive behaviour of zirconium-2.5%niobium pressure tube material at high rates of tensile strain.

摘要

A direct-tension split Hopkinson bar apparatus was used to perform experiments on Zr-2.5Nb pressure tube material at tensile strain rates in the range of 500 to 3000 s-1, temperatures between 20 and 300°C, and fluences up to 8 x 1025 n/m 2. Tests were performed on specimens manufactured from both the longitudinal and transverse tube orientations. Experimental results on non-irradiated material showed that the effect of increasing the rate of strain was to increase the strength and decrease the ductility for both specimen orientations. The effect of irradiation was to further increase the strength and decrease the ductility. All of the irradiated material, however, exhibited a significant amount of plastic deformation and failed in a ductile manner. A bi-linear dependence of flow stress on log-strain rate exists for both the non-irradiated and irradiated material, with the rate sensitivity being greater over the high strain rate regime. Zr-2.5Nb pressure tube material remains anisotropic at high strain rates with the strength being greater in the transverse orientation. The effect of increasing the temperature was to decrease the strength over the range of strain rates and fluences considered.; Results from the mechanical tests were used to characterise the constitutive behaviour in the longitudinal direction with the Johnson-Cook and Zerilli-Armstrong hcp relations. Both constitutive relations closely predict the behaviour of the non-irradiated material for the range of strain rates and temperatures investigated, if the effects of localised deformation and adiabatic heating were incorporated in the derivation of the constitutive parameters. The irradiated responses, however, were not as closely predicted due to the relatively large variability in the data for the irradiated samples. The Zerilli-Armstrong hcp relation produced slightly more accurate results for the conditions considered.; Finite element simulations of the direct-tension split Hopkinson bar experiments were performed using the derived constitutive relations to describe the material behaviour. Results from the simulations showed that the finite element method can be used to predict the longitudinal stress-strain behaviour of as-received and irradiated Zr-2.5Nb pressure tube material at high rates of tensile strain. The onset of a diffuse necked region and the reduction in cross-sectional area to the point of failure were also predicted with reasonable accuracy. The anisotropic deformation behaviour exhibited by the radial and transverse strains in the necked region, however, was not captured.