Failure of Hydrided Zircaloy-4 Under Equal-Biaxial and Plane-Strain Tensile Deformation

Abstract

The fracture behavior of unirradiated Zircaloy-4 containing either solid hydride blisters or hydrided rims has been examined for the contrasting conditions of equal-biaxial and plane-strain tensile deformation at three temperatures (25 o , 300 o , and 375 o C). Cold-worked and stress-relieved Zircaloy-4 sheet containing hydride blisters shows nearly identical failure strains in equal-biaxial and plane-strain tensile deformation for a wide range of blister or rim depths. In all cases, failure strains decrease rapidly with increasing hydride blister or rim thickness, especially in the ≤100 µm range. Test temperature has a significant effect on ductility with failure strains at 300 o and 375 o C being much greater than at room temperature. The results indicate that the ductility of material containing hydride rims/blisters greater than ≈ 30-40 µm deep is limited by crack growth, which occurs in a mode I manner at 25 o C but in a mixed mode I/II manner at ≥300 o C (and at higher failure strain levels).

Fracture of Zircaloy-4 cladding tubes with or without hydride blisters in uniaxial to plane strain conditions with standard and optimized expansion due to compression tests

“…The fracture strain in plane-strain measured by Pierron et al [32] and equibiaxial tests of Glendening et al [12] on sheets are similar to the one obtained in the present study on cladding tube ( figure 12). The present study results only suggest a slightly lower ductility for small blisters at 25 • C (black) and at 300-375 • C (blue).…”

“…Samples with large blisters failed at similar fracture strain regardless of the macroscopic stress state between uniaxial and plane strain. This statement can be extended to equibiaxial conditions based on previous results [32,12]. Nevertheless, samples with small blisters about 50-60 µm in depth did not fail in EDC tests, while they failed with HB-EDC tests.…”

“…Secondly, regarding samples with an hydride blister. Previous studies showed that similar fracture strains were obtained on Zircaloy-4 sheets containing a blister regardless of the loading biaxiality level between plane-strain (Pierron et al [32]) and equibiaxial (Glendening et al [12] - figure 12). Nevertheless, the unfailed EDC tests with 50-60 µm deep blister suggest that a blister depth threshold might exist below which the macroscopic loading biaxiality does impact the fracture strain.…”

“…This fracture strain correlation is valid between 25 and 480 • C, for uniaxial to plane strain conditions for 310 wppm hydrided samples with a blister depth higher than the threshold discussed above. It is even valid up to biaxial loading considering the results of Pierron et al [32] and Glendening et al [12] discussed above. But this correlation is only for plane strain conditions for 310 wppm hydrided material without blister or 310 wppm hydrided samples with a blister depth lower than this threshold.…”

“…Therefore, displacement-controlled mechanical tests should be preferred to characterize the cladding fracture in the PCMI phase. Punch tests techniques [11,12] are suitable to provide information for various stress state conditions. However, these tests can only be performed on sheets, which are generally not fully representative of tubular sample in terms of mechanical behaviour and fracture.…”