Fracture toughness of hydrided Zircaloy-4 sheet under through-thickness crack growth conditions

The susceptibility of fuel cladding to failure in the case of a postulated reactivity-initiated accident may be determined by crack initiation within a hydride blister or rim and subsequent crack growth through the thickness of the cladding. This study has determined the fracture toughness of hydrided coldworked stress relieved Zircaloy-4 sheet subject to through-thickness crack growth at both 25 and 300°C. The experimental approach utilizes a novel procedure in which a narrow linear strip of brittle hydride blister across the specimen width creates a well-defined precrack upon initial loading. The subsequent crack growth resistance is then characterized by four-point bending of the specimen and an elastic-plastic fracture mechanics analysis. At room temperature, the through-thickness fracture toughness (K_q) is sensitive to the orientation of the hydride platelets and K_q≅25 MPa√m for crack growth through a mixed in-plane/out-of-plane hydride field. In contrast, K_q is much higher (≅75 MPa√m) when the hydride platelets are oriented predominantly in the plane of the sheet and therefore normal to both the crack plane and the crack growth direction. At 300°C, the material exhibits greater ductility as the hydride particles within the matrix resist fracture such that K_q≅83 MPa√m, despite the much lower flow stress of the material.