An investigation of cladding effects on shallow-flaw fracture toughness ofreactor pressure vessel steel under prototypic biaxial loading

Potential structural-integrity benefits or liabilities of the stainless ste el cladding on the inner surface of a reactor pressure vessel (RPV) are imp ortant considerations in the effort to refine or improve safety assessment procedures applied to RPVs, Clad-beam tests were carried out to investigate and quantify effects of the clad structure on fracture initiation toughnes s of through-clad shallow surface flaws in RPV material. A cruciform beam s pecimen Was developed at ORNL to introduce a prototypic, far-field, out-of- plane biaxial stress component that provides a linear approximation of the nonlinear stress distribution generated by thermo-mechanical loading transi ents in an RPV. The cruciform specimens (102-mm-thick test section) were fa bricated from RPV shelf segments available from a canceled pressurized-wate r reactor plant, The specimens were tested under biaxial load ratios rangin g from 0.0 (uniaxial) to 1.0 (full biaxial), the ratio being defined as the total load applied to the transverse beam arms divided by that applied to the longitudinal arms, The rest results imply that biaxial loading is effec tive in reducing the shallow-flaw-fracture toughness of the clad/heat-affec ted zone/structural-weld region of the RPV shell below that determined from uniaxial loading conditions. The lowest toughness value from the clad cruc iform specimens tested under biaxial loading is only slightly above the ASM E Section XI K-Ic curve. For all biaxiality ratios, the test results imply that shallow-flaw fracture toughness data from the RPV structural weld mate rial are significantly lower than that obtained from a high-yield strength plate material.