A specimen and method for evaluating the effect of cladding on the behavior of subclad flaws

A specimen that reveals important fracture-related properties of cladding in the presence of a subclad flaw is under development at the Oak Ridge National Laboratory (ORNL). Information developed from testing these specimens, referred to as Jo-Blocks, is being used by the Heavy-Section Steel Technology (HSST) Program in evaluating the behavior of subclad flaws in Pressurized Water Reactors (PWR) pressure vessels during pressurized-thermal-shock (PTS) loading conditions. The cladding can be idealized as a force that holds an otherwise surface flaw ''closed'' at the surface, reducing the stress intensity factor along the portion of the crack front in the base material. This closing force is approximately equal to the average stress in the cladding, which for postulated severe PTS transients is at yield, multiplied by the cladding thickness. There is a critical amount of stretching of the cladding that results in through-clad flaw propagation, i.e., cladding failure, thus converting the subclad flaw to a surface flaw. The Jo-Block specimen consists of two steel (base metal) blocks with ends butted together to form a ''crack'' and with opposite edges clad so that the crack terminates at the two fusion zones. Testing of Jo-Block specimens reveals as a minimum the ''effective yield point'' of the cladding, in the presence of a subclad crack, and the critical value of clad stretching (crack opening displacement). 15 refs., 10 figs