Evaluation of High-Temperature Fracture Toughness of Zircaloy-4 Thin-Walled Specimens Using Internal Conical Mandrel Technique

Clad tubes of pressurized heavy water reactor (PHWR) are made up of Zircaloy-4 material. These clad tubes have a thickness of 0.41 mm with an internal diameter of 14.4 mm for 220 MW PHWR reactor. It is not possible for extracting standard fracture specimen owing to its low thickness and internal diameter. Standard operating temperature of the reactor is around 300 °C and the crack initiation and growth behaviour under these high-temperature conditions are necessary for design and safety analysis of clad tubes. In this work, internal conical mandrel technique has been used for determination of fracture toughness of clad tube material at high temperature. Fracture tests have been conducted on the axially cracked tubular clad tube specimens. The conical mandrel is pushed inside the clad tube specimen which simulates mode-I opening at the crack tip. Since it is not possible to measure the crack growth inside the furnace, multi-specimen method has been used. Multiple tests are carried out on clad tube specimen that is loaded upto different load points and crack growth is measured after the test was completed using digital imaging. Using the crack growth and load-displacement data, J-R curve at different temperatures (25 °C and 300 °C) has been obtained. The effect of friction between the mandrel and the specimen on the load-displacement behaviour of the clad tube specimen has been modelled using 3D elastic-plastic finite element (FE) analysis of the test setup. The crack-tip constraints of the axially-cracked specimens at different temperatures have been evaluated from the results of finite element analysis. The crack initiation toughness at different temperature upto 300°C have been obtained and J-R curve has been evaluated. These data shall be useful in design and safety assessment of clad tubes during operational conditions.