PSI - Issue 71

A. Syed et al. / Procedia Structural Integrity 71 (2025) 82–89

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2. Experimental details Fracture toughness testing was carried out on Zircaloy-4 clad tube at room temperature and 300 °C using internal conical mandrel technique. The test temperature of 300 ˚C is chosen since this temperature is the typical operating temperature of PHWRs under normal conditions. The load-displacement data has been obtained which is further used for evaluating the crack initiation toughness. The details of test setup and specimen design is discussed in subsequent section. 2.1. Design and fabrication of fracture specimen and test fixtures Specimens have been machined from clad tube of 220 MW reactor. The inner diameter of the clad tube is around 14.4 mm and thickness is 0.41 mm. The material was taken from NFC, Hyderabad. Final composition of the Zircaloy 4 clad tubes is shown in Table 1. Table 1: Chemical composition of Zircaloy-4 clad tubes used for fracture testing. Material Zirconium Tin Iron Chromium Oxygen Alloy 690 97.89 1.5 0.204 0.102 0.18 The specimens were fabricated with a/W ratio of 0.5. The crack length (a) is taken as 20 mm and the total length of the specimen (W) is kept as 40 mm. The crack was manufactured using fine molybdenum wire of around 0.18 mm thickness in electro-discharge machine. Details of the specimen are shown in Fig. 1(b). The conical mandrel was manufactured using alloy 625 material. The conical angle is kept at 24°, and the total length of the mandrel is around 100 mm. There is very little gap between the straight portion of mandrel and clad tube in order to prevent buckling of the specimen during compressive loading due to movement of conical mandrel. Smooth finishing of mandrel is done to prevent any excess friction between mandrel and clad tubes. Schematic of test setup is shown in Fig. 1(a). 2.2. Test matrix for evaluation of fracture toughness Tests were carried out at room temperature and 300 °C. Two tests were carried out at room temperature at a/W ratio of 0.5. The crack growth at room temperature was measured using digital imaging after every time intervals. Multiple specimen technique was used for evaluating the fracture toughness at 300 °C in order to measure the crack growth at every load value since taking digital imaging is not possible at this high temperature. Six tests were carried out upto different loading values and the corresponding crack growth were used for determination of crack initiation and propagation toughness of the clad tubes. The details of the test matrix is shown in Table 2. 2.3. Experimental procedure The tests were performed at room temperature and 300 °C. The universal testing machine can operate at temperatures up to 1200°C, with a control range of ±2 °C. It has a load control capacity of ±50 kN and features a servo-based control system. The conical mandrel setup was placed inside the furnace and put on the machine attachments. The mandrel is suitably lubricated before testing the specimen. A split-type furnace with three-zone control was utilized to achieve the desired temperature. Once the target temperature was reached, the specimens were held for a minimum of one hour to ensure thermal equilibrium with the surroundings. During this period, the specimens were maintained under zero load using an inbuilt software program to prevent any loading effects due to thermal expansion. Additionally, water circulation was employed to cool the fixtures and protect external electronics from overheating. Fig. 2(a) shows the machine used for these tests. Table 2: Test matrix used for evaluation of fracture toughness of Zircaloy-4 material. Temperature (°C) No. of tests Maximum load (KN) a/W ratio 25 1 - 0.5 300 6 2, 3, 4, 5, 5.5, 6 0.5 The specimens were then tested under constant displacement mode at a displacement rate of 0.5 mm/min. By compressively loading from the machine attachment, the loading mandrel travels into the specimen. As the mandrel's diameter increases and mode I type force is applied at the crack tip, the axial crack in the fuel clad will start to open. This loading situation replicates the loading brought on by the fuel particle inside the clad hour-glassing. As the mandrel's displacement increases, the fracture will start and spread. The specimens were loaded till the desired load value was reached. The furnace was shut down, cooled and specimens were taken out. Data of the displacement, load, and time were recorded for further post-processing. The crack lengths were recorded using a microscope. The specimen before (Fig. 2(b)) and after testing (Fig. 2(c)) at 300 °C is shown below.