PSI - Issue 71

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

85

Initial crack

Conical mandrel

Initial crack

Crack growth

Clad tube

Clad tube

Base fixture

Base fixture

(a)

(c)

(b)

Fig. 2: (a) Machine used for the tests; (b) Specimen before test at 300˚C (c) Specimen after test at 300 ˚C to a load of 5 kN.

3. Experiment results obtained after fracture testing The fracture tests were carried out at 25 °C and 300 °C to evaluate the crack initiation and propagation toughness of Zircaloy-4 material using the internal conical mandrel technique. The load displacement response is obtained at these temperatures and the crack growth corresponding to some displacement levels is noted. The details of load displacement response and crack growth are discussed in subsequent sections. 3.1. Fracture tests carried out at room temperature To evaluate the crack initiation toughness at room temperature, a specimen with an a/W ratio of 0.5 is prepared and mounted onto a conical mandrel with a half-angle of 12°. The mandrel is pushed inward along the specimen, and the load-displacement behaviour is recorded. The behaviour at 25 °C is depicted in Fig. 3(a). It is observed that the load-displacement curve consists of two distinct slopes. The initial slope reflects the increasing load required to overcome friction between the mandrel and the clad tube specimen. However, after 12 mm of displacement, the load increases more rapidly as the larger diameter of the conical mandrel reaches the crack tip of the specimen. Once sufficient crack growth occurs, the slope decreases due to a reduction in the ligament region ahead of the crack tip. As the mandrel advances, the specimen opens, and crack growth is monitored at various displacement values using a digital camera. Images of the specimen at different displacements are used to measure the crack. The crack growth behaviour, illustrated in Fig. 3(b), shows that initial growth is slow up to a displacement of 10 mm, but it accelerates significantly after 12 mm, corresponding to the rapid increase in load as the cracked specimen opens. 3.2. Fracture tests carried out at 300 °C Fracture tests were conducted at 300 °C to determine the fracture toughness of Zircaloy-4 material. Due to the inability to capture images during high-temperature tests inside the furnace, a multiple specimen technique was employed. Each specimen was loaded to a specific load value, and the corresponding crack growth was measured. Subsequent specimens were loaded to progressively higher load values, and the crack growth was recorded. A total of six tests were performed at different load levels, with their corresponding crack growth noted. The load displacement behaviour obtained is shown in Fig. 3(a), and it is similar to the behaviour observed in room temperature tests. However, the load-displacement curve at 300 °C is lower than that at 25 °C. The increase in slope of the load displacement curve occurs at a displacement of about 16 mm, compared to 12 mm at 25 °C, indicating slower crack growth at 300 °C, as shown in Fig. 3(b). The crack growth curve shifts to higher displacement values as the temperature increases to 300 °C due to increased ductility and plastic deformation at elevated temperatures. These conditions promote crack tip blunting and enhance the material’s ability to absorb energy before crack propagation, resulting in reduced crack extension under similar loading conditions.