PSI - Issue 58

Mirjana Opačić et al. / Procedia Structural Integrity 58 (2024) 87 – 94 M. Opa č i ć et al. / Structural Integrity Procedia 00 (2019) 000–000

90 4

Fig. 3. Some of the results obtained by using ToFD technique.

4. Numerical modelling of defect 5.6 After determining the dimensions and location of the assumed crack using modern UT techniques, the next step was to numerically simulate the behaviour of vessel 970 with the same crack, in order to determine the stress state around the internal crack. As is common practice, these simulations were performed using finite element method, Jovičić et al (2015), Aranđelović et al (2021), Milošević et al (2021), Lostado Lorza et al (2017), Doerk et al (2003), in ABAQUS software (version 6.17). Initial models were made with and the without crack, for comparison purposes. Models were made using a single segment of the pressure vessel mantle, in order to decrease calculation time. Due to this, it was necessary to replace the removed parts of the vessel with corresponding boundary conditions. Geometry and the finite element mesh of the initial models are shown in Fig. 4. The segment that was modelled was 1390 mm long. Finite elements within the welded joint region were much smaller, in order to improve the accuracy of the calculation, as opposed to larger elements further away from the critical area. In this way, the accuracy of the model was ensured, while also decreasing the total number of elements in the model, which contributed to a decrease in calculation time. Of course, in accordance with previous experience related to finite element mesh generation, several different element sizes were tested, in order to ensure the convergence of obtained results. Total number of elements used for the simulation of a model without the crack was 327180, and the number of nodes was 350740. For the model with a crack, the number of finite elements remained unchanged, but the total number of nodes was 403706. Standard linear hexahedral elements were used for both analyses, due to the simple and largely symmetrical geometry of the models in question. Mechanical properties adopted for the simulation of (potentially) plastic behaviour of the pressure vessel model are given in Table 1.

Table 1. Mechanical properties of the materials used in the simulation.

Material

Yield stress [MPa]

Tensile strength [MPa]

Elongation [-]

Nioval 50 (parent materials)

500 560

796.25

0.225 0.183

EVB 65 (weld metal)

840