PSI - Issue 79

Lazar Jeremić et al. / Procedia Structural Integrity 79 (2026) 117 – 123

118

procedure was introduced in [1] based on failure likelihood and consequences as described in [2]. More recently, finite element method was used to evaluate precise stress and strain state around a crack-like defect, [3], whereas advanced UT techniques, such as Phased Array Ultrasonic Testing (PAUT) and Time of Flight Diffraction (TOFD) were applied to evaluate the position and size of a defect more precisely, [4]. Risk-based approach commonly uses definition of risk as the product of probability and the consequence of failure. This approach is based here on Failure Assessment Diagrams (FAD), typically at level 2 and occasionally at level 3 for more ductile materials [6,7]. Detailed estimation of consequence in the case analyzed here is described in [2], considering eventual losses of human lives, materials, production and reputation. 2. Problem definition - pressure vessel 971 description Inspection and testing of pressure vessels in RHPP Bajina Basta were done on several occasions by the ultrasonic technique, according to EN ISO 11666 2018, [8]. In total 9 pressure vessels (PV) for compressed air in Reversable Hydro Power Plant (RHPP) Bajina Bašta, Serbia, were tested, marked 970-978. The PV 971 basic geometry data was: diameter 2150 mm, height 4485 mm, volume 13 m 3 , thickness 50 mm, Fig. 1a. Design pressure was 8.1 MPa, with working pressure 7.7 MPa.

Fig. 1 a) PV 971; b) defect 1.1

It should be noted that the material for inspected vessels was High Strength Low Alloyed (HSLA) steel NIOVAL 50, one the oldest in this steel class, well known for its cracking problems in welded joints, [9]. The vessel 971 was inspected by 100% UT on 2 vertical and 3 circular welded joints, by USM 36XL Krautkramer device, including defect 1.1, analyzed here, shown in Fig. 2b. Welded joints were made by Manual Metal Arc Welding (MMAW) process using the electrode EVB 65 to obtain an overmatching effect, being the common practice for steels with the yield strength up to 500 MPa. Tensile properties of the base metal (BM) and weld metal (WM) are shown in Table 1 as used in the FEM analysis.

Table 1. Tensile properties of the BM and WM, [16]

Yield strength [MPa]

Tensile strength [MPa]

Elongation [%]

Material

BM WM

500 560

650 720

22.5 18.3