Pressure vessels (PV) for compressed air were installed 45 years ago in RHPP Bajina Basta. After 18 years of exploitation, unacceptable defects in welded joints were found during regular monitoring by NDT. Initial assessment of structural integrity, based on conservative fracture mechanics analysis, indicated a possibility to use the vessel in regular operating mode. Monitoring in the meantime was intensified, with regular NDT examinations every 6 months, using conventional NDT methods, Ultrasonic (UT) and Radiographic testing (RT). Recently, more advanced NDT UT methods were introduced, Phased Array Ultrasonic Testing (PAUT) and Time of Flight Diffraction (TOFD), to evaluate defect position and size more precisely. Also, the finite element method was applied to evaluate more precisely the stress state in the PV with a crack-like defect. In this way structural integrity and risk of failure was assessed more reliably. Finally, after 27 years of exploitation these PVs were replaced and welded joints with defects examined once again to get the final verdict.

Recently, during rehabilitation of RHPP in 2025, cracks were discovered in rotor spoke ribs. It was determined that cracking was caused by unexpected overloading due to weight. It was also concluded that cracks cannot be grooved and surface welded due to very limited space and some other limitations. Thus, a different approach had to be taken, leading to a reconstruction of the rotor spoke. To do so, it was necessary to stress release the rotor spoke ribs which was done by hydraulic lifting the whole rotor and then by strengthening the spoke ribs by new additional rib elements. This solution is based on detailed 2D and 3D stress analysis performed by the finite element method.

Orthopaedic plates are long-established medical devices conventionally manufactured from metals, most notably titanium alloys. The introduction of Additive Manufacturing (AM) has created new opportunities to design implants with complex internal architectures, enabling precise control over infill patterns and densities that directly influence mechanical properties and fatigue performance. Biodegradable polymers such as polylactic acid (PLA) have attracted growing interest in biomedical engineering, potentially reducing the need for secondary implant-removal surgery if degradation rates are carefully controlled and clinically approved. Additionally, AM offers the ability to customise internal structure for improved mechanical performance and load-bearing, while also providing the possibility of integrating advanced functionalities, such as controlled drug delivery. Building on previous work by our research group at the University of Belgrade, this study investigates the fatigue behaviour of the best-performing AM-optimised orthopaedic plate design. Numerical models incorporating honeycomb infill structures with the full range of achievable densities were developed to assess structural integrity under fatigue loading. Fatigue crack growth was simulated in ANSYS Mechanical (ANSYS Inc., Canonsburg, PA, USA) software, employing a four-point bending configuration in accordance with the ASTM F382 standard. A validated PLA material model was implemented at a reduced load level (10%) relative to previous studies. Direct comparison with titanium plates was avoided due to fundamentally different material properties, focusing instead on infill architecture to identify optimal AM design strategies for orthopaedic plates.

This research is the continuation of extensive analyses of the behaviour of titanium alloy hip implants under fatigue loads, mainly by using numerical methods such as classic and extended finite element method. The focus in this case was on different loads, both in terms of patient weight and load cases such as walking, going up/ down the stairs and stumbling. By combining three different weights and four load cases, a total of 12 models were obtained. Results for numbers of cycles, critical crack length and stress intensity factors were then compared to each other, in order to determine how different load combinations affected the work life of these implants, i.e. how much their life decreases in the case of the least favourable load combinations. In the next stage of this research, new and improved models were developed with the goal of completing the analysis, by increasing the number of load steps in the simulations up to a point where fatigue crack growth (FCG) actually enters its unstable stage. The findings confirm that the newly developed finite element (FE) models offer a more accurate representation, thereby providing valuable insights into implant behaviour. Notable differences in fatigue life were observed between walking and other loading scenarios. In addition, the more extreme cases were somewhat conservative - such as the assumption that a patient would only walk upstairs over a long time period.

The metallurgical characteristics of high-strength low-alloyed (HSLA) steel and the effects that could lead to crack initiation, especially in the heat affected zone, have to be taken into account during defining welding technology. Primary aim of this study is dealing with the thermal effect caused by repair welding of HSLA Nioval 47, along with the damage analysis of a water supplying pipeline made of this steel and the circumferential welded joints. Analysis has shown the involvement of different damage mechanisms on reconstructed pipeline. Thermal cycle during repair welding with a focus on cooling time (t8/5) and with heat input (E) was thoroughly defined, along with recommended technological measures. After repair welding (using E 50 6 1 Ni B 42 electrode), microstructure analysis was performed on the surfaces at the most critical locations, i.e., the repaired circumferential welds A and B. In addition to martensite structure in the coarse-grain heat affected zone, crack initiated in the weld metal, ending at the fusion line, was detected, despite the adequately defined welding technology. One of the major remarks is related to the importance of available data needed for analysis and failure prevention during the exploitation period, guarantying reliability and safety of the pipeline.

This paper presents an experimental and numerical analysis of the mechanical behaviour of orthopaedic implants with crack-type defects, considering the principles and advantages of the modern X-FEM method, which was used due to limitations of traditional FEM in terms of crack growth simulation, especially for complex geometries. In X-FEM, the finite element space is enriched with discontinuity functions and asymptotic functions at the crack tip, which are integrated into the standard finite element approximation using the unity division property. Though rare, femoral component failures are well-documented complications that can occur after hip prosthetic implantation. Most stem fractures happen in the first third of the implant due to the loosening of the proximal stem and fixation of the distal stem, leading to bending and eventual fatigue failure. The main goal of this paper was to obtain accurate and representative models of such failures. Experimental analyses of the mechanical behaviour of implants subjected to physiological loads, according to relevant standards, using a new combined approach, including both experiments and numerical simulations was presented. The goal was to verify the numerical results and obtain a novel, effective methodology for assessing the remaining fatigue life of hip implants. For this purpose, the analysis of the influence of Paris coefficients on the total number of cycles was also considered. Hence, this simulation involved defining loads to closely mimic real-life scenarios, including a combination of activities such as ascending stairs, stumbling, and descending stairs. The tensile properties of the titanium alloy were experimentally determined, along with the Paris law coefficients C and m. The finite element software ANSYS 2022R2 version was used to develop and calculate the three-dimensional model with a crack, and the resulting stresses, stress intensity factors, and the number of cycles presented in the figures, tables, and diagrams. The results for the fatigue life of a partial hip implant subjected to various load cases indicated significant differences in behaviour, and this underscores the importance of analysing each case individually, as these loads are heavily influenced by each patient’s specific activities. It was concluded that the use of numerical methods enabled the preliminary analyses of the mechanical behaviour of implants under fatigue loading for several different load cases, and these findings can be effectively used to predict the possibility of Ti-6Al-4V implant failure under variable cyclic loads.

This paper will present a novel approach to supporting a piece of process equipment subjected to long-term exploitation conditions, with the main goal of improving its reliability and safety. Optimising the supports of the process equipment (in this particular case, 16 autoclaves used for coal drying) began by measuring the load at the support points. It was followed by an analysis based on good engineering practice to develop a new technical solution. The old support solution represented a rigid connection between the autoclave envelope and the supporting structure. Meanwhile, the new approach introduced spring supports, thus providing flexible connections between the Autoclave and the structure. This flexibility ensures that the load on the vessel's shell is reduced significantly and that stress distribution at the support points is uniform. Simultaneously, the load distribution in the structure's support zone is significantly more favourable. The economic benefit of such an approach and a reflection on sustainability are also discussed.

The 12th Annual Conference of Society for Structural Integrity and Life (DIVK12), organized between 17th and 19th of November 2024, at the Faculty of Mechanical Engineering of the University of Belgrade, Serbia, gathered more than 160 participants (both in person and on-line) from all over the world, with more than 25 nationalities demonstrating the vitality and importance of this new event. This Special Issue gathers the 71 papers presented at the conference, including some keynote lectures and regular presentations. Awards for special contribution in certain topics were delivered attributed during the conference. The Organizing Committee of the DIVK12 conference sincerely thanks all contributing authors for playing a significant role in the overall success of this event, with their exciting presentations. The members of the International Scientific Committee are also fully acknowledged for their support of the DIVK12 event. Special thanks to the Keynote Speakers for their dedication and knowledge and energy brought to this event. The Organizing Committee would also like to express their gratitude to the sponsors for their support without which the conference would be impossible to organize. Finally, chairmen sincerely thank the tireless efforts of Organizing Committee members, as well as Faculty of Mechanical Engineering, IMS institute and Innovation Center of Faculty of Mechanical Engineering staff.

Failure analysis of a hydropower plant turbine shaft was performed by using the Finite Element Method (FEM) to assess its structural integrity and remaining life. Static and dynamic loading was applied to assess relevant fracture mechanics parameters using FEM for stress analysis and its extended version (XFEM) for simulation of fatigue crack growth. Application of XFEM to turbine shaft crack growth problem is in focus of this paper, in combination with material properties and its expected behavior under amplitude loading. The goal of the research was to determine the remaining life of a turbine shaft that has failed in service. Such an approach provided clear answer to why the cracked shaft failed in a short period of time. Based on that, suggestions to prevent such a failure are given.

Hip implants are nowadays typically high-quality and reliable components which rarely fail. Nevertheless, they still sometimes do, due to static, impact or amplitude loading, under hostile environment such as our bodies. Therefore, we still have to analyse factors that may affect both hip implant integrity and life, such as body weight, implant geometry and material. In this paper, a review of recent investigation of hip implant fracture and fatigue behaviour is presented with focus on the effects of body weight, implant geometry and material. It is shown that body weight plays an important role, especially for fatigue life, since the relation between remaining life and stress (proportional to weight) is exponential (typically m = 3 in Paris equation). Regarding implant geometry, it is demonstrated that the main issue is stress analysis. In the case of modular (full cross-section) implants, stress state is more favourable than in the case of implant with holes for fine adjustment according to patient bone configuration. It is also shown that stress level in this case can be surprisingly high, especially with younger, active patients. Finally, the interaction between influencing factors should be taken into account, being a relatively simple task when numerical methods are used.

Structural integrity and life assessment of railway and aeronautical components, critical for the safety of these two important means of transport, is analysed using four case studies, two for each of them. In respect to railway components, a brief failure analysis of a fractured wheel is presented, with the idea to use this experience to improve structural integrity of wheels in general. Second case study is focused on coupling link used in freight wagons, once again providing failure analysis to improve its safety. Case studies of aeronautical components comprise torque links and High Temperature and Pressure (HTP) casings, as crucial components of airplane landing gear and jet engine, respectively. In both cases residual fatigue life was in focus with an aim to replace damaged torque link with a new one, produced by Additive Manufacturing (AM), and to check if damaged HTP casing can remain in operation for a certain period o time.

Cracks are found during regular periodical NDT (MT and UT) in a large spherical tank, produced in 1974 of low carbon high quality steel of yield strength 385 MPa, similar to A516 Gr.60. Crack length was between 5 and 150 mm, with depth up to 13.8 mm, more than half of vessel thickness (26 mm). Repair was performed in the usual way, by grinding and surface welding. The waterproof testing of the repaired vessel was performed according to standard procedure with additional strain measurements using strain gauges positioned at surface welded regions to assess the vessel structural integrity. Also, acoustic emission was used to follow vessel behaviour during the waterproof test. Since no signal was recorded and strains were linear during the test, it was concluded that the repair was successful, and structural integrity is proved as well.

Hip implants are nowadays typically high-quality and reliable components which rarely fail. Nevertheless, they still sometimes do, due to static, impact or amplitude loading, under hostile environment such as our bodies. Therefore, we still have to analyse factors that may affect both hip implant integrity and life, such as body weight, implant geometry and material. In this paper, a review of recent investigation of hip implant fracture and fatigue behaviour is presented with focus on the effects of body weight, implant geometry and material. It is shown that body weight plays an important role, especially for fatigue life, since the relation between remaining life and stress (proportional to weight) is exponential (typically m = 3 in Paris equation). Regarding implant geometry, it is demonstrated that the main issue is stress stress state is more favourable than in the case of implant with holes for fine adjustment according to patient bone configuration. It is also shown that stress level in this case when numerical methods are used.

The main focus of research presented here is the influence of cooling time t8/5 on impact toughness of A516 Gr. 60 steel specimens. Experiments include simulation of heating and cooling during welding (thermal simulation) in order to achieve desired microstructures that would correspond to those obtained in actual welding. Specimens simulated in this manner are then subjected to impact testing, and obtained results are analysed in terms of total impact energy and its components in order to provide detailed insight into the behaviour of these specimens. By obtaining diagrams that show mutual dependence of force, time, energy and displacement (deflection), it is possible to determine how impact toughness is affected by cooling times. Hence, the research provides valuable input for the selection of welding parameters.

This paper presents the effect of stress concentration due to notch or cracks on the impact toughness of duplex steel S32750. This analysis is based on the results obtained by Charpy instrumented pendulum, enabling the separation of total energy into crack initiation, Ei, and crack propagation energy, Ep. The crack sensitivity factor (CSF) was determined, defined here as the ratio of total impact energy (KV value), obtained by testing standard ISO-V specimens, and KV1 value, obtained on a same type of specimen but with 1-mm long fatigue cracks: CS = KV/KV1. Testing was conducted in accordance with standard EN ISO 148-1:2017 at different temperatures: +20 degrees C, -40 degrees C, -60 degrees C, and -80 degrees C. Fatigue crack lengths ranged from 1 to 5.5 mm, as measured from the notch root on ISO-V specimen. Based on KV vs. crack length diagrams, KV1 values are obtained by interpolation of all results, providing data for CSF determination. Fractography was also done to clarify the fracture behavior of notched and cracked specimens under impact loading and different temperatures.

This work presents the method for monitoring the integrity of oil methanol tanks located in Serbia and Norway. Considering the importance of safe operation of such equipment, it was necessary to use advanced non-destructive testing inspection for this purpose. The inspection, which involved state-of-the-art ultrasonic testing method, was completed in order to collect data required for evaluation of structural integrity and fitness for continued service of the tanks in question. Inspection and structural integrity analysis will include assessment of the most critical elements of these tanks, including their foundations, shell, roof and floor/bottom of the methanol tanks. This analysis included the calculation of thickness in various parts of the tanks, and it was determined that they were all within the allowed limits, implying that both tanks were safe for continued service.

Belgrade fair hall 1 is well known for its dome, which is still the world largest self-supported construction made of pre-stressed concrete, with its diameter of 106 m. In this paper the Finite Element Method (FEM) was performed to analyze different loading and supporting conditions. At the same time, crack growth in a support column was simulated by the extended FEM (XFEM). Results of numerical calculations indicate ingenious design of such a complex structure which was based on “hand” calculation decades ago, without computers. In addition to classical engineering and more advanced numerical calculations, risk based analysis was performed taking into account artificially introduced crack and Failure Analysis Diagram, obtained using stress intensity factor and net stress, as well as fracture toughness and critical stress. This analysis, made for the first time for Belgrade fair hall 1, proved that its structural integrity is jeopardized only when a crack reaches half the thickness of a steel bar.

The paper deals with damage mechanisms affecting the piping and piping elements during working life of the well collector as an integral part of the upstream plant. An initiated crack has propagated in the vertical piping elbow and caused leakage. The damaged elbow has a sensitive cut, and a new elbow is welded onto the pipe by GTAW and SMAW processes. Necessary NDE was performed after repair welding activities showing that remediation of the line is properly conducted. The line is successfully put into service.

The idea behind the idea of developing this method was to introduce reference points at important locations, such as the fusion line and heat-affected zones, the displacement of which would be monitored during the uniaxial testing, and then measured at key moments. The uniaxial tensile test process was recorded with a highresolution camera so that changes could be observed during the test. The reason why this approach was chosen was that the crucial zones could be adequately marked and thus allows the allocation of the appropriate frame in order to monitor the strain of each welded joint zone individually.

The goal of the research presented is to combine reverse engineering methodologies with a numerical approach to analyse the structural integrity of artificial hip implants made of Ti-6Al-4V alloy subjected to different types of loads. In this way, numerical models validate the adopted methodology for obtaining implant geometry using 3D scanning, while also providing valuable insight into the behaviour of hip implants under different static loading cases. Since 3D scanning is proven as efficient and reliable for obtaining accurate geometry of various types of implants, it is applied in this research. Following a detailed development of a hip implant model geometry, involving 3D scanning and refining the obtained point cloud to a level that would realistically represent the actual hip implant, numerical models are made based on the obtained geometry. Results of these simulations using the finite element method in ANSYS (R) software have provided realistic values of stresses in most critical areas of the hip implant. The precise value of load that would produce plastic strain on the implant is also determined and is used as the limit criterion for selecting load cases for further analysis. This analysis would involve the assessment of fatigue life of hip implants with the same geometry and the same material while assuming the presence of a crack in the most critical area.