# Assessment of the Durability of Threaded Joints  
[PERSON]  
1 Department of Transport Engineering, Faculty of Mechanical Engineering and Design,  
Kauras University of Technology, 51424 Kaunas, Lithuania; [EMAIL_ADDRESS] (Z.B. ); [EMAIL_ADDRESS] (L.R. )1 Department of Mechanical and Materials Engineering, Faculty of Mechanics,  
Vilrius Gediminas Technical University (VILNIUS TECH), 03224 Vilnius, Lithuania; [EMAIL_ADDRESS]  
[PERSON]  
2 Department of Mechanical and Materials Engineering, Faculty of Mechanics,  
Vilnius Gediminas Technical University (VILNIUS TECH), 03224 Vilnius, Lithuania; [EMAIL_ADDRESS] (C.  
If a structure is free of defects or defects have occurred during operation, the defects shall not reach the critical values to ensure safe operation of the structure. Evaluation of the degree of defects in threaded joints is a complex process that requires both theoretical and experimental investigation after the identification of the defect or crack [8; 9; 10]. An increase in the dimensions of the threaded joint may result in not only greater dimensions of the assemblies but also considerable changes in the force flows within the assembly as a result of which the critical internal forces may increase instead of decrease. The degree of loading on the connecting elements of the pressure vessels and the effect of adjacent parts of the actual structure can lead to plastic deformation in the threaded joints. In the event of cyclic variation thereof, the parameters of the non-elastic strain process limit the service life of the structure. If the residual strains resulting from plastic flow at the beginning of loading no longer accumulate after a certain number of cycles, this means that the structure has adapted to the loads [11; 12; 13]. The research articles are mostly focused on axial tensile fatigue questions [14; 15; 16]. [PERSON] et al. [17], to investigate the fatigue performance of M24 grade 8.8 high-strength bolts in constant amplitude, tested 21 high-strength bolts in axial tensile fatigue at a stress ratio of 0.5. Based on the experimental results, S-N curves were obtained by regression analysis, and a fractographic analysis was performed on representative specimens. The results were compared with existing experimental results and standards for fatigue. It was concluded that the area of the fatigue propagation region is essentially inversely proportional to the stress ratio. [PERSON] et al. [18] presented a meta-study on bolts and bolted connections where a few thousand fatigue tests on these elements were evaluated. The results of this study have been implemented in the new revision of the European standard EN 1993-1-9. This paper provides the background for the modifications. [PERSON] et al. [19] mainly studied the low-cycle fatigue life of the pre-tightened bolts working at a high temperature. A novel test fixture was designed for fatigue tests, and low-cycle fatigue tests of pre-tightened bolts were conducted at the temperatures of 550 \({}^{\circ}\)C and 650 \({}^{\circ}\)C. The research results provided a theoretical basis for the low-cycle fatigue life prediction of pre-tightened bolts. The article published by [PERSON] et al. [20] is insightful as it deals with the issues of fatigue strength improvement and the anti-loosening performance of JIS M16 bolt-nut connections. Three different root radii were considered coupled with three different pitch differences. The extension of the root of the radius of the bolt root improved the fatigue limit of the bolt by more than 30% because both the stress amplitude and the mean stress can be reduced. Other articles deal with anti-loosening problems [21; 22; 23; 24; 25] or analyze the performance of specific threaded joint fatigue strength [26; 27; 28; 29]. Some research focuses on the application of finite element modelling to the study of threaded joints. [PERSON] et al. [30] provided analytical calculations according to Verein Deutscher Ingenieur (VDI) Recommendation 2230 and numerical analysis using the finite element method (FEM).