# Assessment of the Durability of Threaded Joints
## 6 Conclusions  
Based on the results, we make the following observations:  
1. Short cracks (up to 0.5 mm deep, up to 5 mm long) do not reduce the static strength of threaded connections. This is the size of the crack easily measurable by nondestructive inspection methods, which can be considered as the maximum allowed under service conditions. 2. The crack propagation characteristics are different in cyclically bending and tensile threaded joints. At a certain fixed nut-and-pin position (the bending plane is aligned with the most loaded position in the thread recess), the crack is one-sided. Otherwise, i.e., at any other position of the nut relative to the stud, the crack is two-sided. The adaptability of the crack decreases with the progression of a two-sided crack, i.e., the crack is more dangerous. 3. In prestressed and cyclically bending threaded joints, a relationship was found between the loosening and the parameters of the failure process, such as the shape and depth of the crack. The propagation of the crack is mainly due to the energy of the cyclic bending, so the stresses in the propagation plane of the crack increase as the stress is only slightly reduced. When the crack reaches a critical size, the rest of the crack breaks rapidly due to stress after a small pulse of cyclic bending energy. 4. In order to obtain a stable tension of the threaded joint and thus a stable clamping of the elements to be joined, it is necessary to apply a repeated (3-4 times) turn of the nut, irrespective of the method used to assemble the threaded joint. This prevents stress reduction during the assembly process. 5. Shakedown analysis of the threaded joints was performed. The safety margin of the progressive shape change in the experimentally tested joints was determined, and a statistical evaluation was performed. The safety factor ranged from 1.06 to 1.79 for the joints analyzed, with the probability varying from 1% to 99%. 6. Where the tightening was close to 0.8\(\sigma_{y}\), (\(\sigma_{y}\)--yield strength) and the bending was 0.4\(\sigma_{y}\), the safety factor for the progressive change in shape was close to 1. These safety factors are considered to have insufficient reliability in the elastic-plastic area analyzed. 7. The statistical investigation carried out as part of the study showed that to develop a reliable definition of the safety margin for progressive change in shape in threaded joints, an experimental curve of 50% must be developed, which, in the case analyzed, reflects a safety factor equal to 1.322. **Author Contributions:** Conceptualization, Z.B. and M.L. ; methodology, Z.B. and M.L. ; software, V.L. ; validation, Z.B., M.L. and V.L. ; formal analysis, Z.B., M.L. and V.L. ; investigation, Z.B., M.L., V.L. and L.R. ; resources, L.R. ; data curation, Z.B., M.L, V.L. and L.R. ; writing--original draft preparation, Z.B., M.L. and V.L. ; writing--review and editing, Z.B., M.L. and V.L. ; visualization, V.L. ; supervision, Z.B., M.L., V.L. and L.R; project administration, L.R. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** This article does not contain any studies with human participants or animals performed by the author. **Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article. **Conflicts of Interest:** The authors declare no conflict of interest. **Nomenclature**