# Assessment of the Durability of Threaded Joints
## 3 Experimental Results and Their Analysis
### Influence of Tightening and Cyclic Bending  
An investigation of the patterns of force distribution was carried out according to the testing scheme depicted in Figure 2. The loading modes of the investigated threaded joints were (1) \(\sigma=(0.6\pm 0.35)\sigma_{y}-10\) cycles; and (2) \(\sigma=(0.6\pm 0.45)\sigma_{y}-200\) cycles. The force distribution after tightening the threaded joint to \(\sigma_{t}=0.6\sigma_{y}\), as seen in Figure 3, corresponded to the characteristic general pattern of the joints subjected to tension (tensioned stud and compressed nut). The strains that occurred in the tensioned layers due to  
Figure 2: Scheme (**a**) and operating principle (**b**) of the special designed bending equipment. bending in the turn cavities during the first load cycle also corresponded to the general patterns of force distribution of the tensioned joints. The general stress and nut effect in the compressed layers caused distortion in the known patterns. The strains of the tensioned region of the stud in the turn cavity located at the pitch distance from the nut are also shown in Figure 3, which depicts a free turn. The experiment showed that the increase in stress caused by bending from \(0.35\sigma_{y}\) to \(0.45\sigma_{y}\) proportionately did not lead to an increase in forces in turns 1 and 2 of the threaded joint. Force distribution patterns did not change during further application of strain up to 200 cycles. Maximum stress was obtained in the bending plane regardless of the position of the nut-stud with respect to each other. However, the position of the nuts had an effect on the conditions of initiation and propagation of the crack. In view of the force distribution, one of the nuts was placed in the appropriate position, while another was positioned according to the tightening value. The value of looseness was important for both individual and group joints, as it led to lower tightening of the connected parts and a change in the loading conditions on the assembly. Figure 4 presents the experimental data for the threaded joint. Tension stress in the tested threaded joints varied from \(\sigma_{t}=0.95\sigma_{y}\) to \(\sigma_{t}=1.2\sigma_{y}\). The maximum tension stress values caused by the bending varied from \(\sigma_{b}=0.35\sigma_{y}\) to \(\sigma_{b}=0.55\sigma_{y}\). In the case of initial stress \(\sigma_{t}>0.95\sigma_{y}\), a more pronounced reduction in stress was observed at the beginning of the cyclical strain. In the case of a strong tightening \(\sigma_{t}>\sigma_{y}\) (curve 3, Figure 4) and single-time stud tension during joint assembly, stress was considerably reduced during the first cycles. Further cyclic bending to crack initiation was virtually insignificant for the stability of tightening. Residual strain was recorded in the failure layers, outlying layers. The durability of the studs was reduced both in terms of crack initiation and in the terms of final failure in case of the same stress value and increasing bending stress. With the pitch increasing, the stress was reducing faster during the first load cycles. With the crack propagating, a more considerable reduction in the stress would not be observed in the case of increase in the pitch. The stress of the stud began to gradually reduce in the case of crack propagation (Figure 4, curves 1' and 2'). When the initial stress was \(\sigma_{t}\approx\sigma_{y}\), the tightening was considerably reduced during the first cycles. Subsequently, the stress remained stable during crack initiation, and the curves straightened out. The strains increased in their environment only after the crack had reached a certain value, while the stress reduced. The experiment showed that the increase in stress caused by bending from \(0.35\sigma_{y}\) to \(0.45\sigma_{y}\) proportionately did not lead to an increase in forces in turns 1 and 2 of the threaded joint. The force distribution patterns did not change during further application of strain up to 200 cycles.