# Assessment of the Durability of Threaded Joints
## 5 Discussion  
To evaluate the structural performance of components, it is not sufficient to schematize the actual parts; their overall deformation and the local stress-strain behavior need to be considered, using dependencies that relate the displacements of the model to the external loads due to the occurrence and propagation of a fatigue crack, to the accumulation of plastic deformations, or to the adaptation of the component to cyclical effects, which are influenced by the interaction between the components.  
The experimental and theoretical investigation of the resistance of threaded joints to low-cycle and multicycle loading shows that the existing calculation methodologies are not sufficiently valid, and therefore the existing methods of investigation have now been refined and improved, taking into account structural, technological, and operational factors, using decay mechanics criteria and theories of shakedown.  
The analytical dependences of plastic failure and progressive shape change for studs without a crack, taking into account parameters of the failure process obtained in this work such as crack shape, depth, and stress reduction have been validated experimentally. The plastic degradation limit state is unrealistic in threaded joints used to join assemblies and flanges.  
The analytical shakedown conditions are accurate as they are obtained using static and kinematic methods, i.e., the result is approached from two sides. The condition of progressive change of shape can be used as a limit state, which occurs in the boundary layers of the stud if the sum of the tensile and bending stresses is close to 1,2. When this condition is exceeded, unidirectional accumulation of plastic deformation begins.  
Using the analytical expressions obtained and the shakedown diagrams derived therefrom, the paper presents a new methodology for the calculation of the margin of progressive change in shape for threaded joints without a crack and with one-way and two-way cracks.  
A completely new methodology for the calculation of the progressive change in shape is suggested to be performed prior to the calculation of the cyclic strength to form, together with the resulting or refined dependencies, a unified methodology for the calculation of the cyclic strength and the shakedown of the responsible threaded joint.  
In the paper the shakedown analysis of the threaded joints was performed using the static and kinematic methods, i.e., the result was approached using a two-sided tech  
Figure 20: Statistical assessment of threaded joints with a two-sided crack: \(\bullet\)–experimental data.  
nique. The shakedown conditions were obtained for crack-free studs in view of the experimental data, i.e., the failure process parameters such as the shape, size of crack, and stress reduction.  
Tightening and cyclic bending of the threaded joints may cause accumulation of plastic strain in the outer layers of the stud. In case a favorable residual stress field has developed, the cyclic plastic strain does not accumulate, i.e., the shakedown takes place.  
The limit state conditions determined for crack-free studs and studs with one-sided and two-sided cracks show that studs with two-sided cracks have the smallest safety margin for progressive shape change.  
Calculation of the cyclic strength and shakedown is an important stage in the design of the critical threaded joints and connecting elements and crucial for service life extension. The calculation methods designed and improved within the study were based on theoretical and experimental investigations and simplified for convenient application to engineering practice.