# Assessment of the Durability of Threaded Joints
To verify the FEM analysis, the forces in the bolts were measured under preload and under service load on a test bench. On the basis of analytical and numerical results, the influence of the point of application of the working load on the bolt load and its fatigue life were analyzed for different cases. In [31] a finite element model was proposed for the variation of the wear profile of a threaded surface to simulate the self-loosening of the bolted joints under transverse loading. The finite element model successfully simulated the phenomenon by which the clamping force gradually decreases due to the loosening of the wear as the distribution and magnitude of the contact stresses between the threads change. The predicted results were in good agreement with the experimental results. The paper [32] presented the finite element analysis model applying the elastic-plastic finite element analysis for bolts under variable combustion pressure. The stress history was then used to calculate the stress intensity factors and the fatigue life of the bolts. The numerical results show that the existence of a crack at a depth of 0.35 mm is the source of premature fracture failure. Article [33] presents the self-loosening mechanism of the bolt in the curvic coupling due to structural ratcheting under cyclic load. Finite element simulations were performedunder several loading conditions with different preloads and cyclic torque loadings in the curvic coupling. Calculation methods for the cyclic strength of threaded joints in critical objects have not been sufficiently substantiated using theoretical and experimental investigations, or investigations have not been conducted at all. Plain, volumetric, and calculated model investigations using photoelasticity, the finite element method, and other methods do not reproduce the actual operating conditions of the threaded joints. The kinetics of short cracks in threaded joints has virtually not been investigated. The shakedown in one of the joint elements (bolt or stud) has not been investigated. Theoretical and experimental investigations in the area of resistance to cyclic failure and shakedown are insufficient to support the calculation methods and guarantee the strength of the structural elements. Therefore, improving cyclic strength and shakedown assessment methods is important to ensure the greater reliability and better occupational safety of objects designed and already in service. The paper followed the objective of determining the patterns of resistance of heavily loaded threaded joints and elements of pressure to low-cycle and multi-cycle failure and developing the improved, well-substantiated cyclic strength and shakedown calculation methods. The aim of the paper is: (i) to develop an original methodology for an experimental investigation of threaded joints; (ii) to experimentally explore the stress state of defective threaded joints and elements and the crack initiation conditions in the concentrator environment; to compare the experimentally investigated threaded joints to the normative calculations; to predict the patterns of crack initiation and propagation in threaded joints; (iii) to analytically and experimentally analyze the shakedown of threaded joints in view of the cyclic failure process parameters.