The influence of the working load on the dynamic loading of the bolt was investigated in our study for two cases of flange joints. The analytical calculation according to the Verein Deutscher Ingenieure (VDI) 2230 recommendation and the numerical analysis using the finite element method (FEM) were performed for a model of a four-bolt joint. To verify the FEM analysis, the forces in the bolts were measured during preloading and during the application of the working load on the test rig. Based on the analytical and numerical results, the influence of the working load application point on the bolt load and its fatigue life was analysed for different cases. Comparison of the results shows that the analytical method overestimates the additional bolt stresses at low working load, mainly due to the extremely large fraction of bending stress. As the working load increases, the differences between the two methods decrease, but only for the reason that the analytical method can only linearly scale the overestimated results at lower working load, and FEM analysis, on the other hand, shows a progressive increase of the additional stress in the bolt at higher working loads due to the spreading of the flange. It is also shown that a high washer significantly increases the fatigue life of the bolt for two reasons: (i) a high washer reduces the additional stress in the bolt, and (ii) the high washer shifts the critical fatigue point from the thread area to the transition of the bolt shank to the head.

The amplitude of a bolt load in dynamically loaded bolted flange joints depends on several factors: the resilience of the bolt and the clamping parts, the magnitude of the working load, the point of action of the working load, the way the working load is transferred from the structure to the bolt, the preload, and the geometrical imperfections of the contact surfaces of the joint. These factors are analysed in many papers, and they are covered in the VDI 2230 guideline and in standards. Fatigue curves (S-N curves) of bolts are determined by tests in which an ideal axial load is usually applied to the bolts. The effects of the bolt strength class, the thread manufacturing process, the surface protection, and the cross-section size on the fatigue strength of bolts are precisely defined. The main problem in the evaluation of bolted joints is the calculation of the actual stress, which is compared with the fatigue curves. Despite extensive research, fatigue-related bolt failures still occur in practise. This article provides a systematic overview of the influences that affect the fatigue of bolts. The conclusions are based on the research results of many authors and on our own analytical, numerical, and experimental investigations. The effects are illustrated using two practical examples of flange bolting. The assessment of fatigue according to Eurocode 3 and the VDI 2230 guideline is discussed in more detail.