One of the most exciting graphs in the engineering. It shows at the same time the compression of the clamped elements and the elongation of the screw, shifting them on the horizontal axis so that they cross each other at maximum pre-load. Supposing that due to external load the screw and the compressed elements share this load in the same proportion, as much the elongation of the screw grows as much the compression of clamped elements decreases (into minus). It is easy to illustrate the effect of the external load on the joint by the elongation of the line in the diagram which represents the compliance of the screw. The cyclicality of the external load causes repetitive stress and strain. It can be easily accepted that it is essential to define the exact stiffness and resilience of the screw and the compressed elements to be able to calculate the exact impact of the stress/fatigue load. Since the amplitude that we are looking for is only a small part of the pretension force, only a small change in the applied parameters can have a huge impact on the screw’s estimated strength. It can be seen in the case of finite element simulations as well that a small change of the iteration interval is enough to calculate a different lifespan to the same bolt on the same model, with the same load. One of the reasons is friction, which you can read about here. Considerable experience is necessary to conduct reliable analytical calculations and FE simulations on bolted joints.