Optimal design of prestressed bolt thread

The design of bolt and nut thread comes in many variations, typically the designs are specified in standards like the metric ISO thread. The thread design is important for the connection strength, and bolted connections are used for many applications. In the present paper focus is on the strength of prestressed bolts, that is, where the preload is typically more than 75% of the proof strength. The objective is in the present work to reduce the stress concentration in the thread. The stress is found numerically using a full 3D finite element model, and a simplified 2D axisymmetric model. It is shown that the 2D model is sufficient for evaluating the maximum stress. Applying a symmetric design it is shown that a reduction in the maximum stress in the thread of 14% can be achieved. Allowing for an unsymmetrical thread design the maximum stress is reduced by 33%. With the unsymmetrical design some of the stress reduction is due to a more evenly load distribution along the thread. geometrically, all optimized designs are such that they can be manufactured by the same methods as a standard ISO thread.