The bolt strength is determined based on the concentrated bolt stress at the thread roots. The allowable stress is determined so that the thread root will not yield by the pretension and the external loads, using the stress concentration factor obtained as 3 to 5 from experiments. However, the concentration factor is not clear so far, as it is quite difficult to measure the stress at such a localized region. On the other hand, structural analysis, namely finite element analysis, has the possibility to provide the most-likely stress at the thread root. In this paper, a special technique, a.k.a. submodelling, is used to calculate the stress distribution at thread surfaces very precisely. The result will be useful to solve any stress related problems.

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