Abstract: Threads are small in size and complicated in structure, and cause great difficulty in finite element analysis of the nonlinear contact interaction. For this reason, threads are usually neglected during the strength simulation analysis of mechanical parts. In fact, parts bearing heavy cyclic loads usually fatigue and fracture from the internal threads, thus their fatigue strength is necessary to be checked in the design stage. To solve this problem, an approach based on stress conversion is proposed to analyze the high-cycle fatigue strength of the internal threads. In this approach, the stress at the thread roots is decomposed into near-source stress and far-source stress. The contact pressure and friction calculated from simplified model without threads are conversed into near-source stress and far-source stress and then superposed into the maximum stress of thread roots. Afterwards, taking the influence of stress gradient, survival probability and surface roughness into account, the fatigue safety factor of the internal threads are calculated employing Haigh diagram. Comparison is conducted between the proposed approach and the finite-element simulation and fatigue analysis of detailed model with threads, and shows the safety factor result of the proposed approach is 7.6% lower than the corresponding result of the detailed model. The proposed approach is also applied to a main bearing wall of a crankcase, its results is 8.2% lower than the fatigue test result, validating its feasibility.

Key words: fatigue strength, safety factor, stress concentration, internal thread