基于应力转换的内螺纹高周疲劳强度分析方法

Stress-conversion-based Approach of High-cycle Fatigue Strength Analysis for Internal Threads

螺纹具有特征尺寸小、结构复杂的特点,其非线性接触的有限元分析具有很高的难度,因此机械零件强度仿真分析时螺纹通常被忽略,只根据经验对螺纹作保守设计。事实上承受循环重载的零件极容易从螺纹部位发生疲劳断裂,在设计阶段有必要进行疲劳强度校核。针对此问题,提出了一种基于应力转换的内螺纹高周疲劳强度分析方法。此方法把螺纹根部应力分解为近源应力和远源应力两部分,通过无螺纹简化模型仿真获得螺纹接触压力和摩擦力之后转换为近源应力和远源应力并叠加得到螺纹根部最大应力,最后考虑应力梯度、存活率和表面粗糙度等因素的影响系数,借助Haigh图计算各扣内螺纹的疲劳安全系数。把这种方法与带螺纹的细节模型有限元仿真和疲劳强度分析结果进行对比,所提方法的安全系数结果与细节模型的相应结果相差–7.6%;把这种方法应用于某型号缸体主轴承壁,其分析结果与疲劳试验结果相差–8.2%,证明了它的可行性。

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.