Optimizing fatigue life prediction of high strength bolts in bolted spherical joints

Bolted spherical joints (BSJs) are integral to large-span space grid structures commonly used in expansive public and industrial buildings. BSJs are susceptible to fatigue damage in high strength bolts due to variable random loads. This study summarized findings from prior fatigue tests, detailing the fatigue failure patterns and S-N curves for grade 10.9 M20, M30 high strength bolts, and grade 9.8 M39 high strength bolts in BSJs. By employing finite element analysis, the study determined the stress concentration factors for these bolts, which were then utilized in fatigue life estimations using the Heywood model and the local strain approach (LSA). Both methods could yield satisfactory predictions, but when it came to the Heywood model, particularly when modifying it in areas of high stress, it demonstrated superior accuracy in predicting the performance of M20, M30, and M39 high strength bolts. Additionally, incorporating initial defect considerations, fatigue life was predicted using the damage tolerance design method (DTDM). Finally, when comparing the fatigue life prediction results obtained from various methods, it became evident that the modified Heywood model outperformed the other methods. This not only provided a superior solution but also demonstrated user-friendliness, thereby establishing itself as the preferred option for predicting the lifespan of high strength bolts in BSJs.