Flared tube fittings are extensively utilized in pipeline systems due to their effective connection and sealing capabilities. However, during practical service conditions, transversal vibration frequently induces thread loosening, subsequently leading to seal failure and other malfunctions. Current research lacks a systematic investigation into the loosening behavior of flared tube fittings under transversal vibration conditions. This study establishes a precise finite element model of the flared tube fitting and systematically examines its loosening behavior under stress redistribution, plastic deformation, and fretting wear conditions by simulating the assembly process and applying cyclic transversal vibration loads. The research findings demonstrate that the loosening process of flared tube fittings occurs in two distinct stages. The initial stage primarily involves preload reduction caused by non-rotational factors such as stress redistribution, while the subsequent stage features continuous preload attenuation resulting from relative rotation between internal and external threads. Notably, a critical amplitude has been identified.
When the actual transversal amplitude remains below this critical value, only non-rotational loosening occurs in the flared tube fitting, with no rotational loosening taking place. Further investigation into factors affecting the critical amplitude, including preload, friction coefficient, material properties, and thread type, reveals that preload, friction coefficient, and material elastic modulus significantly influence the critical amplitude, whereas thread type demonstrates a negligible impact. These findings provide valuable insights for enhancing the reliability of flared tube fittings in vibration-prone applications.