Loosening and fracture behavior of hybrid titanium-to-steel threaded connection under cyclic loading condition

Hybrid threaded connection involving titanium alloy and conventional steel components is introduced and studied in the face of lightweight metals application trend in modern machinery design. Pre-tightening procedure and loosening test were conducted based on specially designed specimen regarding cyclic pulling load condition. The loosening and fracture behavior were intentionally investigated through both mechanical analysis and related microstructure obser-vation. The loosening evolution tendency along with the corresponding low cycle fatigue process was obtained, which was then further related with the examined structural mechanical responses and thread wear mechanism. Results have shown that in the early stage of fatigue process the pretension would experience a rapid and mass reduction accompanied with a high rate cyclic hardening response of structure. The cyclic load induced thread wear occurred mainly in the titanium nut component, which involved abrasive wear, fatigue wear and oxidative wear mechanism. Besides, the thread root turn closest to the bearing surface was demonstrated as major position of stress concentration and strain accumulation, making it critical source for loosening initiation and fatigue fracture, which should be properly optimized in both designing and fabricating process.