Contact stiffness evolution mechanism and experimental study of bolted joints under repeated tightening

Accurate analysis of bolted joint stiffness is critical for assessing the dynamic characteristics of mechanical equipment structures. Under repeated tightening conditions, thread contact stiffness is the predominant factor affecting joint stiffness and structural dynamic response. This study establishes a spring oscillator model for bolted joints incorporating thread contact stiffness. A computational method for asperity stiffness on contact surfaces is developed by integrating Hertz contact theory with a cosine contour model. The thread surface topology is characterized using non-contact 3D laser scanning microscopy, enabling acquisition of high-resolution point cloud data. The analysis of point cloud data reveals a progressive enhancement of contact stiffness with increasing tightening cycles. A hammer-impact modal test is conducted to study the vibration response of the bolted joint structure. The frequency shifts in structural modes reveal the effects of repeated tightening and torque variations on bolted joint stiffness. Experimental validation confirms the consistency between the proposed model and the contact stiffness of bolted joints.