U-bolted joints are widely used in heavy-duty vehicle suspension systems to ensure structural integrity under dynamic service conditions. The preload applied during assembly is the key factor that keeps the joint secured and prevents premature separation. However, under repeated transverse loading, preload can gradually decrease, leading to self-loosening of the joint and potential safety risks. This study aims to develop a predictive framework for identifying the self-loosening behavior of unsecured U-bolted joints, focusing on preload degradation under realistic transverse loading conditions. A finite element analysis (FEA) model representing a typical U-bolt connection between a tube axle and suspension system was developed. Cyclic transverse displacements, derived from combined vehicle operating loads, were applied to simulate vibration-induced loosening. The preload decay trend was monitored and extrapolated to determine the loosening threshold. The numerical results reveal a clear progressive reduction in preload under cyclic shear loading, allowing estimation of the vibration cycles leading to critical torque loss. The study demonstrates that unsecured U-bolt joints exhibit predictable loosening behavior when subjected to sustained transverse excitation. The novelty of this work lies in introducing a cycle-based predictive FEA framework specifically tailored for unsecured U-bolted joints under realistic multi-directional service conditions. By correlating numerical predictions with representative road test behavior, the approach enables reduction of costly experimental campaigns while improving the reliability of maintenance planning. This engineering-oriented methodology supports early detection of loosening risk, establishment of optimized re-torque intervals, and implementation of proactive predictive maintenance strategies, thereby enhancing the durability and safety of heavy-duty vehicle suspension systems.
Fatma Dilay AKSOY (Tue,) studied this question.