Loosening mechanisms and preload prediction of chassis threaded fasteners under transient loads

The operational reliability of automotive systems relies heavily on the structural integrity of chassis threaded fasteners. However, under dynamic road impacts, these fasteners frequently experience severe preload loss, leading to joint failures and structural degradation. This paper proposes a systematic framework to predict fastener loosening under vehicular conditions. First, analytical boundaries for frictional slip and localized plastic yielding are established. Next, a detailed finite element analysis (FEA) is conducted to uncover the loosening mechanisms. Results show that under a 1 g axial transient impact, stress concentrations propagate from the first to the second and third engaged threads within 8 ms. Furthermore, single transverse impacts induce a relative residual nut rotation of up to 15 mrad, triggering a combined degradation mode of rotational slip and cumulative plastic deformation. Based on these mechanisms, a nonlinear dynamic model is developed. This model directly links external accelerations to internal energy dissipation, enabling the cycle by cycle prediction of the residual clamping force. Finally, the framework is applied to a representative M12 Grade 8.8 chassis assembly using 1600 s measured road spectra cycles from the Dingyuan proving ground. The engineering evaluation reveals a strict tradeoff effect regarding the initial preload: while excessively high preloads suppress macroscopic slip, they exacerbate root plasticity, causing the clamping force to drop by over 20% prematurely. An optimal initial preload of 20 kN is identified, which balances frictional resistance and plastic damage, ensuring the clamping force remains above 80% of its initial value over the 5000 h design life. This study provides a strong theoretical basis and practical guidelines for the anti-loosening design of critical chassis components.