Influence of creep on preload relaxation of bolted composite joints: Modeling and numerical simulation

Preload relaxation can greatly influence the durability of bolted composite joints. This paper focuses on the effects of material creep and contact creep between rough surfaces on preload relaxation. An efficient finite element model is proposed to predict bolt preload relaxation in ABAQUS. First, a creep constitutive model of the composite based on elastic-viscoplastic theory was established and the contact stiffness of uneven interfaces of the joints, featuring multi-asperity contact at the micro perspective, were derived using fractal contact theory. Then, fractal contact theory was extended to address the time-dependent elastic-viscoplastic contact problem. The models were implemented within ABAQUS software using the UMAT and UINTER subroutines. Finally, a comparison of the numerical and experimental results showed that calculation error decreased from 1.4–2.0% to 0.1–0.5% after 35 h when effects of rough surface contact were considered, thereby significantly improving model accuracy. Furthermore, the results suggest that thicker jointing materials could effectively reduce preload relaxation. In addition, according to surface fractal parameters D and G, rougher surfaces facilitate contact creep and preload relaxation. Thus, controlling surface topography could be an effective way to improve surface contact properties in practical applications.