Phenomenological Modeling and Numerical Simulation of Relaxation in Bolted Composite Joints

The effects of various bolt preloads, viscoelasticity, and external applied static and dynamic loads on bolt load relaxation in a carbon/epoxy composite bolted joint have been studied. Both phenomenological modeling and finite element analysis (FEA) of bolt-connected three-point bending specimens were employed in the studies. Relaxation of 1.25—4.25% over a period of 30 h was observed depending on the initial preload and applied external loads. Both static and dynamic applied loads were considered. It was observed that for any magnitude of external load the bolt load relaxation decreases with increasing initial preload. These findings emphasize the importance of the magnitude of the preload. Comparing the bolt load relaxation in steel and composite joints for the duration of 30 h, it was concluded that only about 1/3 of the relaxation in composite specimens is due to viscoelastic behavior of the polymer matrix in the composite, and the remaining 2/3 of the relaxation is due to other mechanisms such as bolt thread slip, plasticity and/ or external excitation.