Relaxation behavior of clamping force in bolted joints of pultruded GFRP laminates

The relaxation behavior of clamping force in friction-type FRP bolted joints is one of the significant subjects in application of FRP to structures. The aim of this paper is to obtain the factors of relaxation behaviors in the GFRP bolted joints based on the experimental results. The relaxation behaviors are due to both steel bolt and GFRP laminate. Steel bolt dominates the relaxed force immediately after initial loading but not reloading. The relaxation factors of GFRP laminate except matrix polymer could be both laminate surface geometry and laminate bond, considering the variance of relaxed force and more relaxed force for bonded laminate than single laminate observed in the experimental results. Boltzmann superposition principle can be hardly applicable to the relaxed forces for initial loading and reloading, suggesting that less relaxed force for reloading could be obtained by the strain hardening of bolt threads through the decrease of cross section area due to shear plastic deformation and that surface geometry could be modified by plastic and irreversible viscous deformation considering the viscoplastic property of matrix polymer. More relaxed force could be obtained for the surface geometry farther from the flat surface and the surface geometries could be categorized into the surface with either equal height protuberances or random height ones. Laminate bond dominates the relaxed force and its lower viscosity could give relaxed force more. The viscosity of laminate bond could be more variable than matrix polymer and increase through the modification of structure by plastic and irreversible viscous deformation. The minimum elapsed time could be required to complete the relaxation behavior for the flat surface and the bond viscosity as high as matrix polymer. ©2010 The Society of Materials Science,.