Investigation of the effect of pre-tightening forces on bolted connection for FRP-steel joints

This paper experimentally and numerically investigates the effect of pre-tightening forces on bolted connection for double-lap FRP-steel joints, considering varied levels of pre-tightening forces. In the experimental phase of this study, six bolted double-lap pull-pull specimens with varying pre-tightening degrees (preloaded of 0, 3, 6, 7, 8, and 9 N.m, respectively) were examined. Test results indicated that all specimens exhibited bearing failure of FRP plates and the load capacity was significantly influenced by the pre-tightening forces applied through bolts. The highest loading capacity (27.1 kN) was recorded for the specimen with a preloaded of 6 N.m. In order to study the stress distribution of FRP plate for the bolted specimens, three-dimensional finite element (FE) models were conducted. Based on the FE analyzing results, the pretightening coefficient P-c (the coefficient of the preload increment per pre-tightening torque of bolts) of 750 m(-1) was concluded and further proved by the analysis of the stress in contact zones for specimens preloaded by 0, 3, and 6 N.m.