Research On Bolt Connection Quality Detection Technology Based On Nonlinear Damping Identification

The quality of bolted joints is the key to the quality of mechanical structure assembly.It is important to study fast and effective detection technology.In this paper,based on the conservation of energy,the nonlinear damping identification formula of the bolted contact surface is derived.The sensitive natural frequencies of the system for damping identification are determined,based on the combination of system dynamics and Empirical Mode Decomposition(EMD).The L-beam bolt connection model and the casing bolt connection model are used as the research objects in the bolt connection quality test was carried out with.Firstly,the distribution of stress characteristics of bolted joints are analyzed by the finite element analysis.The finite element simulation results and modal experimental results are compared to determine the sensitive natural frequencies of damping identification of different experimental objects.The L-beam is used as the research object,and the viscous damping ratio and nonlinear damping ratio of the system are obtained by using the Hilbert transform and the established nonlinear damping identification formula.It is shows in the research that the change of bolt torque has little effect on the natural frequency of the system,but has a significant influence on the damping of the system,especially the viscous damping ratio.When the bolt is loose,the viscous damping ratio of the system will be significantly nonlinear.The change,from 0.1225 to 0.2587,increased by 111.18%,thus verifying the effectiveness of detecting bolted joints based on nonlinear damping changes.The damping and frequency of the casing bolt connection are found to be the same as those of the L-beam when the number of loose bolts is increasing.In terms of frequency variation,loose bolts at the region with ribs will cause greater frequency variations than loose bolts at the region without rib.In terms of damping variation,the damping caused by the loosening of the bolts at the position without ribs is greater.But no matter at which region the bolt is loose,the change of damping is significantly greater than the frequency.In addition,this paper also compares the data detected by different detection points when the machine bolts are loose.It was found that due to the reinforcing effect of the ribs on the structure,the data detected in the casing bolt connection at region without the ribs is more sensitive.Therefore,the detection point should be selected away from the ribs when testing the quality of the bolt connection.