Monitoring The Connection State Of Bolted Spherical Joint In Space Structures Based On Time Reversal Method

Bolted spherical joints are widely used in long-span space steel structure engineering because of their strong adaptability,no additional eccentricity,avoiding a large number of field welding workload and convenient transportation and installation.However,in the practical application and monitoring process,bolted spherical joints are also faced with some difficulties,including:(1)Ignoring the stiffness of bolted spherical joints in the design stage;(2)Dislocation of bars and bolt false twisting in the installation stage;(3)Material aging inevitably occurs in the use stage;(4)Bolted joints are located in the interior of the joints and can not be observed directly from the outside.It is difficult for the traditional monitoring methods.Because the joint is the place where the stress is concentrated in the space structure,compared with the bar damage,the joint damage is more likely to cause safety accidents.Therefore,using piezoceramic sensors and time reversal technology to monitor the loosening state of bolted spherical joints in new and in-service space structures in real time and accurately is of great significance.At present,the research on bolted spherical joints at home and abroad mostly focuses on semi-rigidity,applicability,mechanical properties,construction and assembly,and little is known about the connection status of bolted spherical joints in service.Therefore,this paper proposes a method based on time reversal technology to monitor the connection status of bolted spherical joints.The theoretical analysis,numerical simulation and model experiment are used to study the connection status of bolted spherical joints.The connection status of bolted spherical joints is judged by extracting the peak value of the focused signal received on the piezoceramic sensor(PZT):(1)Based on the contact theory of bolt joint surface,the relationship equation between contact stiffness and contact area function is deduced by analyzing the contact state of contact surface,and the relationship model between contact area and connection state is established.It is concluded that the contact stiffness of contact surface increases as the contact area increases.Because contact stiffness can reflect the connection state of joints,The change of contact area in the joint area can represent the connection state of thejoint.(2)The finite element model of bolted spherical joints is established,and the simulation of bolt preloading force is carried out.The change trend of contact area of joints with bolt preloading force is obtained.The relationship between joint state and contact area of joints is deduced,and the correctness of theoretical deduction is further verified.(1)Firstly,the finite element model of single bolted spherical joint is established according to the experimental model,and the preloading force is gradually applied to the bolt,which is incremented by 2.5 k N and applied to the end of 20 k N.Then,the contact area change cloud map of the joint connection area is derived.Through comparative analysis,it is found that the contact area of the joint connection area increases with the increase of the bolt preloading force.(2)Summarizing the results of numerical simulation,it is concluded that with the increase of bolt preload,the contact area of joint connection area becomes larger and larger,and the connection of joint becomes closer and closer.This is consistent with the theoretical deduction of the contact surface of bolt joint.It can be concluded that the connection state of the joint can be reflected by the contact area of the joint area.(3)Through the theoretical deduction of the time reversal focusing principle,it is obtained that the peak value of the focusing signal,the transfer function of the signal,and the contact area of the connection area are in turn positively correlated,which can be obtained: according to the peak value of the focused signal received by the sensor,the contact area of the joint connection area can be indirectly reflected.Taking into account the previous theoretical derivation and numerical simulation conclusions,that is,the contact area through the joint area can reflect the connection state of the joints.Combined with the deduced results of the time reversal focusing principle,it can be obtained that the connection state of the joints can be monitored by extracting the peak value of the focusing signal on the sensor.(4)Based on the above theoretical basis,it is proposed to use the time reversal technique to monitor the connection state of the bolted spherical joints.A scale model of atriangle-on-triangle offset grid is designed and fabricated,and eight damage conditions under different torque values are defined.The effectiveness of the proposed method is verified by model experiments.The details are as follows:(1)The experiment is carried out on the upper chord,i.e.the No.5 bar,which is connected with the center bolted spherical joint.The piezoceramic sensor PZT1 is pasted at the central bolted spherical joint as the generator,and the piezoceramic sensor PZT2 is pasted at end of the bar near the central bolted spherical joint as the sensor.The electrical signal of PZT1 is stimulated,and the stress wave signal is detected to be transmitted along the bar.After receiving by the sensor PZT2 on the bar,the time is inverted,and the signal is re-transmitted along the original path.Finally,the signal is focused at PZT1.The stress wave signals under eight kinds of torques are collected in turn.After signal conversion and data processing at PC,the stress wave focusing signals of bar 5 under eight kinds of damage conditions are obtained.It was found that the focusing peak value increased with the increase of the torque value,and the trend of the broken line gradually became flat.Through the analysis of the experimental results,it can be concluded that with the increase of the torque value,more and more stress wave signals are received by the sensor,and the focus peak value is getting larger and larger,which is close to the healthy state,and the connection of the joints tends to be close.(2)In order to verify the effectiveness of the method,five repetitive experiments were carried out on the other five upper chords connected with the central spherical joints in the experimental model.The stress wave signals received by each bar under different torques are collected sequentially.It is found that under each torques,the signals show different degrees of focusing.The peak values of the focusing signals under different torques are extracted and the images are drawn.Through the images,it is found that the peak values of the focusing signals of the five bars are different under different torques,but with the increase of the torques,the peak values of the focusing signals are all the same,there is an upward trend.It can be concluded that with the increase of the torque value,the signal received by the sensor becomes more,the peak value of focus becomes larger,and theconnection of joints becomes closer.The results of repeatability experiment and bar 5experiment are basically the same,Therefore,the validity of the experiment is further verified.(3)In order to clearly characterize the different loosening states of joint connections,based on the stress wave energy signals received by sensors,the damage index,loosening index,is defined as RMSD(Root-Mean-Square-deviation)of the received stress wave energy.The loosening index of the energy received by the sensor under different tightening degrees is calculated,and the change maps of the loosening index of six bars under different torque values are drawn.Through the image,it is found that with the focus signal at 20 lb·ft state as the reference line,with the increase of loosening index,the parallel measurement values at other torque levels are farther and farther away from the reference line,and the data differences are more and more obvious.The increase of loosening index means that the joints are becoming looser.With the increase of loosening index,the difference between the looser state and the healthy state is getting bigger and bigger.Therefore,the change of the loosening index can represent the different loosening states of the joints.In summary,through theoretical analysis,numerical simulation and model experiment,the state monitoring method of bolted spherical joints based on time reversal technology is studied.For structures under construction and in service,monitoring signals are often affected by uncertainties such as environment,load and human factors.Time reversal technology has a certain anti-noise and good robustness.Therefore,the monitoring method proposed in this paper has a certain reference value for the health monitoring of bolted spherical joints in practical engineering.