Research On Strand Tension Recognition Based On ARX Identification Model

Prestressed steel strands are widely used in civil engineering buildings.They are also the most important mechanical components of long-span bridges,such as the main prestressing tendons of girder bridges,Suspenders of arch bridges and cable-stayed bridges.The steel strand is in a high stress state for a long time,which is very sensitive to environmental erosion and structural creep.The actual retained stress value often decreases,resulting in structural cracking,deflection and other diseases,reducing the bearing capacity and durability of long-span structures.Usually,the steel strand is in layer-by-layer protection because of its anticorrosion needs.It is wrapped with concrete or high density polyethylene sheath.As its anticorrosion performance improves,it also increases the difficulty of daily detection and monitoring of the steel strand.In-service steel strand stress detection and evaluation has always been a technical problem in the field of civil engineering.The relatively mature steel strand stress measurement technology focuses on the detection of stress increment and needs to be embedded in the early stage,which can not meet the application needs of a large number of in-service prestressing and cable structures.In this paper,the propagation model of ultrasonic guided wave in steel strand is established based on ARX system identification method,and the propagation model coefficients of guided wave in different tension states are extracted to establish the identification of tension force of steel strand.This provides an effective method for the study of propagation characteristics of guided wave in complex waveguide structures,and also provides a non-destructive method for ultrasonic guided wave in steel strand.The application of detection provides theoretical support.Firstly,guided wave propagation experiments under different stress conditions are carried out,and the guided wave propagation process in steel strand is assumed to be an independent system.Then,a more sensitive tension identification index is constructed by identifying the variation of model parameters with the stress of steel strand.Finally,the sensor placement and steel strand are discussed.The effects of line loading,unloading and different excitation frequencies are discussed.The following conclusions are drawn:（1）The system identification method can effectively identify the change of system guided wave propagation characteristics caused by strand tension.Taking ARX system identification model parameters as eigenvectors,the identification index I_stf of prestressing strand tension is constructed.With the increase of strand tension,the identification index values show obvious monotonous linear law.Linear fitting by least squares has good results.（2）The functional relationship between tension F and measured index I_stf based on finite element data modeling and experimental data is very small.Because attenuation is not considered in the finite element simulation process,attenuation has little effect on identification index,and the obvious change of longer strand is attenuation problem.Therefore,it is feasible to apply the identification index of the model to longer strands in practical engineering.（3）The tension identification index I_stf based on the measured guided wave data at the end and the side presents obvious monotonic linear variation with the increase of tension.Relatively speaking,the linear fitting k of the sensor arranged at the end increases by 8.6%,the determination coefficient increases by 5.5%,and the sensitivity and fitting degree of the index value are better.（4）The identifying index has good consistency in the process of repeated loading and unloading.The k value of straight line in the two loading and unloading cycles decreases by 1.3%and 0.9%respectively in the loading and unloading stages,and the identifying index is less affected by the loading path.（5）The I_stf of steel strand tension identification index still has the same linear variation rule under different length guided wave propagation distance.In comparison,the sensitivity coefficient k of strand tension identification index with a length of L=6.0m and a length of L=5.5m respectively increased by 5.0%and the certainty coefficient R² increased by 4.5%.The research results showed that the longer the guided wave traveled,the richer the tension information carried and the more significant the tension identification index.（6）The identification index of tension force of strand under different single frequency excitation still has the same linear variation law.In comparison,the tension identification index I_stf of the two groups with a single excitation frequency of 200k is larger than the identification index k and R² of 300k and 250k.Therefore,the experimental results show that when the single excitation frequency is 200k,the identification index is the most obvious,followed by the single excitation frequency of300k and 250k.When the excitation frequency is limited in the state of hp,the single excitation frequency of 200k can be preferred.