| Steel strands are the critical load-bearing components of prestressed concrete structures.It is necessary to measure the effective prestress,whether the formed components that have not yet been put into use or finished products that have been put into operation.In prestressed concrete structures,nondestructive stress detection of steel strands has attracted widespread attention in engineering and research fields.Because steel strands are embedded in reinforced concrete structures,it is very difficult to test the stress of prestressed steel strands with the dynamic stress measurement method,the ultrasonic stress measurement method and the stress release method,which are respectively suitable for the cable force measurement,the residual stress measurement and the exposed structures.Therefore,in this paper,a stress measurement method for steel strands based on magnetoelastic effect is explored,and the formula of stress-permeability-frequency mathematical physics(mechanics)relationship is deduced,and the stress measurement model of electromagnetic oscillation for steel strands based on magnetoelastic effect is established,and the model testing validation in laboratory is completed.Different from the conventional stress measurement methods of ferromagnetic steel based on the coupling effect of force and magnetism,in this paper,the effect of stress-induced magnetic property change in magnetoelastic effect is used.Without external magnetic field excitation,steel strands are directly set as component elements in LC oscillation circuit,and stress can be measured throuth the electromagnetic resonance frequency of the oscillation circuit.The main research contents are as follows:(1)Domain rotation equivalence of stress field and external magnetic field is analyzed,and a theoretical relationship between stress and permeability of steel strands is established.Permeability is taken as the main parameter of magnetization effect of steel strands domain rotation(domain wall movement and rotation)under tension.Based on the consistency of the mechanism and effect of domain wall motion(deformation)of ferromagnets under stress field magnetoelastic effect and external magnetic field,the equivalent analysis of stress-induced magnetic anisotropy and domain rotation magnetization of ferromagnets under external magnetic field is carried out.A theoretical relationship between stress and permeability of steel strands is established based on domain rotation equivalence,and the effect of stress on initial permeability is quantified.(2)The steel strand is considered as an inductance element with resonant inductance effect in LC oscillation circuit.The influence of structural parameters such as stress,length,section form,size,spiral number and spiral twist distance of steel strands on the inductive characteristics is analyzed.Furthermore,two inductance models of steel strands,single circular large diameter finite length conductor model and single layer long column model,are proposed.For different inductance changes controlled by different stress levels,the first-order linear approximation is carried out by the partial differential method of multivariate functions,and the factors of inductance variation are summed up as three main factors:stress,length and spiral angle.Finally,the application scope of the two models is discussed.(3)Research on frequency-stress measurement theory based on stress-magneto-electric resonance characteristics of steel strands is implemented.The magnetoelastic effect of steel strands under external force and the mechanism of inductance resonance effect in LC oscillation circuit are organically integrated,and a stress-resonance frequency relationship model is established.Furthermore,the stress-frequency resonance theory based on the principle of LC electromagnetic resonance is formed.(4)A prototype of stress measurement system for steel strands based on magnetoelastic effect is developed.Specifically,A hardware architecture based on LC electromagnetic oscillation circuit is developed,and the software of signal acquisition,processing and analysis of oscillation circuit is programmed by adopting PGFA processor.The resonant frequency measurement of electromagnetic oscillation circuit with prestressed steel strands is realized by using the measurement system.(5)A stress measurement model of electromagnetic oscillation for steel strands based on magnetoelastic effect is established.The electromagnetic oscillation stress tests of bare steel strands and unbonded steel strands embedded in prestressed concrete beams with lengths of 5 meters,10 meters and 15 meters are designed and implemented.By using the measurement system,the experimental fitting curve of stress and resonance frequency of steel strands is established.The test results of bare steel strands and unbonded steel strands in prestressed concrete beams are in good agreement with the theoretical model. |
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