Three-Dimensional Absolute Stress Measurement Inside Steel Components Based On T-Wave Refringence

In recent years,numerous large and complex steel structure systems have emerged with the development of steel structure buildings.Subsequently,numerous steel components with huge sizes appeared,which are steel-shaped members mainly composed of steel plates,such as H-shaped steel components and box-shaped steel components.The safety evaluation and inspection of steel structures is the key to the development,construction and continuous operation.Scholars pay attention to the research of non-destructive measurment methods for steel components,aiming at the non-incremental absolute stress.They systematically explored the acoustoelasticity of transverse waves(T-waves)under the unidirectional and plane stress state,established theoretical models and proposed methods to measure the internal unidirectional and plane absolute stress of components by identifying the acoustic time that changes with stress.However,with the increase of component size,steel plates in the component is not only subjected to in-plane plane stress,but also out-ofplane stress.Steel plates are actually under three-dimensional stress condition.The existing methods fail to reflect the real stress of steel components.It is necessary to study the T-wave propagation mechanism and polarization law inside steel components under three-dimensional stress state,establish theoretical models based on multi-time and space information,and propose methods of acoustic time recognition and stress measurment.Based on the T-wave refringence,this paper aims to establish the constitutive relations of three-dimensional stress and acoustic time,use the spectral characteristics for acoustic time recognition,and propose a method for the three-dimensional absolute stress measurment inside steel components.The main research work of the paper is as follows:Constitutive relations of three-dimensional stress and acoustic time based on Twave refringence.Utilizing the polarization law of T-waves propagating in different directions inside the steel member,the acoustoelastic formulas of the polarized wave velocity and three-dimensional stress were derived,the correlations of the stress and the quasi-shear wave(q S wave)velocity difference propagating obliquely,the quasilongitudinal wave(q P wave)velocity propagating obliquely and the shear wave velocity difference propagating vertically were revealed.To separate the influence of stress components on the wave velocity,the quantitative relations of the six components of three-dimensional stress and wave velocity were established by combining q S waves and q P waves propagating obliquely on multiple paths.Meanwhile,the quantitative relations of the four components of three-dimensional stress and wave velocity were established by combining shear and longitudinal waves(P-wave)propagating in a single vertical path.Then,taking the time-of-flight(TOF)as the characteristic quantity of stress measurment,the TOF and stress constitutive relations of q S and q P waves,and the TOF and stress constitutive relations of shear waves and P-waves under three-dimensional stress were established.Acoustic time recognition and measurement method based on spectral characteristics.The time-domain signal expressions of the T-wave refraction polarization wave component based on the broadband pulse transmission signal were deduced.Transforming the time-domain signal into the time-frequency domain by Fourier method,the spectrum functions of the received signal were deduced,from which the influence of the q S wave TOF difference,q P wave TOF difference,T-wave TOF difference and P-wave TOF on the functions of magnitude spectrum and phase spectrum of the received signal was clarified.The frequency-domain characteristic extraction method based on the change of the initial polarization direction of the Twave was proposed,and TOF recognition formulas based on the amplitude and phase characteristics were derived.Furthermore,a three-dimensional absolute stress measurment system was established,the method for determining the dectection coefficients based on unidirectional compression was given,the calibration experiments for determining the dectection coefficients of the specimens were conducted,and the three-dimensional absolute stress measurment method for steel components was proposed.Research on influencing factors of three-dimensional absolute stress measurement.For the issue that acoustoelasticity does not consider the dispersion,several frequencies were selected for frequency research and q S wave polarization angle research according to the frequency range of the amplitude spectrum curve of the received signal.For the issue that the q S wave polarization angle may change with the stress and thus affect the measurment coefficient value,three trends of the q S wave polarization angle the under the trirefringence were found.The polarization angle decreases with the propagation direction angle increase,and increases first and then decreases with the frequency increases,and decreases with the stress increases.The results show that the polarization angle varies greatly with frequency and propagation direction,but little with stress.It is clear that after the frequency and propagation direction are determined,the influence of polarization angle change on the measurment coefficient is negligible.In addition,comparing the stress and TOF fitting results of the frequency,the optimal frequencies based on T-wave trirefringence and birefringence were determined,and the correlation coefficients of the corresponding fitting results were all higher than 0.9.Based on the optimal frequency,the three-dimensional absolute stress coefficients for steel specimens of different materials and thicknesses were determined.Experimental research on three-dimensional absolute stress measurement.Three-dimensional absolute stress measurement tests based on T-wave refringence were carried out on Q235 B and Q355 B specimens with different thicknesses.Threepoint loading was performed on the specimen to construct four components of threedimensional stress,and oblique loading was applied to construct six components of three-dimensional stress.The reliability of the proposed method was verified by using the strain gauge method as a comparative experiment method and combined with the finite element simulation.The results show that the results of ultrasonic method,strain gauge method and finite element method have good consistency.The maximum absolute deviation of the results based on T-wave birefringence is 12.1MPa(the relative deviation is 6.03%),the maximum relative deviation is 10.26%(the absolute deviation is 6.29MPa),and the average relative deviation is 5.4%.The maximum absolute deviation of the results based on T-wave trirefringence is 19.44MPa(the relative deviation is 13.81%),the maximum relative deviation is 18.87%(the absolute deviation is 5.76MPa),and the average relative deviation is 9.1%.The ultrasonic results are generally higher than the experimental results,and the proposed method is feasible.