Measurement Of Absolute Uniaxial Stress In Steel Members With Ultrasonic Wave Methods

In recent years,a large number of steel structures have been built in China.Measurement of the absolute stress in structural steel members is of great significance in studying the failure mechanism of large and complex steel structures.However,it is difficult to obtain the absolute stress information of in-serviced steel structures directly using the current structural health monitoring technologies.The existing stress detection methods are unsuitabl e to be applied to evaluate the absolute stress in steel members.It is urgent to develop related methods to evaluate the absolute stress in steel members using a nondestructive way.The ultrasonic method,which links the stress in solids and ultrasonic time-frequency signals,provides a promising way to measure the absolute stress in steel members.However,the methods of measuring the absolute stress in steel members using the ultrasonic method are not clear yet.It is critical that relevant research should be carried out.Therefore,this paper focuses on measurement of the uniaxial absolute stress in steel members using ultrasonic methods.Specifically,the method of measuring the uniaxial absolute stress in steel members using the longitudinal wave time-domain signals is presented,and then a system is developed to evaluate the uniaxial absolute stress field in steel members.Based on the shear-wave birefringence and interference effects,the method of measuring the uniaxial absolute stress in steel membe rs using the shear wave frequency-domain signals is proposed,and then a corresponding system is proposed to measure the uniaxial average absolute stress of cross section in steel members.The main work is as follows:The method of uniaxial absolute stress measurement in steel members using the longitudinal wave time-domain signals is presented.The sensitivity of different type of ultrasonic waves to the uniaxial stress in steel members is compared on the basis of the acoustoelasticic theory,and the longi tudinal critical refracted wave(Lcr wave)is chosen as the optimum wave to detect the uniaxial stress in steel members.The generation of the Lcr wave is analyzed and the theoretical relationship between the Lcr wave time-of-flight(TOF)and the uniaxial stress in a steel member is optimized.A simplified model of the stress measurement in variable cross-section steel members is established,and a formula of measuring the uniaxial stress field in variable cross-section steel members is deduced using multi-sensor strategy.To accurately identify the Lcr wave TOF,the correlation of two Lcr wave series affected by the noise is analyzed based on cross-correlation algorithm,and then a method to identify the Lcr wave TOF is proposed on the basis of waveform cor relation.The above research lays a foundation for the use of ultrasonic longitudinal wave time-domain signals to measure the uniaxial absolute stress in steel members.A system based on longitudinal wave TOF method is developed to measure the uniaxial absolute stress in steel members,and then the uniaxial stress field in stressed steel member is evaluated.A system,aiming at identifying the Lcr wave TOF,is established to measure the uniaxial absolute stress in steel members.The ultrasonic TOF parameters are calibrated using the cross-correlation algorithm and the peak value method,respectively.The cross-correlation algorithm shows better advantage and is determined as the candidate method to evaluate the Lcr wave TOF.The linear relationship between the average uniaxial stress in steel members and the Lcr wave TOF difference is verified.Thus,the average uniaxial stress in a constant cross-section steel member is measured using the presented method.Furthermore,a single-emission and double-receiving sensor group is designed,in which the Lcr wave propagation length can be adjusted and the sensors can be adsorbed on the surface of steel members by embedded high-strength magnets.On this basis,the ultrasonic TOF parameters is calibrated at different Lcr wave propagation length,and then the stress field in a variable cross-section steel member is measured and the results are consistent with the strain gauge method.Finally,the error of measuring the uniaxial absolute stress in a steel member using the ultrasonic TOF method is analyzed in the aspects of test point location,steel member material and the Lcr wave propagation length.The method of measuring the uniaxial absolute stress in steel members using the shear wave frequency-domain signals is clarified.The interference phenomenon of two shear-waves propagating in a stressed steel member is analyzed on the basis of the birefringence theory.The theoretical expression of the shear-wave pulse echo spectrum is deduced when propagating in a stressed steel member.The shear-wave pulse echo spectrum contains both amplitude spectrum and phase spectrum,which contains the stress information of steel members.The interference factor is analyzed theoretically.Specifically,the influence of the amplitude of the interference factor on the shear-wave pulse echo amplitude spectrum is discussed,and theoretical expressions of the characteristic frequencies and the shear-wave optimal polarization angle are derived.Combined with the shear-wave acoustoelastic theory,a theoretical relationship between the uniaxial section stress and the characteristic frequency is established.To accurately extract the characteristic frequencies from the spectrum,the effect of the uniaxial stress on the shear-wave phase spectrum is further discussed.The non-linear phase-shifting characteristics of a shear-wave propagating in a stressed steel member is revealed,and the characteristic frequency can be extracted from the first derivative curve of phase difference function.The above research provides a way to measure the average stress on cross section in steel members.A system based on shear wave spectrum method is designed to measure the uniaxial absolute stress in steel members,and then the average stress on cross section is measured.A system is designed to measure the uniaxial absolute stress in steel members based on the shear wave spectrum method,in which the core of the system is to extract the shear-wave pulse echo signal.The influence of shear-wave polarization angle on the shear-wave pulse echo amplitude spectrum curve is compared and analyzed.The experimental results confirm that the shear-wave pulse echo amplitude spectrum curve changes with the uniaxial stress in steel members.Then,the target shear-wave polarization angle is obtained.A linear relationship between the uniaxial stress in a steel member and the inverse of the characteristic frequency is experimentally verified.Furthermore,the uniaxial stress evaluation of a stressed steel member is carried out and the measurement results are reliable.Further experiments are implemented to investigate the non-linear phase-shifting phenomenon of a shear-wave propagating in stressed steel members.And experimental results show that the characteristic frequency can be extracted from the derivative curve of shear wave phase difference.The difference of the characteristic frequencies extracted from the shear-wave amplitude spectrum and phase spectrum is compared,and the results show that the spectrum parameters calibrated u sing the two methods are consistent.Finally,the influence factors of calibrating the spectrum parameters using the spectrum method are analyzed in the aspects of test point location,steel member material and steel member thickness.