Ultrasonic Damage Detection Of Concrete Defects Based On Reverse Time Migration And Travel Time Tomography

Ultrasonic non-destructive testing(NDT)technology has been widely used for defect inspection of concrete structures in civil engineering.However,most of the current data processing methods can only provide qualitative information regarding the existence of concrete inner defects.In this study,an ultrasonic inner defects inspection approach with a high-resolution imaging method which combines tra-vel time tomography(TTT)and reverse time migration(RTM)is proposed for concrete and concrete-filled steel tube(CFST)columns.TTT estimates a reasonable distribution of ultrasonic velocity over the cross-section of the concrete and CFST columns from the first arrival time of the ultrasonic transmission signal.The velocity distribution is used as an input of the initial model for RTM to image the defects inside the concrete and CFST column cross-sections with a high resolution.Numerical experiments demonstrate that the air cavity inside the concrete and CFST columns,and the debonding between the concrete core and the steel tube of the CFST column can be identified clearly,and that the location,size and shape of both defects can be determined accurately.It is concluded that the proposed defect detection approach with a high-resolution imaging method is efficient for the non-destructive inspection of concrete and CFST structures using ultrasonic waves.The main task of this paper is to image the defects in concrete and concrete-filled steel tubular structures.Chapter two introduces the theory of acoustic wave equation and the acoustic equation we use,simultaneously,this article explain the propagation characteristics of ultrasound wave.Then,in view of the curved ray tracing method is used in this paper,we apply improved Coppens’ method in order to extract the first arrival time from the measured or simulated data,as a result,the coefficient matrix related to the nonlinear equation and the slowness matrix(the reciprocal of the velocity matrix)is calculated.It is necessary to calculate the coefficient matrix when the known slowness model is used.According to the coefficient matrix and time matrix,in this paper,the least squares iterative algorithm is applied to inverse computation,and the velocity distribution of the internal structure of the concrete model is iteratively reversed.Then,this article introduce the steps and principles of the inverse time migration imaging algorithm.This article give a simple explanation of the method proposed by us.Specifically,this article use the velocity distribution estimated by the concrete model obtained by travel time tomography as the initial velocity model of the reverse time migration imaging.Chapter three mainly introduces the concrete structure model based on the above inverse time migration algorithm and the traveling time tomography algorithm.Three models are mainly used,that is,a concrete model with an air void,a CFST column model with an air void and a CFST column model with an interface debonding defect between concrete core and steel tube.The fourth one is the concrete model of two holes.At the same time,this article choose the pressure point source for wave excitation and put it on the outer surface of the measured structure.This article analyze the advantages and advantages of using this wave source.In the numerical simulation,this paper considers the impedance mismatch between concrete and air defects,and the mismatch between the concrete and the surrounding environment.The emitter and receiver are arranged.In Chapters 4 and 5,the homogeneous and stochastic models in Chapter 3 were simulated based on acoustic wave equations and elastic wave equations,respectively,and relatively good numerical results were obtained.Finally,this paper concludes that the use of traveltime tomography in conjunction with reverse-time migration imaging allows for concrete embedded in concrete structures and concrete-filled steel tubular columns,even if the air interface de-bonding thickness in concrete-filled steel tubular column members is only 10 mm.Defects and interfacial delamination defects between the concrete column and the steel pipe in the concrete-filled steel tubular column member can be easily imaged while maintaining high resolution.At the same time,through the proposed defect detection imaging method,the position,shape,and size of the air defects in the concrete structure as well as the debonding of the concrete core and interface of the concrete-filled steel tubular column member can also be accurately determined.