Study On Ultrasonic Coda Wave Properties Of Damaged Concrete Under Uniaxial Loading

Concrete is one of the most widely used building materials in civil engineering.With the increase of the service life of concrete engineering structures,the monitoring of structural safety and stability becomes more and more important.Therefore,it is of great significance for the safety of engineering structures to determine and quantitatively evaluate the damage state of concrete structures through reliable nondestructive testing techniques.Coda Wave Interferometry（CWI）is a nondestructive evaluation technique for monitoring micro velocity changes in multiple scattering media.It is based on the sensitivity of the coda wave to small changes in the scatterers in the media.In fact,concrete is a complex multiphase composite material,which provides a good condition for the development of the coda wave.Based on experimental research,theoretical analysis and numerical simulation,this thesis systematically studies the ultrasonic coda wave characteristics of concrete with different strength under uniaxial monotonic loading and cyclic loading,correlation between ultrasonic coda wave characteristic parameters and concrete damage degree of concrete,the whole process of concrete failure under uniaxial compression is simulated numerically by discrete element particle flow method,simulation of concrete ultrasonic field based on digital image processing.The main research results are as follows:（1）Carried out the different strength of concrete in uniaxial monotonic loading and cyclic loading under the action of ultrasonic coda wave interference experiment,the different deformation stages of concrete by using the wave expansion ultrasonic coda wave signal analysis and processing,obtained with load percentage increase or decrease the corresponding ultrasonic coda potter character parameters（relative change rate of wave velocity and the maximum tensile factor）.（2）The experimental results show that there is a certain corresponding relationship between the change rate o f coda velocity and the damage development law of concrete structure.The damage state of concrete can be divided into four distinct intervals:loading strengthening stage,loading stability stage,loading weakening stage and loading instability stage.Finally,the damage model between the change rate of wave velocity and the percentage of maximum load is constructed.（3）The discrete element method(PFC^2D)was used to simulate the failure process of concrete under uniaxial compression,and the effect of granular meso-parameters on the mechanical properties and parameters of concrete model was analyzed in detail by“trial and error method”,so as to determine the values of meso-parameters.By comparison,it is found that the mechanical properties of concrete obtained by numerical simulation are basically consistent with the damage condition,and the whole process of concrete crack formation,propagation and damage damage can be simulated realistically.（4）Based on PFC^2D numerical simulation,different degrees of concrete damage maps are obtained.Combined with digital image processing technology,a two-dimensional numerical model of ultrasonic field of damaged concrete is established.The propagation characteristics of ultrasonic wave in concrete with different degrees of damage are simulated and studied,and the wave velocity trend diagrams of concrete with different degrees of damage are obtained.The results show that they are consistent with the experimental results.In summary,the overall trend of wave velocity variation of damaged concrete obtained by numerical simulation is basically consistent with the experimental results,which is in line with the expected results of simulation,and has certain guiding significance for practical engineering testing.This thesis has 84 pictures,15 tables,119 reference.