Detection On Concrete Microcrack Propagation With Nonlinear Ultrasonic Modulation Method

General concrete structures can work with cracks,but micro-cracks(width of less than 100-150 microns)are not allowed for some important buildings or special buildings or buildings or Bridges under fatigue load.Because the micro-cracks in the destruction of external factors,on the one hand,will produce new micro-cracks,on the other hand will lead to the expansion of micro-cracks into a great harm of macro cracks,further lead to the failure and damage of concrete material performance.Therefore,how to use an effective method to monitor the dynamic growth of micro cracks in real time is the focus of scholars at home and abroad.This paper presents a nonlinear acoustic field modulation technology based on broadband excitation micro cracks of concrete expansion method,established the micro crack propagation length and nonlinear modulation method defined in the relationship between the damage index of the peak,via ultrasonic piezoelectric ultrasonic detection of micro crack propagation simulation and experimental study on the damage detection of crack propagation.It is proved that this method is feasible to detect the micro-crack propagation damage of concrete materials,so as to realize the real-time dynamic monitoring of the micro-crack propagation trend of concrete,which has important theoretical significance and engineering application value for evaluating the performance and life of concrete structures in service and improving the safety and durability of concrete.Based on the extended finite element method(EFEM),the nonlinear fracture mechanics theory of concrete and the propagation criterion of concrete cracks are studied.On this basis,the modeling process of discontinuities in EFEM is deduced by using the two-node method.In this paper,the solution of nonlinear ultrasonic wave equation under the excitation of broadband signal in solid medium is studied emphatically,and then the modulation phenomenon of nonlinear acoustic field is explained.On the basis of the dual frequency excitation,an extended finite element method based on ABAQUS built-in broadband excitation of concrete structure under a single micro cracks extend nonlinear acoustic field modulation signal is the study of numerical simulation.The sideband peak counting method is adopted to define the damage factor,the different damage degree is obtained by finite element analysis of micro cracks extend the response signal.The characteristics and differences of single micro crack propagation under static load and fatigue load are emphatically studied.The piezoelectric nonlinear ultrasonic experiments were built to detect the response signals of the micro-cracks at the crack tip of a four-point bending concrete beam with a single prefabricated crack under different propagation lengths.The accuracy of the numerical simulation model and the engineering applicability of the proposed method were verified.The results show that the amplitude of the damage factor increases with the increase of the length of a single micro crack when the width of the micro cracks changes slightly under either static load or fatigue load.Compared with the static loading process,the damage factor amplitude increases more uniformly with the crack propagation,indicating that the crack propagation speed is more uniform under the fatigue loading process,which is consistent with the actual situation,and verifies the feasibility of this method in the detection of concrete micro cracks propagation.In order to further study the superiority of the broadband excitation nonlinear modulation method for the extended micro cracks of concrete,the influence of the number of micro cracks on the damage factor was analyzed through numerical simulation and experimental research,and the feasibility of the proposed method for multiple extended micro cracks under static load and fatigue load was discussed.The results show that the damage factor peak value increases with the increase of the micro cracks length,and the damage factor peak value in the process of multiple microcracks growing simultaneously is always greater than that in the process of single crack growing,which further proves the feasibility and engineering applicability of this method in the detection of concrete micro cracks growing.