| At present,the ultrasonic stress wave testing technique of concrete stress has attracted more and more attention from researchers at home and abroad.The research object has gradually expanded from the initial one-way tensile test block to the pressured test block and simple force component.The content is to establish the correspondence between the coda parameters and the specific concrete stress field.In this paper,the reinforced concrete I-beam is taken as the research object,the content of the “Experimental Test of the Relationship between Time Domain Signal and Three-Point Bending Stress Fields” of the National Natural Science Foundation of China(No.51478072)is studied,the experimental route of using the first-order signal parameter to characterize the stress change is proposed,and the finite difference format algorithm program is developed,an ultrasonic coda wave test scheme for three-point bending stress field of concrete beams is designed and implemented.Under the 22 load levels,eight sets of repeated tests have been performed at each load level.Based on the stability of the test data,a finite difference format algorithm program has been compiled,the first-order components of the changes in coda wave amplitude(linear component)have been extracted,an approximate linear relationship between the first-order variation coefficient of the I-beam amplitude and the load levels has been established,and the variation coefficient of linear relationship coefficient(slope)has been analyzed.The main research work is as follows:(1)The investigation of ultrasonic coda testing for concrete stress fields has been conducted in two aspects.Firstly,research results on the relationship between the parameters of the coda wave and the stress changes of the concrete have been investigated.Then the difference algorithm was investigated and the forward differential format algorithm program has been explicitly adopted.Based on these,the technical route of using the first-order signal parameters to characterize the stress changes is proposed,and the test position layout,instrument parameter setting and loading system of the test are preliminarily determined.(2)Based on the principle of Taylor expansion,a MATLAB data processing program of finite difference format has been developed.Based on the Taylor expansion formula deduced from the three-point bending load level as an independent variable,an algorithm program for the forward difference scheme has been developed to analyze the energy ratio of the signals at each order(first-order to fourth-order)of the coda wave from the viewpoint of normalized energy.After examining the rationality of the unfolding order,the effects of higher-order components were removed,the first-order coefficient of coda wave amplitude has been extracted,and the linear relationship curve with the stress is established.(3)Experimental Study on model parameters of Time Domain Measurement of ultrasonic coda waves in I-beam three-point bending stress field has been implemented.Firstly,the coda wave test for the stress field of the concrete beam was carried out,and the repeatability of the test data was analyzed from the perspective ofSNR,and the stability of the results was examined from the perspective of the energy amplitude.On this basis,a three-point bending stress field wave test test for I-beams has been designed and implemented.The data of 22 load levels was collected,and the first-order variation coefficients of each state waveform in the time domain have been extracted by using the aforementioned algorithm program.Within the scope of the test,the linear relationship between the first-order coefficient of change and the load level of the I-beam has been obtained.The variation coefficient of the linear relationship coefficient(slope)in each test has been calculated,and the stability of the test process has been analyzed by the test data. |
