Theoretical And Experimental Study On Characteristics Of Nonlinear Ultrasound Propagation In Concrete

The propagation of ultrasonic wave in concrete is an extremely complex process,which contains remarkable nonlinear characteristics after received.The ultrasound will carry a lot of information about the internal structure of the concrete after passing through the material.Based on the head wave amplitude and the wave velocity,traditional ultrasonic test is widely used in quality evaluation and damage characterization of concrete.However,the traditional ultrasonic method based on the linear elastic assumption is incapable of characterizing the initial damage or micro structure change of concrete.Thus,the resolution and accuracy of the traditional ultrasonic method are obviously insufficient.Recently,the development of the nonlinear ultrasonic method provides a theoretical possibility in detecting the initial damage in concrete.In view of the insufficient study on the nonlinear ultrasonic technique,multiple approaches(theoretical analysis,numerical simulation,experimental test and signal analysis)are adopted,and the regularity of high harmonic generation in concrete is studied in this paper.The main researches and conclusions are as follows:(1)Based on the bi-linear stiffness model,the higher harmonic generation at the contact type interface is studied.The three crack-wave interaction models,i.e.hysteresis,rough surface contact and bi-linear stiffness are reviewed,and the advantages and disadvantages are compared.It can be concluded that the bi-stiffness assumption is more suitable for the theoretical study of second harmonic generation in concrete.Based on the bi-stiffness assumption,beginning with the nonlinear ultrasonic respond of a single contact interface,the one-dimensional wave equation of an isotropic and homogeneous solid is derived,and the generation mechanism of higher harmonics at the contact interface is revealed.In addition,nonlinear parameter?characterizing the single contact interface is defined.The study provides theoretical basis for the quality characterization of the weak joints in material.(2)By the finite element method and the nonlinear ultrasonic test,the propagation of ultrasound in mortar model containing the single contact interface is studied,and the nonlinear ultrasonic regularities of the contact interface is obtained.It is shown that when the excitation amplitude is small,the nonlinear parameter?is close to zero.With the increase of the excitation amplitude,the nonlinear parameter?gets larger.Then the nonlinear parameter?remain stable on condition that the excitation amplitude is at a high level.For the mortar samples with varies crack length and angle,the increase of the crack length will lead to the larger nonlinear parameter?,and the increase of crack angle will lead to the smaller nonlinear parameter?.It is noted that the conclusions obtained by the numerical method are coincident with the experimental results.(3)Considering the attenuation effects of the ultrasonic waves,the generation mechanism of the higher harmonics in concrete is studied based on the simplified analysis model of the concrete.The one-dimensional wave equation of the solid with randomly distributed micro cracks is derived,and the nonlinear parameter?is defined based on the bi-stiffness assumption.The attenuation effects are further considered,and the analytical expression of the attenuation coefficient and the nonlinear parameter?is obtained.The effects of the main frequency of the ultrasonic wave,the diffusion angle of the transducer and the propagation distance on the nonlinear parameter?is also studied.(4)By the finite element method,the isotropic and homogeneous solid with randomly distributed cracks is established,and the influence of the excitation amplitude and the crack density on the nonlinear parameter?is studied.It is shown that the nonlinear parameter?gets larger with the increase of the excitation amplitude.Then the nonlinear parameter?remains stable on condition that the excitation amplitude is at a high level.Furthermore,the nonlinear ultrasonic test of the concrete samples with different coarse aggregate size is carried out.The influence of the excitation amplitude and the crack density on the nonlinear parameter?is also obtained experimentally.It is noted that the conclusions obtained by the numerical method are coincident with the experimental results.(5)The nonlinear ultrasonic test of the concrete samples with different water to cement ratios was carried out under uniaxial compression.The nonlinear ultrasonic effects of the damage evolution in concrete were obtained.By the established nonlinear ultrasonic and uniaxial loading system,the relation between the load F and the nonlinear parameter?can be obtained.Three stages are divided according to the F-?curve.P1,P2 and P3 correspond to the compaction process,the elastic and the stable growth process of crack,the stable growth and the unstable growth process of the initial crack in concrete under the uniaxial compression.The nonlinear parameter?is stable when the external load is in a low level in P1.With the load increase,the nonlinear parameter?increase sharply and then decrease in P2.At last,the nonlinear parameter?show an oscillatory trend in P3.(6)The wavelet-EEMD joint method is proposed to improve the accuracy of the nonlinear ultrasonic method.By comparing the denoising effects of three wavelet bases(Symlet,Daubechies and Coiflet)under the condition of the decomposition layer2?4,the optimal wavelet base and decomposition layer are determined.Then,the EEMD decomposition is carried out,the first two inherent modal components are discarded,and the remaining components are reconstructed.At last,the denoising signal is obtained.The results show that the signal-to-noise ratio is increased from 4.97to 14.18 by the proposed method.The relative error of the fundamental amplitude and the second harmonic amplitude are reduced to 4.29%and 1.10%,respectively.(7)In order to further validate the wavelet-EEMD joint method,the nonlinear ultrasonic test of the complete and the pre-cracked concrete samples is carried out.In addition,the regularity of the nonlinear parameter and the damage degree in concrete is studied.It is shown that the nonlinear parameters?and?measured by different points are obviously varied,due to the strong randomness and discreteness of the micro structure.When the damage become more serious in concrete,the average change rates of the nonlinear parameters?and?reach 185%and 247%,respectively.It can be concluded that the sensitivity of the third order nonlinear parameter?is higher than the second order nonlinear parameter?.Overall,the nonlinear parameters measured at 100mm height point is obviously larger than the rest points.The corresponding average velocity change rate of the UPV increases from 4%to 10%.According to the experimental results of the nonlinear ultrasonic test of the concrete samples under the uniaxial compression,the signal amplitude ratio A/A_maxdecreases by 55%.Therefore,the ultrasonic attenuation has a great influence on the higher harmonic generation,which further leads to the irregularity of the nonlinear parameter when characterizing the macro damage.