| Concrete material is relatively complex composite material,which is widely used in civil and hydraulic engineering.The detection methods based on concrete materials have been receiving extensive attention.Among them,nonlinear ultrasonic detection methods have great potential and advantages.And non-linear phenomena such as higher harmonics,mixing method has been discovered.With practical applications,although there have been a lot of researches on the high-order harmonic method,it is not good at directional control and locating the damage location.For such problems,a non-collinear mixing method is proposed.At present,the non-collinear mixing method is mostly used in the field of metal detection,and its introduction into the detection of heterogeneous materials such as concrete has great potential.In this paper,theoretical derivation and experimental exploration are carried out for the problems of concrete non-collinear mixing detection method.The main research contents and results are as follows:(1)By deriving the nonlinear ultrasonic dynamic equation in the isotropic medium,the solution of the nonlinear ultrasonic dynamic equation after the interaction and superposition of two series of incident waves is discussed.And the influencing factors of the nonlinear ultrasonic mixing wave generation process are analyzed.Based on this,the type of nonlinear ultrasonic mixing wave and resonance conditions are determined.(2)Based on the theory of non-collinear mixing,the relationship between the incident angle and frequency ratio of the two rows of incident fundamental ultrasonic waves is obtained.And the reasonable frequency ratio range is calculated to be [0.54,1.87].Based on this,a pair of incident angles is determined.Then a concrete non-collinear mixing test device is built,the test method and analysis method of the device are proposed,and the test accuracy and influencing factors are analyzed.(3)Numerical simulation calculations are carried out on the process of two rows of incident fundamental waves scattering out a mixing wave in the concrete.The time domain diagram and frequency domain diagram of the mixing wave are obtained.By analyzing the frequency components of the two rows of incident fundamental wave signals and the mixing wave signals,it is verified from the perspective of numerical simulation that the nonlinear ultrasonic non-collinear mixing effect of concrete is produced and the feasibility and reliability of the test device meet the requirement.(4)By carrying out the non-collinear mixing acoustic characteristics test of the intact concrete sample,the study found that the overall trend is that the mixing nonlinear coefficient gradually decreases with the increase of the fundamental frequency and amplitude,and the amplitudes of the fundamental wave and the mixing wave increase with the increasing frequency of the fundamental wave,at the same time,the amplitude of the mixing wave increases with the increasing amplitude of the fundamental wave;further comparison and analysis with the test results of the higher harmonic method show that it is non-collinear mixing nonlinear coefficients are more sensitive to changes in fundamental frequency and amplitude.(5)The non-collinear mixing acoustic characteristics test of concrete specimens with different lengths and different angles being carried out,firstly,the volatility of the test results caused by different excitation amplitudes is compared,and the best excitation in the non-collinear mixing detection is discussed to be 100 V.Further analysising of the effect of cracks on the nonlinear acoustic characteristics of concrete,it is found that with the expansion of the crack length and angle,the mixing nonlinear coefficient gradually increases,and the mixing amplitude also has a similar trend.The comparative analysis of the test results of the harmonic method shows that the nonlinear coefficient of the noncollinear mixing method is more sensitive to the change of crack length and angle,and the trend is more obvious. |
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