Experimental Study On Micro-crack Detection Of Thermal Damage Concrete Based On Nonlinear Ultrasonic Technology

As a building material with wide application and large dosage,concrete has the advantages of wide source of raw materials,simple construction process and good durability.However,the concrete structure is easy to form micro-cracks after being heated or fired,which will seriously affect the durability and strength of the concrete.Therefore,it is necessary to detect and evaluate the micro-cracks in the heat damaged concrete in order to take corresponding measures to control its development.At present,the ultrasonic method for concrete non-destructive testing is widely used in engineering.Traditional ultrasonic testing usually uses linear features such as reflection and scattering of waves to evaluate large damage and is not sensitive to micro-cracks in thermally damaged concrete.Since the change of micro-cracks in concrete has a great influence on material nonlinearity,it is of great significance to use this property to detect and evaluate micro-cracks in heat-damaged concrete.At present,the method of detecting concrete micro-cracks based on nonlinear coefficients is more to use the high-order harmonic method,which is highly demanding and susceptible to environmental influences.This paper establishes the relationship between the residual mechanical properties and the peak of the damage index defined in the nonlinear modulation method.The feasibility of detecting the micro-cracks in thermal damage concrete based on nonlinear sound field modulation method is proposed,and nonlinear acoustic field resonance method is introduced.The test results are compared and the feasibility of detecting nonlinear micro-cracks in thermal damage by nonlinear sound field modulation method is demonstrated by experimental research.Based on the mechanism of concrete thermal damage and the theory of nonlinear ultrasonic wave,according to the solution of nonlinear wave equation under double frequency excitation,this paper studies the solution of the equation when wide-band signal passes through solid materials,and cons tructs the nonlinear wave equation when single frequency ultrasonic signal and wide-band signal act,and then the mechanism of the modulation signal generated in the nonlinear sound field modulation method is analyzed.When the local characteristic parameters of the micro-crack are increased from the microscopic point of view,the amplitude of the modulation term will increase accordingly.According to the concept of damage index in the proposed sideband peak counting method,the characteristics of the dama ge index increase with the increase of the nonlinearity of the solid material,that is,the theoretical results are consistent with the experimental results.The heat damage concrete was prepared by high temperature method and the linear change rate of con crete at different temperatures was measured.It is found that the linear change rate of concrete increases with the increase of test temperature,and by scanning electron microscope quantitative analysis,the area proportion of micro-cracks will increase with the increase of temperature.The feasibility of detecting micro-cracks in thermally damaged concrete based on nonlinear sound field modulation method is studied experimentally.The feasibility of damage index defined by sideband peak counting method is verified.It is proved that the theory of nonlinear acoustic field modulation is effective to evaluate the micro-cracks of heat damaged concrete.It is known from the impact of impact force on nonlinear sound field modulation that a certain range of impact force values will not affect the peak of the damage index.At the same time,the relationship between the peak value of the damage index and the test temperature is established.It is found that the peak value of the damage index increases with the increase of the test temperature,and the damage index obtained by the nonlinear sound field resonance method also increases with the increase of the test temperature.The sound field modulation method and the sound field resonance method are used to detect t he micro-cracks of thermal damage concrete.It is found that the dispersion degree of the sound field resonance method is obviously larger than that of the sound field modulation method.It has good sensitivity and stability to evaluate the concrete micro-cracks caused by temperature by the sideband peak counting method in the acoustic field modulation method.At the same time,the operation requirements of acoustic field modulation method for external excitation are significantly lower than that of acoustic field resonance method.In order to further verify the feasibility and universality of nonlinear acoustic field modulation method for detecting micro-cracks of thermally damaged concrete,the effects of water-cement ratio,fine aggregate ratio and heating rate on the damage index measured by thermal damage concrete were studied.At the temperature,the peak value of the damage index increases as the concrete water cement ratio or the fine coarse aggregate ratio increases.With the increase of temperature,the influence of water-cement ratio on micro-cracks becomes weaker,and the influence of fine coarse aggregate on micro-cracks becomes more and more obvious.At the same temperature,the degree of thermal damage of the concrete after rapid heating is sign ificantly less than the degree of thermal damage after a slow temperature rise.At the same time,the compressive strength of the residual core of the concrete gradually decreases with the increase of temperature,and the ratio of the reduction of the resi dual axial compressive strength is linear with the peak normalized value of the defined damage index,and the influence of water cement ratio and the ratio of fine to coarse aggregate can be ignored,and the relationship between the damage index peak value and the macro mechanical properties is finally established.