Study On Damage Evolution Of 2D C/SiC Composites Based On Alternating Current Impedance

Ceramic matrix composites are widely used in thermal protection structures to ensure bearing capacity and improve the structural efficiency.However,the materials show complex damage evolution process under loading.Before the final failure of the composites,the damage is usually invisible,but it will lead to the reduction of the bearing capacity of the structure.If the material damage is not found in time and the failure warning is not carried out,it will cause catastrophic consequences.Therefore,it is of great academic significance and engineering value to develop insitu and online damage monitoring methods to realize the monitoring and early warning of composite material damage status.Electrical-based damage monitoring method has unique advantages because it takes the material itself as a sensor,and the operating environment is not limited by additional sensors,which is suitable for insitu online monitoring and offline detection of damage.Electrical-based damage monitoring method can be divided into the direct current(DC)resistance method and the alternating current(AC)impedance method.The AC-based damage monitoring method contains the full complex plane information of electrical properties,so it has a more promising prospect.However,the current research on AC-based damage monitoring method is still in its infancy,and the law of AC impedance characteristic change caused by material damage is not clear.In this paper,a typical 2D C/SiC ceramic matrix composites are studied,and the corresponding relationship between the damage evolution process of the material and the change of AC impedance characteristics is explored from both the experimental and theoretical perspectives,as follows:(1)Using meso-scale mechanical damage analysis method,the damage evolution process of 2D C/SiC composite materials was studied.Establish a microscale unit cell model of carbon fiber bundles,predict the stiffness of the bundles,obtain the equivalent elastic parameters of the bundles,and provide more accurate material parameters inputs for mesoscale damage analysis;Establish a meso-scale unit cell model of 2D C/SiC composite materials,apply periodic boundary conditions and mechanical loads to the model,carry out damage analysis based on the damage initiation criteria and damage models of the matrix and fiber bundles,so as to obtain the damage evolution process of the composite material;The stress-strain response is obtained by the homogenization method.The above method was applied to the damage analysis of 2D C/SiC composite materials under uniaxial tensile loading,and the damage evolution process and stress-strain response of the material were obtained.The calculated results were consistent with the experimental results,verifying the effectiveness of the method used in this paper.(2)Establish impedance response monitoring and feature extraction methods for2 D C/SiC composites,and clarify the corresponding relationship between the damage evolution process of the materials and the changes in AC impedance characteristics from an experimental perspective.Determine the damage mode of the material by monitoring the acoustic emission signal;Conduct morphology scanning on the material fracture surface to verify the damage evolution process of the material;The manifold learning algorithm based on linear tangent space arrangement(LLTSA)is used to extract the characteristics of multi-frequency impedance data.The above method was applied to uniaxial tensile tests of 2D C/SiC composite materials under monotonic loading and cyclic loading/unloading conditions.The results showed that the damage evolution process of the material can be divided into four stages:microcrack expansion,macroscopic matrix crack formation and interface debonding,stable damage propagation,and fiber fracture;The manifold learning algorithm used can effectively remove test noise and redundant features from multi-frequency impedance data;The phase angle tends to flatten before the occurrence of fiber fracture damage and begins to decrease during the fiber fracture damage stage,indicating that the phase angle is suitable for failure warning of composites.(3)Propose a modeling method for the electromechanical coupling behavior of2 D C/SiC composite materials.From a theoretical perspective,predict the AC impedance response characteristics of the material under mechanical loads,and explain the impedance characteristic changes of the material during the experimental process.Establish an AC impedance model for composite material meso-scale unit cell based on Maxwell Ampere’s law,which is used to calculate the multi-frequency impedance characteristics of the cell;Propose a calculation method for macroscopic equivalent impedance,expanding the obtained multi-frequency impedance characteristics from the meso scale to the macro scale.The above method was used to calculate the multi-frequency impedance characteristics of uniaxial tensile specimens,and the calculation results were in good agreement with the experimental results in Chapter 3,verifying the effectiveness of the above method.Establish a composite material electromechanical coupling analysis model by combining the mechanical damage analysis method in Chapter 2.The above model was used to calculate the impedance characteristic changes of 2D C/SiC composites under uniaxial tensile load conditions.The influence of coupling parameters in the electromechanical coupling model on the impedance characteristic calculation results was analyzed,and the coupling parameters were optimized based on the experimental results.The results indicated that the electromechanical coupling model and optimized coupling parameters established in this article can accurately predict the changes in AC impedance characteristics caused by damage evolution of 2D C/SiC composites under uniaxial tensile load conditions,especially the characteristic phenomenon of phase angle values tending to flatten before fiber fracture and phase angle decreasing during fiber fracture stage.(4)On the basis of the above research,experimental and theoretical research on2 D C/SiC composites under biaxial tensile loading is carried out to obtain the damage evolution process and impedance characteristic changes of the material.Biaxial tensile tests with a loading ratio of 2:1 was conducted,and the results showed that the damage evolution process of the material can be divided into three stages:microcrack expansion,macroscopic matrix crack formation and interface debonding,and stable damage propagation.The material ultimately breaks instantaneously without a sustained fiber fracture stage;Throughout the entire damage process,the impedance magnitude and phase angle continue to increase.Before the final failure of the material,the increase rate of phase angle significantly decreases and the value tends to flatten,indicating once again that phase angle is suitable for failure warning of 2D C/SiC composites.Conducting mechanical damage analysis under experimental conditions,the calculated material damage evolution process and stress-strain curves in the x and y directions were consistent with the experimental results,verifying the effectiveness of the method used.Based on the electromechanical coupling model established in Chapter 4 and the optimized coupling parameters,a mechanical electrical coupling analysis was carried out under experimental conditions.The predicted material impedance response characteristics were consistent with the experimental results,especially the typical characteristic phenomenon of the phase angle increasing rate slowing down and numerical values tending to flatten before final failure,which verified the effectiveness and scalability of the proposed electromechanical coupling modeling method.The impedance response monitoring and feature extraction method for 2D C/SiC composite material damage proposed in this paper can obtain the corresponding relationship between the damage evolution process and impedance feature changes of the material from an experimental perspective.The results of tensile tests indicate that the phase angle feature in AC impedance is suitable for material failure warning;The modeling method for the electromechanical coupling behavior of 2D C/SiC composites proposed in this paper can predict the impedance response characteristics of 2D C/SiC composites under tensile loading conditions from a theoretical perspective,and provide ideas for modeling the electromechanical coupling behavior of composites under complex loading conditions.