Ultrasonic Nondestructive Evaluation Of Micro Defects In Medical Magnesium Alloy Coatings

As a bone repair material for human bodies,medical magnesium alloys are often modified with organic coatings to prevent rapid corrosion and inflammation in body fluids,in order to slow down the corrosion rate.However,the existence of micro defects in the coating can greatly reduce the corrosion resistance of the coating,making it difficult to accurately control the degradation time of magnesium alloy parts,threatening the health of patients.These defects are generally small in size and difficult to detect using conventional testing methods.Electron microscopy is often used for microscopic testing of coatings,which requires high technical personnel and cannot achieve batch testing in a short time;Alternatively,electrochemical testing is used to evaluate the corrosion resistance of the coating.This method is mostly sampling testing,which requires a harsh experimental environment and cannot meet the non-destructive testing requirements of the coating specimen.Therefore,it is of great significance to develop an effective,fast,and non-destructive detection method for micro defects in medical magnesium alloy surface coatings.This project is supported by the National Natural Science Foundation of Henan University of Technology.In order to solve the problem of nondestructive testing of micro defects in functional coatings on medical magnesium alloys,polylactic acid coated magnesium alloy specimens with different defect levels were prepared using AZ31 b magnesium alloy as a metal substrate and polylactic acid as a coating material.A series of tests,including nonlinear ultrasonic nondestructive testing,microscopic observation,and electrochemical corrosion testing,were conducted on the specimens,Based on the experimental results,the relationship between the ultrasonic nonlinear coefficient and the degree of micro defects and corrosion resistance of the coating was comprehensively analyzed.In addition,the problem of signal processing in nonlinear ultrasonic testing and the influence of micro defect changes on nonlinear characteristic parameters were analyzed and studied.The main research contents are as follows:(1)A highly reliable nonlinear ultrasonic longitudinal wave detection system has been established.According to the actual requirements of the testing system and the characteristics of the test piece,a special testing fixture was made,and a testing signal acquisition and processing software based on the Lab VIEW platform was developed.An electrochemical detection system was established to quantitatively analyze the corrosion resistance of the coating.(2)A signal processing method based on truncation least square method is proposed.Aiming at the problem that the second harmonic wave is more susceptible to noise interference due to the small number of excitation signal cycles,and there is a frequency offset,an improved signal processing method based on the least square method is proposed,and the accuracy and reliability of the method are verified by simulation signals and practical tests.(3)Based on the nonlinear ultrasonic testing and experimental parameters,a finite element model for the defects of magnesium alloy polylactic acid coating was established and numerical simulation analysis was conducted.By changing the length,width,and number of micro defects,the effect of micro defects on nonlinear characteristic parameters such as DC component and second harmonic wave was studied,and the ability of these parameters to characterize coating micro defects was evaluated.(4)Ultrasonic nondestructive evaluation of micro defects in magnesium alloy coatings was carried out.Two sets of poly(lactic acid)coated magnesium alloy specimens with different microdefects were fabricated.Using the established detection system,nonlinear ultrasonic longitudinal wave detection of coating microdefects was performed on the specimens.Combining with quantitative evaluation of coating corrosion resistance in electrochemical corrosion tests,the relationship between ultrasonic nonlinear coefficient and coating corrosion resistance was obtained;Further,the microscopic observation results quantify the degree of micro defects in the coating,and establish a relationship between the defined uniform coating coverage and the ultrasonic nonlinear coefficient.The change trend is consistent with the numerical simulation results.Finally,the correlation between the ultrasonic nonlinear coefficient of the specimen,the electrochemical corrosion resistance test results of the coating,and the microscopic observation test results was analyzed.