Research And Development Of Capacitive Ultra-high Cycle Fatigue Test System

Special application environments such as aerospace,defense and military industries place high demands on the fatigue life of alloys and non-metallic composite materials,and ultra-high cycle fatigue is the main form of material failure.In order to ensure the safety of equipment operation,the material must pass the ultra-high cycle fatigue test to check its fatigue performance before being put into use.The ultra-high cycle fatigue test lasts a long time.For example,it takes 138 hours to complete a 10¹⁰-cycle test at a test frequency of 20 kHz,which puts high requirements on the long-term stability of the testing equipment.The traditional ultra-high cycle fatigue test system usually uses a fiber-optic vibrometer to measure the high-frequency resonance of the tested material.However,the laser light source is prone to aging and the output light is unstable,which further deteriorates the measurement accuracy of the vibration displacement.In view of the above problems,this thesis designs a high dynamic,wide bandwidth ultra-high cycle fatigue vibration measurement overall scheme based on the principle of amplitude modulation capacitors,develops a high-precision measurement method of vibration parameters,and develops a prototype of a capacitive ultra-high cycle fatigue test system for symmetrical tension and compression test and three-point bending test,and calibration,precision comparison experiment and error analysis were carried out.The main research contents are as follows:1.Aiming at the requirement of long-term and high-dynamic measurement of high-frequency resonance of the material under test,an overall scheme for ultra-high cycle fatigue vibration measurement based on the principle of amplitude modulation capacitor is designed.This scheme uses conductive materials such as alloys and carbon fiber composite materials as moving plates,and high-stability capacitive sensors as fixed plates,and then measures the amount of capacitance change based on the principle of amplitude-modulated capacitance measurement;according to the mapping relationship between the capacitance change and the vibration displacement,the long-term,high-stability and high-precision measurement of the high-frequency resonance displacement of the tested material specimen is realized.2.Aiming at the problems of high resource consumption and low accuracy when the FFT spectrum method is used to solve the vibration amplitude of the tested material,a high-precision measurement method of vibration parameters that combines variable sampling rate,adaptive band-pass filtering and Hilbert transform is developed.This method uses the single frequency characteristic of the vibration signal,adaptively changes the sampling rate according to the real-time frequency estimation result,realizes the"free adjustment"of the adaptive band-pass filter parameters,and further realizes the accurate estimation of the vibration amplitude through the Hilbert transform.The simulation results show that this method takes up less resources and the relative error of amplitude estimation is less than 0.06‰.3.An ultra-high cycle fatigue capacitance sensor is designed,and then a sensor calibration device is designed for the nonlinear characteristics of capacitance gap measurement,and a piecewise polynomial calibration model of vibration displacement is established to achieve high-precision calibration.The hardware processing circuit with FPGA as the core is designed,and the host computer software based on Qt platform is developed,which realizes real-time data collection,processing,transmission,data storage and display,system parameter configuration and other functions.4.Through modeling and experimental tests,the factors that affect the measurement accuracy of the system vibration amplitude in the symmetric tension and compression test and the three-point bending test are analyzed,and an error compensation model is constructed.A prototype of the capacitive ultra-high cycle fatigue test system was developed,and the system precision comparison experiment was carried out.The experimental results show that when the vibration frequency is20 k Hz,the absolute error of the system within the measurement range of 50-70μm amplitude does not exceed±0.17μm.