Research On Design Theory And Experiment Of Piezoelectric High Frequency Fatigue Testing Machine

The main failure mode of the parts under alternating load is fatigue failure. Currently,the main effective mean to prevent fatigue failure of materials or parts is carrying out fatiguetest on the subject, getting subject’s anti-fatigue fracture property, and getting the S-N curveto calculate its fatigue life. Now, there are two kinds of fatigue testing machine: fatiguetesting machine driven by electromagnetic and fatigue testing machine driven byelectro-hydraulic servo. Due to the restriction of system’s impedance and reluctance, theoperating frequency of them is lower than200Hz generally. Meanwhile because of badamplitude controlling ability and resonance stabilization, low loading accuracy, the abovetwo fatigue testing machine are not suitable to do high frequency fatigue test on small andhard and brittle specimen. With the development of industry and technology, the applicationof small, hard and brittle mechanical parts in the condition of high frequency, smallamplitude stress gradually increases, which makes it more and more important to develop afatigue testing machine to test materials or parts with small amplitude, high frequency load.The paper is based on the National Natural Science Foundation Project named “Thestructure design and performance research of piezoelectric high frequency fatigue testingmachine”. By using piezoelectric vibrator as the driving source and the method of systemresonance, a piezoelectric high frequency fatigue testing machine to test materials or partswith small amplitude, high frequency load is developed. The corresponding research ondesign theory, dynamic analysis and simulation and prototype measurement is carried out,the main contents are as following:1. Based on shell theory of the elastic material, the vibration mode of the circularpiezoelectric bimorph vibrator is established, and the first order vibration modal and thecorresponding natural frequency is solved; Applying the finite element software to do themodal analysis of piezoelectric vibrator; Applying impedance analyzer to measure theimpedance characteristics of the piezoelectric vibrator in order to obtain its natural frequency,comparing the above three results to verify the reasonable theoretical model and the reasonable solving process.2. Applying the lumped-mass method, the4-dof dynamic model of the piezoelectrichigh frequency fatigue testing machine is established. According to Newton second law, thesystem’s differential equation group of mechanical motion is established in the paper, thesteady-state response is solved, and the designing principle of the plate spring’s parameter isdetermined: based on meeting the strength requirements, it should have good flexibility andsoftness; it is concluded that426Hz and829Hz are modal conversion frequencies. Whenoperating frequency is below426Hz, the fatigue testing machine works in the main vibrationmode; When the operating frequency is close to these two values, the rack of the fatiguetesting machine produces a greater vibration, and the force loading on the elastic loader andspecimens force is low.3. According to the structural parameters in Table4.3, the solid model of piezoelectrichigh frequency fatigue testing machine and the finite element model of prototype is built,and the vibration modals and the corresponding natural frequencies are solved. The14thvibration mode is determined as the main vibration for the fatigue testing machine whoseworking frequency is330.4Hz; By changing the parameters of the nine main components offatigue testing machine, the impaction of these different parameters on its main modalnatural frequency、vibration mode is researched. The result shows that when the thickness ofpiezoelectric vibrator’s metal substrate, plate spring and the mass of loader increase，thenatural frequency of main vibration mode changes significantly; When the input voltage is165V, the main vibration work frequency is330.4Hz (330.0Hz to330.9Hz), the harmonicresponse analysis on the machine is carried out. It is verified that when the fatigue testingmachine works in frequency of main vibration modal, the load on specimen is stable, andamplitude of the frame of fatigue testing machine is little.4. Components with different parameters are made in the paper, and four prototypes areassembled. By changing nine parameters of the main components of the prototype, themaximum dynamic load on the specimen when prototypes work in six different drivingvoltages is obtained with a series tests, and the influence of different parameters onmaximum dynamic load acting on the specimen is analyzed. The measured results show thatthe maximum dynamic load acting on the specimen becomes larger as the increasing of themass of the elastic loader and the driving voltage. By adjusting the mass of the elastic loaderand the driving voltage, the different requirements of load acting on specimen is meetcorroding the requirements of load table of fatigue testing.5. Using the prototype to carry out the tensile fatigue test for HT100under the cyclecharacteristic of R=0.1and the working frequency of352.4Hz. The S-N curve of HT100isdrawn, and the fatigue limit (23.049Mpa) is get under the cycle times of1.18×10~8. The microstructure of the fatigue fracture of HT100is observed by scanning electronicmicroscope, the crack source and momentary interruption, as well as the crack propagationare analyzed.6. A fatigue testing machine driven by the single rectangular crystal piezoelectric isdesigned and manufactured in this paper. The prototype could provide small, high frequencyand precision load, and carry out fatigue test for small scale materials. Using the prototype,33groups of alternating stress is loaded on dragonfly with the cyclic characteristics R=0.1,the loading precision of0.01N and the working frequency of207.4Hz. Based on thepictures and videos in the test, the appearance and expansion mechanism of dragonfly wingscleft caused by fatigue is analyzed in the paper. The fatigue cleft appears in the position ofpostal vein or front vein of dragonfly wings at first, and then it expands with the increasingof loading cycle times. Wing membrane almost can’t prevent the cleft extending, but the veincan significantly slow down or stop the extension of the cleft.