| The stress state of the hydraulic pipeline of aircraft is very complicated in the service process.It is not only subjected to low-cycle load constrained by joints and clamps,but also subjected to high-cycle load caused by uneven flow field in the environment.Therefore,the fatigue of aircraft hydraulic pipeline is actually a combined high and low cycle fatigue(CCF)failure problem.At present,there are few researches on CCF of aircraft hydraulic pipelines.In order to improve the reliability of hydraulic system and further enhance the safety of aircraft,the life of CCF problem is studied in this dissertation.Since the outer diameter of the conduit is mainly less than 20 mm,this dissertation analyzed the conduit with the outer diameter of 8 mm and 12 mm.Meanwhile,combined with the actual complex working conditions,the CCF test was conducted,and fatigue life prediction was carried out based on crystal plastic finite element method.The main research work and achievements are as follows:1.The CCF test platform was built,and the specimens conforming to orthogonal biaxial loading were designed to conduct the CCF test.The CCF test data of materials were obtained by the CCF test with different frequency ratio and different stress amplitude ratio.The fracture of the specimen was analyzed,and the section law of the specimen under different frequency ratio and different stress amplitude ratio was summarized.Under the low stress amplitude ratio,there is obvious relatively smooth crack propagation zone at the fracture.With the increase of stress amplitude ratio,the crack sources change from single to multiple,and the smooth crack propagation area becomes smaller and smaller.Under the same stress amplitude ratio,the fracture morphology under different frequency ratio is similar.Based on the experimental results,the damage accumulation rule of CCF was investigated,and the CCF life prediction model was established.2.Fatigue test and finite element analysis were performed on four different types of conduits to obtain the corresponding strain life data of the four types of conduits.Among them,the overall life trend of 8 mm conduit and 12 mm conduit is similar,and the strain of the conduit with a 90° U-notch is higher than that of the conduit with a 45°U-notch under the same life.The average ratio of crack propagation life to total life of the four kinds of conduits is less than 7%,and the fatigue crack propagation life of the conduit with 45°U-notch is slightly lower than that of the other three kinds of conduits.The maximum principal stress direction of the 45°U-notch conduit is 12° with the axial direction of the cracked pipe at the crack initiation stage,and the fatigue crack propagation life of the conduit is slightly lower than that of the other three kinds of conduits.Three correction coefficients were proposed from the loading type,surface quality and sample size,and the crack initiation lives of 8 mm and 12 mm conduits are predicted based on the Manson-Coffin formula of improved four-point correlation method.Compared with the experimental results,the prediction error of crack initiation life of 12 mm conduit is less than 30%.The fatigue crack initiation life can be well predicted by introducing fatigue notch sensitivity and fatigue notch coefficient for Unotch conduit,and the maximum error is 34.2%.The CCF life of the pipe under pressure pulsation and ambient vibration is analyzed by using the proposed CCF damage accumulation model.The predicted CCF life and frequency ratio show a linear relationship in single logarithmic coordinate.3.The fatigue crack propagation life is also an important part of the fatigue life.The crack propagation rate of the conduit was analyzed,and the parameters of the crack propagation were obtained by combining the fatigue test and finite element analysis.The propagation of conduits with initial cracks at different angles under internal pressure and vibration loads was analyzed by numerical method.In the analysis of the initial crack propagation at different angles,the larger the angle between the initial crack surface and the direction of maximum principal stress,the faster the crack propagation.Under the combined action of pulsating internal pressure and vibration,when the pulsating internal pressure is large,the crack propagation rate increases with the increase of vibration amplitude,and the crack propagation rate increases with the increase of the angle between the crack propagation direction and the conduit axial direction.The higher the stress intensity factor amplitude ratio and frequency ratio of the conduit,the faster the crack propagation rate under the combined cycle.4.The microscopic constitutive model of polycrystalline materials was established based on crystal plasticity theory,and the microstructure modeling of polycrystalline materials based on Voronoi diagram was realized by Python.The calculation amount and accuracy of two-dimensional and three-dimensional polycrystalline model were compared,and the two-dimensional polycrystalline model was selected for analysis while ensuring the computational efficiency.Considering the relevant factors affecting the calculation accuracy of polycrystalline model,and the number of grains,grain orientation,mesh type and mesh number were quantitatively analyzed.When the number of grains reaches 200 and the number of single grain mesh reaches 100,the calculation accuracy can be guaranteed.The smooth grain boundary corresponding to the free grid is more prone to deformation than the stepped grain boundary corresponding to the regular grid.In microscopically,the local stress and strain will be different,and the local stress and strain corresponding to the free grid will be larger.The micromechanical parameters of the material were optimized and calibrated by Isight,and the deviation between the calculated results and the experimental results was less than 1%.The relationship between the cumulative slip and cumulative dissipated energy and the CCF life was numerically calculated,and a life prediction model was established.The predicted results and experimental results are within 3 times of life error.Under high stress amplitude ratio,the two predicted lives are higher than the experimental values,the deviation is large,and the predicted lives are 2～3 times of the experimental average.When the stress amplitude ratio is relatively low,the predicted life of the two methods is close to the experimental results. |
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