Development And Experimental Research Of Non-proportional Biaxial Tensile Device For Charactering Mechanics Of Materials

In the aeroengine,the failure of the critical life-limiting component under the complex stress state has become a bottleneck problem that limits its performance.The multiaxial stress experienced by the material during actual service is considered to be the main cause of the damage and failure.The traditional testing methods only deal with failure of materials under uniaxial loading,which are not able to test the mechanical properties of materials under multiaxial stress.The non-proportional biaxial tensile test device has been widely considered and developed in the research of multi-axis performance testing of materials.In this paper,the development and experimental research of non-proportional biaxial tensile device for charactering mechanics of materials are carried out for the low cycle fatigue life under biaxial stress state.What this paper has done is as follows:Firstly,the whole machine design of the non-proportional biaxial tensile test device was carried out.According to the test requirements,the whole machine program and main technical indicators of the test device are formulated,including the mechanical main frame,the servo hydraulic system(hydraulic station,electro-hydraulic servo actuator,digital servo controller,etc.)special fixtures for thin-plate specimens.And it is assembled and debugged to finally obtain a non-proportional biaxial tensile test device that meets the requirements.Secondly,the design of the biaxial quasi-static tensile and fatigue test specimens was carried out.The finite element analysis and parameter optimization was executed to design the cruciform specimens which meets the test requirements.The specimen ensures that the yield and fracture of occur in the biaxial stress zone of the specimen under the biaxial tensile state.In addition,the biaxial tensile fatigue specimen that meets the test requirements is designed,which means the crack initiation,expansion and fracture occur in the biaxial stress zone under the cyclic loading.Subsequent test results show that the design of the specimen has achieved the expected results.Finally,based on the constructed non-proportional biaxial tensile test device,the quasistatic tensile test and low cycle fatigue life test of GH4169,TA19 and TC11 under uniaxial and biaxial conditions were carried out.The quantitative and qualitative relationship between static strength properties under different stress states and low cycle fatigue life under different loading conditions.The quasi-static tensile test results show that the static strength properties of the three materials are lower than those of the same material under different ratios of biaxial tensile.The results of low cycle fatigue life test show that: 1)the results of low cycle fatigue life test have a certain degree of dispersion;2)whether uniaxial or biaxial low cycle fatigue life test,the fatigue life of the material presents logarithm decrease with the stress increase,3)Under the case 2,the low cycle fatigue life of the material shows a linear decrease with the increase of the load phase difference,4)only when the loading is proportionally loaded,ie,the case1 the biaxial low cycle fatigue life of the material is distributed within the error zone of the uniaxial low cycle fatigue life,otherwise it is lower than the uniaxial fatigue life.In addition,based on the comparison of common multiaxial fatigue equivalent stress expressions,it is found that the parameters in these equivalent stress expressions fail to reflect the unique stress trajectory of multiaxial load conditions,which means they are not able to accurately characterize the loading conditions.The analysis of loading condition process based on Von Mises equivalent stress shows that the cyclic parameters of Mises equivalent stress under multi-axis load cycle are different from the cyclic parameters of each axial direction.