| The acquisition method of fatigue properties for small millimeter-scaled specimens is significant for safety evaluation and life assessment of in-service structures and small components,With the development of nuclear power,wind power,aerospace,bathyscaphe,high-speed railway,new energy vehicle,bioremediation,artificial intelligence,the invention and application of new materials and complex service environment,the test conditions of fatigue properties become more and more strict.For fatigue tests of thin plates,welding materials,thin-walled tubes,micro materials from in-service structures and small components,traditional specimen can not be used because of the size limit.Therefore,it's important in academic and engineering field to develop a new method for measuring mechanical properties of materials based on millimeter-scaled specimens.The works compeleted in this paper are listed as follows.(1)A millimeter-scaled funnel slice specimen was designed to accomplish the low cycle fatigue tests for small size sheet materials,components and micro materials from in-service structures.Due to the difficultly in clamping the specimen,different loading fixtures and test schemes were designed for millimeter-scaled specimens with different size.The low cycle fatigue tests were then realized.(2)A universal prediction model to obtain cyclic constitutive relationship(UPCCR)of power-law materials based on an energy theory was proposed.For materials satisfying the Ramberg-Osgood law which is usually used in analysis process of fracture and fatigue,the strain energy of a loaded specimen can be expressed in terms of the separation equation,and the elastic strain energy and plastic strain energy separation function in deformation domain of millimeter-scaled specimens were given.On the basis of work-energy principle and Castigliano's theorem,the relationship of cyclic constitutive elastic-plastic parameters,elastic deformation,plastic deformation and loading was obtained.After parameters of the model were calibrated by finite element aided calculation,the prediction model which has universal validity for different power-law materials and different dimensions of the millimeter-scaled specimens was established.At last,finite element analysis were performed to prove the prediction accuracy.(3)Based on equivalent principle of fatigue life,stress amplitude and strain amplitude conversion equation for most metals were obtained by elastoplastic finite element calculation of millimeter-scaled funnel slice specimen.Two sets of equations were conducted to convent the testing strain amplitude and average stress amplitude of funnel slice specimens to axial strain and stress amplitude at the notch root.The RVE(representative volume element)fatigue life of material at the root of funnel notch and parameters of Manson-Conffin life prediction model were obtained.(4)The variable-amplitude symmetric-strain low cycle fatigue tests of 316L stainless steel and TA17 alloy for millimeter-scaled specimens with different sizes were accomplished.Based on load-displacement curves in cyclic stable stage,cyclic constitutive relationships of 316L stainless steel and TA17 alloy predicted by UPCCR model were in good agreement with experimental results obtained from traditional round bar specimens.The good prediction accuracy of UPCCR model was verified by experiments.(5)The constant-amplitude strain low cycle fatigue tests of 316L stainless steel and TA17 alloy for millimeter-scaled specimens with different sizes were carried out.Based on the cyclic constitutive relationships of two materials predicted by UPCCR model,the fatigue life of the materials and parameters of Manson-Conffin model were obtained by stress amplitude and strain amplitude conversion equations.Compared with the testing results of straight round bar specimens,the data points of millimeter-scaled specimens fell within 2 times of safety line.Therefore,it indicates that the millimeter-scaled specimen can be used to test the fatigue life of materials. |
PDF Full Text Request