| On the basis of the research on the plastic deforming ability and the anisotropy of commercially pure titanium(CP-Ti)Grade 1 and Grade 2 sheets,the stress-strain responses of the two Grades sheets under various loading strain paths were investigated.Uniaxial tension,monotonic tension/compression,reverse loading and cyclic loading tests were conducted on samples cut from different directions.Based on the mechanical behaviors obtained from the tests,EBSD measurements of samples with various strain states were used to analyze the micro crystal orentations and the texture.After that,the relationship between macro mechanical properties and micro twinning activities were investigated.At last,the Schmid Factor of the as-received sample of CP-Ti Grade 1 sheet was calculated using theoretical methods,which agreed with the results of the experiments and measurements very well.The main research works and contributions are concluded as follows:(1)Under uniaxial tension loading,Digital Image Correlation(DIC)was used to record the flowing of the materials and to analyze the fracture characteristics for RD and TD samples in CP-Ti Grade 1 sheet.Two kinds of material flowing and the fracture mechanisms were investigated.After that,the stress-strain curves with different loading speed for RD and TD samples were obtained to analyze the changing of the mechanical properties.(2)DIC was used to measure the strain in width and longth in real-time on RD,TD and45°samples in CP-Ti Grade 1 sheet.Distribution of R-value calculated at the unloading point was obtained.The changing behavior of the R-value in samples cut from different directions was compared,which verified the the anisotropy in CP-Ti Grade 1 sheet.(3)Monotonic loadings were conducted on samples with RD,TD and 45°samples in Grade 1 and Grade 2 sheets.The influence of the pressure in the thickness direction and two kinds of lubricants,including moly paste and mineral oil-based hydraulic oil,were considered for the stress-strain curves.As for the mechanical responses,the work-hardening behaviors of the two Grades sheets were investigated under monotonic loadings to compare the difference under the same loading conditions.(4)Reverse loadings were conducted on samples with RD,TD and 45°samples in Grade1 and Grade 2 sheets.The difference in the mechanical behavior on two Grades of sheets was examined.Analysis was focused on the stress peak phenomenon under reversed tension and the two-stage work-hardening behavior under reversed compression on RD sample in CP-Ti Grade 1 sheet.On the basis of the results obtained,the work-hardening curves were plotted on all three kinds of samples to analyze the variation tendency of the work-hardening rate.After that,the Baushinger Effect of the two Grades of sheets were investigated.Non-dimensional parameter?=(-~?)/was created to compare and analyze the difference of Baushiger Effect between Grade 1 and Grade 2 sheets.(5)Cyclic loadings,including strain oscillation loading and loading-unloading tests,were conducted on samples with different directions in Grade 1 and Grade 2 sheets.The variation tendency of the flowing stress changing with the cyclic number was checked,and the work-hardening behavior at the end of loading was examined.The mechanical responses during the cyclic loading under conditions with different strain amplitudes and pre-strains were investigated.During the loading-unloading tests,three-stage fitting straight lines could be observed.The influence of pre-strain on the mechanical properties and the stress peak was examined.A parameter for plastic deformation m was created to analyze the resistance ability for reducing the thickness on RD samples.(6)The distribution of the original crystal orientations was obtained using EBSD measurement.Then the micro structures of samples with various strain states were examined to investigate the{10(?)2}and{11(?)2}twinning activities.The twinning behaviors of RD,TD and 45°samples under the same loading condition were compared.With the help of the pole figures,inverse pole figures and extracted twinning boundary figures,the relationship between the work-haredening behavior obtained in experiments and the twinning activities was investigated.The particularity of the RD sample in Grade 1 sheet was explained.(7)Two space coordidate systems were built,based on the the sheet plate and the hcp-structured crystal,respectively.The distribution of the c-axis of crystals was confirmed.The coordidates of the external loading,the normal direction of twinning plane and the twinning direction of{10(?)2}and{11(?)2}twinning were calculated to obtain the SF-?curves for the six patterns of twinning systems.(8)Through the geometry of the six patterns of twinning systems,the influence of the tilted angle of c-axis on the variety of acivatd twinning was analyzed.Twinning activities for RD samples in plane a and TD samples in plane b were investigated using the SF-?curves obtained.The reason why different kind of twinning happened in RD and TD samples under the same monotonic loading was explained,which agreed well with the results from experiments and EBSD measurements. |
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