Damage Evolution And Ductile Fracture Prediction In Power Spinning Of Pure Titanium And Titanium Alloy

As an effective way to manufacture tubular workpieces with large diameter and thin thickness,spinning forming technology has been used in the forming process of titanium and titanium alloys.However,it is difficult to manufacture tubular workpieces of hard-to-work materials,such as titanium and titanium alloys,through the spinning process.Ductile fracture is a serious problem often occurring in titanium and titanium alloys which has restricted the application of the spinning technology.This thesis focused on the ductile fracture in titanium and titanium alloy during power spinning process.The ductile fracture behavior for TA2 titanium under various stress triaxiality were studied.Based on the uncoupled ductile fracture criteria,the spinning limits and damage evolution of TA2 titanium tube and sheet spinnability tests were predicted,and the the mechanism of spinning crack was reserched.In order to control surface crack during tube spinning of Ti-15-3 alloy,under various thinning rates,the damage evolution for different thickness layers were systematically investigated and the mechanisms of surface cracks occurring in the tube spinning process for material with low ductility were analyzed.A series of tests on TA2 titanium blank providing clues to fracture ductility for a wide range of stress triaxiality were carried out.Based on the analysis of the deformation characteristics(modes)for various physical tests,the effect of stress states on ductile fracture behavior for TA2 titanium was researched.The result showed that the fracture strain obtained the maximum value at negative stress triaxiality.At high stress triaxiality,the fracture strain decreased monotonically with stress triaxiality and reached the minimum value in the tensile test with smallest notch radius.In addition,a lower fracture strain existed in the pure shear test,only higher than the minimum value at high stress triaxiality.Ductile fracture criteria were coupled into FEA software(ABAQUS)to simulate the process of crack initiation,propagation and rupture during various physics tests of TA2 titanium,the applicability of DFCs is evaluated and the damage limits for various stress state were obtained.Through the experiments of tube spinnability test,the initialization and location of ductile fracture were observed,and then the spinnability of TA2 titanium tube was confirmed.Based on the uncoupled ductile fracture criteria,the damage distribution of tube spinnability test was predicted.The result showed that except for the Freudenthal and Ayada models,the other ductile fracture criteria correctly predicted the damage distribution on TA2 titanium tubes in spinnability tests.To reveal how the crack initiated in tube spinnability test,the stress,strain and stress state at fracture location were analyzed.Two important parameters,the periodic stress triaxiality and periodic Lode parameter were introduced firstly to analyze the evolution of stress state with tube spinning process.The outer layer was prone to fracture firstly due to large equivalent plastic strain and higher stress triaxiality.Effect of aspect ratio on prediction accuracy and damage accumulation for different damage models were discussed.The C-L criterion provided the highest prediction accuracy on the spinnability of TA2 titanium tube and was little influenced by the aspect ratio.While the prediction accuracy of R-T and McClintock model changed greatly with the aspect ratio.In addition,compared with the upsetting test,the tensile test yielded higher prediction accuracy on the spinnability of TA2 titanium.In order to analyze and control surface crack during tube spinning of Ti-15-3 alloy,ductile fracture criteria were used to simulate the damage evolution in the tube spinning process under various thinning rates,and the mechanisms of surface cracks occurring in the tube spinning process for material with low ductility were analyzed.The results show that only the prediction yielded by McClintock model was consistent with the experimental results.The McClintock model was more sensitive to the changes of stress state on the inner layer due to the consideration of both the first and second principal stresses.Especially under moderate thinning rate,the McClintock model had captured the symbol changes of the second principal stress in the uplift and thinning deformation stages,thus producing a safe zone in which the surface crack in tube spinning of the Ti-15-3 alloy could be eliminated successfully.The damage evolution in sheet spinnability test was predicted by ductile fracture criteria and the spinnability experiments of TA2 titanium sheet were conducted.The vital factors such as the equivalent plastic strain and stress state which influenced the damage accumulation were analyzed.Although the inner layers own higher stress triaxiality during the entire spinning process,the maximum shear stress had little influence on the fracture ductility of inner layers.For the outer layers,the stress triaxiality was lower than the inner layers,but the outer layers yielded lager plastic deformation and the higher maximum shear stress existed in the outer layers could accelerate the shear-linking up of voids.Therefore,for the ductile fracture criteria that focus on the principal stress or the stress triaxiality,the maximum damage value predicted by these damage models were located on the inner layers,which was not consistent with the experimental results.Only the Freudenthal model,LeRoy model and the model proposed by Lou and Yoon could predict the crack initiation on outer layers of TA2 titanium sheet.