| Tube hydroforming (THF) is an attractive manufacturing process in automotiveindustry, and forming limit diagram (FLD) is a significant strategy to assess the formabilityof THF. The FLD for THF is proved to be a scientific and reasonable tool to assess actualproduction, and has great value for research. The forming limit strain state of an element atthe middle of the tube can be reflected in the FLD for THF. The geometric shape of FLDfor THF is various due to the influences of many factors especially the strain path.Therefore, it is badly need to discuss the establishment method of FLD for THF and todiscuss the change rules of FLDs for THF under various strain paths has practicalsignificance.In present study, the forming behavior of the thin-walled tube (1Cr13Mn9Ni1N) inthe THF process, the equivalent strain rates at potential fracturing and its adjacent nodesand the influence of the influences of changing strain path for THF were investigated onthe combination of theoretical analysis, experimental study and finite element (FE)simulations. The main contents of the research in present study were concluded asfollowing.(1) The deformation condition and loaded condition of THF process wereanalyzed; the common failure patterns during THF process were listed; and the specifyconditions that the failure patterns during THF process to be taken place were discussed.(2)The FE models of the THF process system were built based on the experimental setups;and the reliability of the FE models was confirmed by comparing the sectional geometricalparameters and the forming failure of the components which are respectively deforming inthe FE simulation and the experiments.(3) A method of predicting the FLD for THF wasdeveloped based on finite element (FE) simulation with the strain rate change criterion(SRCC) as a failure criteria of identify localized necking; and the results show that thisprediction method bears good agreement with experimental data.(4) The FLD for THFconsidering the effect of changing strain path were discussed; and the influences ofchanging strain path are revealed.The main results of present study can be summarized as following.(1) Using the tubefree hydroforming with axial feeding as an example, the stress and strain states during theprocess were analyzed. When the tube attains the yielded and limit state, the characteristiccurves between axial pressure and internal pressure were obtained through the theoreticalcalculation, respectively. The relationship between maximum radius of middle section andinternal pressure, the relationship between the extent of deformation and internal pressurewere illustrated.(2) The FE models are reliable that they can accurately calculate the sectional geometrical parameters, the strain path of an element at the middle tube andsimulate the forming failures of components.(3) Strain rate change criterion (SRCC) usesas the failure criteria to identify critical equivalent strain rate, when the difference ofequivalent strain rates at potential fracturing and its adjacent nodes over100times, thetube attains bursting, and the fracture strains required for establishing the FLDs are issuccessfully determined under various strain paths based on the simulation results of THFprocess.(4) Through the comparison of these FLDs obtained from various strain paths, theinfluences of changing strain path were revealed. The position of FLDs for THF will bedrifted with changing the strain paths. Compared with the position of FLD establishedalong simple strain path, the position of FLD moves to upper left direction under two-stagelinear strain path with initial uniaxial tensile strain path, the position of FLD moves tolower right direction under two-stage linear strain path with initial biaxial tensile strainpath.The establishment method of FLD for THF and the influences of changing strain pathachieved by the present study are useful for improving the formability of THF process, andwide application of other methods of THF process. |
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