WANG, QIAOLING (2019) Modelling plastic behaviour and fracture occurrence in HCP metal sheets deformed in a wide range of temperatures and stress states. [Ph.D. thesis]
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Abstract (italian or english)
Titanium and magnesium alloys both have hexagonal closest packed (HCP) structure with limited slip systems. This structure lead to two features: low formability at room temperature and the anisotropy influence on the mechanical characteristics and fracture behaviour. Therefor, the plastic behaviour and fracture occurrence need to be investigated at elevated temperature and different specimen orientations.
The approach comprises a joint use of experimental and numerical techniques. In terms of experiments, on the one hand, the plastic behaviour was studied and analyzed on the basis of a large number of tensile tests carried out on smooth specimens in a wide range of temperature and strain rate; the fracture behaviour was studied and analyzed by carrying out tensile tests on different specimen geometries as well as Nakajima tests, in order to reproduce various stress states, identified by the related stress triaxiality and deviatoric stress parameters; on the other hand, the alloy microstructure was analyzed on the tested specimens to explain the mechanical characteristic and anisotropy behaviour, including Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Electron Back Scatter Diffraction (EBSD) techniques and micro-hardness testing. In terms of numerical modelling, on the basis of the large number of experimental results, numerical simulations were implemented in the Ls-DynaTM code environment using a dynamic-implicit analysis with a coupled thermo-mechanical solution procedure to reproduce the sheet anisotropic plastic behaviour and fracture characteristics. The yield criterion and damage model were studied and modified to predict the experimental results.
Specifically, for Ti6Al4V titanium alloy, the heating of the alloy below the β-transus temperature is recognized to enhance its formability, reducing the flow stress and increasing the ductility. However, to investigate the effect of the sheet anisotropy on the material flow behaviour and plastic instability at varying temperature and strain rate, uniaxial tensile tests were carried out in a wide range of testing temperatures (from room temperature to 800°C) and strain rates (0.01, 0.1, 1 s-1) to assess the anisotropy effects. Strain hardening, strain rate sensitivity, and Lankford coefficients were evaluated as a function of the testing parameters and specimen orientation. Furthermore, a numerical model of the uniaxial tensile tests was developed and calibrated making use of the Barlat-Lian-1989 yield criterion and a hardening rule, which was adapted to take into account the anisotropic behaviour at different temperatures. It was proved that the developed model was capable of predicting the strain localization in the specimen gauge length due to plastic instability as well as its thickness distribution at varying temperature and strain rate. To investigate the influence of the sheet anisotropy on the material failure, tensile tests on smooth and notched specimens, shear tests and Nakajima-type tests carried out at varying temperature and rolling direction. The fracture strain is measured and the effect of the specimen orientation and stress state is identified. The coupling of the Barlat-Lian 1989 anisotropic yield criterion and GISSMO damage model was introduced to predict the fracture occurrence and capture its anisotropic characteristics. The two models were calibrated on the basis of an extensive experimental campaign. In order to validate the proposed approach, tests not used in the calibration phase were used, and the comparison between experimental and numerical results was carried out in terms of fracture characteristics at varying temperature and rolling direction. It was proved that the proposed modelling was able to satisfactorily reproduce the different fracture characteristics arising as a consequence of the sheet anisotropy and testing temperature.
Regarding AZ31B magnesium alloy, the same tests carried out on smooth specimen at the range of temperatures from room temperature to 300°C at 0.1 s-1 to evaluate the anisotropy influence on the mechanical and microstructural characteristics. The Lankford coefficients, ultimate tensile strength, diffuse necking strain and fracture strain values were evaluated as a function of the testing temperature and specimen orientation. Furthermore, microstructural features were analyzed as well as micro-hardness was measured for each testing condition to assess the post-deformation characteristics. To describe the fracture behaviour of AZ31 magnesium alloy sheets as a function of the temperature, stress state, and rolling direction. The same fracture tests were carried out at room temperature, 100°C and 300°C using different specimen geometries cut at different orientations with respect to the sheet rolling direction. The fracture strain values were measured making use of the Digital Image Correlation (DIC) approach and the fracture surfaces qualitatively characterized by means of stereoscopy and SEM. Afterwards, the AZ31 fracture locus as a function of the stress state, temperature and rolling direction was constructed. To analytically predict the fracture locus, a temperature-dependent parameter was added to the Modified Mohr Coulomb (MMC) fracture criterion; the newly proposed criterion was calibrated on the basis of the experimental data obtained from the tests previously mentioned, proving to be suitable to predict the AZ31 fracture locus for tension-dominated regions at varying temperature.
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