The aluminum alloy has been used widely during the past decade in many fields for media strength and good formability. During manufacturing and applying, a variety of problems may be caused by fracture, so its ductile fracture mechanism is still a hot spot. The fracture can not be totally explained by the classic damage constitutive models, reflecting that the damage evolution and ductile fracture mechanism of metal under complex loading is insufficient. The damage evolution and ductile fracture mechanism under plastic deformation are systematic studied by theoretical analysis, numerical simulations and experimental study for Al-alloy 5052BD-H14 and 5052P-H34, combining with the latest research results in the continuum damage mechanics.
In chapter 1, both the research background and purpose of this study will be introduced. The constitutive model is the fundamental to deal with its mechanics behavior, while the fracture criterion is the key technique to judge fracture. So, the conventional ductile fracture criteria are reviewed. Then the state of damage mechanics for metals based on I₁-J₂-J₃ framework is briefly reviewed. The main research contents and outline are also given in this chapter.
In chapter 2, by analyzing the behavior of metal containing voids under tension and shear deformation, the applicability of original Rousselier model is discussed. A modified Rousselier model is proposed by incorporating the recent extended damage evolution model by Nahshon and Hutchinson, in which the non-dimensional metric ω(σ̳) (or Lode parameter) and the shear damage coefficient kω are employed. The physical meaning of the new damage evolution rule will also be interpreted in theory of probability. The analytical solution of the modified damage evolution equation under shear was obtained, and its ability to describe shear fracture of material is discussed.
In chapter 3, the numerical implementation of modified Rousselier model in finite element analysis (FEA) will be conducted. Firstly, the backward Euler scheme based stress integration algorithm will be briefly developed within computational plasticity framework to solve the proposed model and the kernel derivation is carried out. Secondly, the integration algorithm is implemented and embedded into the commercial finite element software Abaqus/Explicit via its user material subroutine interface VUMAT by using Fortran coding language. Thirdly, some benchmark simulations will also be conducted to verify the stress integration algorithm and correspondingly developed program.
In chapter 4, the tensile tests of smooth round bar and notched round bars of Al-alloy 5052BD-H14 with different sizes were performed and the ductile fracture mechanism was analyzed by the macroscopic fracture phenomenon via scanning electron microscope. The mechanism can be concluded that the material failure under tension is caused by the nucleation, growth and coalescence of some micro-voids and micro-cracks. While for shear specimen, a shear fracture mechanism combining with void deformation was found. So the kernel of damage evolution is the mechanical behavior of micro-voids under complex stress state. Consequently, the material parameters of the classical Rousselier model were identified by an inverse method using these experimental data. A shear test was also performed to calibrate the new shear damage coefficient in the modified Rousselier model. For the shear test, the simulations show that although shear failure can be predicted by the Rousselier model, the ductility was over-estimated. However, the modified Rousselier model can give more accurate results. The simulations on uniaxial tension of the round bars also confirm that the modified Rousselier model can well predict the cup-cone fracture mode. The results indicate that the Lode parameter in the new damage evolution model is important to capture the cup-cone fracture mode transition.
In chapter 5, the ductile analysis of Al-alloy 5052P-H34 under different loading will be carried out by both physical experiments and numerical simulations. The physical failure mode was concluded and fracture mechanism was analyzed. Consequently, the material parameters were identified by an inverse method using these experimental data. A shear test was performed to calibrate the new shear damage coefficient kω. The Sandia test was also performed to verify the model's applicability. The crack path was investigated by the modified model, the results show that the fracture process and crack propagation under complex loading can be predicted by the modified model.
In conclusion, the ductile fracture mechanism of Al-alloy 5052BD-H14 and 5052P-H34 are studied by a modified Rousselier mode. The applicability of this model on shear failure is enhanced by the new damage evolution rule in which possible link-up of nearby voids under shear stress is considered. The simulation results show that the modified model can give more accurate results for both of the tension and shear failure.