|Title:||Process modeling of fine-blanking using thermo-mechanical coupling method|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xxvi, 248 leaves : ill. (some col.) ; 30 cm|
|Abstract:||The primary objectives of the research project are to investigate into the failure mechanism of fine-blanking process and to develop a methodology that can be used to predict failure in fine-blanking. The scope of this research work is to study the forming mechanism and failure characteristics occurring during fine-blanking based on theoretical modelling, numerical simulation, and experimental investigation. Since intensive plastic deformation localized in a narrow shear zone, characteristic of the fine-blanking process has been considered as a thermo-mechanical coupled process. Therefore, attention is mainly paid to investigate the combined effects of strain-hardening, thermal-softening, and material damage. In the numerical aspect, a step-wise and staggered decoupling strategy was adopted to handle coupling between mechanical deformation and temperature variation. Based on this strategy, an updated Lagrangian thermo-mechanical finite element programme together with a special designed local remeshing procedure has been successfully developed to solve large deformation problems. Using the programme, the fine-blanking process has been simulated. The thermal effect on the material properties has also been taken into account in the constitutive equation. In order to ensure the accuracy of simulation, the major process attributes such as veering, ejector and the edge radii of the tools have been incorporated into the finite element model. From the numerical results, it has been realized that drastic variation of stress triaxiality during fine-blanking processes can cause material damage in the form of microcrack initiation, growth, and coalescence. By applying the concept of damage mechanics, the evolution of damage at different stages of fine-blanking has been estimated. Moreover, an energy-based criterion derived from the classical damage theory was employed to determine the critical value of fracture at the final stage of fine-blanking. The predicted value of damage energy density agrees with which has been published in related literature. In order to measure the strain distribution for validating the numerical findings, the effective strain has been measured experimentally on the meridian plane of fine-blanked specimens. By using the photochemical etching method, a chessboard pattern mesh has been pre-etched on the cross-section of the specimens. After fine-blanking, the coordinates of specific points on the meridian plane of the fine-blanked specimen were recorded digitally and thus the deformation gradient at the points could be estimated. Furthermore, the effective strain was calculated in terms of the deformation gradient tensor. To make clear the evolution of microstructure in the shear zone, examinations of metallurgical microstructure by means of optical microscopy and SEM have been carried out. It has been observed that the grains were highly rotated and elongated in the plastic zone, whilst in the other regions equiaxed fine-grained microstructures remained approximately unchanged. The presence of voids and microcracks proved that material damage developed at the final stage of fine-blanking. Moreover, the existence of local recrystallized microstructure may imply that the severe plastic strain and temperature rise could cause recrystallization in fine-blanking processes. It is almost the first time that temperature effect as well as various kinematic boundary conditions has been taken into account to simulate the complete fine-blanking process. Compared with the experimental findings, the accuracy of simulation is acceptable for engineering design purposes. The derived approach can be applied practically in estimating the fine-blanking force, and together with the damage criterion, fracture failure in fine-blanking can be predicted.|
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