| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 91 === To investigate the fracture on the free surface in upsetting process, five different ductile fracture criteria are evaluated in this research. The evaluated criteria include equivalent stress, maximum tensile stress, normalized maximum tensile stress, void growth and coalescence, and hydrostatic stress. After the most adoptable fracture criterion being obtained, we can identify the billet fractured or not by this criterion.
In this research, we used the limit strain data measured in the literature to evaluate the most adoptable fracture criterion. After the most adoptable fracture criterion being obtained, it is then used to create simulation model by the DEFORM-3D software to predict the forming limits under different process conditions. The simulation of cylinder compression for the Al-2017F alloy was performed in different friction conditions and height/diameter ratios.
The evaluated results show that the Cockcroft and Latham ductile fracture criterion is the most suitable for the fracture prediction in upsetting process. From the simulated results, friction coefficient and the height/diameter ratio will not change the fracture locus. The suddenly change of friction coefficient during experiments will affect the strain path, thus affecting the formability of the billet. With the proposed method, the construction of the forming limit diagram (FLD) can be simplified by selecting proper ductile fracture criteria during finite element simulation and comparing the simulated result with a few experimental results. Thus, the identification of fracture during process parameter optimization can be made.
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