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유한요소법과 실험을 통한 고압 비틀림 공정의 소성변형에 대한 해석

유한요소법과 실험을 통한 고압 비틀림 공정의 소성변형에 대한 해석
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Recently, various methods of sever plastic (SPD) deformation have been developed to fabricate bulk nanocrystalline metallic materials. High-pressure torsion (HPT) has become an especially attractive process among various SPD processes because it involves the largest strain to impose a possibility of producing nano and ultra-fine grained materials than the other SPD processes. Theoretical analyses as well as experimental approaches of the process are necessary in order to understand the HPT process. The purposes of the thesis are as follows: the first is analyses of strain and stress imposed by HPT process. The second is verification of FEM simulation using the constitutive model by experiments. In this thesis, the finite element method was applied to an analysis of plastic deformation behavior during HPT process. Numerical simulations dealt with a phenomenological constitutive model based on microstructure and dislocation density evolutions. The mechanical properties and microstructure of commercial purity copper subjected to various pressures and turns in HPT process were investigated. Compression before torsion significantly increased the hardness and low angle grain boundary in the center as well as in the intermediate and edge regions. It was found that the compressive strain was the reason for the increased strength and refined grain size in the center region during HPT. During torsional process, effective strain and strain rate kept decreasing around edge of specimen in radial direction. These phenomena made the ‘dead metal zone’ or ‘less deformed zone’ at the corner of specimen. In the case of Mg alloy, the shear fracture was occurred at dead metal zone having 0.1-0.2 s-1 of effective strain rate compared with FEM. The average grain size decreased with increasing the number of turns measured by electron. In addition, high angle grain boundaries (HAGBs) increased and low angle grain boundaries (LAGBs) decreased from 4/16 turns. The fraction of HAGBs at the 4/16 turns was 0.165 which had the smallest value. After 1 turn, the fraction of HAGBs was 0.849 which meant that copper reached at steady state for grain refinement after 1 turn and the average grain size with 253 nm was achieved similar as calculated cell size with 150 nm.
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