Author: Leung, Wing Yan
Title: Design and realization of structural materials with high strength and high ductility
Advisors: Zhou, L. M. (ME)
Shi, S. Q. (ME)
Ruan, H. H. (ME)
Degree: Ph.D.
Year: 2017
Subject: Hong Kong Polytechnic University -- Dissertations
Nanostructured materials -- Design and construction
Materials -- Analysis
Department: Department of Mechanical Engineering
Pages: xxi, 189 pages : color illustrations
Language: English
Abstract: The SMAT has been widely studied through experimental approach in the past decades. After the process, the microstructure of metal can be scaled down to nano-scale mechanically by numerous balls striking to the metal. So, the strength of metal can be enhanced drastically. However, lots of parameters can affect the result in SMAT. Investigating the effects experimentally is not an effective approach. Through the understanding of the strengthening mechanism of SMAT, numerical computation can then facilitate the investigation. A virtual SMAT chamber with numerous balls was designed and the number of ball impacts was counted in a given processing duration. The position of each ball at each time increment was checked to detect collision. If there was a ball-ball collision or ball-boundary collision, the velocity of ball would be updated. The impact positions and velocities of the balls in a predetermined processing duration were used for the simulation of SMAT. Johnson-Cook (JC) model was widely used in the simulation of high-velocity-impact. However, the original JC model only considered the equivalent plastic strain, strain rate and homogenous temperature. As the microstructure of metal was continuously refined in SMAT, a new constitutive model was developed to include the effect of grain and twin spacing refinements in SMAT. Both microstructure refinements were both influenced by the growth of dislocations, which was in turn determined by the equivalent plastic strain. By updating the equivalent plastic strain in each impact, the flow stress in SMAT was predicted by the micro and macro parameters. Utilizing the information of impact positions and velocities of the balls in the virtual chamber and the new constitutive model, a simulation model was developed to predict the mechanical properties in SMAT. It was found that the original JC model would underestimate the stress in SMAT, while the stress predicted by the new constitutive model agreed well with the experimental results reported in the literature. In the new constitutive model, a varying yield stress was used as one of the input parameters. A 100-balls SMAT process lasting for 10 minutes was simulated. The simulated maximum tensile stress was 1.26 GPa while the measured tensile stress reported in the literature was 1.23 GPa, proving the effectiveness of the new constitutive model. Using the new computational model, different parameters can now be investigated first by simulation, which will provide valuable guidelines to the design of experiments.
Rights: All rights reserved
Access: open access

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