The application of oscillation to the deformation of an elastoviscoplastic material

Huang, Zhihong (2000) The application of oscillation to the deformation of an elastoviscoplastic material. PhD thesis, University of Glasgow.

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Abstract

The research reported in this thesis demonstrates the benefits of applying coaxial vibration to forming tools in soft solid forming processes using Plasticine as a model material. In the study of vibration assisted upsetting and indentation (conical and spherical), finite element models under kinematic loading were first developed to gain insight into interface mechanics. FE simulation of the model material included the effects of elasticity, viscoplasticity, strain rate, large strains and a coulombic stress boundary condition in the presence of a lubricant. Agreement was achieved between the FE results and those obtained from upsetting and indentation experiments with respect to the force-displacement curves and deformed configurations for a range of friction coefficients, specimen sizes and platen velocities. The FE models, subsequently developed to simulate processes under superimposed vibration loading of the forming tool, predicted an apparent reduction in the mean forming force. The reduction in mean force is largely dependent on the vibration amplitude and shows a weak dependence on frequency. The results illustrate the phenomenon of stress superposition, where a cyclic stress is superimposed on a non-oscillatory stress. However, a reduction in the mean force alone is not necessarily beneficial since the maximum stress under idealised superimposed vibration loading will follow the same stress-strain curve as under static loading, with both the mean and minimum stresses following paths parallel to the non-oscillatory stress-strain curve. In fact, in the case of strain rate dependent materials, the maximum stress can be greater under vibration loading, and this overstress is correctly predicted by the FE model. However, more importantly, experiments under vibration loading using Plasticine have shown both a reduction in the mean forming force, and a maximum stress which is less than the static stress. The reduction in maximum stress achieved is related to the friction condition at the die/specimen interface. The relationship between vibration condition and soft solid material flow is investigated. It is established that vibration assisted forming can result in a significant reduction in resistance of the forming material to deformation, by a combination of stress superposition effect and a reduction in interface friction.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Margaret Lucas
Keywords: Mechanical engineering
Date of Award: 2000
Depositing User: Enlighten Team
Unique ID: glathesis:2000-73238
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56
URI: http://theses.gla.ac.uk/id/eprint/73238

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