Papers by Author: K. Lei

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Authors: K. Lei, Kai Feng Zhang, M.J. Tong
Abstract: Scale effects in the high temperature gas pressure forming of electrodeposited fine-grained copper thin sheets were investigated by a series of tests at various forming temperatures and die apertures. The average as-deposited copper grain size was 5 μm. The geometrical parameters of the bugling die system and the thickness of copper sheet varied in proportion. Different radius hemisphere parts from 0.5mm to 5mm were obtained at a strain rate of 5.0×10−4 s−1, which was controlled by pressure forces curves determined in terms of a finite element method (FEM) based on constitutive equation proposed by Backoften in 1964. The experimental relative bulging height (RBH) values were measured, and compared with that predicted by the same finite element method (FEM). It was found that the experimental values of large scale parts approach to simulated values, whereas the experimental values of small scale parts were quite different from simulated values. In order to explain these phenomena, a grain-rotation-weakened mechanism was proposed.
Authors: K. Lei, Kai Feng Zhang
Abstract: In order to describe scale effects in rigid-viscosity-plastic deformation at micro scale, a strain rate gradient model with couple stress is proposed. In the constitutive equation, couple stress, strain rate gradient are introduced on the assumption that the potential energy depends upon the scalar invariants of the strain rate tensor and strain rate gradient tensor, at the same times, the influences of length scale of body, grain size and temperature are also considered. A FEM-based program based on this strain rate gradient model is used to simulate the process of high temperature gas pressure forming, and the value of length scale 2 l is evaluated by simulation pilot calculation. It is indicated that rigid-viscosity-plastic deformation can be more accurately simulated by the present strain gradient rate model as long as the material parameters in constitutive equation are selected suitably.
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