Effect of crystallographic texture and dislocation hardening on limit strain in sheet metal forming

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Effect of crystallographic texture and dislocation hardening on limit strain in sheet metal forming


Author: Wen, Xiyu
Title: Effect of crystallographic texture and dislocation hardening on limit strain in sheet metal forming
Degree: Ph.D.
Year: 2000
Subject: Sheet-metal work
Strain hardening
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Manufacturing Engineering
Pages: xv, 142, [58] leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1535351
URI: http://theses.lib.polyu.edu.hk/handle/200/1119
Abstract: In the metal industry, sheet metals are widely used to produce packaging materials for consumer goods, for structures such as automobilse, and for building construction and transportation. The desired shape of the products is imparted by plastic deformation in either the cold or hot state. Traditionally, the prediction of the forming limit of sheet metals is based on tensile tests, simulation tests and continuum mathematical models. Continuum models used in the prediction of the plastic behavior of sheet metals are based on average values of mechanical properties such as elongation, yield strength, work hardening and work-hardening rate, which are usually derived from tensile tests. Although attempts have been made to abandon the phenomenological description of the yield function by applying the theory of crystal plasticity to calculate the yield surface of texture polycrystals and hence the limit strains, only the average properties of the microstructure (e.g., the crystallographic texture of the bulk sheet) have been taken into account. So far, there has been no model for the prediction of the strain path and the limit strain of sheet metals that takes into account the effect of individual grain orientation and the dislocation property. In this thesis, different approaches in the study of plastic deformation are reviewed from the view-point of both macroplasticity and microplasticity. Instead of relying on a unique flow rule to describe the stress and strain relationship, the role of work hardening in the instability process of sheet metal and hence the flow localization phenomenon is explored from a study of the changes in the orientation of the constituent crystallites and from the changes in the dislocation density associated with different grain orientations during the course of large biaxial deformation. The changes in the crystallographic textures of an aluminium sheet sample deformed under various stress states from plane-strain tension to equi-biaxial tension have been followed. From X-ray diffraction and ODF(orientation distribution function) data, the orientation hardening characteristics as well as the dislocation hardening characteristics of the sheet samples as well as the major texture components have also been determined. The changes in both the orientation and dislocation hardening at the grain level are complex and no simple generalization can be drawn over the range of strain and stress states used in the study. During the course of deformation, a grain can be hardened or softened, depending on the level of applied strain and the external imposed stress state. The traditional classification of grain orientations into "hard" and "soft" may need to be interpreted with care. Unlike the case in plane-strain compression, no single texture group could be identified to dominate over the plastic instability and flow localization process. The theoretical strain path that a grain orientation will follow has also been derived for some common texture components in FCC sheet metals. By applying the flow rule to a crystallographic based yield criteria, a system of non-linear equations is obtained to solve the strain components. The predicted strain ratios deviates significantly from those of isotropic deformation. When the rate sensitivity of slip is included, there is a further difference in the strain path taken by most grains compared with those without taking into account the effect of dislocation hardening. The effect of dislocation hardening is incorporated into a rate sensitive crystal plasticity model for the prediction of limit strain in biaxial deformation. The assumption of a preexisting surface groove defect in the Marciniak-Kuczynski method of limit strain prediction is relaxed, and the surface defect is seen to arise from the differential deformation between two colonies of grains with different crystallographic orientations. With the use of a modified localized necking criterion which incorporates the changes in the dislocation density, the limit strain of a sheet metal with two ideal orientations is predicted. The forming limit diagrams of a sheet metal with various combination of two major texture components have been computed. The theoretical limit strains calculated for a number of orientation pairs lie within the bounds of the experimental limit curves of two aluminium alloys published by other researchers. The proposed dual orientation model gives a more realistic prediction of the shape of the FLD as compared with the single crystal model of Zhou and Neale. The proposed dislocation model of limit strain prediction does not only arrive at a more realistic limit strain curve and provide a quantitative basis for the understanding the effect of crystallographic texture on the stretchability of sheet metals, but also provides an alternative to other plastic stability methodologies in the study of the large deformation of metals.

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