Physical origins of shear bands
Abstract
Shear bands are planar regions of microstructure that have undergone collective shape change by simple shearing. Both the plane and the direction of shearing are noncrystallographic. Several theoretical models have been proposed to explain the formation of shear bands (based on either continuum mechanics or theory of dislocations), but as yet there is no agreement of the responsible physical mechanism. Experimental data obtained by different investigators in the literature suggests that there could be more than a single mechanism for shear band formation. This thesis proposes that shear bands form as a result of two processes of widely different time and length scales. The first process involves evolutionary localization of continuum-scale plastic flow within a planar band triggered by prior nonuniform straining. Such shear bands are defined as sample-scale shear bands and their formation leads to a decrease in the fraction of volume of the sample contributing to overall shape change. Sample-scale shear bands initiate heterogeneously at locations where geometrically necessary shear is maximum. Such locations are believed to undergo maximum flow softening due to lattice rotations. The second process involves rapid multiplication and expansion of dislocations from many concurrently active dislocation sources present on multiple slip systems. This process creates planar bands of localized shearing, called crystal-scale shear bands, having a thickness of the order of few microns. Lattice rotations trigger a sudden loss of mechanical equilibrium among previously stored dislocation segments creating large dislocation avalanches. The above process is the mechanism of crystal-scale shear band formation. A mechanics framework is developed to analyze the unstable operation of dislocation sources, which provides a basis for understanding the mechanics of coarse slip, microbands and crystal-scale shear bands. Experimental observations made during the necking of a commercial Al-Mg-Mn(5182-O) alloy show the development of both types of shear bands.
Degree
Ph.D.
Advisors
Bowman, Purdue University.
Subject Area
Metallurgy
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