Slip processes that produce damage during forming
Abstract
Material connectivity is lost within a plastically deforming sample by microstructural damage. Three types of damage processes exist: microvoiding, interfacial separation, and slip-produced separation. This thesis investigates mechanisms that cause surface microstructure to progressively slip apart during ongoing plastic flow. Infinite straining ahead of a crack or notch, nonuniform thinning of microstructure lying adjacent to a free surface, and single or alternate glide plane decohesion have been identified as four processes involving slip that can potentially separate microstructure at a free surface while leaving underlying microstructure connected. None can operate in a sustained manner without being directly coupled to ongoing sample-scale shape change. A mechanism couples one of the four known processes of slip-produced separation to ongoing sample-scale deformation. Mechanisms are presented for nonuniform thinning, and single or alternate glide plane decohesion that can operate when plastic flow is confined within a sample-scale shear band. Experimental measurements of internal shape change within sample-scale shear bands of a duplex steel agree completely with model predictions. The findings of this part of the study are likely to apply in situations during upset forging, sheet forming, impact extrusion, projectile penetration, metal removal by a cutting tool, and metal shearing. Slip-produced damage initiates during plane strain necking by alternate bursts of intragranular coarse slip on intersecting planes originating from a common location. Alternate glide plane decohesion creates small intragranular V-shaped notches, termed surface ruptures, in near-surface microstructure. Surface ruptures initiate in locations of the neck where crystalline lattice rotation is maximum. Intragranular surface ruptures grow and coalesce in an end-to-end manner producing deep transgranular surface ruptures in locations of the neck where the gradient of sheet thinning continues to increase at the most rapid rate. Plastic flow localization ahead of deep transgranular surface ruptures leads to selective growth and coalescence. Extensive crystalline lattice rotation in conjunction with large stress concentration heterogeneous initiates sample-scale shear bands at deep transgranular ruptures. These findings are applicable to development of slip-produced damage during bending.
Degree
Ph.D.
Advisors
Bowman, Purdue University.
Subject Area
Metallurgy
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