Abstract

Continuous bending under tension (CBT) has been shown to increase room temperature elongation-to-failure (ETF) in various sheet metals past that of simple tension (ST). In commercially pure titanium (CP-Ti) Grade 4, up to 3x extended elongation over ST has been achieved. A greater understanding of deformation mechanisms in CBT would allow for its elongation-enhancing effects to be more fully exploited in HCP and other metals, creating potential for new forming strategies. While most of the extended ETF has been attributed to delayed localization via incremental deformation inherent to the CBT process, together with compressive stabilization and relaxation of mechanical strain fields, contributions of microscale components relating to damage evolution, defect structures, and slip system activity intrinsic to the process are also likely to play a role. CBT-induced cyclic bending/unbending stresses combined with applied macroscopic tension create complex through-thickness stress profiles, where differing hardening behavior is expected near the surfaces compared with the middle of the sheet. This work uses high resolution EBSD characterization of geometrically necessary dislocation (GND) density together with X-ray diffraction (XRD)-based evaluations of total dislocation density and in-plane digital image correlation (DIC) to provide an in-depth analysis of through-thickness dislocation development and associated hardening rates throughout the CBT process in CP-Ti Grade 4 sheet metal. It was found that dislocation density is relatively uniform across the sheet at lower cycles, increases in the sheet center at higher cycles, and eventually approaches saturation near failure. Namely, dislocation accumulation occurs more slowly in the ratcheting, bending/unbending portions of the sheet (i.e., near the surfaces) from cyclic load reversals, and develops faster in the central tensile portion, where dislocation density up to 1.43x higher than near the surfaces was observed. The fraction of 〈c+a〉-type dislocations stayed below ~27% within the sheet, decreasing with increased strain, suggesting that the texture evolves such as to favor 〈a〉-type slip. Indications of stronger texture evolution occurring in the ratcheting (cyclic) regions were observed, with central texture resembling that of a sample deformed in ST. High dislocation densities in the sheet center were found to precede significant central void accumulation, concentrating damage away from peak surface stresses, presumably contributing to delayed failure.

Degree

MS

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

https://lib.byu.edu/about/copyright/

Date Submitted

2024-04-12

Document Type

Thesis

Handle

http://hdl.lib.byu.edu/1877/etd13130

Keywords

dislocation density, elongation-to-fracture, strain hardening, cyclic bending under tension, commercially pure titanium (CP-Ti)

Language

english

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