Multiscale characterization of dislocation development during cyclic bending under tension in commercially pure titanium

Continuous bending under tension (CBT) has been shown to increase room temperature elongation-to-failure in various sheet metals beyond that of simple tension. A greater understanding of deformation mechanisms in CBT would allow for its elongation-enhancing effects to be more fully exploited, creating potential for new forming strategies. CBT-induced cyclic bending/unbending stresses combined with applied macroscopic tensile stresses 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 electron backscatter diffraction characterization of geometrically necessary dislocation density together with X-ray diffraction-based evaluations of total dislocation density and in-plane digital image correlation 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. 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.