Large plastic flow in metals, rocks, and polymers manifests as shear bands, folds and superplastic flows. Each emerging under distinct but specialized conditions of pressure, strain rate, and temperature. Among these, shear bands represent an important mode of strain localization for understanding extreme deformation. Accurate prediction and modeling of these bands are essential for tribology, materials processing, and impact science. However, due to the specific conditions required for their formation, they remain challenging to study. Several mechanisms—including thermal softening, grain-boundary sliding, and dynamic recrystallization—have been proposed, yet no consensus has been reached. A continuum-scale understanding is also lacking due to the difficulty of experimentally obtaining local material parameters for validation.

 

In this work, the formation of a shear band is isolated using a metal cutting apparatus and marker-based technique is employed to investigate its formation mechanisms across a wide range of materials. High-resolution microstructural and inscribed markers reveal the shear band structure at both macroscopic and microscopic scales. Local temperature calculations from chip formation mechanics indicate that bands form even at low temperature rise (~35 ºC in cold-worked Naval brass) strongly suggesting that thermal softening is not a prerequisite for initiation. However, initiation strains remain high (γ~1).

 

Microscopic studies show a hierarchical shear band structure with localized displacement steps in seemingly continuous marker lines. Chip free surface exhibit elongated voids, yet fracture is absent, indicating that voids act as precursors to shear band formation. Based on common evidence across three distinct polycrystalline alloys (FCC, BCC, and HCP), a void formation and closure-based mechanism is proposed for shear band evolution. The evolution of shear band gives rise to a macroscopic structure, whose displacement fields exhibit characteristics of a fluid boundary layer. Shear bands formation is modeled using a Bingham plastic flow model and it excellently predicts the band displacement profiles across all the alloys using a single parameter.

 

The study is further extended with observations of large plastic flow in machining and indentation, revealing kink bands, shear bands, and folds. Characteristic features of the proposed mechanism are also observed in two pure metals, a metallic glass, a polymer, and rocks, suggesting a universal mechanism of large plastic flow. Based on this new understanding, two control strategies for large plastic flow are presented, focusing on (i) process design, and (ii) material design. The study’s implications span various fields, including material science, mechanics, and geology.