Probing the origin of surface defects in large strain deformation processes

Abstract

Surface quality is the direct result of tribological interactions accompanying large-strain deformation processes such as cutting or machining. Surface topography and mechanical properties are strongly influenced by near-tool-tip deformation mechanisms that govern defect formation and residual strain distribution. In this work, we investigate these phenomena using an in situ imaging framework of a prototypical surface generation process, analyzed using a recently developed image correlation method termed Ensemble Averaged Digital Image Correlation (EADIC). This approach enables high-resolution analysis of strain fields near tribological contacts and free surfaces. Kinematic analysis near the tool tip reveals that deformation progresses through three distinct stages: material pinning at the tool tip, internal shear leading to dead zone formation, and subsequent dead zone growth. Correspondingly, three types of surface defects are classified—type 1 defects lacking a specific morphological profile are associated with material pinning, type 2 defects characterized by step-like profiles arise from partial dead zone shearing, and type 3 defects with pronounced steps are formed during complete shearing of dead zones till tool tip. The evolution of these defects is tied to transient stages of near-tip deformation. Investigations reveal that transient force signature during cut can serve as probable indicators of defect type. The experiments also allow us to quantify residual strain near the machined surface, confined to a fraction of the cutting depth, with defects exhibiting elevated strain due to separation from the dead zone. These findings directly link transient deformation dynamics to surface quality, providing a framework for optimizing machining processes to reduce defects and improve performance.