Evaluation of the forming limit of incremental sheet forming based on ductile damage

摘要

Forming limit is a key issue for metallic sheets. As a flexible and dieless manufacturing technology, incremental sheet forming(ISF) process evidently improves the formability of sheet metals. In the present work, based on the assumption of simplified proportional loading in ISF, original formulations of force equilibrium of a micro element under isotropic yield criterion are revisited. Fracture forming limit(FFL) model in contradistinction to the traditional necking one is selected to describe failure mode according to the failure mechanism for AA1050-H111 in ISF. Ductile fracture criterion considering the scope of stress triaxiality is employed for the generalized tensile stress state in ISF process. By combining the equivalent plastic work principle with stress components expressed in the form of process parameters, the proposed modified forming limit model reveals the influence of process parameters, which is in good agreement with experimental results. As for a consistent preoccupation of fracture, the assessment of damage accumulation is presented. Forming height is also evaluated as formability metric in ISF. The modified FFL model is proved to catch practical demand as a guideline of process window design on damage evaluation in ISF.