Finite element based formability predictions of sheets subjected to the incremental forming process

The forming limit in the single Point Incremental Forming (SPIF) process of metal sheets is larger than what can be expected from the forming limit curve for monotonic strain paths. From Finite Element (FE) studies¹, it was found that the final strain is achieved through non-monotonic, incremental deformations that consist of in-plane stretching and out-of-plane bending. Recently, experimental evidence was found for the presence of out-of-plane shear, and this was proposed as an explanation for the increased forming limits of incremental sheet forming processes². In this paper, the presence of out-of-plane shear is investigated through FE simulations. A finer meshing leads to more pronounced out-of-plane shear, especially in the forming tool movement direction. Secondly, it is investigated how the frequent strain path changes affect the limit strain prediction of a Marciniak-Kuczynski (MK) model, in which the possible influence of out-of-plane shear is neglected, as well as out-of plane bending. Several material models are adopted to assess the influence of anisotropy and anisotropic hardening. The complex strain paths lead to higher forming limit predictions when anisotropic hardening is assumed, but are still not high enough to be in good agreement with experiments. Better predictions may be obtained by including out-of-plane shear, out-of-plane bending, and/or the elastic part of the deformation in the MK analysis.status: publishe