On the Assessment of the Forming Limit Diagram at Necking and Fracture for Polymer Sheets

In recent years, there has been increasing interest in the mechanical characterization of polymers, including their formability and failure within the forming limit diagram (FLD), as incremental sheet forming (ISF) technologies have been applied to these materials. This interest is connected with the experimental verification that, as with metallic sheet, ISF of polymers can achieve levels of principal strains well above the material's forming limit curve (FLC), allowing for stable deformation up to the fracture forming limit (FFL). However, unlike metallic sheet, there is no systematic procedure for constructing the FLD for polymeric materials. Therefore, the aim of this experimental work is to establish a critical procedure for determining the FLD for polymeric materials at necking (FLC) and fracture (FFL). To achieve this, methodologies widely validated for sheet metals have been adapted and applied to two polymeric materials, ultra-high-molecular-weight polyethylene (UHMWPE) and polyether ether ketone (PEEK).