Experimental Investigation and Plasticity Modeling of SS316 L Microtubes Under Varying Deformation Paths

Abstract

In this paper, results for SS316 L microtube experiments under combined inflation and axial loading for single and multiloading segment deformation paths are presented along with a plasticity model to predict the associated stress and strain paths. The microtube inflation/tension machine, utilized for these experiments, creates biaxial stress states by applying axial tension or compression and internal pressure simultaneously. Two types of loading paths are considered in this paper, proportional (where a single loading path with a given axial:hoop stress ratio is followed) and corner (where an initial pure loading segment, i.e., axial or hoop, is followed by a secondary loading segment in the transverse direction, i.e., either hoop or axial, respectively). The experiments are designed to produce the same final strain state under different deformation paths, resulting in different final stress states. This difference in stress state can affect the material properties of the final part, which can be varied for the intended application, e.g., biomedical hardware, while maintaining the desired geometry. The experiments are replicated in a reasonable way by a material model that combines the Hill 1948 anisotropic yield function and the Hockett–Sherby hardening law. Discussion of the grain size effects during microforming impacting the ability to achieve consistent deformation path results is included.

Superposing tensile stresses into single point incremental forming to affect martensitic transformation of SS304

“…A novel incremental forming process variant introduced in [12] is tensile stress-superposed incremental forming (TSSIF), which uses a custom frame to pre-stress the sheet in-plane to affect the final part properties, e.g., residual stress development. TSSIF is an example of stress superposition, which is generally defined as the incorporation of additional stresses into an existing manufacturing process during a single operation [13,14]. By adjusting the intensity of the stress superposition, i.e., changing the amount of pre-stress imposed on the clamped blank, the final part properties can be tailored for the intended application of the final product.…”