Anisotropy of thin-walled tubes by a new method of combined tension and shear loading

We propose an experimental method for generating combined tension and shear stress-states in thin-walled tubes, with the purpose of calibrating anisotropic yield functions. The method involves extracting wide rings from the parent tubes and machining two notches that form the test-section between them. The rings are then fitted over two semicircular, or D-shaped, mandrels that are parted in a universal testing machine. The test-section remains on the top mandrel and, by virtue of its geometry, it experiences plane-strain deformation during this stretching. Hence we term the specimens as Ring Plane-Strain Tension specimens, or RPST. By controlling the orientation of the notches and thus the test-section, infinite combinations of tension and shear on the test-section can be created. We begin by describing the design of a family of RPST specimens using numerical simulation and comparing them to existing similar designs for sheets. We then proceed to describe RPST experiments that we performed on extruded Al-6061-T4 tubes. We use Digital Image Correlation to probe the strain fields in the test-section and establish the validity of the plane-strain conditions. Subsequently, we describe the post-processing method that we propose for the RPST specimens. Finally, we calibrate two non-quadratic anisotropic yield functions to the data. We demonstrate the value of the proposed method and highlight that due to its simplicity, it can be used in conjunction with existing experimental methods to generate a more complete picture of the material anisotropy. (C) 2015 Elsevier Ltd. All rights reserved.