Abstract 3

Title:

Hydraulic Bulge Testing of CP Gr2 Ti to Quantify Size Effects on the Forming Limit Curve when Undergoing Miniaturization

Abstract Draft 3:

This study aims to get close to the tension-tension portion of the Forming Limit Curve (FLC) of 38 um thick CP Gr2 Titanium with the use of the bulge test. The FLC helps to predict the forming behavior of sheet metal by showing safe and failure strain zones with minor and major strains as the axes. Various tests, including the tensile test, limited dome height test, cruciform test, and bulge test are used for obtaining data for different portions of the FLC. With the miniaturization trend of technology, the forming abilities of Ti have to be evaluated because it is well known that the formability changes when moving from macro to micro scale.

The hydraulic bulge test involves clamping a flat foil sample to obtain a fixed boundary condition and then applying pressure on one side to promote material deformation. If the boundary is circular and secure, pure biaxial tension takes place at the top of the bulge as the material deforms. Micro limited dome height tests already produce excellent FLCs, but the equibiaxial portion is difficult to obtain due to frictional effects. The bulge test is essentially friction free due to the use of hydraulic pressure, and its data can be used to complete the FLC for CP Gr2 Ti. In order to ensure that pure biaxial tension has taken place, the strain history will be recorded by taking measurements of the strain at specific pressure intervals from the start up to the burst pressure. If the strain history has equal major and minor strains, pure biaxial tension is confirmed.

Strain is measured by studying the deformation of a grid of 50 um in diameter circles that are on top of the thin foil. Since the Ti is 38 um thick, the strain at the top of the specimen is assumed to be approximately the same throughout the thickness. A scanning electron microscope takes pictures of the strain zone of interest. After pictures are taken, ImageJ software is used to fit ellipses to the deformed circles. The major and minor axes of the fitted ellipses are used to calculate the major and minor strains. 

This thesis tests four different bulge diameters of 20 mm, 15 mm, 10 mm, and 2 mm. When bulge diameter decreases, size affects are expected to increase. Theoretical equations for the bulge test exist, which include parameters such as pressure, thickness, dome height, diameter, radius of curvature, and material constants. Theoretical calculations will be compared to LS-Dyna simulations and experimental results to check whether the theory continues to apply when moving to the smaller diameter thin foil. The wide range of diameters tested from 20 mm to 2 mm will allow for clarification of any trends when undergoing miniaturization in forming. This study will compare theoretical, numerical, and experimental results in order to bridge the gap between the macro and micro scale.