Enhancing the hydraulic bulge-test using full-field DIC data

Abstract

The bulge test is an important experimental technique to identify the properties of materials and, in particular, metals. One of the main advantages of such technique, in testing metals, is the possibility of reaching high levels of deformation before fracture. For instance, a tensile test allows to evaluate the hardening curve only up the occurrence of necking, i.e. at the maximum force in the load vs. displacement curve. A bulge test can be performed using mechanical punches or the hydraulic pressure of a fluid. When hydraulic pressure is used, the test is not affected by friction. The hydraulic bulge test (HBT) is therefore a very efficient method to evaluate the properties of metals at large strains. Usually, the outcome of the HBT is the hardening curve in the equi-biaxial stress state that occurs at the top of the dome, during the expansion. In order to obtain the stress value is important to measure the curvature of the dome, this involve a second derivative of the vertical displacement field that can lead to measurement errors, especially if an optical technique is used to obtain the shape of the bulge. If DIC is employed as full-field measurement technique to evaluate the displacement and the strain field of the sheet metal during the HBT, only few measurement points in the center of the dome are actually used in the evaluation of the stress-strain curve. Accordingly, there is a large part of information that is not used in the identification procedure In this paper, the whole full-field measurement is exploited to derive the hardening behavior using an adaptation of the Virtual Fields Method (VFM). Suitable virtual fields are used to write the equilibrium equation in a large zone of the sheet metal during the test. All the strain data measured with DIC in such zone are then used in the identification procedure. The method is illustrated through simulated experiments on stainless steel sheet metals.