Calibration of ductile fracture criterion from shear to equibiaxial tension using hydraulic bulge test

Abstract

Phenomenological ductile fracture criteria have defining parameters to be determined from experimental fracture tests. For a precise calibration of a ductile fracture criterion, fracture experiments should cover a wide range of stress states. To this end, sheet metal specimens with different geometries are tested using various experimental setups, such as uniaxial tension, Nakazima, and butterfly tests. Usually, a combination of these tests is performed to capture a wide range of stress states, which makes the calibration procedure more difficult and tedious. In contrast, in the present research, only the hydraulic bulge test with a circular die is utilized to calibrate a ductile fracture criterion with three defining constants. To cover a wide range of stress triaxiality from the pure shear to the equibiaxial tension, circular blanks with different geometries are designed and developed with the aid of the finite element (FE) simulation. Among the presented geometries, there is an innovative specimen for reproducing the in-plane shear state using the hydraulic bulge test. In the present research, the ductile fracture of the AA6061-T6 aluminum alloy sheet is investigated. By comparing results of the FE modeling and the experiment of the hydraulic bulging, it is shown that the pressure, the bulging height, and the sheet thickness at the instant of fracture onset can be predicted with a reasonable accuracy by the ductile fracture model calibrated using the proposed test. Therefore, the designed specimens allow the hydraulic bulge test to be implemented for investigating the ductile fracture of sheet metals under a wide range of stress conditions.