Deformation behavior of aluminum alloy sheets in biaxial in-plane tensile stressing

A biaxial tensile testing was conducted using cruciform specimens made of an aluminum alloy sheet of the 6000 series, and the testing was successfully kept up to a large deformation in its plastic region. Then, the work hardening exponent n* (n* value) in the central region of the specimens under in-plane biaxial stress was calculated using the strain and the measured value of the load from the device equipped for the testing. Thus, the biaxial tensile testing with cruciform specimens has an advantage over the biaxial testing with a semi-spherical rigid punch, such as the load value can be measured correctly without being affected by bending or friction. Yet, it is very difficult to make the cruciform specimens that make it possible to measure a large strain in plastic region, and there are few reports on the studies about the biaxial stretch forming area using cruciform specimens. In this study, how to make the cruciform specimens which enable the measurement of a large strain, and how to calculate the n* value is explained. Also, the deformation behavior of aluminum alloy sheets of the 6000 series in biaxial stretch forming was attempted to be researched by the experiment on the three stages of strain ratio using the cruciform specimens and two types of devices: a hydraulic type stress control machine and a mechanical type strain control machine.