The ultra-high strength boron steel has been intensively used in the
hot stamping process to produce complex-shaped structural components in transportation industries. Forming limit diagram (FLD) is a fundamental and useful tool to evaluate the formability of metallic
materials under various forming conditions. Since the standardized Nakajima test and Marciniak test are not applicable to perform
formability tests for hot stamping applications due to the
complex heating and cooling processes required, an in-plane testing method, in which cruciform specimens are deformed under hot tamping conditions in a Gleeble materials simulator combined with a multi-axial
tensile rig to convert an input force to an output biaxial force, has
been successfully applied to assess the formability of aluminium alloys
at elevated temperatures. However, it is challenging to apply this in-plane testing method for boron steel due to higher nonuniformity
of temperature distribution in gauge region of the cruciform specimen
at a higher temperature. In this paper, a new type of cruciform specimen, together with a new specimen heating strategy, has been
proposed to improve the temperature distribution in the gauge region.
The dimensions of the newly-designed cruciform specimen have been optimised by a thermo-electrical finite element model embedded with
a UAMP subroutine in ABAQUS to improve the niformity of temperature
distribution in the gauge region. In order to validate the new design
of cruciform specimen, biaxial tensile tests were conducted under hot stamping conditions by using the in-plane testing method.