Material plastic deformation has been predominantly characterised and modelled using the uniaxial tensile test, since it is one of the simplest and most universally-embraced practices. Yet this does not alter the fact that multiaxial loading conditions, experienced in sheet metal forming operations, require more complex tests, such as the controlled biaxial test. The is particularly accentuated in modelling quick plastic and superplastic forming operations, due to the large plastic strains, high strain rate sensitivity of the material, and the possible presence of anisotropy. Efforts on controlled biaxial testing have been bounded by the complexity of testing instrumentation; yet more importantly, by the lack of viable cruciform specimen designs where large plastic strains can be reached before failure. Most available cruciform geometries are based on, and therefore suitable for, ambient-temperature biaxial stretching to limited plastic strains. This work is devoted to filling the aforementioned gap by developing viable biaxial cruciform geometries, through a comprehensive integration of both experiments and finite element simulations. A balanced biaxial testing apparatus, capable of multi-rate stretching at elevated temperatures, has been designed and built. Multiple and rather diverse cruciform geometries are prepared from AZ31 magnesium and 5083 aluminium sheets; the specimens are then stretched at various temperatures and rates until failure. Plastic deformation in each specimen is tracked and recorded via a series of images, which are thence used to quantify the developed planar strains by the use of digital image correlation. The viability of each design is finally validated based on the true biaxiality of measured strains, and ultimate failure, within the gauge section of the specimen. Furthermore, FE analyses are carried out on multiple variations of each tested design, to investigate the effects of the associated geometrical parameters on the performance of that particular design. The study presents several cruciform-shaped specimens that succeed in delivering true biaxial deformation: localised within the gauge section, and progressing to fracture through the centre of the specimen. What is more, the results provide key insights into the influences of certain geometrical parameters on the degree of deformation-biaxiality in a cruciform specimen.
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