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Design of an Experimental Device for Biaxial Tension Tests Used in a Uniaxial Test Machine

In the present study, a set of novel clamping apparatus that could deliver biaxial stretching motions with the use of a uniaxial tensile testing machine was designed and manufactured. The conversion of uniaxial motion into biaxial stretching motions is achieved by a sliding mechanism that consists of two blocks sliding in two mutually perpendicular grooves, respectively. During the biaxial tension test, a cross-shaped specimen sitting in the grooves are stretched by the two blocks driven by a pulling rod. The different stress ratios could be obtained by adjusting the groove surface shape and the lengths of specimen wings. In the clamping apparatus design stage, the finite element simulations were performed to examine the validity of the sliding mechanism and the frictional force generated between the sliding blocks and the grooves. The coefficient of friction was determined afterwards from the comparison of the pulling forces obtained in the experiments with those calculated by the finite element simulations. In addition, the optimum geometry and dimension of the cross-shaped specimen used in the biaxial tension tests were investigated by the finite element analysis as well. The slotted specimen proposed by Kuwabara et al. was taken as the basic design. A sufficiently large area in the central region of specimen where the principal stress directions aligned with the groove direction was obtained for gluing the strain gauges to the specimen for the biaxial stretching tests. The number of slots and associated slot widths were also examined to optimize the shape of the specimens. The proposed clamping apparatus was manufactured and the biaxial tension tests were conducted with cross-shaped specimens made of advanced high strength steel sheets. The validity of the designed clamping apparatus used for biaxial tension tests was confirmed and the congruence of various yield criteria applied to the advanced high strength steel sheets subjected to biaxial stress states was discussed.