Novel Design of Cruciform Specimens for Planar Biaxial Testing of Soft Materials

Cruciform specimens have long been used in planar biaxial testing of inanimate materials such as metals and composite materials. The efforts to improve the geometric design of cruciform specimens have focused on maximizing the degree of uniformity of stress and strain in the gage section. The standardization of the procedure for the determination of the mean stress in the gage section is lacking, however, because the exact load transferred from the grippers to the gage section during testing is unknown. Here, we introduce a novel split-arm design for cruciform specimens by taking into account three important factors: i) the effectiveness of load transfer from the grippers to the gage section, ii) the uniformity of normal stress (in the loading direction) over the symmetry line, and iii) the compatibility between the nominal stress and the true stress. By ensuring these conditions, one can estimate more accurately the mean stress in the gage section based on the measured force at the grippers and the deformed configuration of a reference length. A genetic algorithm coupled with finite element analysis was utilized to optimize the geometric shape of the novel cruciform design. The identified optimum design provides a load transfer effectiveness of 100 %. The calculated nominal stress deviates from the true stress at the center of the specimen by only -0.49 %. A numerical experiment was conducted to validate the substantially improved performance of the optimized new design. Experiments were also conducted for natural latex rubber to demonstrate the application of the proposed design.