Biaxial Extension of Cruciform Specimens: Embedding Equilibrium Into Design and Constitutive Characterization

Background: Main desired features of biaxial tests are: uniformity of stresses and strains; high strain levels in gauge areas; reliable constitutive parameters identification. Despite cruciform specimen suitability to modern tensile devices, standard testing techniques are still debated because of difficulties in matching these demands. Objective: This work aims at providing rational performance objectives and efficient cruciform specimens shapes in view of constitutive characterization. Methods: Objective performance is evaluated along particular lines lying on principal directions in equibiaxial tensile tests. A rich specimen profile geometry is purposely optimized via finite elements analysis by varying cost function and material compressibility. Experimental tests, monitored via digital image correlation, are carried out for validation. Results: New shapes are designed and tested in a biaxial tensile apparatus and show to perform better than existing ones. Elastic parameter identification is efficiently performed by only exploiting full field strain measurements along statically significant lines. Conclusions: Small gauge areas and small fillet radii cruciform specimens approach the ideal deformation behaviour. For the constitutive parameters identification in planar tensile experiments, it suffices to monitor strains along the gauge lines.