Biaxial measuring device and method for determining normal and shear stress-correlated material parameters

A method for determining normal stress- and shear stress-correlated material parameters using a biaxial measuring device comprising a frame for clamping a planar sample of a bendable and flexible material, a plurality of drives for applying normal and shear stresses to the sample, and an optical measuring apparatus operatively coupled to a data processing unit, the method comprising:

• a) applying a predefined character grid over the planar sample, wherein an orientation of the character grid is either irregular or correlates with directions of orthotropy of the planar sample, and clamping the planar sample in the frame in accordance with the directions of orthotropy of the planar sample,
• b) collecting, by the optical measuring apparatus, a first set of data that describes a first position of the character grid and sending the collected first set of data to the data processing unit,
• c) applying, simultaneously, predetermined normal and shear stresses to the planar sample by the plurality of drives, thereby bringing the planar sample into a deformed state and changing the position of the character grid,
• d) collecting, by the optical measuring apparatus, a second set of data that describes a second position of the character grid and sending the collected second set of data to the data processing unit,
• e) determining a relative position change of the character grid by correlating the collected first set of data and the second set of data and determining therefrom, by the data processing unit, a relative displacement and a current distortion state of the planar sample,
• f) determining, from the relative displacement and the current distortion state of the planar sample, a deformation equilibrium of the deformed state of the planar sample, wherein the deformation equilibrium correlates with the material parameters, and
• g) calculating the material parameters from the deformation equilibrium and outputting the material parameters.