Many materials in modern civil engineering applications, such as coated carbon reinforcement grids or PVB-interlayers for laminated (safety) glass, are polymer-based. Polymeric materials are showing typical viscoelastic (time or strain-rate) and temperature dependent behaviour. Beside that, polymeric materials also may need to have constitutive equations which include hyperelasticity (nonlinear stress-strain behavior in a quasi-static condition) when undergoing large deformations. In the context of the theory of hyperelasticity, the stress-strain relations are derived from the Helmholtz free energy. To examine the constitutive hyperelastic behaviour at large strains, different tests such as the uniaxial, biaxial and pure shear tests can be applied to the polymer in order to assess the geometry of the Helmholtz free energy surface in the plane of invariants. Within the context of this paper, uniaxial, biaxial and pure shear tests are performed on a standard PVB-interlayer to assess the respective stress – strain distributions. Inspection of the Helmholtz free energy surface based on the experimental findings allows the evaluation of a proper approach for the Helmholtz free energy against commonly used mathematical forms such as the Neo-Hook or Mooney-Rivlin material model.