R. Beer, J. Eberhardsteiner, F. Pulay, R. Reihsner:
"Zweiachsiges Testen anisotroper Werkstoffe - Konstruktion geeigneter Prüfmaschinen";
ÖIAZ - Österreichische Ingenieur- und Architektenzeitschrift, 137 (1992), 11; S. 544 - 549.
In general, the experimental determination of mechanical properties is strongly influenced by these properties: Materials with higher strength (higher values of Youngs's Moduli) require a basically different testing concept compared, e.g., with soft biomechanical materials.
The main problem associated with biaxial testing of anisotropic materials arises from the fact, that the principal axis of the state of stress and of the state of strain are generally not coincident. Therefore, displacement controlled testing of anisotropic materials is preferred compared to load controlled devices. These test configurations are, however, characterized by the occurance of shear stresses that are to be determined.
For materials with higher values of Young's Moduli, a device was developed for the direct measurement of the components of the normal and shear stresses in the principal directions of the applied state of strain. For soft biomechanical tissues, however, a multidirectionel loading device was developed for application on circular shaped specimen. Six differerent states of strain in six different directions with a spacing of 30 degrees are subsequently applied and the necessary loads are measured. From the overdetermined measurement, the constitutive equations can be determined using a least square procedure.