J. Eberhardsteiner, M. Gingerl, L. Ondris:
"Biaxial Testing Equipment for Plain Wood Specimen with Inclined Grain Directions";
Poster: 11th Danubia-Adria Symposium on Experimental Methods in Solid Mechanics, Baden, Austria; 29.09.1994 - 01.10.1994.
Realistic finite element ultimate load analyses of plates and shells made of wood require suitable constitutive equations for the description of the deformational behavior of biaxially loaded wood. However, there is a lack of adequate biaxial experimental data, particularly if loading situations are considered where the principal load directions do not coincide with the principal material directions of the wood specimen. In order to eliminate these deficiencies a series of biaxial strength tests of plain wood specimen will be carried out. The mechanical parameters of this experimental study are the ratio between the applied principal strains as well as the angle between the principal load direction and the grain direction.
Biaxial testing of anisotropic materials is characterized by the fact that the principal axes of the state of stress and of the state of strain do not coincide in general. Therefore, a displacement-controlled testing of anisotropic materials is preferred compared to a load-controlled one. This test configuration, however, is characterized by the occurance of normal and shear stresses, which have to be measured.
As it can be seen from the Figure the wooden specimen will be deformed by implementation of prescribed displacements at 12 discrete loading points (3 at each border) through 24 hydraulic cylinders, each pulling or pushing under approximately 45^\circ towards the border of the specimen. This test concept allows the application of two independent in-plane displacement components at each loading point. Moreover, the resulting normal and shear forces can be measured by means of load cells integrated in each load axis. The distance between two loading points was minimized with respect to the homogeneity of the displacement distribution in the measuring field of the specimen. The load carrying capacity of one hydraulic cylinder amounts to \pm 17 kN; the accuracy for the displacement-controlled cylinder positioning is about 2 \mu m.
In order to determine technological inhomogeneities or problems with the load application the distribution of deformations and strains on the specimen has to be analysed. For this reason, an electronic speckle interferometer was installed for the contactless quasi-simultaneous measuring of three components of the deformation states. The optical set-up consists of one out-of-plane measuring direction and two dual-illumination-setups. The light of a 150 mW frequency doubled NdYAG-laser (\la=532 nm) is split into three parts, providing the light sources for each measuring direction. The light is switched computer controlled at each loading step and for each step three holograms are taken by a CCD-camera. The evaluation of the measuring results is done by an image processing system installed on a personal computer.