Application of the fields of objects formed on the basis of unit tensor objects to describe the tensile and compression failure stresses of pine wood in the principal orthotropy plane

The experimental investigation into tension and compression for softwood along grain direction shows the linear influence of wood density on the tensile and compression failure stress. For other directions determined by grain angles 25°, 45°, 90° for compression and 30°, 45° and 90° for tension the influence of density on failure stress was not shown. The investigation into the damaged structure of wood allows identification of the correlation between the tensile and compression failure stress and various failure mechanisms occurring for these grain angles. In the work the author proposed a new approach to description of failure stresses of the anisotropic materials. Usually this problem is solved on the basis of the formulated criteria for plane or complex stress states. Nonetheless the description of failure stresses may be formed by mutually compatible fragmentary descriptions. In order to describe the tensile and compression failure stress the objects formed on the basis of the unit objects of second and fourth rank tensors were used. The four models based on these objects or field of these objects in the form of the second degree monomials were used to determine the failure surface of normal stresses versus grain angle and density. The density was introduced to descriptions by dependence of model parameters on density using the least square estimation.