Liczne doświadczenia pokazują, że propagacja szczeliny w jednorodnym, izotropowym materiale, w warunkach złożonego stanu obciążenia, następuje często w innej płaszczyźnie niż pierwotna płaszczyzna szczeliny. Inaczej jest w przypadku materiału ortotropowego, dla którego kryterium pękania powinno uwzględniać fakt, że w materiale istnieją uprzywilejowane kierunki propagacji szczeliny (kierunki wzmocnienia), wzdłuż których najczęściej następuje pękanie. W związku z tym, odporność na pękanie materiału ortotropowego silnie zależy od układu propagacji szczeliny. Ortotropię drewna charakteryzują trzy osie symetrii, oznaczone odpowiednio literami: L (kierunek wzmocnienia, wzdłuż osi pnia drzewa), R (prostopadle do kierunku tzw. słojów, widocznych w przekroju pnia) i T (stycznie do kierunku tzw. słojów).
Numerous fracture tests shows, that a crack propagation in a homogeneous, isotropic body under complex loading condition takes place in a direction, that don’t coincide with a crack direction. Contrary to that, it is known, that a crack oriented along the reinforcement direction in a orthotropic body propagates self–similarly, i.e. it don’t leave its original direction, even though the mode II load component may be significant. The fracture toughness of orthotropic material is highly dependent on both the crack propagation direction and the crack plane orientation. In other words, due to the large difference in fracture toughness between systems of principal material axes, cracks are often found in some of them, whereas they are extremely rare in other systems. The described above fracture phenomenon is well known in fracture mechanics of orthotropic materials such as wood or unidirectional reinforced fibrous composites. At a macroscopic level, wood is treated as a cylindrically orthotropic material with three main directions of the symmetry, namely longitudinal L, along the tree trunk, radial R, perpendicular to the year rings and tangential T, parallel to the year rings. The objective of the present work is to apply the non-local stress fracture criterion to mixed mode fracture in wood in the RL system for arbitrary oriented cracks. On the basis of a failure model taking into account the occurrence of microcracks and a assumption of the existence of the critical plane, the non-local stress fracture criterion of wood was derived for the RL extension system (the first letter alludes to the crack plane normal, the second one indicates the direction of crack propagation). In the most general terms, when the crack is notched with a inclination to the main orthotropy axis, the non–local stress fracture criterion in the system of critical stress intensity factors KI, KII is a rotated ellipse that is located in the center of the KI, KII system. Only when the crack is oriented along the main orthotropy axis, the non-local stress fracture criterion in the KI, KII system is a plain ellipse, whose semimajor and semiminor axes coincide with axes of the KI, KII system. In order to evaluate the derived non-local fracture criterion, a experimental investigation of the mixed mode fracture toughness of pine wood was made. Experiments have been carried out using specimens with single-edge crack. The mixed mode conditions were controlled by varying of the quotient between the stress intensity factors associated with mode I and mode II. This quotient depended both on the inclination of single-edge crack respect to main orthotropy axis and on the tensile and shear components of the loading force applied by machine. The mode I and II stress intensity factors used to characterize singular stress field near the crack tip have been evaluated using the finite element method and singular elements. Experimental data in terms of critical applied force were recalculated with respect to critical stress intensity factors associated with mode I and mode II and next they were compared with results obtained with the derived non-local fracture criterion. In this way, a high efficiency of the proposed fracture criterion of wood was demonstrated and proved.