cylindrical orthotropy with three principal directions termed L (longitudinal, along the axis of the stem), R (radial) and T (tangential). In terms of a mechanical and fracture mechanical description anisotropy, hierarchic and cellular structure cause substantial differences compared to metals or artificial composite materials and require the modification of established concepts for describing fracture and crack propagation especially those using LEFM principles [1, 2, 3]. Recording the load and displacement data for a fracture mechanical experiment under conditions of stable crack propagation was found to be a very powerful method to investigate the fracture behaviour of wood. Material parameters like elasticity, fracture toughness or global strength as well as the energy consumed during the fracture process can be derived from the load displacement curves. Especially the determination of energy portions assigned to the different phases of crack propagation provides useful information about the fracture process itself. To obtain load and displacement data it is necessary to apply experimental techniques which guarantee stable crack propagation until the complete separation of the specimen. For the mode I loading case a experimental technique was developed and applied to obtain fracture mechanical data for several wood species [4]. There is less information about in plane shear fracture of wood and only a few studies dealing with stable crack propagation under mode II loading conditions are available in literature [5, 6, 7] The aim of this study is to present a new experimental procedure to record load and displacement data of fracture mechanical experiments on spruce wood (Picea abies [L.] karst.) under mode II loading conditions until ultimate fracture. The results of a first test series are described and some principal differences between mode I and mode II fracture are discussed. EXPERIMENTAL TECHNIQUE Specimen A new technique has been developed using a notched bending beam (NBB) specimen as shown in Figure 1a. A beam fixed on one side and supported on the other is loaded by a single load close to a necked down cross section close to the support generating a vertical shear stress. After applying a sufficiently high vertical force a crack will start propagating throughout the cross section (ligament) until the pieces are separated. The fixed part is generating an elastic bending moment acting against the applied load which is preventing a too fast transfer of the energy elastically stored in the specimen and therefore supporting slow and stable crack propagation. The experiments were performed on a conventional material testing machine which was equipped with the devices for fixing the beam at constant cross head displacement rates. Force and displacement are measured with a load cell and linear displacement sensor and recorded on a PC for later evaluation. a.) F b.) 100 mm 130 mm Figure 1: (a) Principle of the notched beam specimen; (b) geometry of the notched beam specimen consisting of a bending part and a notched specimen part
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