As previously mentioned, on a macroscopic scale, wood is regarded as an orthotropic material exhibiting, in each point, three distinct directions of symmetry: the longitudinal (L) direction following the tracheids or fibre arrangement; the radial (R) direction regards the concentric growth rings and the tangential (T) direction (Figure 3.1). Hence, for a complete mode I fracture characterisation of wood, the critical energy release rates should be determined for the six independent fracture systems, i.e. LR, LT, RT, RL, TL and TR [with the first letter indicating the normal direction of the crack plane and the second specifying the direction of crack propagation (Figure 3.2)]. However, in the context of structural application of wood, the RL and TL fracture systems are the most important ones owing to propensity of crack propagation parallel to grain, with several experimental tests being proposed to estimate the critical energy release rate in mode I, i.e. https://www.w3.org/1998/Math/MathML"> G Ic RL https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781351106979/092077e0-99a0-4060-996e-d1d71d68c3ab/content/c003_1.tif"/> and https://www.w3.org/1998/Math/MathML"> G Ic TL https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781351106979/092077e0-99a0-4060-996e-d1d71d68c3ab/content/c003_2.tif"/>. Hence, among the most common fracture tests, one may point the double cantilever beam (DCB), the single-edge-notched beam loaded in three-point-bending (SEN-TPB), the tapered double cantilever beam (TDCB) and the compact tension test (CT) that will be analysed in the following sections.