To investigate the mechanism of fracture in softwood at cellular level, a coupled experimental and numerical modeling approach was implemented. On one hand, a method was developed for microscopic observation of the fractured surfaces which provided instructive information on crack initiation and propagation. This method was implemented on fractured spruce specimens in mode I, RL orientation to understand the mechanism of fracture at cellular level. On the other hand, a 3D mixed lattice-continuum fracture was developed to study the wood fracture while the porosity and heterogeneity of the microstructure were taken in to account. The microstructure of wood was simulated by a network of interconnected beam element and its heterogeneities were introduced by introducing the earlywood (EW) and latewood(LW) fibers with beam elements which have different geometries and mechanical properties. These elements were connected to each other by some direct and diagonal beam elements which represented the ray cells and the bonding medium between wood tracheids. Other heterogeneities and microstructural defects were considered by randomly choosing the failure criterion of each element through a normal distribution with different standard deviations. Standard deviation represented the variability of this criterion due to defects. Step by step removing of the critical elements (reached elements to the failure criterion) from the lattice mesh showed the process of development of microcracks and crack propagation during the simulation of fracture test. The proposed model was used to investigate the mode I fracture of a small softwood sample in RL orientation and the results were compared to the experimental measurements and observations. The pre-peak and post-peak behavior of the obtained stress-displacement curve and also the crack opening trajectory in cross and longitudinal section in model and experiments had a good agreement. Both model and microscopic observation showed that in mode 1 fracture and RL orientation, the main trajectory of crack propagates in EW ring. Investigating the fracture in a 3D and heterogeneous geometry, which represented the structure and mechanical properties of wood, allowed the development of distributed microcracks and bridging mechanism which provide the fracture stability.
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