Study of wood and wood bonded joints behaviour under mixed-mode loading

Wood is nowadays increasingly used in structural applications. In fact, the interest of applying renewable resources in structural design is growing due to ecological reasons and energy shortages. In Portugal, wood industries are economically important, as is the case of furniture industries.

The mechanical behaviour of wood is strongly affected by the complex anatomy of the material. From a macroscopic point of view, wood is generally considered as a cylindrically orthotropic material, with the principal axes of orthotropy (L, R, T) given by three directions: longitudinal (L) of the grain, radial (R) of the growth rings and tangential (T) to the growth rings. There are large differences in stiffness, strength and fracture behaviour between these directions. Therefore, the fracture toughness is highly dependent on both the crack propagation direction and the crack plane orientation. Indeed, six different crack propagation systems are usually distinguished: LR, LT, RT, normal to the crack plane.

It is common that notches and cracks in wood structures are frequently subjected to mixed mode loading, which result not only from the imposed structural loads but also from the wood anisotropy. Negligence of mixed mode interaction effects in the design of wood structures may lead to significant errors in strength predictions. Mixed mode criteria are thus of great importance for predicting failure of notched wood components.

To avoid some disadvantages of solid wood related to its great variability and heterogeneity, several engineered wood-based materials have been developed over the years. Many of these materials are produced by cutting solid wood in smaller pieces and bonding them. Moreover, bonding wood can contribute to preserve forests, allowing combinations of several species. Among such reconstituted materials is the glued laminated timber (glulam), which is extensively used in structural applications. Hence, the knowledge of fracture behaviour of wood-wood bonding under mixed loading conditions is a crucial requirement to improve the mechanical efficiency of wood based structures.

The main objective of the proposed research project is the study of mixed mode fracture criteria suitable for wood and wood bonded joints. The following combinations of pure fracture modes will be examined: I-II, I-III and I-II-III. The accuracy of different fracture tests and data reduction methodologies will be first evaluated through finite element analyses. The selected experimental tests will be performed, and the experimental data will be compared with the predictions of existent mixed mode failure criteria. Eventually, new mixed-mode failure criteria will be proposed for wood. This extensive research work is a novelty in the area of fracture mechanics of wood, and even in the area of fracture mechanics of other orthotropic materials.