Initiation and growth of delamination in wood and wood-based composites, a fracture mechanics approach

Abstract

Reliable prediction of delamination growth is still proving to be problematic, leading to the use of large safety factors and reticence in using wood-based composites in safety critical applications. This has led to composite structures being perceived as expensive to fabricate and needing frequent inspection and repair. The recognise approach to study delamination has been the fracture mechanics. The theory of fracture mechanics has been successfully applied to wood, wood products and wood-based composites since more than 50 years and provided valuable concepts for evaluation of the influence of cracks, notches or other stress raisers in structural elements. The space-time multi-scale nature of the delamination process in wood can be related to the prediction of crack nucleation, growth and arrest. Crack tip displacement is related to crack growth and propagation. The definition of the damage zone ahead of a crack tip is crucial for the studies of wood fracture. If the fracture process zone is small compared to the length of the crack, linear elastic fracture mechanics (LEFM) methods yield an accurate prediction of the load level at which a crack in a structural component will grow. Any deformation of the crack can be described through a combination of three fracture pure modes: Mode I-opening mode in tension, Mode II-the in plane shear mode and, Mode III-the out of plane shear mode. However, mixed fracture modes can be recognised also. The anisotropic nature of wood allows the development of six different fracture system orientations. For the situations where the fracture process zone is not small compared with the length of a crack, the energy methods and the concepts of nonlinear fracture mechanics (NLEFM) can be used. This approach can be used to accurate prediction of wood fracture behaviour through laboratory tests and in reliable interpretation of the mechanical capacities of notched small dimension timbers, or structures with mechanical connections made with fastenings (nails, bolts, shear plates, split rings), etc. A range of failure criteria have been developed based on the physics of delamination fracture in wood and in wood-based composites. These criteria included parameters that relate to the influence of loading, material characteristics and environmental factors. Experimental investigation and predictive (analytical and numerical) modeling are linked through microfractographic studies. The fundamental knowledge on fracture behaviour of wood can have relevance for structural use of timber, in pulping industry, for wood drying technology, or in processes of machining and cutting.