Fracture behaviour of softwood

Abstract

In design wood is regarded as a brittle material, depending on the stress direction, duration of loading and moisture content. The usual presumption is that wood is perfectly brittle–elastic (linear elastic fracture mechanics, LEFM), or that its behaviour mimics other materials such as concrete. Attempts to verify modelling assumptions have been very limited. To date the authors have focused on opening mode (mode I) behaviour of softwood. Real-time microscopic observations have been made in the vicinity of crack tips. Small end-tapered ‘double cantilever beam’ specimens were loaded within a scanning electron microscope and direct measurement made of surface strain fields near cracks. This revealed that a ‘bridged crack’ model mimics behaviour best. Non-linear bridging stresses depend on the crack opening displacement and fall to zero once crack faces are separated. Such precise modelling is necessary only for short cracks in proximity to boundary conditions, e.g. in mechanical connections. Simplified fracture-based design methods can be employed for certain common problems. For example, a closed-form LEFM design equation was developed to predict critical load levels for notched bending members.