Fracture mechanics in timber engineering – Strength analyses of components and joints

Abstract

Traditional timber engineering design approaches use for simple geometries stress- or strain-based criteria for strength analyses. For more complex cases purely empirical methods are also used. Although practical, purely empirical methods are not general, and could lead to estimations on the unsafe side, if extrapolated outside their original scope. It is furthermore known that stress- or strain-based methods are not useful in situations where large stress or strain concentrations arise, such as close to holes or notches, in dowel joints or in glued joints. Furthermore, such phenomena like size effects cannot be predicted, unless stochastic methods are introduced.

The paper discusses some fracture mechanics strength analysis methods that in recent years have been proposed in relation to timber engineering strength design. The methods discussed range from simple hand calculations based on linear elastic fracture mechanics and useful for simple geometries to finite element analyses for general cases taking into account non-linear performance of the material during fracture. Fracture mechanics results in rational strength design, which is based on mechanics, as opposed to purely empirical methods, and with parameters with a clear physical meaning. In order to obtain material characteristics needed for this approach examples of test methods are discussed.

Application examples include structural components, dowel joints and adhesive joints. Design formulae for beams with notches at the support and for the pull out strength of glued-in rods are presented.