Perpendicular to grain fracture analysis of wooden structural elements - Models and applications

Sammanfattning: This thesis concerns experimental and theoretical work regarding perpendicular to grain fracture in wooden structural elements. The experimental part concerns strength tests of glulam beams with a hole. The theoretical parts concern development of two models for strength and fracture analysis, both based on fracture mechanics approaches, and their application to analysis of beams with a hole and dowel-type connections loaded perpendicular to grain. The experimental tests of glulam beams with a hole include investigations of influence of beam size, bending moment to shear force ratio and hole placement with respect to beam height. A strong influence of beam size on the nominal strength was found, increasing the beam height and length dimensions by a factor of 3.5 gave about 35% reduction in mean nominal strength. Eccentric hole placement with respect to beam height gave about 5-15% reduction in beam strength compared to tests of holes with centric placement. A 2D probabilistic fracture mechanics method for strength analysis is further presented. This method is based on a combination of Weibull theory and a mean stress method, which is a generalization of Linear elastic fracture mechanics. Combining these two methods means that strength predictions are governed by both material strength and fracture energy and also that the stochastic nature of the material properties is taken into account. The method was applied to strength analysis of glulam beams with a hole. Based on comparison to results of experimental tests, the method appears to have fairly good ability to predict beam strength for large and medium-sized beams but overestimates the capacity of small beams. A 3D cohesive zone model is further presented, formulated using theory of plasticity and accounting for orthotropic material behavior. The material model is applied to a predefined potential crack plane, within which a fracture process zone may initiate and evolve. The Tsai-Wu criterion is used as criterion for initiation of softening, meaning that all six stress components are allowed to influence the local softening behavior and hence also the global response. The material softening performance after the instant of softening initiation is governed by the three out-of-fracture-plane stress and deformation components, corresponding to crack opening and crack shear slip in two directions. The highly nonlinear global response, often including snap-back, is solved in an incremental-iterative fashion using either a Newton-Raphson method or an arc-length type of path following method. The cohesive zone model was used for fracture analysis of beams with a hole and symmetrically and asymmetrically loaded dowel-connections. Results relating to structural element global strength and fracture course, including the 2D extension of the fracture process zone, are presented. Results of numerical analyses are compared to results of experimental tests, showing overall good agreement both in terms of global strength and general characteristics of the fracture course.