• Danielsson, Henrik
• Jonasson, Johannes
• Serrano, Erik

Abstract

Softwood species are the most commonly used species in structural applications in north-<br/>ern Europe. However, in the last couple of years, utilization of hardwoods has gathered<br/>increased momentum, and different hardwood species, such as birch, have even been uti-<br/>lized in cross laminated timber and glued laminated timber [1]. In general hardwoods<br/>show an increase in strength and stiffness as compared to softwoods. However, the in-<br/>crease in fracture toughness is less pronounced, and, consequently, increased brittleness<br/>can be expected.<br/>Many structural failures stem from introducing a new material, where the behavior is not<br/>completely understood or characterized [2]. Consequently, ahead of using a material with<br/>increased brittleness in structural applications, such as birch, understanding and char-<br/>acterizing the failure and/or fracture behavior is highly important. Research regarding<br/>mechanical behavior of birch has previously been carried out in e.g [3], where sawn birch<br/>timber boards were examined under compression loading, both parallel and perpendicular<br/>to the grain.<br/>The aim of the present work is to characterize the fracture behavior of birch in tension<br/>perpendicular to grain. The experimental work to evaluate the fracture energy has been<br/>carried out according to the standardized Nordtest method [4], where a single edge notched<br/>beam (SENB) is loaded in three-point-bending. Softwood structural timber of class C24<br/>was used as a reference.<br/>In addition to the experimental investigations, numerical evaluations were carried out<br/>with several finite element models, both two- and three-dimensional, corresponding to<br/>the experimental set-up. Crack propagation was modeled along a predefined crack path<br/>where the strain-softening behavior was modeled by discrete nonlinear springs with intial<br/>length equal to zero, see Figure 1. Bi-linear, tri-linear and linear-exponential curves of<br/>the stress (σ) versus deformation (δ) response were used to model the strain-softening<br/>behavior. All types of stress-deformation curves were then evaluated and calibrated after<br/>the experimental results with a cross-validation calibration approach

Topics

• impedance spectroscopy
• grain
• laser emission spectroscopy
• crack
• strength
• fracture behavior
• fracture toughness