Svetlana Vasic

Bridging crack model for fracture of spruce

Abstract An analysis is presented of how cracking in wood can be predicted using fracture mechanics. In situ, real-time, scanning electron microscopy was used as a tool to estimate the physical mechanism of fracture in softwoods using end-tapered double cantilever beam specimens. It was found that bridging behind the crack tip is the main toughening mechanism, which contributes to non-linear wood behavior in the presence of stress concentrations. A new bridging crack model is presented that mimics the observed cracking mechanism. Intrinsic flaw size is found to correspond to the fiber length of spruce.

Coupled experiments and simulations of microstructural damage in wood

In this paper, we explore ways to couple experimental measurements with the numerical simulations of the mechanical properties of wood. For our numerical simulations, we have adopted a lattice approach, where wood fibers or bundles of wood fibers are modeled as discrete structural elements connected by a lattice of spring elements. Element strength and stiffness properties are determined from bulk material properties. Damage is represented by broken lattice elements, which cause both stiffness and strength degradation. The modeling approach was applied to small specimens of spruce subjected to transverse uniaxial tension, and mode I transverse splitting. The model was found to be good at predicting the load-deformation response of both notched and unnotched specimens, including the post-peak softening response. In addition, the damage patterns predicted by the model are consistent with those observed in the experiments.

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.

Finite element techniques and models for wood fracture mechanics

Numerical models for wood fracture and failure are commonly based on the finite element method. Most of these models originate from general theoretical considerations for other materials. This limits their usefulness because no amount of complexity in a model can substitute for lack of an appropriate representation of the physical mechanisms involved. As for other materials, wood fracture and failure models always require some degree of experimental calibration, which can introduce ambiguity into numerical predictions because at present there is a high degree of inconsistency in test methods. This paper explores avenues toward achieving models for wood fracture that are both appropriate and robust.

Failure mechanisms in wood-based materials: A review of discrete, continuum, and hybrid finite-element representations

Abstract Challenges arise in finite element (FE) analyses that predict mechanical failure in wood-based materials because their structural complexity is difficult to mimic. When considered at the macro scale, wood and engineered wood composites can reasonably be assumed to behave as homogenous continua. However, accurate meso- and micro-scale representations require a different approach. Models employing discrete FEs are robust tools for detailed failure analysis, because the elements can be made to mimic the functions of morphological structures in the material. Hybrid models that meld continuum and discrete FEs also show good promise as generalised analysis tools, but as yet their development is in its infancy. In the future, beyond mechanical damage, other energy sinks also need to be included in models, and computational efficiency should be improved. In this overview, the advantages and limitations of alternative FE representations are demonstrated in terms of failure processes in wood-based materials via case analyses.

Static bending and toughness of wood polymer composites (yellow birch and basswood)

Wood polymer composites (WPC) were made from basswood and yellow birch using six cell lumen type polymer formulations. The study was designed to get insight into the influence of wood density and polymer formulation on certain WPC mechanical properties. Small specimens were tested for toughness, stiffness, hardness and bending strength using standard ASTM methods. Results showed that stronger and stiffer polymers produce tougher and stronger WPC, but the effect is small. Thus, there is a wide range of polymer properties which produce useful WPC properties. Study of the fracture surfaces using Scanning Electron Microscopy (SEM) showed that WPC made with different polymers fractured differently and polymer containing a coupling agent bonded to the cell wall. However the cell wall bonding had no noticeable influence on WPC mechanical properties.

Contact-crack problem with friction in spruce

This analysis presents of how friction in wood-to-metal interfaces can affect data from wedge-splitting tests on Double Cantilever Beam (DCB) spruce specimens. Cases evaluated were short cracks, and cracks emanating from a long notch. A numerical analysis is performed to clarify the influence of frictional characteristics at the wood-aluminium wedge interface on stress intensity factors. This demonstrates a correlation between fracture toughness (KIc) and contact friction (μ) in fracture mode I for short cracks. However, the effect of friction diminishes, when the crack tip moves away from the contact area. Analysis and conclusions are relevant to other situations where components develop cracks in close proximity to contact regions, for example, in predicting the strength with dowel fasteners.ZusammenfassungDiese Analyse zeigt, wie sich Reibung auf Holz-Metall-Grenzflächen auf die Messergebnisse bei Keilspaltungversuchen von DCB Fichtenholzproben auswirkt. Ausgewertet wurden kurze Risse und Risse, die von einem langen Einschnitt ausgehen. Eine numerische Analyse wurde durchgeführt, um die Einflüsse der Reibungscharakteristika an der Grenzfläche zwischen Holz- und Aluminium-Keil auf die Spannungs-Intensitäts-Faktoren zu klären. Das weist einen Zusammenhang auf zwischen Bruchfestigkeit und Kontaktreibung im Bruchmodus I bei kurzen Rissen. Die Reibung verringert sich jedoch, sobald sich die Rissspitze von der Kontaktfläche weg bewegt. Analyse und Schlussfolgerungen sind relevant für andere Situationen, in denen Konstruktionselemente Risse in unmittelbarer Nähe von Kontaktflächen entwickeln, z.B. beim Abschätzen der Festigkeit von Dübelverbindungen.