Jean-Luc Coureau

Elastic layer model for application to crack propagation problems in timber engineering

A fracture mechanics model for analysis of crack initiation and propagation in wood is defined and applied. The model has the advantage of being simple, yet it enables reasonably general and accurate analysis commonly associated with more complex models. The present applied calculations are made by means of the finite element method and relate to progressive cleavage fracture along grain. The calculations concern a tapered double cantilever beam specimen and an end-notched beam. Comparisons are made of experimental test results. The fracture properties of the wood are modelled by means of a very thin linear elastic layer located along the crack propagation path. The properties of the layer are such that the strength and fracture energy of the wood are represented correctly. This makes a single linear elastic calculation sufficient for strength prediction. Both crack development and pre-existing cracks can be analyzed. Both material strength and fracture energy and stiffness are taken into account, their relative influence on structural strength being different for different elements. The fracture layer is in the finite element context represented by joint elements. Propagation of a crack can be analyzed either by a series of elastic calculations corresponding to different crack lengths or by use of a finite element code for non-linear analysis. The computational results include sensitivity analysis with respect to the influence of the various material parameters on structural strength.

Mixed mode fracture of glued-in rods in timber structures

Glued-in-rods in timber structures lead to overcome the use of traditional bolted connections, preserve a large part of the original timber and offer aesthetic benefits. Several research programs were achieved to improve the mechanical knowledge of this technique, exhibiting experimentally the influence of materials and the effect of the geometric configuration. From these experimental results, some design rules predicting the axial strength are available, but a common criterion is still lacking. This paper relates to experimental investigations and finite element computations on glued-in rods, with the aim of providing a better knowledge about their mechanical behavior until failure. An experimental campaign is carried out on single glued-in rod connections. The finite element modeling reproduces the experimental configuration: it exhibits significant normal stress (to the interface) at the onset of the bonding, in comparison with shear stress. Within the framework of equivalent linear elastic fracture mechanics, resistance curves in mode I and mode II are established for each specimen. Finally, a mixed mode fracture criterion (I/II) is used to describe the fracture process zone development at the wood-adhesive interface (failure zone). An analytical formulation is then proposed allowing the evaluation of peak load of each specimen, which highlights a new approach for the design of such connections.

Wood heterogeneities and failure load of timber structural elements: a statistical approach

Abstract In this work, the effect of spatial location and geometrical characteristics of wood defects (only knots) on the failure load of timber structural elements is addressed. For a large set of timber structural elements (188 Maritime pine full size beams), spatial location and geometrical characteristics of knots were estimated from the analysis of external images of each element, and then, modulus of elasticity (MOE) and failure load obtained from a four-point bending test were experimentally estimated. From this large database, correlations between the failure load, MOE and geometrical characteristics of knots were studied on the basis of a polynomial regression and a nonlinear regression based on neural network. It was shown that accounting for simple geometrical characteristics of knots, such as the area, the circularity and the location, in addition to the MOE, significantly improves the prediction of the failure load of structural elements.

Influence of moisture content on mode I fracture process of Pinus pinaster : evolution of micro-cracking and crack-bridging energies highlighted by bilinear softening in cohesive zone model

This paper is dedicated to the study of the effect of moisture content on fracture properties of wood through the corresponding effect on the softening function used in cohesive zone model to describe quasi-brittle failure of wood. Bi-linear softening parameters of cohesive zone model are estimated from equivalent linear elastic fracture mechanics resistance curve obtained from a significant number of fracture tests performed in mode I for a wide range of moisture contents (from 5 to

Cohesive zone model and quasibrittle failure of wood: a new light on the adapted specimen geometries for fracture tests.

30\%

Effect of temperature on the mechanical performance of glued-in rods in timber structures

30% moisture). The evolution of the cohesive zone parameters as a function of the moisture exhibits a moisture dependence of the fracture properties of wood and especially the increase of the cohesive energy related to the crack-bridging phenomenon.