Per Johan Gustafsson

Influence of bondline brittleness and defects on the strength of timber finger-joints

Numerical finite element results on the mechanical behaviour of finger-joints for laminated timber products are presented. Simulations are based on a nonlinear fracture mechanics description of the bondlines in the finger-joints. It is shown that, for a commonly used resorcinol–phenol adhesive, there is potential of increasing substantially the finger-joint strength by increasing the fracture energy of the bond. However, even a single, small glueline void can have a considerable influence on reducing the joint strength.

DOL behaviour of end-notched beams

The long-term loading strength of end-notched beams made of glulam and LVL was tested. The beams were of various sizes, with and without a moisture sealing at the notch. Tests were conducted in open shelter climates, and at constant and cyclic relative humidity. The short-term strength was tested after conditioning in the various climates. Both the short-term and long-term strength of beams without moisture sealing was significantly affected by transient moisture conditions, e.g. the moisture variations due to change of the time of the year. The strength was only slightly affected by the magnitude of the humidity, if this was kept constant. Duration of load strength reduction factors were evaluated for 6 months of loading. Average reductions in ultimate failure strength ranged from 0.68 for small LVL beams without moisture sealing tested during spring and summer to 0.81 for large glulam beams with a moisture sealing at the notch.ZusammenfassungDie Zeitstandfestigkeit keilverzinkter Träger aus Glulam und LVL wurde geprüft. Die Balken waren unterschiedlich bemessen und mit oder ohne Endversiegelung versehen Die Prüfung erfolgte sowohl in natürlichem, geschützten Klima als auch bei konstanter Feuchte und im Wechselklima. Die Kurzzeitfestigkeit wurde jeweils nach dem Konditionieren geprüft. Alle Ergebnisse an nicht versiegelten Proben waren deutlich beeinflußt durch Feuchteübergänge, z.B. die Feuchtewechsel innerhalb eines Jahres. Die Festigkeit hing nur in geringem Maße vom Wassergehalt ab, wenn dieser konstant gehalten war. Reduktionsfaktoren (kmod) wurden für ömonatige Belastung ermittelt. Die durchschnittlichen Werte für die Bruchlast lagen im Bereich von 0,68 für kleine LVL-Träger ohne Feuchteversiegelung (geprüft im Frühjahr und Sommer) bis zu 0,81 für lange Brettschichtholz-Träger mit Feuchteversiegelung der Keilzinken.

Shear strength of beam splice joints with glued-in rods

Splitting failure in beam splice joints with glued-in rods parallel to grain in endwood subjected to pure shear is considered. A simple theoretical expression based on beam-on-elastic-foundation theory and quasi-non-linear fracture mechanics is presented for calculation of the joint strength. Tests were conducted on jointed beams in a four-point bending test setup in which the joints were located at the point of pure shear force. Hardwood dowels with a diameter of 12 mm and a glued-in length of 120 mm were used as rods, and various beam cross sections and dowel configurations were tested. The theory presented is found to agree well with test results in all cases in which the edge distance of the glued-in rods is relatively small. Some test results indicate that the theory may be conservative in case of large edge distances.

Material and structural failure criterion of corrugated board facings

Abstract A failure stress criterion for corrugated board facings is presented. The failure criterion is based on material failure and structural local buckling failure, which are evaluated in a combined analysis procedure. The failure stress is compared with collapse experiments on corrugated board cylinders and the failure stress presented herein is seen to be in much better agreement with the measured stresses than the Tsai–Wu failure criterion alone. The fluting wavelength of the corrugated board is also varied for the purpose of strength sensitivity analysis of corrugated board.

Experimental and numerical determination of the hygroscopic warping of cross-laminated solid wood panels

Abstract The moisture-induced warping of three-layered cross-laminated solid wood panels made of Norway spruce was studied. The panels were exposed to different climate conditions at 65% and 100% relative humidity at the two panel faces. The results showed increasing cup deformation with an increasing relative thickness of the outer layers. The annual growth ring orientation was found to have a significant influence on the magnitude of the cup deformation. Measurements and numerical simulations of the moisture distribution within the panel were made in order to provide data for numerical simulations of the warping. A distinctive moisture profile with a conspicuous influence of the adhesive bond lines was found. The coefficient of diffusion of the adhesive bond lines was determined from the measurements and simulations. The mechanical material model used for the warping simulations takes into account elastic strain, moisture-induced swelling, and mechano-sorptive strain. The simulations showed good agreement with the warping test results. The most important material parameters for the cup deformation, which were identified in a parametric study of a panel with vertically oriented annual rings, are the moduli of elasticity and the swelling coefficients in the longitudinal and radial direction. Furthermore, the mechano-sorptive coefficient in radial direction was found to have a significant influence on warp.

A probabilistic fracture mechanics method and strength analysis of glulam beams with holes

A probabilistic fracture mechanics method is presented and applied to glulam beams with holes. The method is based on a combination of Weibull weakest link theory and a mean stress method which is a generalization of linear elastic fracture mechanics. Combining these two methods means that the global strength will be governed by both fracture energy and material strength and also that the stochastic nature of the material properties are taken into account. The probabilistic fracture mechanics method is evaluated by comparison to experimental test results. The method shows good ability to predict strength, with the exception of very small beams where the capacity is overestimated. The comparison to experimental tests deals also with other methods for strength analysis including code design methods.ZusammenfassungEin probabilistisches Bruchmechanikverfahren wird vorgestellt und auf Brettschichtholzträgern mit Durchbrüchen angewandt. Grundlage dieser Methode ist eine Kombination der Weibull Theorie des schwächsten Gliedes und der Methode der mittleren Spannung, einer Verallgemeinerung der linear-elastischen Bruchmechanik. Die Kombination dieser beiden Methoden bedeutet, dass die globale Festigkeit sowohl von der Bruchenergie als auch der Materialfestigkeit bestimmt wird und dass die stochastische Natur der Materialeigenschaften berücksichtigt wird. Das probabilistische Bruchmechanikverfahren wird durch Vergleich mit Versuchsergebnissen überprüft. Das Verfahren erweist sich als gut geeignet zur Vorhersage der Festigkeit mit Ausnahme von sehr kleinen Trägern, deren Tragfähigkeit überschätzt wird. Mit den Versuchsergebnissen werden auch andere Methoden der Festigkeitsanalyse einschließlich normierter Bemessungsverfahren verglichen.

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.

The Hygroscopic Warping of Cross-Laminated Timber

This chapter focuses on moisture-induced deformations in three-layered cross-laminated timber with a symmetrical build-up, where the fibre direction of the middle layer is oriented perpendicular to that of the outer layers. Dimensional stability, i.e. the ability to resist warping, is of main interest for the application of such wood panels. The cross lamination of the layers is advantageous to warping. The moisture-induced expansion/contraction of each single layer is partly restrained by the adjacent layers. The free swelling and shrinkage of adjacent layers differ approximately by a factor of 10 (radial/longitudinal) to 20 (tangential/longitudinal). As a consequence of this difference, stresses and even cracks may occur. In large-scale panels warping was observed. This reduces the serviceability in the practice. Due to a climate gradient considerable distortions (warp) in the form of cup and twist may occur. Models to calculate the moisture and stress fields are provided and validated to experimental tests.

Impact of growth characteristics on the fracture perpendicular to the grain of timber

The natural material wood is commonly graded with regard to the parallel to the grain strength and stiffness properties and taking into account different growth characteristics such as knots and grain deviations. In this paper the impact of knots and grain deviations on the fracture perpendicular to the grain of timber is analysed by means of numerical models. The results are used for the calibration of an analytical model. With this model it is possible to evaluate the impact of growth characteristics on the perpendicular to the grain fracture and compare the results with test data from literature. The evaluation shows that certain growth characteristics increase the strength perpendicular to the grain. This is in contrast with current grading procedures, where such growth characteristics are considered as being strength decreasing. The results are compared with a model for the description of the effects of growth characteristics on the distribution characteristics of the strength perpendicular to the grain. This strongest link model can be used to describe phenomena with a parallel system of failure events.

Shear fracture characterization of green-glued polyurethane wood adhesive bonds at various moisture and gluing conditions

Abstract The shear fracture properties of green-glued one-component polyurethane (PUR) wood adhesive bonds subjected to kiln drying were investigated. The local shear strength and fracture energy of the wood adhesive bonds were determined from experimentally recorded complete shear stress versus deformation curves of the bond line. A stable test set-up and small specimens that were anti-symmetrically loaded were used in order to get a uniform and pure state of shear stress. Different moisture contents (MCs) and pressing times were investigated. The fracture properties of conventionally dry-glued wood adhesive bonds and of solid wood were used as reference. The results show that the fracture energy of green-glued bonds with PUR adhesive is dependent on the MC of wood and on the pressing time. The same fracture energy and strength can be obtained by green gluing as by dry gluing, but there seems to exist a maximum MC of sapwood, in the range between 78% and 160%, and a minimum pressing time, in the range between 3 h and 48 h, for which it can be achieved. Both dry- and green-glued polyurethane adhesive bonds were more ductile than solid wood.