Preben Hoffmeyer

Evaluation of density and strength of Norway spruce wood by near infrared reflectance spectroscopy

The non-destructive evaluation of wood properties by Near Infrared Reflectance Spectroscopy (NIR) has been assessed. The surfaces of specimens of clear wood (Picea abies) were NIR-scanned, and the results compared to such properties as moisture content, density, compression strength and chemical and biological degradation. In addition, the NIR-scans of clear wood specimens were compared to the bending strength of the structural timber from which it had been cut. The NIR dependency of surface roughness was investigated and found to be of minor importance.NIR calibrations for moisture content (MC<30%), density, compression strength and chemical degradation proved that the NIR technique is an excellent non-destructive method (r2≧0.9; independent test set). Even biological degradation was determined with a most promising accuracy (r2=0.75; independent test set). For the prediction of the bending strength of timber NIR proved less efficient. However, NIR still contributed to timber strength prediction at the same level as annual ring width, the parameter which is presently visually assessed by timber graders.It is concluded that the NIR method is very versatile in the non-destructive evaluation of wood. The results merit further investigations in order to develop proper models and instrumentation for commercial use.ZusammenfassungDie Möglichkeit, Holzeigenschaften mit Hilfe der zerstörungsfreien NIR-Spektroskopie zu bestimmen, wurde überprüft. Die Oberfläche von fehlerfreiem Fichtenholz wurde mittels NIR abgetastet. Die Meßwerte wurden zu folgenden Holzeigenschaften in Beziehung gebracht: Feuchte, Druckfestigkeit, Dichte sowie chemischen und biologischem Abbau. Zusätzlich wurden die NIR-Ergebnisse noch verglichen mit der Biegefestigkeit des Bauholzes aus dem die Proben entnommen worden waren. Auch die Rauheit der Holzoberflächen wurde mit einbezogen. Sie erwies sich als rel. unbedeutend. Die NIR-Messungen erwiesen sich als hochkorreliert (r2≧0,9) mit der Feuchte (MC<30%), der Dichte, der Druckfestigkeit und dem Abbaugrad (durch Chemikalien), so daß die NIR-Technik als vorzügliche zerstörungsfreie Bestimmungsmethode anzusehen ist. Sogar der biologische Abbau konnte mit vielversprechender Genauigkeit (r2=0,75) bestimmt werden. Für die Vorhersage der Biegefestigkeit des Bauholzes war die NIR-Methode weniger geeignet. Immerhin erlaubt die NIR-Technik das Abschätzen der Biegefestigkeit mit der gleichen Zuverlässigkeit wie das Abschätzen der Jahrringbreiten, wie es zur Zeit noch bei der visuellen Holzsortierung benutzt wird. Die NIR-Spektroskopie erweist sich als vielseitig nutzbare Methode zum zerstörungsfreien Ermitteln von Holzeigenschaften. Die Ergebnisse ermutigen weitere Untersuchungen, um daraus geeignete Modelle und Geräte für industrielle Anwendungen zu entwickeln.

Equilibrium moisture content (EMC) in Norway spruce during the first and second desorptions

Abstract It is a commonly accepted notion that the equilibrium moisture content (EMC) of wood at a given relative humidity (RH) is highest during initial desorption of green wood due to an irreversible loss of hygroscopicity during the 1st desorption. The basis for this notion is investigated by assessing how drying and saturation procedures influence the differences between the 1st and the 2nd desorption curves for Norway spruce (Picea abies (L.) Karst.) sapwood. The study establishes 1st and 2nd desorption isotherms for a variety of initial conditions and it covers the RH range from 60.1% to 99.9%. The state of the water is not affected by oven-drying and rewetting as demonstrated by time domain low field NMR relaxometry. The results challenge the conclusions of earlier studies and indicate that in these studies the 2nd desorption was initiated at much too low EMC and therefore fails to describe a boundary desorption isotherm. Instead, it becomes an intermediate desorption isotherm starting at the adsorption boundary curve and crossing over to eventually meet the desorption boundary curve. The results also show that vacuum drying at room temperature only gives a modest loss of hygroscopicity compared to the green state. Conversely, oven-drying at 103°C results in a more significant loss of hygroscopicity, except for RH above 96% where an increase in EMC surprisingly is seen.

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.

Compressive fatigue in wood

Summary An investigation of fatigue failure in wood subjected to load cycles in compression parallel to grain is presented. Small clear specimens of spruce are taken to failure in square wave formed fatigue loading at a stress excitation level corresponding to 80% of the short term strength. Four frequencies ranging from 0.01 Hz to 10 Hz are used. The number of cycles to failure is found to be a poor measure of the fatigue performance of wood. Creep, maximum strain, stiffness and work are monitored throughout the fatigue tests. Accumulated creep is suggested identified with damage and a correlation is observed between stiffness reduction and accumulated creep. A failure model based on the total work during the fatigue life is rejected, and a modified work model based on elastic, viscous and non-recovered viscoelastic work is experimentally supported, and an explanation at a microstructural level is attempted. The outline of a model explaining the interaction of the effect of load duration and the effect of the loading sequences is presented.

Non-linear creep caused by slip plane formation

SummaryThe minute compression failures of the cell walls known as slip planes are quantified with respect to their occurrence in the compression side of beams subjected to mechano-sorptive creep. It is shown that slip planes occur at values of stress/strain lower than previously assumed. The slip plane intensity proves to be linearly related to creep and non-linearly related to stress. A permanent increase, ΔS, of longitudinal shrinkage /swelling results from slip plane formation, and this parameter therefore serves as a substitute for slip plane intensity, which is not easily measured. Slip plane intensity is considered a measure of accumulated damage, and the substitute parameter ΔS is used to quantify the accumulated damage of structural timber beams. The load level experienced by structural timber beams subjected to mechano-sorptive creep is shown to be reflected in the accumulated damage as assessed by ΔS.

A simple size effect model for tension perpendicular to the grain

The strength in tension perpendicular to the grain is known to decrease with an increase in the stressed volume. Usually this size effect is explained on a stochastic basis, that is, an explanation relying on the increased probability of encountering a strength reducing flaw when the volume of the material under stress is increased. This paper presents an experimental investigation on specimens with a well-defined structural orientation of the material. The experiments exhibit a large size effect and the nature of the failures encountered suggests that the size effect can be explained on a deterministic basis. Arguments for such a simple deterministic explanation of size effect is found in finite element modelling, using the orthotropic stiffness characteristics in the transverse plane of wood.

Duration of load behaviour of different sized straight timber beams subjected to bending in variable climate

This paper is the second in a series which sums up the results of an extensive project to quantify the duration-of-load (DOL) effect on different sized timber beams in different climates. The paper deals with straight (unnotched) beams. The results of various DOL-tests of stepwise and constant bending of LVL and glulam beams are reported and results of modelling outlined. It is concluded that in cyclically varying climate large cross-sections are less affected by the DOL-effect than smaller ones. The results do not show marked difference between LVL and glulam in susceptibility to the DOL-effect.ZusammenfassungDiese Arbeit ist der zweite einer Artikelreihe über ein ausgedehntes Projekt zur Quantifizierung des DOL-Effektes bei Tägern unterschiedlicher Abmessungen unter dem Einfluß verschiedener Umgebungsbedingungen. Hier werden nicht keilverzinkte Träger untersucht. Die Ergebnisse verschiedener DOL-Tests unter stufenweiser und kontinuierlicherBiegebelastung von LVL- und Glulam-Trägern werden vorgestellt. Ebenso werden Modelier-Ergebnisse beschrieben. Es ergab sich, daß große Querschnitte in zyklischem Wechselklima weniger vom DOL-Effekt betroffen sind als kleine Querschnitte. Die Ergebnisse zeigen keinen besonderen Unterschied zwischen Glulam und LVL bezüglich ihrer Empfindlichkeit gegenüber dem DOL-Effekt.

Size effect of glulam beams in tension perpendicular to grain

The strength of wood is reduced when the stressed volume is increased. The phenomenon is termed size effect and is often explained as being stochastic in the sense that the probability of weak locations occurring in the wood increases with increased volume. This paper presents the hypothesis that the lower strength is caused by stress concentrations. The stress concentrations arise from the anisotropic structure of wood, and are therefore deterministic. The hypothesis is substantiated through extensive FEM-calculations and experiments. A reasonable agreement between ultimate stresses determined experimentally and in FEM-computations was found.

An experimentally validated fatigue model for wood subjected to tension perpendicular to the grain

This study presents an experimental investigation of fatigue in wood subjected to tension perpendicular to the grain. The study has been designed with special reference to the influence of the frequency of loading. The investigation reveals an interaction between number of load oscillations and accumulated time under load to failure. This interaction corresponds to frequency dependent fatigue. Current models for damage accumulation and failure modelling are screened with respect to their ability to account for such “two-source” damage. The Damaged cracked viscoelastic material model proved to give a good basis.