Surface modification of wood using friction

The potential of linear vibration friction as an innovative means of producing increases in both surface density and surface hardness was explored. The influence of processing pressure and time on the degree of surface densification, surface hardness and surface elasticity was investigated. It was found that surface hardness (measured as Brinell hardness) was positively correlated with densification ratio. Furthermore, surface elasticity, that is the ability of the surface to recover elastically after indentation during the Brinell hardness test, could be increased by up to 33% depending on the degree of surface densification. The temperature rise due to friction was also studied. During processing, it was found that the temperature rise on both the radial and tangential surfaces was positively correlated with the processing pressure and time

Properties and set-recovery of surface densified Norway spruce and European beech

The chemistry and wetting behaviour of surface densified wood were investigated using FT-IR spectroscopy and contact angle analyses. Furthermore, set-recovery of the surface under conditions of fluctuating humidity was measured and quantitative microscopy analyses were undertaken. FT-IR indicated that no significant chemical changes took place during the densification process. However, the wettability of the densified surfaces was significantly lower than unmodified surfaces. Following several high humidity-oven dry cycles, it was found that this densification process was almost completely reversible, i.e., there was full set-recover

Surface modification of wood using friction

The potential of linear vibration friction as an innovative means of producing increases in both surface density and surface hardness was explored. The influence of processing pressure and time on the degree of surface densification, surface hardness and surface elasticity was investigated. It was found that surface hardness (measured as Brinell hardness) was positively correlated with densification ratio. Furthermore, surface elasticity, that is the ability of the surface to recover elastically after indentation during the Brinell hardness test, could be increased by up to 33% depending on the degree of surface densification. The temperature rise due to friction was also studied. During processing, it was found that the temperature rise on both the radial and tangential surfaces was positively correlated with the processing pressure and tim

Influence of grain direction in vibrational wood welding

Wood grain orientation differences in the two surfaces to be bonded yield bondlines of different strength in no-adhesives wood welding. Longitudinal wood grain bonding of tangential and radial wood sections yields an approximately 10% difference in strength results of the joint. Cross-grain (±90°) bonding yields instead a much lower strength result, roughly half that observed for pieces bonded with the grain parallel to each other. These differences can be explained by the very marked effect that homogeneity of fibre orientation is known to have on fibre–matrix composites. Oak yields lower results than beech and maple and is more sensitive to welding conditions. Differences in both anatomical and wood constituent composition can account for this difference in performance. Contrary to the other wood species, oak always presents joint bondlines where little or no increase in density at the interface is noticed. This explains its somewhat lower strength results. This is based on the different mode of bonding predominant in this species, while the other species present two different modes of bonding. Thus, two types of bondlines are observed by scanning electron microscopy (SEM): (i) bondlines where entangled fibre–matrix composites are formed at the interface and (ii) bondlines in which direct welding of the cell walls occurs, just by fused intercellular material or cell surface material. In this latter case the cells remain flat, without an entangled fibre–matrix composite being formed. This is the almost exclusively predominant case for oak. Both cases and even hybrid cases between the two have also been observed in beech

Parameters of wood welding: A study with infrared thermography

Welding of wood is a well-known joining procedure that offers several advantages over traditional mechanical fasteners or gluing. During welding, extensive solid-state transformation phases occur in the so-called melting zone and the heat-affected zone. The nature and the extension of such transformations are correlated to the energy input and thus to the heat generated during the process at the wood joint interface. In the present work the influence of the welding parameters and wood grain orientation on the temperature profile and distribution and final strength of welded connections was investigated. For this purpose, the characteristics of the joints were evaluated with both destructive and non-destructive techniques. Non-destructive evaluation was performed with infrared thermography, which allowed measurement of the maximal and average peak temperature, temperature profile and distribution, and rate of temperature increase. Thus, this technique can also be used to detect welding defects and to provide information on material modification during welding

Parameters of wood welding: A study with infrared thermography

Welding of wood is a well-known joining procedure that offers several advantages over traditional mechanical fasteners or gluing. During welding, extensive solid-state transformation phases occur in the so-called melting zone and the heat-affected zone. The nature and the extension of such transformations are correlated to the energy input and thus to the heat generated during the process at the wood joint interface. In the present work the influence of the welding parameters and wood grain orientation on the temperature profile and distribution and final strength of welded connections was investigated. For this purpose, the characteristics of the joints were evaluated with both destructive and non-destructive techniques. Non-destructive evaluation was performed with infrared thermography, which allowed measurement of the maximal and average peak temperature, temperature profile and distribution, and rate of temperature increase. Thus, this technique can also be used to detect welding defects and to provide information on material modification during weldin

Solid wood joints by in situ welding of structural wood constituents

Mechanicallyinduced wood flow welding, without any adhesive, is here shown to rapidly yield wood joints satisfying the relevant requirements for structural application. The mechanism of mechanicallyinduced vibrational wood flow welding is shown to be due mostly to the melting and flowing of the amorphous polymer materials interconnecting wood cells, mainly lignin, but also some hemicelluloses. This causes the partial detachment of long wood cells and wood fibres and the formation of an entanglement network in a matrix of melted material which then solidifies. Thus, it forms a wood cell/fibre entanglement network composite having a molten lignin polymer matrix. During the welding period, some of the detached wood fibres no longer held by the interconnecting material are pushed out of the joint as excess fibre. Crosslinking chemical reactions of lignin and of carbohydratederived furfural also occur. Their presence has been identified by CPMAS C-13 NMR. These reactions are, however, relatively minor contributors during the very short welding period. Their contribution increases after welding has finished, explaining why relatively longer holding times under pressure after the end of welding contribute strongly to obtaining a good bond

Sviluppo e applicazioni di nuove resine melammina-urea-formaldeide (MUF) per del legno

This report concerns the research focused on the study of structural wood adhesives properties. The research was conducted on four levels namely: 1 Formulation, synthesis and characterization of new MUF wood adhesives. 2 Development of a new honeymoon MUF adhesive system for exterior grade glulam and finger-joints. 3 Experimentation of new chemical additives able to improve the bonding properties of commercial adhesives. Bonding of high moisture content wood. 4 Comparative performances of fast-setting single and separate application wood adhesives for structural glulam.Cette recherche se situe dans le cadre d'une collaboration franco-italienne visant à développer les connaissances sur les adhésifs pour usages structurels. La recherche a été conduite en intégrant l'étude de nouveaux systèmes d'agents collants par le biais de la formulation, de la synthèse et de la caractérisation des résines MUF.Voici les quatre lignes de recherche abordées : 1. Formulation, synthèse et caractérisation de nouvelles résines aminoplastiques MUF à hautes performances et faible impact sur l'environnement. 2. Développement d'une nouvelle technique de collage bi-composant et durcissement rapide à température ambiante, permettant pour la première fois l'emploi de résines MUF pures. 3. Test de nouveaux additifs capables d'augmenter les performances et de diminuer le degré de toxicité des résines MUF de production industrielle. 4. Evaluation des performances des trois différentes techniques de collage utilisées pour l'assemblage du bois caractérisé par une haute humidité.La presente ricerca si inserisce nell'ambito di una collaborazione italo-francese volta ad ampliare le conoscenze sugli adesivi per usi strutturali. La ricerca è stata condottaintegrando la sperimentazione di nuovi sistemi collanti (migliorati o no attraverso l'uso di additivi) con la formulazione, la sintesi e la caratterizzazione di specifiche resine MUF prodotte in laboratorio.L'incollaggio deI legno è uno dei principali mezzi che la modema tecnologia mette a disposizione per valorizzare questa importante materia prima, ecologica e rinnovabile. La scelta del sistema adesivo e della specifica tecnologia d'incollaggio condiziona perà sempre pesantemente la qualità, il costa e l'impatto ambientale dei prodotti. Le crescenti esigenze del mercato indirizzano quindi l'attuale ricerca scientifico-tecnologica versol'introduzione di collanti efficaci economici e compatibili con l'ambiente. In tale contesto si inserisce il presente lavoro i cui obbiettivi riguardano proprio la messa a punto di adesivi e sistemi d'incollaggio innovativi allo scopo di ottenere un migliore equilibrio tra qualità, ecocompatibilità e costa dei prodotti realizzati con l'incollaggio

Long-term behaviour of pre-stressed timber-CFRP composites

The application of pre-stressed CFRP-Iamella to timber beams is difficult because the concentrated shear stresses at the beam ends often lead to delamination of the lamella or to timber failure due to tension perpendicular to the grain. An alternative method involving decreased pre-tension force towards the beam ends shows promising results, but requires large and heavy equipment and is therefore not suitable for on-site applications. In order to post-strengthen historic wooden beams with CFRF-Iamella, a different approach of prestressing was developed where the timber beam is cambered prior to the bonding of the CFRP-Iamella with an epoxy resin. In short-term bending tests the potential efficiency of the method was confirmed by increased bending stiffness (+14%) and bending strength (approx. + 30%). In order to assess the long-term behaviour of timber after pre-stressing and CFRP reinforcement, six loaded and three unloaded spruce battens were exposed to seven climate regimes of one week duration each. Although the exposure time of each cycle was too short to reach moisture equilibrium, the cycling of temperatures between 19° and 49°C and relative humidities between 36% and 70% induced average wood moisture changes of up to 3.3%. Creep deflection of the beams showed a progressive trend in the first three cycles, where the timber is supposed to undergo plastic deformation, the deflection gradient reduces in the second part of the exposure time. The comparison of the creep factor of the loaded and unloaded specimens and the unchanged MOE of the battens after loading in cycling climate provided evidence that the reinforcing system, especially the applied epoxy resin, is able to withstand cyclic changes in climate class 2 without relevant stress relaxation

Strengthening of timber structures using pre-stressed carbon fibre lamellas

The refurbishment of old buildings often comes hand in hand with an increase of the dead- and live load. This and the higher safety factors often make a reinforcement of the old beams necessary. The use of steel is often not regarded as an adequate solution in a timber construction. This because of the increase of the structures self weight, the appearance and the corrosion in marine environments. The use of CFRP allows methods, which are nearly invisible. In order to gain the needed contribution of the reinforcement to the service limit state a solution involving pre-stressing is requested otherwise the amount of carbon fibre would be very high and therefore the method not cost efficient. Pre-stressed carbon fibres are commercially used to strengthen concrete and steel structures (especially old structures in cast iron). Until now the methods were not successfully applied to timber this mostly because of the delaminating of the carbon fibre strip due to the concentrated shear stress at the beams end. Moreover, compared to the steel, the FRP composites show a different structural behaviour and their utilisation for the in-situ external bonding has not yet been fully explored. In order to gain quantitative and qualitative knowledge on structural adhesive and fibre-reinforced polymer (FRP), suitable for in-situ post-strengthening of timber structures, the authors investigated in Cooperation with SIKA and EMPA an application method using pre-stressed carbon fibre strips. In this method the timber beam is cambered prior to the installation of the FRP lamella. This cambering is done with a in the height adjustable prop placed in the middle of the beam. The pre-stress force in the FRP is not constant. It peaks in the middle of the beam were it is mostly needed and is zero towards the ends were high stress would causes delamination. A calculation model was developed and verified. Thus, this paper reports on the latest progress made on the development of the in-situ strengthening of timber beams with pre-stressed carbon fibres