Measurement of moisture-related strain in bonded ash depending on adhesive type and glueline thickness

Structural wood-adhesive bonds (WAB) have to be durable while subjected to considerable stresses caused by mechanical loads and moisture content changes. To better understand the moisture-related durability of WABs, knowledge is important of how moisture changes generate strain in the bond. In this paper, strain on end-grain surfaces of bonded ash specimens was analyzed by means of digital image correlation. Strains were generated by wood shrinkage, and the evaluation was focused on shear strain (SStr). The bond lines were studied depending on the adhesive type - phenol resorcinol formaldehyde (PRF), melamine urea formaldehyde (MUF), polyurethane (PUR), and emulsion polymer isocyanates (EPI). Moreover, three different glueline (GL) thicknesses of MUF were taken into consideration. Comparing the adhesive types, SStr distributions (SStrD) were strongly influenced by adhesive elasticity. MUF and PRF bonds were quite rigid and were associated with pronounced strain amplitudes in and close to the GL together with strain dissipation reaching deep in the wood. PUR and EPI adhesives were more elastic and therefore allowed for smoother strain transition showing less distinct strain peaks. GL thickness had significant impact on SStrD. A high strain level and direct strain transition between adherends was found for the 0.01 mm GL, whereas a pronounced strain decrease was observed in the 0.1 and 0.2 mm GLs. This indicates different stress levels in the wood-adhesive interface dependent on GL thickness

Anwendung geführter Wellen aus der Überprüfung der Sensorkopplung für die Bestimmung von Prüfkörpereigenschaften und Schädigungsgrad

Im Rahmen quasistatischer und zyklischer, bruchmechanischer Biegeversuche an verklebten Holzprüfkörpern wurden diese mit resonanten Schallemissionssensoren überwacht. Zweck der Schallemissionsanalyse war, neben visueller Beobachtung und Auswertung der Nachgiebigkeit ein weiteres, unabhängi- ges Kriterium für Rissbeginn und Rissfortschritt zu nutzen. Zusätzlich wurde beim Beginn und am Ende jedes Versuchs die Kopplung der Schallemissionssensoren mittels elektrischer Pulsanregung jedes Sensors und Aufzeichnung der Wellenformen mit den übrigen Sensoren überprüft. Die dadurch in den balkenförmigen Prüfkörpern entstehenden Lambwellen können, neben der Kontrolle der Sensorkopplung, auch für die Bestimmung des Elastizitätsmoduls der Prüfkörper in Ausbreitungsrichtung der Wellen vor und nach der Rissausbreitung und somit auch für eine vergleichende Abschätzung des Schädigungsgrads genutzt werden. Der Beitrag diskutiert ausgewählte Ergebnisse dieser Analysen. Diese zeigen das Potential der Methode geführter Wellen in Kombination mit Schallemissionsüberwachung zur Bestimmung der Schädigungsentwicklung

A timber guardrail for highways made with hardwoods

A timber guardrail made of sustainable tropical hardwoods has been developed in the Netherlands. The guardrail is an environmentally friendly alternative for zinc-coated steel barriers. The guardrail is made of a combination of two durable hardwood species: angelim vermelho (Dinizia excelsa) from Brazil and azobé (Lophira alata) from Africa. Full-scale tests have shown that the guardrail is able to withstand the impact of a 13000 kg bus driving at a speed of 70 km/h and an impact angle of 20° as well as that of a car of 900 kg having an impact speed of 100 km/h and same angle. Steam bent curved boards are used as energy absorbers from the passenger car impact. After the full-scale tests with the bus, no damage was found in the timber elements, and the guardrail had only to be straightened, saving repair costs during the service life of a guardrail. The guardrail fulfils the requirements specified in European standard EN 1317 Road Safety Systems for the H2 level with accident severity index of 1.0

Timber sheet pile-vegetation model for stream bank retaining structure

Timber sheet piles are widely used to protect canal and stream banks. Quite often, riparian vegetation also grows along these retaining structures. Roots of riparian vegetation mechanically reinforce the soil with their root systems. A timber sheetpile- vegetation model is developed taking into account the mechanical reinforcement of the vegetation roots. The model uses easy to obtain physical parameters, which makes it suitable to have a preliminary estimate of how the forces on the bio engineered structure would evolve

Strength grading of hardwoods using transversal ultrasound

Detection of local wood inhomogeneities is important for accurate strength and stiffness prediction. In hardwood specimens, visual characteristics (e.g. knots or fibre deviation) are difficult to detect, either with a visual surface inspection or by the machine. Transversal ultrasound scan (TUS) is a non-destructive evaluation method with high potential for hardwoods. The method relies on differences in ultrasound wave propagation in perpendicular to the grain direction. The aim of this study is to estimate and analyse the capabilities of TUS for defect detection in hardwoods and prediction of mechanical property values. In the current paper, the TUS was applied to the hardwood species European ash (Fraxinus excelsior) and European maple (Acer sp.). In total, 16 boards of both specimens were completely scanned perpendicular to the grain using a laboratory scanner with drycoupled transducers. The measurements were processed to 2D scan images of the boards, and image processing routines were applied to further feature extraction, defect detection and grading criteria calculation. In addition, as a reference for each board, all relevant visual characteristics and mechanical properties from the tensile test were measured. Using the TUS global fibre orientation, the size and the position of the knots can be detected. Knottiness correlates to the strength properties similarly or even better compared to the manual knottiness measurement. Between the global fibre orientation (measured using TUS and measured on the failure pattern) no correlation could be found. The ultrasound MOE perpendicular to the grain does not show any meaningful correlation to the elastic-properties parallel to the grain. In overall, TUS shows high potential for the strength grading of hardwoods

Measurement of moisture-related strain in bonded ash depending on adhesive type and glueline thickness

Structural wood-adhesive bonds (WAB) have to be durable while subjected to considerable stresses caused by mechanical loads and moisture content changes. To better understand the moisture-related durability of WABs, knowledge is important of how moisture changes generate strain in the bond. In this paper, strain on end-grain surfaces of bonded ash specimens was analyzed by means of digital image correlation. Strains were generated by wood shrinkage, and the evaluation was focused on shear strain (SStr). The bond lines were studied depending on the adhesive type – phenol resorcinol formaldehyde (PRF), melamine urea formaldehyde (MUF), polyurethane (PUR), and emulsion polymer isocyanates (EPI). Moreover, three different glueline (GL) thicknesses of MUF were taken into consideration. Comparing the adhesive types, SStr distributions (SStrD) were strongly influenced by adhesive elasticity. MUF and PRF bonds were quite rigid and were associated with pronounced strain amplitudes in and close to the GL together with strain dissipation reaching deep in the wood. PUR and EPI adhesives were more elastic and therefore allowed for smoother strain transition showing less distinct strain peaks. GL thickness had significant impact on SStrD. A high strain level and direct strain transition between adherends was found for the 0.01 mm GL, whereas a pronounced strain decrease was observed in the 0.1 and 0.2 mm GLs. This indicates different stress levels in the wood-adhesive interface dependent on GL thickness.ISSN:0018-3830ISSN:1437-434

Machine strength and stiffness prediction with focus on different acoustic measurement methods

Strength grading is an important step for the production of homogenous and high-quality solid wood material. In particular, for hardwoods, the use of non-visible characteristics is indispensable. Dynamic MOE (MOEdyn) is an important parameter widely used for grading of softwoods and applicable to hardwoods as well. There are two common ways to measure MOEdyn – ultrasound (US) wave propagation and longitudinal stress wave (LSW) propagation. Both methods are used in practice, however, due to the different measurement techniques behind them, the results differ. Current paper analyses the stiffness and strength prediction accuracy for several temperate European hardwood specimens and stress the differences between the two measurement systems. The performance was analysed with regard to grading techniques, testing modes for the mechanical properties (tension and bending) and wood qualities. For more than 2861 pieces of European ash (Fraxinus excelsior), European beech (Fagus sylvatica), European oak (Quercus spp.) and maple (Acer spp), the MOEdyn was measured using both techniques, and destructive tests (tension and edgewise bending) were applied. The results show that LSW has higher prediction accuracy compared to the US MOEdyn. The prediction accuracy for both methods and tensile application can be increased by calculating MOEdyn with average density. Furthermore, the results support the species independent strength grading of hardwoods. Further research on the effect of different wood qualities and sawing pattern is required