At present, the evaluation of wood–adhesive bonds lacks a method that is able to predict the long-term load carrying capacity in shear of a bond in a comparatively short testing time. For this reason, a new test approach was investigated to determine the time to failure of wood–adhesive bonds. In our research, lap joint specimens were prepared with a melamineurea- formaldehyde (MUF) adhesive at two mixing ratios (100/100 and 100/20 [resin/hardener]). The specimens were subjected to tensile shear stresses at load levels between 30 and 90 percent of their mean wet short-term strength while being immersed in water at temperatures of 608C and 908C. The time to failure and the wood failure percentage were determined. The analysis showed good correlations between time to failure and load level as well as between time to failure and temperature. The adhesive mixing ratio, however, showed no influence on the failure characteristics. The wood failure percentage highly depended on the test duration. With prolonged test duration, the mode of failure increasingly changed from wood failure to adhesion failure. Overall, the test method proved to be promising for a detailed performance evaluation of wood–adhesive bonds.@en

The resistance to delamination of cross-laminated timber (CLT) made from spruce has been investigated. The main goal was to examine if delamination results are influenced by the shape of test specimens as both square and round specimens may be evaluated according to the CLT standard EN 16351. Furthermore, the influence of the production parameters number of layers, layer thickness and bonding pressure on delamination behavior were investigated. The results show that specimen shape and number of layers affect the resistance to delamination. Square specimens and a high number of layers show significantly higher delamination than round specimens and specimens with low number of layers. Layer thickness and bonding pressure, however, have no influence. As the number of specimens which exceeded delamination or wood failure requirements given in the CLT standard was very high it is questionable, if the applied delamination procedure is suitable for evaluating the bond integrity of CLT. As a result, it is recommended to eliminate the option to use square or round specimens in delamination tests and to revise the CLT standard accordingly. Furthermore, we suggest either to adjust the delamination test parameters or to develop a new test method to be able to assess bond durability more realistically.

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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.@en