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.

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.