In this paper, acoustic emission (AE) signals obtained during quasi-static crack propagation in adhesively bonded beech wood were classified using an unsupervised pattern recognition method. Two ductile one-component polyurethane (1C-PUR) adhesives with the same formulation except for one system being reinforced with short polyamide (~ 1 mm long) fibers were compared to a relative brittle phenol–resorcinol–formaldehyde (PRF) adhesive. Using only localized AE signals, it was shown that the signals originating from the crack propagation could be classified into two different clusters. Comparing the AE signals with a new fractography method, it was estimated that different clusters are due to distinct failure mechanisms, with signals of cluster 1 being associated with wood failure and signals of cluster 2 with adhesive failure. The obtained results suggest that for the PRF adhesive the wood fibers help to slow down the crack propagation. A similar but lesser effect was noted for the polyamide fibers added to the 1C-PUR adhesive matrix.@en

The feasibility of using the modified Hartman–Schijve (HS) equation to analyze the fatigue fracture performance of adhesively bonded wood specimens under cyclic mode II loading was investigated in comparison with the Paris crack growth equation. Wood joints prepared with three different adhesives have been subject to cyclic Mode II testing at room-temperature (23∘C and 50% relative humidity) in a four-point End-Notched-Flexure configuration, determining the crack length from specimen compliance. It was shown, that the HS-equation can be successfully applied to adhesively bonded wood and that it successfully estimates threshold and maximum energy release rate (ERR) values for three different adhesive systems. Since a limited number of tests were performed for investigating the feasibility, scatter sources and possible scatter reduction methods are analyzed and discussed in detail. Also, a new, automated data reduction method was developed for estimating the maximum and the threshold ERR (G _thr) values. The main advantage of the HS-equation appears to be the application in design standards. However, before the maximum ERR and G _thr values derived here can be used in design applications or for drafting a design guideline, additional testing is required for understanding how the number of cycles, the related measurement resolution; the corresponding ERR value influence the threshold value G _thr and how and to what extent its scatter can be reduced; and to further explore the link between cyclic ERR and the critical ERR value measured during quasi-static fracture tests.

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