Influence of Interlayer Connection Type on the Flexural Performance and Failure Mechanisms of Three-Layer Timber Composites

The development of adhesive-free layered timber composites requires effective mechanical connections capable of ensuring sufficient composite action between individual layers. This study investigated the effects of interlayer connection type (polyurethane adhesive bonding and wooden dowels) and annual-ring orientation of inner layer on the flexural performance and failure mechanisms of three-layer Scots pine (Pinus sylvestris L.) composites. Four configurations were evaluated through bending tests, considering apparent modulus of elasticity (MOEapp), apparent modulus of rupture (MORapp), apparent bending stiffness (EIapp), energy absorption capacity (EC), and density. A two-way analysis of variance was performed to assess the effects of both experimental factors and their interaction. Connection type was the dominant factor influencing mechanical performance. Adhesive-bonded composites exhibited MOEapp values of 8.35–9.82 GPa and MORapp values of 134–141 MPa, whereas dowel-connected composites achieved 1.36–1.40 GPa and 71–74 MPa, respectively. On average, dowel connections were associated with an approximately 85% reduction in MOEapp and a 47% reduction in MORapp compared with adhesive bonding. Conversely, dowel-connected specimens exhibited approximately 2.1–2.7 times greater energy absorption capacity. Annual-ring orientation of the boards forming the composite’s middle layer had a statistically significant but relatively small effect on MOEapp, while its influence on the remaining mechanical parameters was not significant. No statistically significant differences in density were identified among the investigated configurations. Adhesive-bonded specimens predominantly exhibited timber failure without visible bond-line damage, whereas dowel-connected specimens showed more complex failure behaviour involving timber rupture, connector damage, and interlayer displacement. No clear differences in failure mechanisms were observed in relation to the annual-ring orientation of the core layer. The findings highlight the dominant role of interlayer connection type in the global flexural response of layered timber composites and indicate the need for further investigation of connector stiffness and partial composite action in adhesive-free systems.