Mode Ii Fracture Behavior of Bonded Viscoelastic Thermal Compressed Wood

The influence of viscoelastic thermal compression (VTC) of wood on bonding performance was studied. Mode-II (shear mode) fracture of the bonded interphase was performed using the overnotched and end-notched flexure methods. The study examined four groups of specimens; a control and VTC specimens with three different degrees of densification (63, 98, and 132%). The specimens were bonded with phenol-formaldehyde (PF) adhesive. Prior to fracture testing, the bonded interphase was examined and the effective penetration (EP) of PF into the capillary structure of wood was measured. The results showed that EP was greatest in the control wood specimens, but in the case of the VTC specimens decreased with increasing degree of densification. The mode-II fracture performance of the VTC wood specimens with PF differed from the control wood specimens. In the control specimens, the mode-II crack propagation occurred in the interphase, while in the VTC specimens the crack diverted away from the interphase into the VTC wood. A hypothesis of relative shear resistance was used to explain the bonding performance of the control and VTC specimens.