| Research aimed at understanding and explaining aspen wood/PF type adhesive failure at the microscopic level was undertaken. Modulus of elasticity (MOE) of resorcinol-formaldehyde (RF) adhesive in relation to preparation temperature was investigated, and it was found that it was not consistent with that of the model phenolics, but was similar to that of amorphous polymers. Neither MOE nor degree of cure was related to curing temperature within the defined range used in this research. The MOE of the RF adhesive was less than wood, a condition opposite to that normally assumed in explaining wood/adhesive joint failure.; Press pressure that was shown to cause elastic and plastic deformation in static compression tests for aspen had actually caused cell wall failure due to time delayed responses in pressing. The ray parenchyma cells facilitated adhesive penetration, which resulted in greater penetration depth in the radial direction than the tangential direction. Aging was shown to adversely affect shear strength and percent wood failure of mechanically prepared aspen surface, and of the 3 mechanically prepared surfaces, planing was preferred based on superiority of shear strength, percent wood failure, glueline thickness, and cleanliness and evenness of surfaces.; Maximum peak stresses at failure of glued aspen wood surfaced by planing with small differences in roughness were similar, but the location of failure with respect to the glueline differed. Failure within the wood seemed to limit abruptness of failure, and in crease the chance of crack arrest. Failure within the glueline was abrupt without the chance of arrest. Differences in surface roughness also influenced the contribution of fiber bridging in the fracture plane.; The wood fiber/adhesive composite layer was identified at the prepared wood surface and is composed of broken and cut wood cell walls and loose and trailing wood fibers embedded in adhesive. It was described in a modified anatomical representation of bonding. Wood/adhesive failure was shown not as a single crack to failure process, but was due to numerous cracks of varying sizes, some of which joined the failure plane, and many of which did not. |
