| Comprehending the anatomical nature of wood and its role in the resultant fracture of glued wood products stressed to failure is paramount in the full, advantageous use of both wood and adhesive in the productin of the strongest possible products. Any relation of wood structure to a measure of fracture resistance, such as strain energy release rate (G(,Ic)), can only aid in the design of stronger and better wood-adhesive composites.The regions of crack arrest and initiation and the path of failure are described on an anatomical basis in specimens constructed with sawn and jointed surface preparations and RL and TL systems of crack propagation. An attempt is made to relate the path of failure to wood structure and G(,Ic) at three structural levels: macroscopic, microscopic, and ultrastructural.Fracture initiation (G(,Ic)) and arrest (G(,Ia)) energies showed no statistically significant responses (5% level) to modification in surface preparations or crack propagation systems.Failure types and their relative occurrence (ranked from most common to least common) were wood failures of the intrawall, transwall parallel to the fiber axis, an transwall perpendicular to the fiber axis types interphase failure interface failure and adhesive failure. These failures exhibited some similarities to failures reported for plywood and solid wood, although the frequency of intrawall failure was higher in this investigation.A contoured double cantilever beam (CDCB) test specimen configuration, manufactured using Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and phenol-formaldehyde adhesive bonds, is utilized and is shown to be an effective means for evaluating G(,Ic).Higher G(,Ic) values were generally characteristic of wood failure near the interface/interphase area in the better bonds, while lower values characterized wood failure away from this area or poor bonding. This trend of decreasing G(,Ic) with increasing distance from the bondline was more consistent for the TL system than the RL system fractures. The conclusion was made that a recognizable difference should be evident in the fraction path or location of the fracture initiation when G(,Ic) values differ greatly. |
