Fracture Behavior of Adhesive Joints in Poplar
Keywords:Cleavage strength, fracture toughness, J-integral, nonlocal theory, wood-adhesive joint, trembling aspen
This study investigated the fracture behavior of poplar-resorcinol gluebonds and determined the respective ranges of applicability of Griffith's brittle fracture theory, Rice's J-integral theory, and Eringen's nonlocal theory. Three-point bending (TPB) specimens of 27 replications for each crack length (0, 3.2, 6.4, and 12.7 mm) for a total of 108 specimens were tested in flexure by the bending normal-to-glueline method. Circular-bar tension (CBT) specimens of 18 replications were tested in tension according to ASTM standard D897-78. Furthermore, compact tension (CT) specimens of 27 replications were tested in cleavage for use as a basis of comparison.
The ultimate bending stress formula corrected for the low span-depth ratio (RA) was found to be applicable to a TPB specimen with a crack of various lengths. The Griffith brittle fracture, Rice's J-integral, and Eringen's nonlocal theories were all valid in application to poplar-resorcinol joints, provided that the appropriate adjustments are taken into account. On this basis, fracture toughness (Kic) of the joint is approximately 255 kPa√m. The percentage of interphase failure was found to correlate well with tensile strength of the gluebonds, but not with cleavage strength.
In addition, microscopic examination of the glue joints revealed that the adhesive flows mainly into the vessels of poplar and that the depth of adhesive penetration depends on gravity. Failure that occurred away from the gluebond in the three-point bending specimens always followed a zone occupied by the largest vessels (weakest link).
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