Effect of finger geometry and end pressure on the flexural properties of finger-jointed tropical African hardwoods

Finger-joints from three tropical African hardwoods, Obeche (Triplochiton scleroxylon), Makore (Tieghemella heckelii), and Moabi (Baillonella toxisperma), were prepared using three finger profiles, three end pressures, and resorcinol-formaldehyde adhesive, to assess the effect of the study variables on the flexural performance of the joints. Finger profile geometry was found to have a statistically significant influence on modulus of rupture (MOR), but not on modulus of elasticity (MOE) of finger-joints form the three tropical African hardwoods. The finger profile F2 (18 mm (0.72 in.) finger length) exhibited the significantly strongest and the most efficient finger-joints among the three finger profiles studies for each hardwood. End pressure, however, was generally found to have no significant influence on MOR and MOE of the finger-joints. Although the strongest finger-joints seemed to have been produced when the profile F2 was combined with the end pressure of 12 N/mm2 (1,740 psi) for each species, end pressure of 12 N/mm2 resulted in lower joint efficiencies than 4 N/mm2 (580 psi) end pressure in Obeche and 8 N/mm2 (1,160 psi) in Moabi. From the viewpoint of economy, therefore, profile F2 combined with end pressures of 4 N/mm2, 12 N/mm2, and 8 N/mm2 seem the best combination of production variables for optimum finger-joints in Obeche, Makore, and Moabi, respectively. Among the three hardwoods, finger-joints from the low-density Obeche exhibited the highest joint efficiency of 88 percent and wood failure of 61 percent, followed by that from the medium-density Makore of 78 and 29 percent, respectively. Finger-joints from the high-density Moabi exhibited the lowest joint efficiency of 60 percent and wood failure of 14 percent. The results have shown that finger-joints of high flexural performance could be produced from the low-density Obeche and the medium-density Makore wood using the type of resorcinol-formaldehyde glue studied.

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