Effects of curing time and end pressure on the tensile strength of finger-joined black spruce lumber

Finger-joined black spruce (Picea mariana (Mill.) B.S.P) specimens from eastern Canada were prepared to assess the effect of curing time and end pressure on the tensile strength of the joints. An isocyanate adhesive cured at room temperature and a feather joint configuration were used for this purpose. The joints were machined at a 18.3 m/min. (60 ft./min) feed rate, 3500 rpm rotation speed, and 0.86 mm (0.034 in.) feed per knife (chip load). A single-face glueline application was used at a spread rate of 110 g/m2. Four curing times (1, 2, 5, and 24 hr.) and six end pressures ranging from 1.38 MPa to 4.82 MPa (200 to 700 psi) applied for 20 seconds were tested. The results showed that curing time and end pressure have a statistically significant influence on the performance of structural finger-joints. After 5 hours of curing time, finger-joints made with isocyanate can achieve more than 90 percent of the reference ultimate tensile strength based on 24 hours of curing time. Analysis also indicated that finger-joined black spruce has the best performance at an end pressure of 3.43 MPa (498 psi). Lower or higher end pressure can result in a lower tensile strength.

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