The Effect of Scarf-jointed Veneer on Mechanical Performance of Thick Plywood

Abstract

For the purpose of effectively utilizing low quality logs of short length, 24 mm-thick, 9-layer 9-ply plywood was produced with longitudinal scarf joint at the center of the strong-axis veneer. The effects of lamination number and arrangement of the veneer-joints on mechanical properties are discussed. Sugi (Cryptomeria japonica D. Don) was used for the veneer and hot-melt adhesive was used for longitudinal joints. A tendency for lowered bond quality was observed with increasing number of veneers with a longitudinal joint. Bending performance of specimens with longitudinal joints in face and back layers decreased significantly. The performance of plywood without longitudinal joints in face and back layers did not show a significant decrease and met the requirements of the JAS standard for structural plywood. The efficiency of veneer joints was calculated with parallel-ply theory. The efficiency for the layup where all strong-axis veneers were jointed showed the lowest values ; 60% for bending stiffness and 35% for bending strength. From the above findings, it was concluded that thick plywood with longitudinally jointed veneer can have practicable bending performance, if the longitudinal joints of veneer are restricted to the inner layers.