Analyzing the impact of veneer layup direction and heat treatment on plywood strain distribution during bending load by digital image correlation (DIC) technique

Abstract

In this study, radiata pine veneers and phenol-formaldehyde resin were used to prepare specimens of 5-ply plywood with different layup directions and heat treatments of veneers. The physical and flexural properties of the plywood specimens were assessed, and digital image correlation (DIC) analysis was employed to determine the strain distribution of the plywood under bending loads. The results of the static mechanical strength and DIC tests showed that the plywood with a small-angle veneer layup ([0]5 and [0,22.5,0,22.5,0]) exhibited a better longitudinal modulus of rupture (MOR//), while [0,45,0,45,0] plywood showed the least bending strength and most strain. Moreover, the results revealed that for plywood composed of veneers that were fully heat treated at 200 °C (5T200), the moisture content was efficiently decreased, and the modulus of elasticity parallel to grain (MOE//) was the highest. The DIC images indicated that the largest strain along the x-direction (εxx) was concentrated on the tensile side of untreated plywood (5N) and on the opposite side of plywood composed of heat-treated veneers, except for the plywood composed of veneers treated at 220 °C (5T220). Of these, 5T200 plywood showed the least strain. In addition, the plywood with 200 °C heat-treated veneers instead of face and core layers (NTNTN200) or crossband layers of untreated veneers (TNTNT200) had larger strain values than 5N and 5T200 plywood specimens, with NTNTN200 plywood having the greatest strain. According to the above results, appropriate layering and heat treatment of veneers can effectively improve the dimensional stability and flexural properties of plywood.