Understanding the effect of PF glue impregnation on the shear strength of wood growth ring boundary

The growth ring boundary substantially affects the mechanical performance of wood. Glue impregnation has been widely applied to wood protection. Shear strength of the growth ring boundary with and without glue impregnation under normal pressure (NP) and vacuum pressure (VP) conditions was measured using the lap-shear test, meanwhile strain distribution was recorded with digital image correlation. Glue distribution and microstructure in the specimens were visualized using scanning electron microscopy. Glue impregnation can switch specimens’ failure location from growth ring boundary to earlywood or notches. Compared to the control specimens, shear strength and breaking elongation in VP specimens are significantly higher. Looking at the ruptured cross section, there are less ruptures in the control specimens and they are sharper and cleaner than in NP and VP specimens. In NP and VP specimens, the relationship between strength and elongation consists of four sections. In control specimens, the relationship between strength and elongation includes two sections. More sections could be due to residual stress from impregnation processes and breakage of glue. Such mechanical performance relates to the fact that tension strain is much larger than shear strain in NP and VP specimens, which protects the specimens from failing in the growth ring boundary.