Fracture behavior of wood bonding interface based on fiber pull-out test

  Objective  The true interfacial shear strength of the wood/phenolic resin (PF) bonding interface was investigated via the fiber pull-out method in composite materials, and the fracture morphology of wood strips and the failure mode of cell wall layers after pulling out were analyzed to provide theoretical support for revealing the fracture mechanism of wood bonding interface.

  Method  Two types of earlywood and latewood strips/PF resin bonding specimens were prepared, respectively, according to the preliminary exploration of test conditions. The average interfacial shear strength was calculated by the data collected from a universal mechanical testing machine and measured by the three-dimensional ultra-depth of field microscope. Fourier transform infrared spectroscopy (FTIR) was used to analyze the intermolecular interaction of polymer in the bonding interface. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were applied to acquire the macroscopic differences in fracture morphology of two types of wood strips after pulling out, the microscopic failure mode of cell wall layers, as well as the initiation and diffusion of cracks.

  Result  The load-displacement curves of the pull-out test showed that brittle fracture occurred within the wood/PF bonding interface. The interfacial shear strength of earlywood/PF resin was (1.23 ± 0.12) MPa, and that of early-latewood/PF resin was (3.66 ± 0.11) MPa, about three times of earlywood/PF resin. FTIR analysis showed that the chemical cross-linking reaction between wood and PF resin polymer molecules was confirmed to enhance the bonding interface property. SEM results showed that the fracture type of the earlywood sample on the top of the embedment was shear failure, while that was combined tension and shear on the bottom of the embedment. The fracture type of the early-latewood sample was mainly splintering tension failure. AFM images showed that the failure of the earlywood strip was the intrawall failure. The initial crack occurred in the S₂ layer of the adjacent tracheid wall with a significant thickness variation and then propagated along the S₁/S₂ interface. The initial crack in the S₂ layer of latewood cells of the early-latewood sample simultaneously expanded along the CML/S₁ and S₁/S₂ interfaces. In addition, stress concentration was prone to be in the cross-field region, and the external tension load made the ray cells peel off or fall off as a whole.

  Conclusion  The difference of PF resin penetration in earlywood and latewood leads to the shear strength difference between the two types of bonding interface. The difference in structure and performance between earlywood and early-late wood is the main reason for the difference in fracture morphology at the interface after pulling out.