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

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ObjectiveThe true interfacial shear strength of the wood/phenolic resin (PF) bonding interphase 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 interphase. MethodTwo 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 using 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 interphase. 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. ResultThe load-displacement curves of the pull-out test showed that brittle fracture occurred within the wood/PF bonding interphase. 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 that 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 interphase 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 S ₂layer of the adjacent tracheid wall with a significant thickness variation and then propagated along the S ₁/S ₂interface. The initial crack in the S ₂layer of latewood cells of the early-latewood sample simultaneously expanded along the CML/S ₁and S ₁/S ₂interfaces. 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. ConclusionThe difference of PF resin penetration in earlywood and latewood led to the shear strength difference between the two types of bonding interphase. The difference in structure and performance between earlywood and early-late wood was the main reason for the difference in fracture morphology at the interface after pulling out.

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