Graduate Thesis Or Dissertation
 

Understanding the durability of Wood-based composites using fracture toughness

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/7d279134t

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  • Wood-based composites (WBCs) are an important sector of the US economy. For a WBC product to succeed, understanding its long-term durability, including seasonal variations in temperature and humidity, is critical. Durability is especially important WBCs because they are predominantly made from wood, which is a biological, hygroscopic material that is susceptible to decay. There has been substantial evidence that exposure to increased moisture and/or elevated temperature affects the physical and mechanical properties of WBCs. However, a key question remains as to the best approach to characterizing the durability attributes of these materials. Several studies have monitored flexural and shear property degradation, along with degradation in the adhesive bond linefailure. These tests address material durability as a whole and are mostly dependent on the early stage of loading up to the initiation of failure. In contrast, recent work suggests that fracture analysis of crack propagation within a single composite material may provide more thorough information than conventional testing. Therefore, this research explored the suitability of crack propagation fracture experiments to access thermal and moisture durability of a few important WBCs predominantly used in the furniture and cabinet industry, namely Medium-density Fiberboard (MDF), Oriented Strand Board (OSB), and Particle Board (PB). Three-layered double cantilever specimens (DCB) with an initiated crack cut in the middle iilayer were prepared. Fracture toughness was determined by measuring energy dissipation during monotonic loading and crack propagation through the middle layer. The load, deflection, and time data were continuously recorded. Objective observation of crack length in wood composites, especially in the presence of crack-tip process zones with fiber bridging, is challenging, but essential for accurate energy release rate measurement. This issue was solved by using Digital Image Correlation (DIC) methods.For thermal durability experiments, MDF, OSB, and PB were exposed to 10 sets of elevated temperature regimes and then tested for crack propagation by fracture toughness values as a function of the amount of crack growth. Such curves are known as the material’s R curve. The toughness of MDF and OSB initially increased as temperature increased, but then decreased at higher temperatures. The toughness of PB remained constant or decreased slightly for small temperature increases, but then decreased at higher temperatures. These data were interpreted by optimized fits to a trilinear model for MDF and OSB and a bi-linear model for PB. The knowledge of thermal durability of these specimens will further help the design of structures that are exposed to elevated temperature in a short duration of time as this study tested the specimens when they reach the elevated temperature. These results also add new information compared to prior thermal degradation studies that tested specimens at room temperature after exposure to high temperature rather than test them at elevated temperature, as done here.For moisture durability experiments, MDF, OSB, and PB specimens were submerged in water for times from 2 hours to one week and subsequently tested for fracture toughness (while still at high moisture content). A trend of decreasing R curves with an increase in immersion time was observed and wet specimens exhibited substantially lower fracture toughness in comparison to control specimens. The toughness of MDF decreased rapidly with exposure to moisture. OSB and PB decreased at about the same rate and both decreased much slower than MDF. The role of fiber bridging in WBC toughness can be characterized by the amount their R iiicurves increase with crack growth. Moisture durability results showed that the fiber-bridging effect of composites deteriorated drastically. All WBCs had very little fiber bridging toughening in the saturated state (i.e., after one week of immersion). Both the thermal and moisture studies made use of full R curves to help in differentiating effects on WBC type or resin type used in the WBCs. Prior methods used to study degradation, such as internal bond testing (IB), are not able to characterize properties learned in R curves. As a consequence, R curve testing is recommended as the preferred approach to characterizing the durability of WBCs.Future work should extend to more fracture tests on each WBC type made with a variety of resin types.
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  • Intellectual Property (patent, etc.)
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  • 2020-12-11 to 2022-01-12

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