A bookshelf example can be used to illustrate the viscoelasticity of wood: A number of books are put on a shelf and, in time, it will have a limited amount of sagging deformation.
When all books are removed from the shelf, it will never return to its original flat condition. Thus, there will be a residual deformation left because of its viscoelasticity. Figure 2 illustrates the viscoelastic behavior of wood, as in the bookshelf example. Compression of wood and wood-based materials plays an important role in almost any construction projects.
If the compression strength or bending strength of a 2-inch by 4-inch beam is not known, deflection due to bearing a load may cause significant deformation, which could even lead to its failure during service life. Therefore, most softwood construction lumber is graded based on allowable load resistance, which can be determined from a stress test.
However, strength properties of hardwood lumber are not that critical because a majority of it is used for furniture manufacturing and is not exposed to substantial loads.
Compression or shear strength of a wood beam or truss used extensively for construction can be calculated based on the following equation:.
Figures 3 and 4 show compression and shear stress developed by a perpendicularly applied load on small wood blocks. In the case of bending a beam, we are dealing with modulus of elasticity MOE and modulus of rupture MOR to evaluate its load resistance.
While MOE is a measure of the stiffness of a body, MOR is related to maximum strength that can be resisted by a member. Both are expressed as stress similar to most of the other mechanical properties of wood. In general, depending on the species, wood has MOE and MOR values of ,—2,, psi and 5,—15, psi, respectively.
If a Red Oak with an approximate MOE value of 2,, psi is used to make the bookshelf mentioned above, its deflection deformation will be less than that of Aspen, which has a lower MOE. Figure 5 also illustrates a typical beam bending with deflection as a result of a central load. Table 1. So, what is this big find? It is the binding, glue-like binding relationship between two of the most abundant substances in plants, cellulose and xylan. They are found in the cell walls of plants and have been found to be the reason timber and some other plant materials are strong and indigestible.
Plant materials are often used for biofuel and to feed animals, but woody materials are not effective for either. This was the original driver of the research. These substances also help make up fibre in other plants, which are good for gut health.
When xylan interacts with cellulose, it is induced to unwind and attach itself to the rod-shaped cellulose molecules. This creates a bond that is resistant to all but the most extreme force or chemical disruption.
But it does open the door for a lot of advances in energy, agriculture and architecture. In the garage there may be a car whose chassis and bumpers could be composed of densified wood rather than steel and plastic—knock on wood. Sid Perkins, who writes most often about Earth and planetary sciences, materials science and paleontology, is based in Crossville, Tenn.
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