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Mechanical properties of wood | Illustrated guide

Mechanical properties of wood | Illustrated guide

25/08/2019

Of all the existing timber, only the most "workable" ones are industrially exploited, which make it possible to obtain the final product with a reduced use of energy and little waste.

The workability of the wood is linked to the way the cutting edge removes the chip and the mechanical properties of the wood. The cutting edge must be as sharp as possible to penetrate and separate the fibers, the smaller the area of force distribution (cutting edge), the greater the pressure applied at the point of separation and, therefore, the sharper the removal is of the chip. The cutting angles are equally important, since with small angles (thick tool) a greater deformation of the chip is obtained with compression and tearing of the fibers that give a poor surface. It is easier to get a good result working hard than soft woods as the former require less finishing work. Let's see in detail what are the mechanical properties of wood.

Mechanical properties of wood - it is useful to know that

Wood is very flexible, since it is made up of bundles of lignin fibers, a very elastic and resistant material. Steam heating makes the wood much more flexible and the imposed shape is maintained in a stable manner after adequate drying.
Flexibility of wood: the flexural strength of wood is high and is greater the more the growth rings are arranged in the same direction as the bending stress. For a floor beam, for example, they must be vertical. If the arrangement is orthogonal, the resistance drops to 86%.

Mechanical properties of wood

  1. Wood texture and grain: the texture defines the distribution of the cellular elements of the wood, it can be fine, medium and coarse. The grain is the direction of the fibers along the longitudinal axis of the wood: straight, inclined, intertwined, wavy, irregular.
  2. Wood workability: it is the generic ability of the wood to let itself work with manual and electric tools. it is maximum in the direction of the fibers, minimum in the transverse direction to them. We can distinguish it in high, medium, low.
  3. Density of the wood: it is the specific weight, it is measured in kg / m3 on seasoned timber to obtain a moisture content of 12-15%. With the same drying the density is relative to the porosity of the wood, or to its compactness.
  4. Hardness of wood: it is an indication of the resistance that a wood opposes to the penetration of a punch. It is calculated from the impression that the punch leaves after a mechanical pressure or from that of a metallic sphere falling from a predetermined height.
  5. Drying of the wood or shrinkage: it is due to the loss of water contained in the tree at the time of cutting, it starts when the humidity of the fibers falls below 30% of its dry weight, ie it begins to lose the "saturation water" "Absorbed.
  6. Durability of wood: live wood protects itself against parasitic attacks, but once cut it becomes prey to insects and fungi. Some types of wood remain impregnated with repellent substances produced by the tree and are more durable.

Wood's cleavage with nails and screws

The slit is the aptitude of the wood to crack in the direction of the fibers when it is subjected to the penetration of a wedge. The woods most suitable for splitting are long ones with no knots such as top quality fir, alder and chestnut. The slit comes into play when it comes to screwing or nailing two pieces of wood together and, in these cases, it is little appreciated. The pressure exerted by the threads can generate cracks, so it is better to make holes for the passage of the screws that do not stress the wood.

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