How to know which species are suitable for curved furniture, dogwood or construction by analysing their mechanical properties

Various forces act on wood used in furniture making or construction and can deform or break it. The response to these forces depends on the species, structure, grain direction, density, water content, growth or processing defects, but also on the nature, duration and intensity of these forces. The response of wood can be predicted on the basis of its mechanical properties. Knowing these helps us to know which species is best suited, for example, to support the weight of a house or to be deformed into curved furniture. Mechanical properties give us answers about the behavior of wood during different processing stages or tell us how strong it will be as part of final structures (houses, furniture).

The resistance to the action of various forces is measurable and can be compared between species. To make this comparison, however, the determination must be made under the same conditions of temperature and humidity, on identical samples of wood of specific shapes. All these determinations are standardized. In the article you will find the main mechanical properties of wood, the size level and how they vary according to species, grain direction or other specific characteristics.

Elasticity, the property that makes timber-framed buildings more resistant to earthquakes

We say about wooden frame houses are more resistant to earthquakes than concrete or brick because wood is elastic. This means that it has the ability to return to its original shape once the force acting on it ceases (seismic force, in the case of houses), provided that this does not exceed the elastic limit. The elasticity of wood is not infinite. Each species has a limit beyond which the wood gives way and the deformation becomes permanent. The elasticity of wood is expressed by modulus of elasticity which is measured in N/m² and is different depending on the fiber orientation, longitudinal or transverse, and the acting force - tensile, compressive, bending or twisting.

A resilient wood is more shock-absorbent and cushions knocks. It's the property behind the choice of certain species for sports goods (maple, beech or bamboo for baseball bats) or tool handles (carpen). A property derived from elasticity is bending strength. For example, in relation to specific weight, resinous have much higher bending strength than fagul. As a result, softwoods are used for the load-bearing structures of houses and beech for furniture.

Plasticity

You may remember Thonet chair, the best-selling piece of furniture of all time. It is a simple but very elegant chair made from cylindrical curved beechwood elements. Plasticity is the underlying property of curved furniture. It is the ability of materials to retain their shape after the force acting on them has ceased. In fact, bent wood is a combination of elasticity and plasticity. It doesn't seem very possible because plasticity is the inverse of elasticity and according to mathematical principles should cancel out. And yet, they don't cancel, they complement each other.

Normally wood is elastic and after a certain limit it gives way, it is brittle. The more elastic it is, the more it deforms without cracking. Beech is one such wood. When subjected to various treatments - steam, chemicals, high-frequency currents (CIF) - it becomes even more elastic and can be shaped without breaking. When fixed on a mould, it will take the desired shape and will keep it even after the treatment has stopped, thanks to the plasticity acquired by the treatment.

Species that curve well are also frasin and ulmul. And the younger it is, the more easily it bends, lignification leading to stiffening.

Compressive strength

Compression is the most important force in construction because it acts on the strength structure of a house. In the case of timber-framed houses, the posts and load-bearing walls must be able to unload this force, given by the house's own weight and additional forces that may occur (thick snow cover on the roof), down to the foundation and ground level without subsidence or deformation. In order for this to happen, the compressive strength of the wooden elements must be greater than the compressive force exerted.

Compression can occur parallel to the grain or transversely (radially or tangentially to the annual rings). In timber frame houses, the compression along the grain is supported by the posts and transversely by the house sole at the contact between the sole and the post. The wood compressive strength in the transverse direction is only 15-201TPTP3T of that in the longitudinal direction. Compressive strength increases with increasing density and decreases with increasing wood moisture content. For example, an increase in wood moisture content of 1% may result in a decrease in compressive strength of 4%.

Species with good compressive strength are softwoods, oak treeash, hornbeam, elm. Larch, considered to be the oak of the resin oak, has even greater compressive strength in the longitudinal direction than oak. Hornbeam has the highest compressive strength, both longitudinally and transversely.

Tensile or tensile strength

In this case the wood is subjected to forces that want to lengthen it, to increase its dimensions. It's basically the reverse of compression. Here again, the response is different depending on the direction of the grain, longitudinal or transverse, but transverse tensile strength is rare in wood uses. And it is good that this is so because the tensile strength in the transverse direction is poor. Longitudinal tensile strength increases with increasing density and decreases with increasing moisture content.

Wood is highly resistant to longitudinal tensile strength, which is practically the highest of all its mechanical strengths. It is double the compressive strength in the same direction. The highest strength is in hardwoods with high density and hardness (oak, acaciaHowever, resistance decreases if defects such as knots or twisted fiber.

Bending strength

It is the response of wood when subjected to bending forces. Static bending strength determines the level of force at which the wood breaks. Very important here is how the wood breaks. The more the breakage is made with large, uneven fiber pulps, the stronger the wood. Static bending strength is associated with elasticity, the more elastic the wood, the stronger it is. Species and defects influence the level of static bending strength.

In timber frame houses, buckling is the force exerted when lateral wind or seismic forces act on beams or posts that are not in braced structures and are rigidly fixed to the floor. As a result of these actions, the element is deformed in relation to its longitudinal axis.

A special type of bending strength is resilience, i.e. resistance to shock bending (dynamic bending). Depending on their resistance to dynamic bending, species are divided into resilient (or tough) and brittle. Resilient species are those recommended for construction and areas where shocks and vibrations may occur (sports goods, tool handles). Resilient species are ash, sprucelarch, elm, and the fragile ones, chestnut or poplar. Growth defects or processing greatly reduce the wood's resilience. Ash is one of the most shock-resistant species.

Shear strength

Shear is another force that occurs particularly in construction. The shearing force acts in a certain plane. Depending on the combination of the way the force acts and the plane, there are 3 types of shear: transverse (force and plane are transverse to the fiber), longitudinal parallel to the fiber (force and plane are parallel to the fiber) and longitudinal perpendicular to the fiber (force is along the fiber and plane is transverse).

Shear strength increases with density and is highest at a wood moisture content of 10%. Some defects, such as creped grain, increase shear strength, others (twisted grain, cracks) decrease it. The shear strength is 8-10 times lower than the longitudinal tensile strength and 6-8 times lower than the compressive strength parallel to the fiber. Beech and oak are 50-75% more resistant to shear than spruce or fir.

Hardness

It is the resistance with which the wood responds when a hard body tends to enter, to penetrate it. Hardness divides species into hard and soft and tells us how easily they work, how they behave when nails or screws penetrate, how hard they can be sanded, how much they resist wear.

In absolute values, there are Janka hardness and Brinell hardness, depending on the method of determination. The highest hardness is on the transverse section, and the hardness on the radial section is comparable to the tangential hardness. According to hardness, spruce, poplar and willow are soft species, birch and larch medium species, and oak and acacia hard species.

Resistance to splitting

It is a property that helps us to choose the right wood for making staves, shingles, shingle or tells us how hard to split firewood. It is the wood's resistance to the force of splitting in the longitudinal direction. This force does not sever the fibers but only separates them. Strength is also important in construction, when assembling components. Thick bolts used to fasten posts, for example, can cause the wood to split.

The wood splits most easily in the axial plane, the plane of the medullary rays. Again, as with shearing, it depends on the plane in which the force acts. The resistance in the radial plane is lower than that in the tangential plane, so splitting occurs axially, in the radial direction. Defects such as twisted fiber, fiber crevice, nodes increase splitting resistance, while cracks decrease it. Split strength is the lowest mechanical strength of wood.

Much can be said about the mechanical properties of wood, but I'll stop there. I just wanted to show you why not every wood can be used everywhere and how the predominant use of one species in a particular area can be explained. Any additions from you are welcome, especially if there are examples of the use of different species. And if you have missed a project because you chose the wrong wood, please share your experience with us, it will be very helpful. Those who have any queries or questions can leave them, as usual, in the dedicated space. I will certainly reply.