Detailed explanation of the causes and basic solutions of wood cracking in woodworking decoration skills

Dry weather is like a double-edged sword, on the other hand, it is also easy to cause dry cracks in wood engineering. Come with me to get to know it!

Forms of wood cracking:

1. Surface crack: refers to the surface crack, and surface crack refers to the crack on the original wood body or the surface of the finished product.

Cracks are usually confined to the surface of the chord and develop along the radial direction. Shallow surface cracks can be removed by planing, but deep surface cracks are not only ugly, but also reduce the strength of wood, especially the shear strength. Surface cracks will also affect the paint quality of wood. After the wood with cracks on the surface is painted, the cracks can be opened and closed due to the change of climatic conditions, resulting in the rupture of the paint film.

Surface cracks are caused by uneven drying of the inner and outer layers of wood, and the difference of radial and chord shrinkage is an important additional factor. When wood is dry, water first evaporates from the surface. When the moisture content of the surface layer falls below the fiber saturation point, the surface layer wood begins to shrink, but at this time, the moisture content of the adjacent inner layer wood is still above the fiber saturation point, and no shrinkage occurs. The shrinkage of the surface wood is limited by the internal wood,

It can't shrink freely, so it produces internal stress in the wood: the surface wood is in tension and the internal wood is in compression. The stronger the drying condition, the greater the difference of moisture content between the inner and outer layers of wood and the greater the internal stress. If the tensile stress of the surface layer exceeds the tensile strength of the wood grain, the wood tissue is torn, and the tensile strength of the tissue along the wood ray is less than that of the adjacent wood fiber, so the crack occurs first along the wood ray. 2. Internal crack: internal crack. Internal cracks are also often called honeycomb cracks.

Internal cracking occurs in the later stage of drying, and sometimes in the storage period of dried materials. It is usually not easy to find from the outside of wood, but it can be judged by the depression on the surface of wood when it is serious. Internal cracking is caused by tensile stress of wood inner layer. At the initial stage of wood drying, under the action of tensile stress, not only elastic deformation but also residual deformation (plastic deformation) occurs. Due to this residual deformation, the size of the external wood is larger than the free shrinkage.

In the later stage of drying, when the moisture content of the inner wood drops below the fiber saturation point, the inner wood begins to shrink, but due to the limitation of the stretched outer wood, it cannot shrink freely, so the internal stress opposite to that in the earlier stage of drying is generated in the material: the inner wood is pulled and the outer wood is pressed. If the tensile stress of the inner layer exceeds the transverse tensile strength of the wood, the wood tissue is torn and internal cracks occur in the wood. 3. End crack: End crack.

End cracking is limited to the end face of wood, or extends to one or both sides of the end, the latter is usually called splitting. The main reason is that the water conductivity along the grain direction of wood is much greater than that along the grain direction. When wood is dry, water evaporates from the end face much faster than from the side face. The moisture content of the end is lower than that of the middle, and the shrinkage of the end is limited by the wood in the middle, so tensile (tensile) stress is generated at the end.

When the tensile stress exceeds the transverse tensile strength of wood, the end face cracks. 4. Wheel crack: This kind of crack develops along the growth ring direction and often extends to several adjacent growth rings. Wheel cracks generally occur in the early stage of drying and appear on the end face of wood. As drying progresses, the cracks deepen and become longer. Sometimes it happens inside, but it happens in the later stage of drying, which is caused by serious internal tensile stress.

Basic crack prevention methods:

1. Mechanical crack prevention: Tie both ends of dry wood with iron wire, and use crack prevention rings, combined nail plates, etc. And mechanically force the wood not to expand or contract, which can also prevent the wood from cracking.

2. Improve the sawing method when making wood: under the same temperature and humidity change, the wet expansion and dry shrinkage coefficient of wood is the largest in chord direction, followed by radial direction, and the longitudinal change is the smallest, so more radial chopping boards are produced during sawing.

Cracking can be reduced. Especially, the pulped board is prone to serious splitting when it is dried, which is caused by the difference of radial and chord contraction near the pulp. It happened in the early stage of drying, and the initial crack only appeared on the end face. With the progress of drying, it can expand to the pith and longitudinally. This kind of crack is difficult to prevent when it is dry. The best way is to avoid making boards with pulp when making wood ("coring and blanking"). 3. Brush waterproof coating: Brush waterproof coating on the end and surface of wood to slow down the evaporation intensity of wood surface, which can reduce the moisture gradient inside and outside the wood and also reduce the cracking of wood.

4. High-temperature qualitative treatment: High-temperature qualitative treatment can be used to reduce the internal cracks of wood, and the tensile residual deformation of the surface layer of wood with internal cracks can be eliminated by high-temperature and high-humidity treatment before the end of the drying process.

In the process of treatment, the compression residual deformation of the wood surface layer is caused by humidification and expansion, which is offset by the original elongation residual deformation. After treatment, the excess water is evaporated and shrinks together with the inner layer of wood, so there is no residual deformation in the wood and the internal cracks of wood are eliminated. 5. Waterproofing agent soaking treatment: A more effective method is to pressurize the wood with waterproofing agent, so that the waterproofing agent can penetrate deep into the wood, thus achieving a lasting and good anti-cracking effect.