Flaws of wood

Flaws of wood

The quality of wood is determined by the presence or absence of defects, which in most cases reduce its strength or worsen its external appearance.

Knots. In cut wood, one encounters healthy knots that have fused with the wood, healthy, partially fused with the wood, and those that have not fused with it, loose, hollow knots that fall out easily (fig. 4). Knots are the basic flaw that determines the quality of wood species and products, as they disrupt the uniformity of the wood, they make it difficult to process, reduce its strength. In case there are knots in the wood, the reduction of its strength depends on the stage of decay of the knot, its size and the place where it is located. For example, knots located in the tension zone and on the edges of the ceiling beams greatly affect their bearing capacity. There must be no knots with a diameter greater than 10 mm in wooden lattice supports.

In sawn timber, the wood is most weakened in the places where there are knots — dead and large fused knots. Knots that are partially fused, and especially those that are not fused at all, disrupt the integrity of the elements, lower the quality of the wood even more than healthy, fused nodes. 


Figure 4. Nodes

a - healthy, hard, fused; b - not fused, partially fused; c - falling out

d - horned; e - twisting; f - knot = half-throat


Abnormal colors rots are divided into internal and external. Internal colors and rots (Fig. 5) appear during the life of the tree in the core of a mature tree or in the core part of the tree. As a rule, they are caused by fungi that destroy wood. Fungal spores enter the interior of the tree trunk, through broken branches and through injuries on the trunk and veins.


Picture 5. Rot on pine


In the cut wood of coniferous and broadleaf species, the life work of the fungus stops. In the wood of leafy bulging species (without core), the development of fungi can continue even in the cut wood.

External colors and rot appear in cut wood as long as there is sufficient humidity in it. During artificial drying, all fungal spores, which cause external colors and rot, die.

Internal redness, freckles i false core they are not of great influence on the change in the strength of the wood. Internal rot, caused by fungi, destroys the wood and makes it unsuitable for use in construction.

Exterior colors i it rots which include yellow rot, ruffling, black and blue stripes, black spots, bruising and plethe dream of growing failure external appearance, but slightly reduces the mechanical properties of wood. Sapwood rot, pox rot, peripheral soft rot destroy wood. Some defects of wood are shown in fig. 6. Wood infected with native fungus loses its mechanical properties and should be burned as a source of infection of healthy wood in warehouses and construction sites.


Figure 6. Rot of different types of wood caused by wood-destroying fungi

a - dancing; b - bruise; c - roughness; d - sapwood rot

Cracks. According to the time when they appeared and the character of the injury, cracks are divided into cracks of growing wood which include: flammability, flammability, cracks from frost, and cracks due to weighting, which were caused by the drying of the wood. Cracks damage the integrity of the wood (fig. 7).


Figure 7. Cracks of mechanical origin

a - flammability; b - environmental friendliness; c - cracks due to drying and weighting on the round piece;

d - cracks due to weighting on the boards


Tree shape errors they consist of curvature, groove, twist, eccentric heart. This type of error reduces the utilization percentage of the material.

Wood structure errors — wire twist, fiber flow irregularity, grain line irregularity, inner double sapwood, double heart, and false heart — are related to the abnormal structure of the tree trunk. 

Twisting, irregularity of the fiber flow and irregularity of the grain line reduce the mechanical properties of the wood.

The volume weight, weight and hardness of twisted wood are higher than that of healthy wood. However, since wood with a small percentage of partial twist does not show significant changes in physical-mechanical properties compared to normal wood, it is allowed for use in construction with certain restrictions. High twisting lowers the quality of large assortments. The double sapwood of the oak in the standard construction also lowers the quality.

The heart of the tree causes spattering of sawn timber during its drying and deteriorates its quality. With a round build, the heart zone is not considered a flaw. Double heart deteriorates the quality of round construction. An eccentric heart often significantly reduces the mechanical properties of wood, which in those cases qualifies in lower classes, up to firewood.

wounds are a group of defects that include mechanical injuries, scars, partial drying and deformations from wounds. Mechanical injuries can reduce the value of the wood, help the wood to become infected with fungi during its growth, and also to round timber during its storage.

Scarring and partial drying they deteriorate the quality, damage the integrity of the wood, cause warping of the annual layers.

Cancer deformities somewhat worsen the mechanical properties and reduce the quality of the wood.

Abnormal secretion — incrustations, pitch and resin bags also deteriorate the quality of the wood.

Related articles