__________ are longitudinal cracks in wood which is usually normal to the annual rings.
Understanding Cracks in Timber
Longitudinal cracks are common defects in timber that can significantly reduce its strength and durability. It is crucial to distinguish between different types of cracks based on their orientation relative to the tree's annual rings. This distinction points to different causes and has different implications for the wood's use.
📝 Detailed Analysis of the Options
(a) Shakes
Shakes are also longitudinal cracks, but they run parallel to the annual rings, essentially separating the wood between two rings. They are caused by stresses in the growing tree, such as wind or frost, and are present before the tree is felled.
(c) Knots
Knots are not cracks. They are the hard, dark remnants of branches that became embedded in the trunk as the tree grew. They disrupt the grain but are a feature of growth, not a separation of wood fibres.
(d) Rind gall
A rind gall is a swollen area on the surface of the timber, formed by the tree's attempt to grow over a wound or an improperly cut branch. It is a surface bulge, not a crack.
(b) Checks
This is the correct answer. Checks are longitudinal cracks that are oriented normal (perpendicular) to the annual rings, running from the surface towards the center of the log. They are typically caused by uneven drying during the seasoning process, where the outer surface of the wood shrinks faster than the interior. These cracks can allow moisture to penetrate easily, which reduces the timber's durability.
📊 Checks vs. Shakes: A Key Distinction
| Feature | Checks | Shakes |
|---|---|---|
| Orientation | Normal (Perpendicular) to annual rings | Parallel to annual rings |
| Cause | Uneven drying during seasoning | Stresses during tree growth (wind, frost) |
| Appearance | Cracks running from surface towards the center | Separation between growth rings |
💡 Study Tips
- Checks Cross the Rings: Remember that "Checks" are cracks that go *across* the circular annual rings.
- Shakes Separate the Rings: "Shakes" are cracks that *separate* the layers of rings, like a layer cake coming apart.
- Link Cause to Defect: Checks are from seasoning (drying). Shakes are from natural forces (while growing).