Understanding Cracks in Wood: Identifying Checks

Wood, as a natural material, can develop various types of defects, often due to drying, growth stresses, or fungal attacks. One common type of defect is cracking or splitting. The question asks to identify a specific type of longitudinal crack that is typically oriented normal (perpendicular) to the annual rings.

Analyzing the Options for Wood Cracks

Let's examine the characteristics of each option provided:

• Star shakes: These are cracks that radiate outwards from the pith towards the circumference of the log. They are usually wider at the surface and decrease towards the center. While longitudinal, their pattern is radial.
• Heart shakes: These are cracks that occur in the center of the log, near the pith, and often separate the annual rings. They are typically caused by shrinkage of the heartwood during drying or by stresses during the tree's growth. They are longitudinal but centered around the pith.
• Cup shakes: Also known as ring shakes, these are splits that occur between adjacent annual rings, causing them to separate. They can extend partially or fully around the circumference. These cracks follow the annual rings, not run normal to them.
• Checks: These are longitudinal cracks or splits that appear on the surface of timber, often caused by uneven or rapid drying. As wood dries, it shrinks more tangentially (along the annual rings) than radially (across the annual rings). This differential shrinkage causes stresses that lead to cracks, typically oriented radially (perpendicular to the annual rings) on the surface. These checks run along the length of the timber and are usually normal to the direction of the annual rings visible on the surface.

Why 'Checks' Fit the Description

The question specifies a longitudinal crack that is "usually normal to the annual rings". Based on the analysis of the different types of cracks:

• Star shakes are radial but originate from the pith.
• Heart shakes are central, longitudinal splits near the pith.
• Cup shakes follow the annual rings, not normal to them.
• Checks are surface cracks caused by drying shrinkage. Due to the mechanics of drying and differential shrinkage, these cracks develop along radial lines, which appear normal to the annual rings on the surface of the timber. They are longitudinal along the length of the wood piece.

Therefore, checks are the type of longitudinal crack that fits the description of being usually normal to the annual rings.

Summary of Wood Crack Types

Based on the definitions and common characteristics, a longitudinal crack usually normal to the annual rings in wood is called a check.

Revision Table: Key Wood Defects

Additional Information: Why Wood Shrinks

Wood shrinks primarily due to the loss of moisture from its cell walls. This shrinkage is anisotropic, meaning it is not equal in all directions. The amount of shrinkage varies significantly depending on the direction relative to the wood grain and annual rings:

• Longitudinal shrinkage: Very small, typically 0.1% to 0.3% along the length of the grain.
• Radial shrinkage: Occurs across the annual rings, typically 2% to 6%.
• Tangential shrinkage: Occurs parallel to the annual rings, typically 4% to 10%.

Because tangential shrinkage is significantly greater than radial shrinkage, as wood dries, stresses build up. These stresses often cause the wood to crack along the weakest planes, which are usually the radial planes, resulting in checks on the surface that run perpendicular to the annual rings.

Understanding these drying characteristics is crucial for working with timber and predicting potential defects like checks.