Which statement identifies a common timber defect and how moisture content affects wood properties?

Which statement identifies a common timber defect and how moisture content affects wood properties?

• A. Cup; moisture content has no effect on modulus of elasticity.
• B. Warp; moisture has no effect on dimensional changes.
• C. Bow; drying increases swelling.
• D. Twist; moisture changes cause swelling/shrinkage, reducing strength and stiffness.

Answer: (D)

The key idea here is how moisture content affects wood properties and what kind of distortion shows up when moisture fluctuates. Wood is hygroscopic, so it takes in or loses water from the air. When moisture content rises, wood swells; when it falls, wood shrinks. This dimensional change can distort the grain, leading to common defects like twist, which is a distortion where the wood twists along its length due to uneven drying or uneven internal stresses. Twist matches the statement because it identifies a frequent timber defect and correctly links moisture changes to swelling and shrinkage, which in turn can reduce strength and stiffness. The other options describe incorrect or inconsistent relationships: moisture does affect stiffness (modulus of elasticity), so saying it doesn’t is false; warp doesn’t imply no dimensional change, and drying doesn’t cause swelling—in fact, swelling occurs with moisture gain, while drying causes shrinkage.

The key idea here is how moisture content affects wood properties and what kind of distortion shows up when moisture fluctuates. Wood is hygroscopic, so it takes in or loses water from the air. When moisture content rises, wood swells; when it falls, wood shrinks. This dimensional change can distort the grain, leading to common defects like twist, which is a distortion where the wood twists along its length due to uneven drying or uneven internal stresses.

Twist matches the statement because it identifies a frequent timber defect and correctly links moisture changes to swelling and shrinkage, which in turn can reduce strength and stiffness. The other options describe incorrect or inconsistent relationships: moisture does affect stiffness (modulus of elasticity), so saying it doesn’t is false; warp doesn’t imply no dimensional change, and drying doesn’t cause swelling—in fact, swelling occurs with moisture gain, while drying causes shrinkage.