What is the key difference between heartwood and sapwood that influences their mechanical properties?
- Heartwood is darker and has more resins, making it stronger.
- Heartwood is dead and no longer conducts water, while sapwood is living.
- Heartwood is less susceptible to decay than sapwood.
- Sapwood is denser and therefore stronger than heartwood.
Correct Answer: Heartwood is dead and no longer conducts water, while sapwood is living.
Correct Answer Explanation: Heartwood is the central, non-living portion of a tree, while sapwood is the outer, living layer responsible for water transport. This difference in function affects the chemical composition and, consequently, the mechanical properties of these wood components.
How does the rate of growth in a tree affect its strength?
- Faster growth results in stronger wood due to larger cells.
- Faster growth weakens wood due to thinner cell walls.
- The rate of growth has no impact on wood strength.
- Faster growth leads to more knots, weakening the wood.
Correct Answer: Faster growth weakens wood due to thinner cell walls.
Correct Answer Explanation: Trees that grow quickly have thinner cell walls, resulting in lower strength. This is particularly true for hardwoods, where faster growth often correlates with lower strength.
What is the impact of cross-grain on a piece of wood?
- Cross-grain makes wood stronger due to increased resistance to bending.
- Cross-grain reduces the wood’s tensile strength and increases its susceptibility to splitting.
- Cross-grain has no effect on the wood’s strength.
- Cross-grain increases the wood’s resistance to compression.
Correct Answer: Cross-grain reduces the wood’s tensile strength and increases its susceptibility to splitting.
Correct Answer Explanation: Cross-grain, a defect where wood fibers are not parallel to the grain, weakens the wood, particularly in tension, making it more prone to failure in bending and splitting.
Which of the following factors significantly influences the mechanical properties of wood?
- The species of the tree.
- The size of the tree.
- The age of the tree.
- All of the above.
Correct Answer: All of the above.
Correct Answer Explanation: The mechanical properties of wood are influenced by a combination of factors, including the species of the tree, its size, its age, and even the location where it was grown.
What is the primary purpose of timber testing?
- To determine the density of the wood.
- To assess the wood’s suitability for specific applications.
- To classify different wood species based on their appearance.
- To identify potential defects in wood samples.
Correct Answer: To assess the wood’s suitability for specific applications.
Correct Answer Explanation: Timber testing is performed to evaluate the mechanical properties of wood, such as its strength, stiffness, and toughness, to determine if it’s suitable for particular construction or industrial applications.
Why does green (moist) wood have a lower strength than dry wood?
- Moisture weakens the wood’s fibers.
- Green wood is more susceptible to fungal decay.
- Green wood is more prone to splitting.
- Drying increases the density of wood fibers, making them stronger.
Correct Answer: Drying increases the density of wood fibers, making them stronger.
Correct Answer Explanation: Moisture within wood fibers weakens the wood. Drying the wood removes the water, making the fibers denser and increasing their strength.
What is the modulus of elasticity, and why is it important?
- It measures the wood’s resistance to bending.
- It measures the wood’s ability to stretch before breaking.
- It measures the wood’s ability to recover from deformation.
- It measures the wood’s ability to withstand impact.
Correct Answer: It measures the wood’s resistance to bending.
Correct Answer Explanation: The modulus of elasticity (or Young’s modulus) is a measure of a material’s stiffness. It describes how much stress a material can withstand before it deforms elastically. Higher moduli indicate greater stiffness.
Which of these defects weakens wood and reduces its strength?
- Knots
- Frost splits
- Shakes
- All of the above
Correct Answer: All of the above.
Correct Answer Explanation: Knots, frost splits, and shakes are all defects that disrupt the continuity of wood fibers, making the wood weaker and more prone to failure.
How does wood behave when heated?
- It shrinks.
- It expands.
- It remains the same size.
- It becomes more brittle.
Correct Answer: It expands.
Correct Answer Explanation: Wood, like most materials, expands when heated and shrinks when cooled. This expansion is influenced by the moisture content of the wood, with drier wood expanding more than moist wood.
What is the term for the ease with which wood can be split?
- Cleavability
- Toughness
- Resilience
- Hardness
Correct Answer: Cleavability.
Correct Answer Explanation: Cleavability refers to the ability of wood to be split along its grain. This property is influenced by factors like grain direction, moisture content, and the presence of defects.
What is the purpose of preservatives used on wood?
- To improve the color of the wood.
- To increase the wood’s strength.
- To protect the wood from decay and insect damage.
- To make the wood more water-resistant.
Correct Answer: To protect the wood from decay and insect damage.
Correct Answer Explanation: Preservatives are applied to wood to inhibit the growth of fungi and insects that can cause decay and damage, extending the lifespan of the wood.
What is the approximate ratio of wood substance to water by weight?
- 1:1
- 1.5:1
- 2:1
- 3:1
Correct Answer: 1.5:1
Correct Answer Explanation: The weight of dry wood substance is about 1.55 times that of water. This is a key factor in understanding the density and moisture content of wood.
Which of these woods is known for its high shock-resisting ability?
- Longleaf pine
- Douglas fir
- Hickory
- Redwood
Correct Answer: Hickory
Correct Answer Explanation: Hickory, particularly wide-ringed varieties, is known for its high shock resistance due to its high density and the arrangement of its wood fibers.
How does the speed of a testing machine affect the measured strength of wood?
- Faster strain rates generally lead to reduced strength.
- Faster strain rates generally lead to increased strength.
- The speed of the testing machine has no impact on the strength of wood.
- The speed of the testing machine increases the strength of wood only when it is dry.
Correct Answer: Faster strain rates generally lead to reduced strength.
Correct Answer Explanation: Wood’s strength can be affected by the rate at which the load is applied. Faster strain rates (loading more quickly) generally lead to lower measured strengths, as wood may not have time to deform fully before failure.
What is a factor of safety in structural design, and why is it used?
- It is the ratio of the actual load to the breaking strength of the wood.
- It is the ratio of the expected load to the breaking strength of the wood.
- It is the ratio of the breaking strength of the wood to the expected load.
- It is the ratio of the breaking strength of the wood to the actual load.
Correct Answer: It is the ratio of the breaking strength of the wood to the expected load.
Correct Answer Explanation: A factor of safety is a design factor that ensures the safety of a structure. It accounts for uncertainties in materials, loads, and design calculations. In wood structures, a factor of safety of 6 or 10 is often used to compensate for wood’s inherent variability.
Which of these wood properties is most directly related to its density?
- Cleavability
- Hardness
- Strength
- All of the above
Correct Answer: All of the above.
Correct Answer Explanation: Density, which is the weight per unit volume, is a key factor in determining a wood’s strength, hardness, and cleavability. Higher density generally correlates with greater strength, hardness, and resistance to splitting.
What is the typical shrinkage range for wood from green to oven-dry?
- 1-5%
- 6-50%
- 51-100%
- Over 100%
Correct Answer: 6-50%
Correct Answer Explanation: Wood shrinks significantly as it dries, with the range of shrinkage varying depending on the species and initial moisture content. Conifers typically shrink around 10%, while hardwoods shrink closer to 15%.
What is the impact of steaming on wood’s strength?
- Steaming increases the wood’s strength.
- Steaming has no effect on the wood’s strength.
- Steaming reduces the wood’s strength, especially when done at high pressure or for extended periods.
- Steaming increases the wood’s strength only when done at high pressure.
Correct Answer: Steaming reduces the wood’s strength, especially when done at high pressure or for extended periods.
Correct Answer Explanation: High-pressure steaming or prolonged steaming can weaken wood by disrupting the structure of its fibers and reducing its stiffness. While some steaming processes are used for bending wood, excessive steaming can permanently damage its strength.
Why is it important to consider compression failure when designing wooden structures?
- Wood is weaker in compression than in tension.
- Wood is stronger in compression than in tension.
- Compression failure is rare in wood structures.
- Compression failure only occurs in wood that is not properly dried.
Correct Answer: Wood is weaker in compression than in tension.
Correct Answer Explanation: Wood, despite its strength in tension, is generally weaker in compression. This means that wooden beams often fail due to compression on their bottom side, especially when loaded heavily.
How does the presence of sound knots affect wood’s strength in compression?
- Sound knots significantly reduce wood’s strength in compression.
- Sound knots have no effect on wood’s strength in compression.
- Sound knots increase wood’s strength in compression.
- The effect of sound knots on compression strength varies depending on the species of wood.
Correct Answer: Sound knots have no effect on wood’s strength in compression.
Correct Answer Explanation: Sound knots, those that are tightly integrated into the surrounding wood, do not significantly affect compression strength. However, they can weaken the wood in bending and tension, where the fibers are under stress.
What is the primary cause of dry rot in wood?
- Bacteria
- Insects
- Fungi
- Extreme temperatures
Correct Answer: Fungi
Correct Answer Explanation: Dry rot, a type of fungal decay, is caused by specific fungi that thrive in warm, humid conditions. These fungi break down the cell walls of wood, reducing its strength and causing it to become brittle and crumble.
Which of the following is NOT a factor that contributes to the variability of wood’s mechanical properties?
- Species
- Rate of growth
- Moisture content
- The color of the wood
Correct Answer: The color of the wood
Correct Answer Explanation: The color of wood is primarily determined by pigments and resins, and while it can sometimes indicate the presence of heartwood, it is not a reliable indicator of mechanical properties. The other factors listed all have a significant impact on wood’s strength, stiffness, and other properties.
What is the approximate modulus of elasticity for wood compared to steel?
- Wood is about 10 times stiffer than steel.
- Wood is about 5 times stiffer than steel.
- Wood is about 5 times less stiff than steel.
- Wood is about 10 times less stiff than steel.
Correct Answer: Wood is about 10 times less stiff than steel.
Correct Answer Explanation: Steel has a modulus of elasticity around 30,000,000 pounds per square inch, while wood typically ranges from 643,000 to 1,769,000 pounds per square inch. This difference in stiffness explains why steel is used in high-stress applications where wood would be unsuitable.
What is the key concept that “The Mechanical Properties of Wood” emphasizes about the use of wood?
- Wood is a sustainable and renewable resource.
- Wood is a versatile material with many applications.
- Wood’s properties are variable, and careful selection and testing are crucial.
- Wood is a strong and durable material that can be used in many construction projects.
Correct Answer: Wood’s properties are variable, and careful selection and testing are crucial.
Correct Answer Explanation: The book “The Mechanical Properties of Wood” highlights the variability of wood’s properties, even within the same species. It emphasizes the importance of careful selection, proper treatment, and scientifically sound testing methods to ensure the safe and efficient use of wood in various applications.
Which of these properties is NOT typically measured in timber testing?
- Strength
- Hardness
- Cleavability
- Color
Correct Answer: Color
Correct Answer Explanation: Timber testing focuses on evaluating the mechanical properties of wood, which include strength, stiffness, toughness, hardness, and cleavability. Color is primarily a visual attribute and is not a reliable indicator of these mechanical properties.
What is the term for the amount of energy a material can absorb before it permanently deforms?
- Resilience
- Toughness
- Cleavability
- Hardness
Correct Answer: Resilience
Correct Answer Explanation: Resilience refers to a material’s ability to absorb energy during deformation without permanently changing its shape. It is an important property in wood, particularly when considering its resistance to impact and shock.
Which of the following statements accurately describes the relationship between specific gravity and density?
- Specific gravity is the density of the wood divided by the density of water.
- Specific gravity is the density of the wood multiplied by the density of water.
- Specific gravity and density are the same thing.
- Specific gravity is the density of water divided by the density of the wood.
Correct Answer: Specific gravity is the density of the wood divided by the density of water.
Correct Answer Explanation: Specific gravity is a dimensionless ratio that compares the density of a substance to the density of water. It is calculated by dividing the density of the wood by the density of water. A specific gravity of 1.0 indicates that the wood is the same density as water.
What is the primary purpose of a “factor of safety” in structural design?
- To ensure the structure is aesthetically pleasing.
- To account for uncertainties in materials, loads, and design calculations.
- To reduce the cost of building materials.
- To increase the efficiency of the structure.
Correct Answer: To account for uncertainties in materials, loads, and design calculations.
Correct Answer Explanation: A factor of safety is a design factor that incorporates a safety margin in structural calculations. It accounts for uncertainties in the strength of materials, the actual loads the structure will experience, and potential inaccuracies in design calculations, ensuring that the structure can withstand unexpected stresses.
How does the location where a tree grows affect its mechanical properties?
- Location has no impact on the mechanical properties of wood.
- Location only affects the color and appearance of the wood.
- Location can influence the growth rate and density of the wood, affecting its strength.
- Location only affects the wood’s resistance to decay.
Correct Answer: Location can influence the growth rate and density of the wood, affecting its strength.
Correct Answer Explanation: The location where a tree grows can significantly affect its growth rate, density, and overall strength. Factors such as climate, soil conditions, and competition with other trees can influence these properties.
Which of the following statements accurately describes the behavior of a wooden beam under load?
- A wooden beam is stronger in tension than in compression.
- A wooden beam is stronger in compression than in tension.
- A wooden beam is equally strong in tension and compression.
- A wooden beam’s strength varies depending on the direction of the load.
Correct Answer: A wooden beam is stronger in tension than in compression.
Correct Answer Explanation: While wood can withstand significant tensile forces (pulling), it is generally weaker in compression (pushing). This is why wooden beams often fail due to compression on their bottom side, especially under heavy loads.
What is the term for a defect in wood where fibers are not parallel to the grain, making it weaker?
- Knot
- Shake
- Cross-grain
- Frost split
Correct Answer: Cross-grain
Correct Answer Explanation: Cross-grain is a defect in wood where the fibers are not parallel to the grain, resulting in weaker and more brittle wood. This defect makes the wood more susceptible to splitting and reduces its overall strength.
How does the presence of a pith affect the mechanical properties of a wooden beam?
- The pith significantly weakens a wooden beam in tension.
- The pith significantly weakens a wooden beam in compression.
- The pith has no effect on the strength of a wooden beam.
- The pith significantly affects the resistance to horizontal shear in seasoned beams.
Correct Answer: The pith significantly affects the resistance to horizontal shear in seasoned beams.
Correct Answer Explanation: The pith, the central core of a tree, does not significantly affect a beam’s strength in tension or compression. However, it can affect the beam’s resistance to horizontal shear, which is a force that tends to make one part of the beam slide past another.
What is the approximate coefficient of linear expansion for oak wood?
- 0.00000492
- 0.0000492
- 0.000492
- 0.00492
Correct Answer: 0.00000492
Correct Answer Explanation: The coefficient of linear expansion of oak wood is approximately 0.00000492, which means that for every degree Celsius increase in temperature, oak wood expands by 0.00000492 times its original length. This expansion is relatively small compared to other materials, such as iron.
Which type of timber testing evaluates the wood’s ability to withstand shock and impact?
- Impact testing
- Bending testing
- Compression testing
- Shear testing
Correct Answer: Impact testing
Correct Answer Explanation: Impact testing is specifically designed to evaluate a material’s ability to withstand sudden, high-energy loads, such as those caused by impacts or drops. This test is crucial for assessing wood’s suitability for applications where it might be subjected to shock, such as flooring, packaging materials, and sports equipment.
What is the primary purpose of seasoning wood?
- To improve the color and appearance of the wood.
- To make the wood more resistant to decay.
- To reduce the moisture content of the wood, increasing its strength and stability.
- To make the wood easier to work with.
Correct Answer: To reduce the moisture content of the wood, increasing its strength and stability.
Correct Answer Explanation: Seasoning wood involves carefully drying it to reduce its moisture content. This process is essential to improve the wood’s strength, reduce shrinkage, prevent warping and cracking, and increase its resistance to decay.
Why is it important to consider the speed of the testing machine in timber testing?
- The speed of the testing machine affects the accuracy of the test results.
- The speed of the testing machine has no impact on the test results.
- The speed of the testing machine affects the strength of the wood.
- The speed of the testing machine only affects the strength of green wood.
Correct Answer: The speed of the testing machine affects the strength of the wood.
Correct Answer Explanation: Wood’s strength can be affected by the rate at which the load is applied. Faster strain rates (loading more quickly) generally lead to lower measured strengths. This is because wood may not have time to deform fully before failure under rapid loading.
What is the term for the ability of wood to resist permanent deformation after being subjected to a load?
- Elasticity
- Resilience
- Toughness
- Hardness
Correct Answer: Elasticity
Correct Answer Explanation: Elasticity is the property of a material to deform under stress and return to its original shape after the stress is removed. It is a crucial property for wood, especially in structural applications, where it needs to resist deformation under load and maintain its integrity.
What is the approximate range for the average rate of growth associated with the greatest strength in Douglas fir?
- 10-20 rings per inch
- 20-30 rings per inch
- 30-40 rings per inch
- 40-50 rings per inch
Correct Answer: 20-30 rings per inch
Correct Answer Explanation: The average rate of growth associated with the greatest strength in Douglas fir is typically around 24 rings per inch. This means that trees with a growth rate of 20-30 rings per inch generally have the best combination of strength and stiffness.
What is the primary benefit of understanding the mechanical properties of wood?
- To identify different species of wood.
- To appreciate the beauty of wood.
- To select and utilize wood effectively for specific applications.
- To protect forests from over-exploitation.
Correct Answer: To select and utilize wood effectively for specific applications.
Correct Answer Explanation: Understanding the mechanical properties of wood is crucial for selecting and utilizing wood appropriately for various applications, from construction to furniture making to packaging. By understanding its strengths, limitations, and the factors influencing its behavior, engineers, designers, and craftspeople can make informed decisions that ensure safe, effective, and sustainable use of this natural resource.