Overview:
This book, published in 1914, delves into the technical properties of wood, primarily for forestry students. It avoids overly technical language, making the subject accessible to anyone interested in wood. The book is divided into three parts:
- Mechanical Properties of Wood: This section introduces basic concepts of stress, strain, and elasticity, focusing specifically on wood. It covers different mechanical properties like tensile strength, compressive strength, shearing strength, bending strength, toughness, hardness, and cleavability. Numerous tables showcase strength values of various American woods.
- Factors Affecting Mechanical Properties: This part explores how various factors influence the mechanical properties of wood. It discusses the impact of growth rate, heartwood vs. sapwood, weight and density, cross-grain, knots, frost splits, shakes, pitch pockets, insect and fungal injuries, and other external factors like locality, season of cutting, water content, temperature, and preservatives.
- Timber Testing: This final section details methods for timber testing, particularly those employed by the U.S. Forest Service. It explains different test types like bending, compression, shear, impact, hardness, and cleavage, and provides instructions on preparing specimens, conducting tests, and analyzing results. An appendix includes a sample working plan from the U.S. Forest Service for timber research, along with strength values for various structural timbers.
Key Findings:
- The mechanical properties of wood are highly variable, even within the same species.
- Rate of growth significantly influences the properties of wood, with faster growth often associated with greater strength in hardwoods.
- Heartwood and sapwood can have different properties, but sapwood is generally as strong as heartwood.
- Weight and density are directly correlated with strength and stiffness, but only when the increase in weight is due to wood substance, not infiltrated substances.
- Drying strengthens wood but can also lead to checking and weakening due to uneven shrinkage.
- Knots, frost splits, shakes, and other defects significantly affect wood’s strength and usefulness.
- Insect and fungal attacks can significantly weaken and degrade wood.
- Temperature and moisture content greatly affect wood’s properties.
- While preservatives can enhance wood’s durability, they can also affect its strength depending on the treatment method.
Learning:
- Understanding Wood’s Structure: This book provides insight into the internal structure of wood, helping readers understand how its different components contribute to its properties.
- Factors Influencing Wood’s Strength: Readers will learn how various factors, both intrinsic and extrinsic, affect the mechanical properties of wood. This knowledge is essential for selecting the right wood for specific applications and understanding its limitations.
- Timber Testing Methods: The book outlines the various tests used to assess wood’s mechanical properties, providing a basis for understanding scientific research and practical evaluation of wood’s suitability for different purposes.
- Wood’s Limitations: This book emphasizes wood’s variability and limitations, highlighting the importance of careful selection and proper treatment to ensure strength, durability, and safe usage.
Historical Context:
This text was written in 1914, a time of significant industrial expansion and increasing reliance on wood for construction and other industries. The United States was experiencing a period of intense timber extraction and utilization, leading to concerns about the sustainability of timber resources and the need for scientific research on wood properties. This book reflects the growing need for a scientific understanding of wood’s behavior to promote efficient and responsible use.
Facts:
- Wood is an organic product: Its structure and properties are complex and variable.
- Wood cells are mostly fibrous: These fibers provide wood’s strength and toughness.
- Late wood is denser and stronger than early wood: It has thicker cell walls and smaller cavities.
- Heartwood is dead wood: It no longer conducts water or stores food.
- Sapwood is thicker in trees with larger crowns: It’s needed to transport water and nutrients.
- Heartwood is often more susceptible to decay than sapwood: This is due to infiltrations of tannins, oils, and resins.
- Wood substance is heavier than water: This is why wood floats when dry but sinks when waterlogged.
- Density is the weight of a unit of volume: Specific gravity is the ratio of density to water’s density.
- Wood shrinks greatly in drying: This is why it’s important to consider shrinkage when evaluating wood’s properties.
- Cross-grain weakens wood: It reduces tensile strength and makes it more susceptible to failure in bending.
- Knots are a defect: They weaken wood, particularly in bending and tension.
- Frost splits are a common defect: They occur near the base of trees due to temperature differences.
- Heart shake results from shrinkage of the heartwood: It can create radial clefts extending from the pith.
- Cup shake occurs at the junction of growth layers: It is more common in trees with uneven growth rates.
- Insect injuries weaken wood: They disrupt the continuity of fibers and can introduce decay.
- Marine wood-borers attack timber in salt water: They create tunnels that weaken the wood.
- Fungal decay degrades wood: It breaks down cell walls and reduces wood’s strength.
- Dry rot fungi thrive in warm, humid conditions: They can damage lumber even after it’s placed in buildings.
- Wood expands in dry air when heated: This expansion is offset by shrinkage when wood contains moisture.
- Steaming weakens wood: High pressure or prolonged steaming can permanently reduce wood’s strength.
Statistics:
- Wood’s modulus of elasticity is significantly lower than steel: Steel has a modulus of approximately 30,000,000 pounds per square inch, while wood ranges from 643,000 to 1,769,000 pounds per square inch.
- Wood’s tensile strength is two to four times its compressive strength: This is why beams often fail in tension on the bottom side.
- Green spruce can be loaded to four times the weight of a dry spruce block: Drying significantly increases wood’s strength.
- Longleaf pine can safely support a permanent load up to three-quarters of its elastic limit: This holds true for dry wood but not for wood with higher moisture content.
- The factor of safety for wood in structures is often as high as 6 or 10: This is due to the variability of wood and the need to ensure safety.
- The speed of the testing machine can affect wood’s strength: Faster strain rates generally lead to reduced strength.
- The average rate of growth for Douglas fir associated with the greatest strength is 24 rings per inch: For other conifers, this rate varies between 6 and 30 rings per inch.
- Wide-ringed hickory (5-14 rings per inch) has the highest shock-resisting ability: This is due to its greater amount of wood substance.
- The weight of dry wood substance is about 1.55 times that of water: This value is nearly the same for all species.
- The specific gravity of wood represents the density of the dry wood: It is not the weight of a cubic foot of green wood minus the water it contains.
- Shrinkage in volume from green to oven-dry ranges from 6 to 50%: Conifers typically shrink about 10%, while hardwoods shrink closer to 15%.
- The strength of longleaf pine from different states varies little: However, loblolly and shortleaf pine show greater variation in strength depending on their growing location.
- Southern hickory is as tough and strong as northern hickory: However, it is more prone to shakes, resulting in greater waste.
- Sapwood is as strong as heartwood: This is true except for sapwood from very old trees.
- The presence of the pith does not affect wood strength: However, it can influence the resistance to horizontal shear in seasoned beams.
- The coefficient of linear expansion of oak is .00000492: This is about one-sixth that of iron.
- Steaming at 20 pounds pressure for 6 hours does not significantly weaken loblolly pine: However, steaming at higher pressures or for longer durations can permanently damage wood’s strength.
- In 840 large timber beams tested, 58% failed in compression: This highlights the importance of considering compression failure in design.
- Sound knots do not weaken wood under compression: However, they can significantly reduce strength in bending and tension.
- More than $100,000 was spent repairing damage due to dry rot in the United States between 1911 and 1913: This demonstrates the significant economic impact of fungal decay.
Terms:
- Stress: A distributed force that tends to deform a material.
- Strain: The distortion or deformation of a material caused by stress.
- Elastic Limit: The point beyond which deformation becomes permanent.
- Modulus of Elasticity: A measure of stiffness, calculated as the ratio of stress to strain within the elastic limit.
- Modulus of Rupture: A hypothetical value representing the unit stress at the breaking point of a beam.
- Resilience: The ability of a material to absorb energy during deformation.
- Cleavability: The ease with which wood can be split.
- Heartwood: The central, darker portion of a tree, often denser and more resistant to decay than sapwood.
- Sapwood: The outer, lighter portion of a tree, responsible for transporting water and nutrients.
- Cross-grain: A defect in wood where fibers are not parallel to the grain, reducing strength and increasing susceptibility to splitting.
Examples:
- Flexure of a Bow: A bent bow demonstrates the combined action of tension and compression on the wood fibers, illustrating the concept of stiffness.
- Breaking a Buggy Spoke: This illustrates failure in endwise compression, where a long, thin piece of wood fails by bending before crushing.
- Shoving Off a Tenon: This demonstrates the principle of shear across the grain, where one portion of the wood slides past another.
- Season Checks in Wood: These cracks, caused by uneven drying, illustrate the importance of careful seasoning to minimize shrinkage and maintain wood’s strength.
- Decay in a Log: This illustrates the destructive effect of fungi on wood, highlighting the need for preservatives and proper storage to prevent rot.
- Impact Testing: Dropping a weight on a small beam illustrates the concept of impact and how it affects wood’s resistance to shock.
- Testing Spike-Holding Power: This test demonstrates the importance of wood properties in applications like railroad ties, where holding power is essential.
- Strength of a Packing Box: This test evaluates the suitability of different woods for packaging applications, where resistance to impact and pressure is vital.
- Testing Cross-arms: This demonstrates how the properties of wood are relevant in applications like power lines, where strength and resistance to bending are critical.
- Measuring the Specific Gravity of Wood: This process, involving submerging a specimen in water, illustrates the concept of density and its relation to wood’s weight and strength.
Conclusion:
“The Mechanical Properties of Wood” provides a comprehensive and accessible overview of the technical properties of wood, highlighting its strengths, limitations, and the factors that influence its behavior. It emphasizes the importance of understanding wood’s inherent variability and the need for careful selection, proper treatment, and scientifically sound testing methods to ensure its safe and efficient utilization. This knowledge is crucial for informed decision-making in various industries relying on wood, from construction to furniture making to transportation. The book’s historical context further underscores the importance of continued research and innovation in the field of timber science.