Physical Properties of Wood
Wood, as a natural material, exhibits a wide range of physical properties influenced by its anatomical structure, moisture content, and environmental conditions. These properties determine its suitability for various applications, from construction to fine woodworking. Understanding these physical attributes, such as density, moisture movement, shrinkage, and thermal conductivity, is crucial for selecting the right type of wood and optimizing its use in engineering, architectural, and craft applications. This section explores the key physical properties of wood, highlighting their implications for performance and usability.
1. Density and Specific Weight
The density of wood, closely related to its apparent specific gravity, plays a significant role in determining its mechanical properties. Despite the variation in density among different species, the true specific gravity of wood is nearly constant, averaging 1.54. This value represents the ratio of the mass of wood substance to the mass of an equal volume of water, excluding voids such as pores.
However, the specific weight and apparent specific gravity vary based on the wood’s density, which is influenced by factors like moisture content. Moisture has a considerable effect on specific weight, meaning accurate comparisons of this property are best made on dry specimens to avoid discrepancies.
The density of wood can also vary within the tree itself. For instance:
- Wood from the lower trunk tends to be denser compared to wood from higher sections of the tree.
- At any specific height within the tree, the densest wood is typically found between the pith (the central core) and the middle growth rings of the cross-section.
These variations in density, influenced by the tree’s growth patterns, can lead to slight differences in the wood’s strength depending on its position within the tree.
2. Bulk Density
Bulk density is a critical physical property of wood that reflects the material’s mass relative to its bulk volume, including its pores. Unlike the true density of wood, bulk density accounts for the volume of void spaces (pores) and the moisture content present within the material.
For most wood species, the bulk density is lower than the true density, as it incorporates the air-filled or moisture-occupied spaces within the wood structure. Bulk density is an important parameter in determining the quality and usability of wood for various applications.
The quality factor is a measure of the efficiency of wood in resisting compressive forces relative to its bulk density. It is calculated as the ratio of compressive strength to bulk density. This factor helps evaluate the strength-to-weight performance of different wood species.
- For pine, the quality factor is approximately 0.6.
- For oak, the quality factor is slightly lower, at 0.57
3. Moisture Movement
Wood naturally interacts with its environment, particularly through the absorption and release of moisture. This moisture movement significantly influences its properties and usability in various applications.
Water is present in three main forms within wood:
- Living Cells:
In living wood, over 90% of the protoplasm in the living cells is composed of water. - Cell Walls:
Water saturates the cell walls, contributing to the wood’s structure and flexibility. - Cell Cavities (Pores):
Water occupies the voids within lifeless cells, either partially or fully.
Moisture movement in wood leads to shrinkage or swelling, depending on whether the wood is losing or gaining moisture. However, the extent of this movement varies based on the wood’s anatomical direction:
- Greatest movement occurs along the plane of the annual rings.
- Movement is minimal along the grain’s length and moderate across the grain.
To minimize moisture-related issues like warping, splitting, or decay, it’s essential to maintain wood at appropriate moisture levels for its intended use:
- Structural Elements: 12–20%
- Doors: 10–16%
- Windows: 10–16%
4. Shrinkage
Shrinkage in wood refers to the reduction in its linear and volumetric dimensions during the drying process. Notably, shrinkage does not occur during the evaporation of capillary water but begins only when hygroscopic moisture is lost. Due to the non-uniform structure of wood, shrinkage and swelling occur unevenly in different directions.
The linear shrinkage along the grain (longitudinal direction) is minimal, typically ranging between 0.1% and 0.3%. In contrast, shrinkage in the radial direction is more significant, ranging from 3% to 6%, while the tangential shrinkage is even greater, varying between 7% and 12%. On average, the radial shrinkage is about 60% of the tangential shrinkage, and longitudinal shrinkage is almost negligible.
As a result of these variations, the volumetric shrinkage of wood is approximately 1.6 times the tangential shrinkage, reflecting the combined effects of shrinkage across all directions.
5. Swelling
Swelling in wood refers to its ability to expand in both linear and volumetric dimensions upon absorbing water. Similar to shrinkage, the extent of swelling varies based on the wood’s structural orientation.
Swelling along the grain (longitudinal direction) is minimal, ranging from 0.1% to 0.8%. In the radial direction, the swelling is more pronounced, typically between 3% and 5%, while the tangential direction exhibits the highest swelling, ranging from 6% to 12%.
This directional variability is a result of wood’s anisotropic structure, where the arrangement of fibers and cellular composition influences its response to moisture absorption.
6. Heat Conductivity
Heat conductivity in wood is relatively low. The coefficient of heat conductivity is approximately 1.8 times higher along the grain (fibers) than across them, with an average range of 0.15 to 0.27 kcal/mh°C. This property varies with the bulk density and moisture content of the wood. As the bulk density increases or moisture content decreases, the air trapped in the wood’s cavities diminishes. This reduction in air leads to an increase in the wood’s heat conductivity.
7. Sound Conductivity
The velocity of sound in wood is significantly higher than in air, ranging from 2 to 17 times faster. This makes wood an effective medium for sound transmission, demonstrating its high sound conductivity. Due to this property, wood is often utilized in the construction of musical instruments and acoustic panels to enhance sound quality and resonance.
8. Resistance to Alkalis and Acids
Wood exhibits good resistance to weak alkali solutions, remaining largely unaffected under such conditions. However, it is vulnerable to decay in acidic environments where the pH drops below 4. This susceptibility makes it essential to protect wood exposed to acidic conditions to prevent structural or material degradation.
Stress-Strain Relationship in Wood
Wood is an anisotropic material, meaning its mechanical properties vary significantly along its three principal axes: longitudinal, radial, and tangential. These axes determine how wood responds to stress and strain under different loading conditions.
Principal Axes and Elastic Properties
- Wood exhibits three distinct values of modulus of elasticity, which can differ by as much as 150:1 depending on the axis.
- There are three shear moduli, with variations up to 20:1.
- Wood also has six Poisson’s ratios, which can vary by 40:1.
Stress-Strain Behavior
- Unlike metals, wood does not have a sharply defined elastic limit but does exhibit a proportional limit.
- The stress-strain curve is relatively linear over a significant range before curving gradually. This behavior classifies wood as a ductile material.
Proportional Limits and Ultimate Strength
- For direct compression and bending, the proportional limit is typically 65–75% of the ultimate strength.
- In direct tension, there is effectively no proportional limit, as the stress-strain curve remains linear until failure.
Modulus of Elasticity and Rigidity
- The modulus of elasticity is nearly identical in direct tension, compression, and bending, provided shear deformation is excluded.
- The modulus of rigidity of wood is much lower, approximately 1/15 to 1/20 of the modulus of elasticity.
- The apparent bending modulus of elasticity may vary slightly with the type of loading and span but is typically 90–95% of the true bending modulus.
Understanding the stress-strain relationship of wood is crucial for engineering applications:
- Designing Load-Bearing Structures: Accurate knowledge of proportional limits and ultimate strength helps ensure safety.
- Predicting Deformation: The modulus of elasticity and rigidity are vital for calculating expected deflections in beams and columns.
- Optimizing Material Use: Anisotropic properties help engineers select the best grain orientation for specific applications.
Mechanical Properties of Timber
Engineers, architects, and carpenters must have a thorough understanding of timber’s mechanical properties to ensure its proper use in construction and craftsmanship.
For Engineers
Engineers need detailed knowledge of timber’s strength and stiffness to design columns and beams capable of bearing loads safely. They must also understand how defects and environmental conditions impact these properties to select the most suitable timber for structural applications.For Architects
Architects must balance aesthetic considerations with functional requirements. This includes appreciating the visual appeal of different timber species, understanding their workability, and accounting for tendencies like shrinkage, warping, and checking. They also need to proportion joints and rafters effectively to prevent excessive deflection under load.For Craftsmen and Wheelwrights
Craftsmen and carpenters must comprehend the mechanical behavior of wood to maximize its potential during construction or woodworking. Wheelwrights, in particular, need to evaluate how the toughness and strength of timber are influenced by its species, growth rate, density, and defects, ensuring the durability of axles, spokes, and shafts.
Following are the Mechanical Properties of Timber :
1. Compressive Strength of Wood
Wood demonstrates exceptional strength when subjected to compressive forces acting parallel to the grain (growth axis), making it one of the most reliable structural materials for columns and posts. However, its compressive strength perpendicular to the grain is significantly lower, necessitating specific design considerations.
Key Properties
Compressive Strength Parallel to Fibres
- At 15% moisture content, the compressive strength ranges from 30.0 to 77.5 N/mm², depending on the wood species.
- This property is critical for structural applications, as it correlates closely with the bending strength of wooden beams. Experiments indicate that the yield point of beams is largely determined by the compressive strength of the wood.
Modes of Failure
- Parallel to Grain: Failures may occur due to:
- Cell Wall Collapse: Common in dry wood or softwoods.
- Lateral Bending of Fibres: More prevalent in wet wood or hardwoods, which contain thick-walled fibres and vessels.
- Cross-Grain Pieces: Shear parallel to the grain often leads to failure in cross-grained wood.
- Parallel to Grain: Failures may occur due to:
Compressive Strength Perpendicular to Fibres
- This is much weaker than parallel compressive strength and becomes critical under concentrated loads, such as at beam supports.
- To avoid crushing, proper measures such as footings or load-distribution elements must be incorporated to spread the pressure.
2. Tensile Strength of Wood
Wood exhibits exceptional tensile strength when forces are applied parallel to the grain, making it one of its strongest mechanical properties. However, practical limitations and specific structural challenges often restrict its use in tension-loaded structures.
Key Properties and Characteristics
Tensile Strength Parallel to the Grain
- Wood’s tensile strength parallel to the grain is the highest among all stress types, ranging from 80.0 to 190.0 N/mm², depending on species and quality.
- Designing end connections strong enough to utilize its full tensile capacity is challenging, as wood resists poorly against shearing stresses and crushing at restrained ends.
- Tension members are rarely used because of these design difficulties and the dominance of compressive strength in beam behavior, where compressive strength governs beam strength.
Factors Affecting Tensile Strength Parallel to the Grain
- Grain Straightness: Straight-grained wood provides higher tensile strength.
- Wall Thickness: Thicker longitudinal cell walls improve tensile strength.
- Knots and Defects: Features like knots, shakes, and irregular grain significantly reduce tensile strength.
- Moisture Content: Tensile strength parallel to the grain is less affected by moisture compared to other mechanical properties.
Tensile Strength Across the Grain
- Tensile strength across the grain is considerably lower and is closely tied to the cleavability of the wood.
- Failures in this direction occur through cell and fiber separation along longitudinal planes, making cross-grained or spiral-grained beams vulnerable.
- Knots and Defects: These significantly reduce tensile strength across the grain, leading to weak points in the structure.
Failure Mechanism
- When subjected to tensile stress parallel to the grain, failure typically occurs through transverse rupture of the longitudinal elements.
- In tension across the grain, failure is caused by separation of fibers, especially in wood with defects like knots or spiral grains.
3. Bending Strength of Wood
The bending strength of wood plays a vital role in its widespread use for structural elements such as beams, slabs, rafters, and trusses. It reflects the ability of wood to resist deformation or failure under static bending forces.
Behavior Under Bending
Initial and Final Failure
- In long beams of uniform width, initial failure is often seen as wrinkling in the compression fibers, similar to the behavior in compression members.
- Final failure usually occurs in tension when the tensile fibers snap as the beam reaches its maximum load.
- In dry specimens, failure in tension may happen suddenly, sometimes before compression fibers show any visible deformation.
- Conversely, green wood fails silently in compression, with no rupture of the tensile fibers.
Short and Deep Beams
- These beams often fail through horizontal shear, particularly in seasoned timber and structural sizes.
- Shear failures are more common when there are defects in the timber.
Impact of Grain and Orientation
- Long, narrow beams must have lateral restraint to prevent column-like failures in compression fibers.
- Beams supported improperly, such as those with a tilted neutral axis, experience reduced strength.
- These issues are mitigated using diagonal bracing or bridging.
Key Factors Influencing Bending Strength
Wood Condition
- Seasoned Timber: Higher strength but prone to sudden shear failure.
- Green Timber: Lower strength and silent failure in compression.
Defects
- Knots, cracks, or grain irregularities significantly reduce bending strength and are common points of failure.
Load Orientation
- Proper orientation with wide sides vertical ensures maximum strength.
4. Shearing Strength of Wood
Shearing strength is a measure of wood’s resistance to forces that cause sliding between adjacent layers. Wood exhibits low shearing strength along the grain, making it a critical consideration in structural applications.
Key Properties
Shearing Strength Along the Grain
- The shearing strength of wood ranges between 6.5 to 14.5 N/mm² along the fibers.
- This weakness is particularly important in structural joints and areas where concentrated forces occur parallel to the grain.
Shearing Strength Across the Grain
- The resistance to shearing across the fibers is significantly higher, approximately 3 to 4 times greater than along the grain.
- This property is advantageous in situations where forces act perpendicular to the fiber direction.
Combined Stresses
- In practice, pure shear rarely occurs. Instead, wood subjected to shear is often simultaneously affected by crushing and bending forces, which can exacerbate its vulnerability to failure.
5. Stiffness of Wood
Stiffness is a crucial property in timber structures, often as significant as strength but frequently overlooked during design. It ensures that structural elements like floors and rafters can resist excessive deflection under working loads, preventing a sense of insecurity or the unsightly appearance of a sagged roof. In general, denser woods exhibit greater stiffness, and seasoned timber is stiffer than green timber. Interestingly, structural sizes of timber maintain stiffness comparable to that of smaller, clear specimens, highlighting the material’s consistency across scales. Proper consideration of stiffness is essential for both functionality and aesthetic appeal in timber construction.
6. Toughness of wood
Toughness in wood refers to its capacity to resist shocks or blows, combining both strength and flexibility. Hardwoods are generally more tough than softwoods, with longleaf pine being a notable exception among conifers. Green wood typically exhibits greater toughness than seasoned wood due to its higher flexibility. The toughness of a wood is best quantified by measuring the energy needed to break a beam under transverse impact, making it an essential property for applications where resistance to sudden forces is critical.
7. Cleavability of wood
Cleavability measures how easily wood can split, typically being higher along radial planes than tangential surfaces in most hardwoods. Splitting occurs as longitudinal fibers are wedged apart, making it closely related to tensile strength across the grain. Wood used with nails or screws should resist splitting to maintain structural integrity. Among conifers, longleaf pine is an exception, as its cleavage strength shows minimal variation between radial and tangential planes.
8. Hardness of wood
Hardness, defined as the resistance to indentation and scratching, is crucial for applications like flooring, furniture, and finishes. It influences the wood’s ability to endure wear without splintering. These attributes, along with indentation tests, help assess the suitability of wood for areas subjected to heavy use, although no comprehensive test exists for overall hardness properties.
Chemical Properties of Wood
1. Cellulose
A long-chain carbohydrate polymer made of glucose units. Forms the primary structural component of wood cell walls. Highly resistant to chemical breakdown and provides strength to plant tissues.
2. Hemicellulose
A long-chain carbohydrate polymer made of glucose units. Forms the primary structural component of wood cell walls. Highly resistant to chemical breakdown and provides strength to plant tissues.
3. Lignin
A complex organic polymer acting as a natural “glue” between plant cells. Provides rigidity and water-repelling properties. Chemically complex with a cross-linked structure that enhances wood’s structural strength.
4. Chemical Reactivity
Wood is combustible and capable of chemical transformations. Undergoes processes like oxidation, pyrolysis, and biodegradation. Can be chemically modified for preservation and enhanced performance.
5. pH Characteristics
Typically slightly acidic, with a pH range of 4-5. Vulnerable to chemical breakdown in highly acidic environments. Demonstrates variable chemical resistance to different substances.
6. Moisture Interaction
Capable of absorbing and releasing moisture. Chemical structure allows for dynamic interaction with environmental humidity. Moisture content significantly influences wood’s chemical and physical properties
Effect of Moisture on the Mechanical Properties of Wood
The mechanical properties of wood, such as strength and stiffness, are significantly influenced by moisture content. As wood dries, particularly after reaching the fiber-saturation point, further moisture loss leads to a substantial increase in tensile and compressive strength as well as stiffness. However, prolonged seasoning or kiln-drying processes can introduce challenges. For example, shrinkage stresses from seasoning may reduce resistance to horizontal shear, while kiln-drying may result in case-hardening—a condition that traps internal moisture and induces internal stresses, negating the typical strength gains. Thus, managing moisture content is critical to optimizing wood’s mechanical performance.
Effect of Temperature on Strength of Wood
The impact of temperature on wood depends significantly on its moisture content. Dry wood expands slightly when heated, whereas wet wood shrinks due to moisture evaporation. Elevated temperatures above room temperature generally weaken most strength properties of wood, with very high temperatures, like those used in vulcanizing, causing slight weakening in dry wood. Conversely, freezing temperatures increase both the strength and stiffness of wood. Moist wood subjected to heat becomes highly pliable but experiences a reduction in strength, highlighting the interplay between temperature and moisture in determining wood’s mechanical behavior.
Frequently Asked Questions
Wood’s key chemical properties include:
- Combustibility
- Ability to absorb and release moisture
- Potential for chemical decomposition
- Slightly acidic pH (around 4-5)
- Capability of chemical modification
The primary chemical properties include:
- Composed of cellulose, hemicellulose, and lignin
- Capable of oxidation
- Susceptible to biodegradation
- Chemically reactive under certain conditions
- Contains organic polymers that can be chemically altered
Key physical properties include:
- Varying density
- Moisture absorption capability
- Ability to shrink and swell
- Thermal insulation properties
- Anisotropic structure (different properties in different directions)
Some notable physical properties are:
- Heat conductivity
- Sound transmission characteristics
- Moisture movement
- Shrinkage and expansion
- Specific weight and bulk density
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