Anisotropic Nature of Wood

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GROUP :01

MS-150501

MS-110513

MS-150516

MS-150521

Student ID:

Definition of wood

Wood is a three dimensional, anisotropic and

hygroscopic raw material which mainly composed of

cellulose hemicelluloses and lignin.

Anisotropy of wood

The dimensional changes in wood are unequal along

the three structural directions (longitudinal, tangential

and radial).

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Why to know about anisotropy

For determining properties of various direction

Wise utilization of wood

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Principle Directions

“With the Grain” =

Longitudinal

•“Cross Grain” or

“Perpendicular to

Grain” = Radial or

Tangential

Strength and

Shrinkage Properties

are different in each

direction

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Orientation of microfibrils: cell wall layers

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Anisotropic Characteristics of Wood

Shrinkage and Swelling Properties

• Tangential shrinkage is about twice as high as that

the radial direction.

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• The restricting effect of the ray on the radial plane

• The difference in degree of lignification between radial and

tangential walls

• Small difference in the microfibrillar angel between the two walls

• The increased thickness of the middle lamella in tangential wall

compared to the radial wall.

Reasons for higher tangential shrinkage:

Anisotropic Characteristics of Wood cont’d

Strength Properties

longitudinal axis are generally significantly higher than

those in the tangential or radial axes. Strength related

properties in the longitudinal axis are usually referred to

as parallel-to-grain properties.

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Anisotropic Characteristics of Wood cont’d

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Thermal conductivity

• Thermal conductivity is nearly the same in the radial and

tangential directions.

• Conductivity along the grain has been reported as greater

than conductivity across the grain.

Coefficient of Thermal Expansion

• The thermal expansion coefficient of ovendry wood parallel to

the grain appears to be independent of specific gravity and

species.

• Thermal expansion coefficients across the grain (radial and

tangential) are proportional to specific gravity.

• These coefficients range from about 5 to more than 10 times

greater than the parallel-to-grain coefficients.

Anisotropic Characteristics of Wood cont’d

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Electrical Properties

• Conductivity is greater along the grain than across the grain

and slightly greater in the radial direction than in the tangential

direction.

• Relative conductivity values in the longitudinal, radial, and

tangential directions are related by the approximate ratio of

1.0:0.55:0.50.

Elastic Properties

• MOE of douglas fir parallel to the grain is 20 times greater

than perpendicular to the grain.

Anisotropic Characteristics of Wood cont’d

Shear strength parallel to grain

• Ability to resist internal slipping of one part upon another

along the grain. Values presented are average strength in

radial and tangential shear planes.

Compressive strength parallel to grain

• Maximum stress sustained by a compression parallel-to-

grain.

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Anisotropic Characteristics of Wood cont’d

Thank You