Tensile Strength of Wood

In physics and engineering, ultimate tensile strength is the maximum stress that a material can withstand while being stretched or pulled before breaking.

The term “tensile strength” is often used in conjunction with materials science, as well as civil and mechanical engineering. Tensile strength is determined by dividing the maximum load by the original cross-sectional area of the specimen.

Tensile strength is an important property to consider when choosing a material for a particular application. For example, a material with high tensile strength would be ideal for use in a suspension bridge, while a material with low tensile strength would be more suitable for use in a chair.

Tensile Strength of Different Woods

When it comes to choosing the right wood for your needs, you need to consider more than just the appearance. The tensile strength of wood is important to consider, as it will affect how strong the wood is and how much weight it can hold. Different woods have different tensile strengths, so it’s important to know which one will work best for your project.

Here is a breakdown of the tensile strengths of some common woods:

  • Oak has a tensile strength of 9,000 psi (~62 MPa), making it a great choice for projects that need a lot of strength and durability.
  • Birch has a lower tensile strength of only 4,000 psi but is still a strong choice for many projects.
  • Alder has a tensile strength of 4,000 psi and is a great choice for projects that need a lot of strength like tables, cabinets and bookcases.
  • Cherry is a very hard wood, with a tensile strength of 2,000-7,000 psi.
  • Walnut has a tensile strength of 1,500-4,000 psi.

Tensile Strength Of 2×4

The direction of the grain does affect the tensile strength of wood. Wood that is cut across the grain is weaker than wood that is cut with the grain. The more the wood is cut at an angle, the weaker it becomes. The direction of the grain should be considered when selecting lumber for a project.

Let’s use tensile strength as an example. Deflections of a beam under load depend on its cross-sectional area in the plane defined by the axis of the beam and the direction of the applied load.

The larger the cross-sectional area, the more the beam is able to resist the load.

For dimensional lumber, the cross-sectional area of a 2×4 is its length, L, times its thickness, 1.5″.

When you stand the 2×4 up, it has a cross-sectional area of L times its width. For dimensional lumber, this is 3.5″.

The tensile strength of wood is commonly termed, measured and represented as a  strength property.

Tensile strength in building lumber is measured in two main ways. The tensile strength parallel to the grain and the tensile strength is perpendicular to the grain.

Additional measurements are often made to assess work-to-maximum-load in bending, impact-bending strength, and hardness. 

These properties are generally broadly grouped according to the categories of hardwood and softwood.

Tensile Strength Perpendicular To Grain

This refers to the resistance of wood to force acting across the grain that tends to split a member. 

Tensile Strength Parallel To Grain

That is the maximum tensile stress sustained Parallel to the grain. Across a variety of species of clear wood, relatively few data exist on tensile strength along the grain. The following table (sourced from Mechanical Properties of Wood: David W. Green, Jerrold E. Winandy, and David E. Kretschmann) provides the parallel-to-grain tensile strength of a few wood species tested at green. Note that decreased moisture content (12%) increases tensile strength from these values up by 32% in hardwoods and 12% in softwoods.

Modulus of rupture is often substituted for tensile strength of small, clear, straight-grained pieces of wood in the absence of enough tension test data. 

Modulus of rupture is usually used as an indicator of tensile strength for clear specimens.

Compression Failures

Products with visible compression failures have low strength characteristics, especially in tensile strength and shock resistance. Compared to matched clear wood, the tensile strength may be one-third lower in wood with compression failures. Even a small amount of weakening, visible only with a microscope, may result in brittle fractures and severe strength reductions. Because compression failures have low strength, many safety codes require certain structural members, such as ladder rails and scaffold planks, to be entirely devoid of these failures.

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