wood being stored

What Is Modulus Of Rupture In Wood

Definition Of Modulus Of Rupture

Modulus of rupture, commonly called MOR, is a measurement of an objects strength before breaking.  It has been called bending strength, flexural strength, and transverse ruptural strength.

The purpose of this measurement is to gauge a load capacity of a material or object to the point of it breaking.

How To Calculate Modulus Of Rupture

The equation σr = 3Fx/yz2 can be used to calculate modulus of rupture.  F would be the force load while x, y, and z are the dimensions of the wood.

Within a three point load test the wood is slightly elevated and supported on both ends.  The load is applied to the center of the board or beam to its ends.

1x1 lumber

While deformation will begin to take place the point of rupture or breaking would be the purpose of discovery.  It is important to keep all units of measurement consistent for these tests.

You can use pounds and inches yet the same units must be applied throughout the process.  The result of this outcome would produce a yield of pounds per square inch lbf/in2.

Generally measurements given for wood assume that it has been dried to at least 12% moisture content unless it is noted otherwise.  Moisture will have a direct impact upon the results.

MOR is great for understanding the strength of wood where force will be applied horizontal to the wood grain.  When considering a build it should be noted that crushing strength is to be considered as well.

Modulus Of Rupture Compared To Compression “Crushing” Strength Of Wood

This comparison is worth noting due to how wood can be applied to objects, structures, and art.  I will use the words horizontal grain for Modulus Of Rupture and vertical grain for Compression Strength.

Wood will always be weaker when force is applied horizontal to the grain.  The wood is being flexed by that force in a direction where it has the least support.

Wood will always be stronger when force is applied to the vertical grain.  In other words the force is applied to the end of a post or board as if it were standing.

In this case the pressure exerted on the wood is in line with the grain structure where it has the most support.  Thus it will take considerably more force to cause a rupture.

Why is this so important?  Consider making legs for a dining table.  A heavy slab could compromise a leg if the grain is running horizontal to the floor.

By comparison when the same table leg is constructed with the grain running vertically to the floor it is much stronger.  The table can support more weight on its four points of contact with the floor.

It is imperative that the modulus of rupture be considered when constructing objects.  Some have said that this material property matters little when in fact it matters a lot.  Do not neglect its function.

The Relationship With Modulus Of Rupture With Tensile Strength

When measuring wood the law of averages is what we refer too.  Why?  These tests often require a homogeneous material for accurate results.

While wood is not truly homogeneous, we can take multiple tests in order to give us an average yield while controlling variables like moisture.

The relationship between modulus of rupture and tensile strength depend upon a homogeneous material.  Nevertheless, with wood we will use these averages.

Tensile strength will measure the force which the wood will accept before breaking.  Modulus of rupture measures the point of breaking.

These two are different metrics to determine different impacts upon the wood.  They are still related to each other but do not confuse their properties or the purpose.

An additional term to not confuse with rupture is modulus of elasticity. This modulus is for measuring deflection.

How Modulus Of Rupture Is Applied In Wood Art

Wood is a unique structure due to how it is grown.  It is a natural fibrous composite held together by a “natural glue”.  It is robust enough to withstand storms but it does have limitations.

Within engineering the application of the modulus of rupture is quite obvious.  You are seeking structural integrity to a built object.

Wood art is a different need.  The closest common items we can associate these needs with commercially is furniture.

Beauty is the focus in these applications.  Mobile wood art may require an internal core of wood with higher MOR.  Built, joined, or glued onto this core will be the more decorative wood for the art.

In this case the wood with the higher modulus of rupture is used much like a skeletal structure in order to provide the needed support.  The exterior “skin” or surface is where the shape and wood carving takes place.