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What Is Flexural Modulus?

Flexural modulus is a measure of how a certain material reacts to weight and force.
Flexural modulus is an important concept in engineering.
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  • Written By: Dorothy Distefano
  • Edited By: Michelle Arevalo
  • Last Modified Date: 28 March 2014
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Flexural modulus is a measure of how a material will deform and strain when weight or force is applied. It describes the ability of a material, with a specific cross-section, to resist bending when placed under stress. This property is important in civil, mechanical, and aerospace engineering and design, and is frequently used to select correct materials for parts that will support loads without flexing.

This term is used to define the relationship between a bending stress and the resulting strain. Strain is a measure of the amount that a material will deform when a stress is applied. Elastic strain is reversible and will disappear after the stress is removed, meaning the material will return to its original state. At high levels of stress, a material will permanently deform and will not return to its original dimensions. This is referred to as plastic strain or yielding.

The deformation can be plotted in a stress-strain curve, which shows how strain changes with applied bending stress. The slope of this curve in the region where elastic strain occurs defines the flexural modulus of the material. The units of measure are pounds per square inch (psi) or Newtons per square meter, also known as pascals (Pa).

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This measurement is determined experimentally in a laboratory, using a sample of material with a specific shape and dimensions. The flexural test measures the force necessary to bend a beam of known dimensions that has a force applied at three points. The beam is supported on the bottom side near both ends and a force is applied to the top at the center point, between the bottom supports. This is known as three-point loading conditions. A force is applied and the deflection or movement of the beam is measured.

Flexural modulus has been determined for a wide variety of structural materials, including metals, wood, glass, concrete, and plastics. It is usually measured at ambient temperature conditions. The properties of some materials, like many plastics, will change with temperature. The flexural test is sometimes conducted at lower or higher temperatures, to simulate the intended end-use environment.

Ductile materials like steel and brass, which can be deformed a great deal before failure, have a well-defined flexural modulus. Brittle materials, like glass and concrete, have little or no plastic deformation before failure. For this reason, many brittle materials do not have a clearly defined measurement, and they are often described by their flexural strength — which is the maximum amount of bending stress that can be applied before rupture or failure of the material occurs.

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Discuss this Article

anon360276
Post 7

anumeya: flexural stress is highest stress within material at its moment of rupture while as flexural modulus is the ratio of stress to strain in flexural defomation.

anon306336
Post 5

@anon295089: It is easier to understand if you look at the stress strain graphs. What you're describing is that some materials don't have a high enough slope (flexural modulus) although they do reach the flexural stress desired (I am assuming it is yield flexural stress). In other words, it is too bendable, even if it doesn't break.

The other case is that the material has the right slope (flexural modulus), meaning it is stiff enough. However, it just isn't strong enough to handle the flexural stress desired.

Disclaimer: I'm only a third year undergrad. What I post is in no way professional advice and I am not giving it as such. Please consult someone else who is qualified if you are going to do job related stuff. I am only giving my 2 cents for our mutual edification.

anon295089
Post 4

I am a construction inspector and I have a problem interpreting the test results submitted to us. For example, the contract specs for mechanical testing call for a flexural stress of at least 4,000 psi. at 5 percent strain, and for a Flexural Modulus of at least 300,000 psi.

My problem is that some materials passed the flexural stress but failed to meet the Flexural Modulus requirement, and for other samples its the other way around.

In a structural perspective, could anyone out there help educate me as to what these test results really mean?

healthy4life
Post 3

@JackWhack – It sounds strange, but glass would have a larger elastic modulus than rubber. This is because elastic modulus measures stress changes when strain is applied.

So, though rubber would bend much further because it is more elastic, it is actually stronger than the glass when tension is being measured. The glass will break long before the rubber will.

JackWhack
Post 2

I can see how glass would yield to stress suddenly instead of deforming. With something as thick and as sturdy as glass, you will either hear it break or hear it do nothing at all.

seag47
Post 1

This flexular modulus definition reminds me of what we did in high school to determine how strong our wooden bridges were. The teacher had divided us into teams and instructed us to come up with bridge designs, and the team with the strongest bridge would win.

We stressed the bridges until they snapped. The ones that could deal with the most strain resulted in better grades.

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