Strength and Modulus - RC Groups
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Feb 25, 2013, 01:58 PM
I am actually really slow

Strength and Modulus

Can someone explain the difference in an easy to understand way? Im forever finding myself getting confused with the terms when comparing resins. Im also having trouble understanding modulus of elasticity, can this be compared to flexural modulus?
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Feb 25, 2013, 03:10 PM
2012 NZ Speedcup - 231 MPH
DownUnderPilot's Avatar
You're not alone there Ben!

As far as I have grasped it, modulus = stiffness ?

So higher the modulus the stiffer it is, but it comes at the cost of inability to bend. Be good to have a guru wade here.....
Feb 25, 2013, 03:36 PM
Spy's sappin' my thermal!
Chris Sutton's Avatar
Modulus is short for Modulus of Elasticity, which tells you how stiff the material is. It's like a spring constant. Everything stretches when you pull on it, though usually it's too little to see. In most cases, when something is simply called the modulus, it is specifically the tensile modulus - how much the material stretches when under tension. Flexural modulus is similar, but relates to how stiff the material is in bending.

Strength is what you'd expect - how much force a material can handle before it breaks. Tensile strength is generally the point at which the material will break in tension. Yield strength (where noted) is the point at which it stretches permanently in tension.
Feb 25, 2013, 04:09 PM
I am actually really slow
Oh okay, now it's making sense. So I now know, modulus is a measurement of stiffness and strength is load before break, but how come the flexural modulus always seems to be several thousand times higher then flexural strength. How can you continue bending something after its broken. Both bits of data are given in the same units, how does that work?

Chris does that mean that tensile modulus and modulus of elasticity are the same thing?
Feb 25, 2013, 11:02 PM
Registered User
"So I now know, modulus is a measurement of stiffness and strength is load before break, but how come the flexural modulus always seems to be several thousand times higher then flexural strength."

Beware of the unit... If you do a quick search you will understand the meaning of each value when related to their respective unit.
Feb 25, 2013, 11:11 PM
Just fly it!
wyowindworks's Avatar
When you apply a force to a material is deforms to some degree. If the material returns to it's original shape when the force is no longer applied the deformation is elastic. If the deformation doesn't completely rebound then the deformation is said to be plastic. Deformation can happen under tension, compression, bending, shear, and torsion. Modulus, in simple terms, describes the materials ability to resist elastic deformation. Things with a higher modulus resist deformation more than a low modulus material.

In a tradition bending wing spar (flanges separated by a shear web)t he bottom flange is under tension (stretched) the top flange is under compression (squished) and the shear web is under shear (tension and compression at approximately 45* to the span of the wing). There is also deformation in tension, compression, and shear. If we need the spar to be stiffer we would either use more material or use materials that are more resistance to deformation under the given load (higher modulus). Carbon fiber stretches and compresses less than glass does under the same load, therefore carbon would make a stiffer spar for less weight.

Yield strength is the point at which the applied force causes the material to move from elastic deformation to plastic deformation (permanent deformation). Composites typically fracture of break before they under go much plastic deformation (low ductility). Many steels on the other hand will undergo a lot of plastic deformation before they fracture (higher ductility).

With carbon fiber the tensile strength typically goes down as the modulus increases. High modulus carbons are not as strong high tenacity (lower modulus) carbons.

The amount that the material deforms under a given load is called strain. Fiberglass will stretch further before failing than carbon will. For this reason fiberglass is often tougher than carbon fiber. The rate of strain (how quickly the material must deform under load) will typically effect how the material performs on impact. Carbon doesn't like to be quickly deformed while materials like kevlar can perform very well under higher strain rates.

A rubber band is a high strain material with a low modulus.
Last edited by wyowindworks; Feb 25, 2013 at 11:44 PM.
Feb 28, 2013, 02:06 PM
Registered User
Modulus is how much force is needed to flex the material a certain amount. It does not consider how much force is required, or at what flex level the material breaks.

Strength is how much force is required to break the material. It does not take into account how far the material has flexed by the time it breaks.

It is important to get a clear divide in your head about how much force it takes until something breaks and how much distance something needs to be flex before something breaks and how they are independent of each other. Then bridge the two ideas with how much force it takes to flex something a certain distance.

So it means that the following combinations are possible:
-low strength, low modulus
Takes low force flex it a given amount and it does not take a lot of force for it to break. Therefore it won't flex much before it breaks. Like a long piece of soft chewing gum perhaps.

-low strength, high modulus
Takes a lot of force to flex it a given amount BUT does not take a lot of force to break it. So the total force to break it is not very much even though it takes a lot of force to bend it a certain amount. THis kind of thing won't flex much before it breaks and is the kind of thing where if you tap it lightly it may shatter. Like a thin cracker.

High strength/high modulus
This kind of material takes a lot of force to flex a certain amount, yet also takes a lot of force to break. It's real hard to flex it a given distance, but it will also flex a large amount before it actually breaks. Like the cookie part of an oreo cookie. Carbon fiber is this category. It breaks after only a little flex but it takes a large amount of force to flex this much.

High strength/low modulus
This material takes a lot of force to break but does not take a lot of force to flex a given amount. Kind of like a rope. You can flex it to your hearts content almost any distance. But to flex it enough to break it is going to take a LOT of flex distance because it takes so little force to flex it, yet takes so much force to break it. Like a piece of string or...liccorice? Steel can sort of be considered this category not because it is actually low modulus. It actually has pretty high modulus but it's strength relative to this modulus is even higher. This modulus:breaking strength ratio means that while it is stiff and takes a lot of force to bend, it can also flex a lot providing a lot of give before it actually breaks. Which makes it strong and tough, rather than strong but brittle (like carbon fiber)
Last edited by DKNguyen; Feb 28, 2013 at 02:51 PM.

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