Need assistance defining Modulus.

[Troy]

I am trying to get a better understanding of Modulus and how it relates in dealing with physical properties of materials. I am not an engineer but in my everyday dealings with engineers and materials I have to compare things like flexural modulus vs flexural strength and tensile modulus vs tensile strength. I have read several engineering definitions in various books and online info but I wonder if I am lacking some additional info to really get a layman's understanding of how it is practically used. For example, I know the difference in practical terms the difference between low modulus and high modulus carbon fiber tows (for spar material), but I still don't have a visual in my brain of how Modulus works out on paper or graphs and how to translate that to applications...

[MCarlton]

As I understand it, modulus in our terms is really at its most useful as a comparitive measure. Modulus for most practical purposes is a measurement of stiffness, expressed as a ratio of an applied force to a resultant deformation.

It is not a measure of "strength" per se, as you could have two spars, one made of high modulus carbon and one made of low modulus carbon. The two may have the same shear strength, being that at which the spar would fail, but a low modulus spar would bend a good deal more than a high modulus one before that happened.

Contrary to some press and advertising, it is not always an advantage to use the highest modulus spar material available, as this results in a very rigid wing which may not give any visual indication of applied forces. A more "flexible" wing can allow a greater degree of flex before failure which can be identified during flight.

However, high modulus spar material can be useful in creating a smaller section spar and thus lighter weight in weight sensitive applications.

Could be wrong, but thats my reading of it.

Cheers

Matt

[MarkusN]

The best simple definition of modulus I have heard defines it as the "generic spring rate" of a material (is 'spring rate' the term used in your part of the world?, Anyway, it's the rigidity or "hardness" of a spring, the force needed to deform it by a certain length.) You can view it as the "spring rate" of a solid block of material.

High modulus material: Little deformation results under high load.

BUT, collateral: High forces result from little deformation. Meaning that in a compound structure the high modulus material takes most of the load. And that shock loads (which are a result of elastic reaction to mass acceleration) lead to much higher stresses in rigid structures.

[mnowell129]

It is a material property that describes for a given load (stress) how much the material stretches or compresses (strain). To make a simple visual example a rubber band has a low modulus and a piece of string has a higher elastic modulus. For the same load the rubber band stretches more than the string. Another example is monofilament fishing line compared to "spider wire", etc. Mono has a much higher stretch for the same load as the other.The modulus is the slope of the curve that you get when you plot the stress applied to a material (in pressure units) against it's strain (unit deformation = change in length / total length).
This only applies to materials in the elastic region, that is the region in which if you load them and unload them they return to their original length. Once you reach the elastic limit, materials fail in some way. Steel goes plastic and stretches or compresses at the elastic limit, carbon fails catastrophically at the elastic limit.(Note that some materials have a different compressive and tensile elastic limits, most notably concrete which has a very low tensile limit. This led to the development of pre-stressed concrete, but that's a whole other story.)
Modulus is important when you mix materials, for example carbon and balsa. Balsa has a lower modulus than carbon so when you, for example, load a wing and the materials strain, the balsa strains more than the carbon and they may create stresses against each other as a result.
The modulus of a material is pretty much the modulus, the modulus of a tow has to do with how you arrange the fibers to control the modulus of the resulting piece. With parallel fibers you get the modulus of the raw material, when you start laying the material on a bias or a weave the resin comes into play and allows a little movement between fibers and the modulus goes down. You do this to match the material it's being mated to otherwise the mating piece will strain easily under load, the carbon won't and the carbon will become more heavily loaded than the mating piece and fail, the piece that it is mated to then receives all the load and fails. If you match modulus then both pieces strain similarly and you don't have the stress rise in one piece or the other.

[ciurpita]

you may find the following of interest

http://ciurpita.tripod.com/rc/notes/compositeSpar.html

[lincoln]

The formal definition is so simple, why not put it here. I'm specifically talking about modulus of elasticity. Most of the materials we're concerned with obey Hooke's law within some range of stress where they don't take a set or break. (taking a set would also be called "plastic" deformation as opposed to elastic) Hooke's law says that the "strain" is proportional to the stress. By strain, we're talking about elongation or compression. If a material has an elastic modulus of 30,000,000 psi (as in steel), and you stress it to 30,000 psi, you'll see a 0.1% stretch or it will become 0.1% shorter, depending on whether it's a compressive or tensile load. Unidirectional carbon might be up to 20mpsi, I think, depending on how it's laid up, aluminum is around 10mpsi, 4 lb balsa of high quality might be 300kpsi or 400kpsi along the grain, much less across it.

There's also a shear modulus that doesn't get talked about much, but I'm too rusty to remember how that works.

[mnowell129]

The formal definition is so simple, why not put it here.
Those who know the formula already understand it, the formula, for those who don't understand it doesn't explain it. I think the value of this forum is to try to explain complex topics in lay terms.
E = (F/A)/(delta l / l) might mean something to you and I but doesn't help give a feel for what it means..... :)