# Shear strain vs shear elongation

Discussion in 'Materials' started by Scuff, Feb 27, 2020.

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### ScuffSenior Member

I'm comparing two pvc foam datasheets. One uses shear strain the other shear elongation at break. Both are listed as astm c 273 and expressed in %. Are the tests equivalent or is there some difference?
Thanks.

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### gonzoSenior Member

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Whilst the 2 terms are, in principle the same, they are also different.

shear strain, is simply the amount of "stretch" divided by the original length, produced by an applied force/load.

So strain is basically how much, as a ratio, a material has stretched when under an applied load - the ratio between the deformation and the original length.

But Shear elongation. The elongation is the 'stretch'... but in this case it is defined differently because - at break - rather than just at any load, in the above definition.

E = L/Lo
E = the elongation
L = elongation at break (when it has failed)
Lo = gauge length - this is typically 25mm for testing.

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### gonzoSenior Member

That diagram is for normal strain not shear strain. The deformation is completely different. In shear strain, as in the example for a sandwich laminate, the skins have forces in opposite direction. The material deforms, not stretching, but by each surface moving in opposite directions.

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Indeed it is - just as the link you provided shows exactly the same. Or did you not know that before you posted that link?
I'm merely explaining the definition of strain (how and why), and how this relates to shear elongation at break, as per ATSM testing. Thought that was obvious to any engineer...

Last edited: Feb 27, 2020
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### ScuffSenior Member

It sounds to me that the two cannot be directly compared, you would need specifics of the shear strain test? Elongation at break seems straightforward .. you apply the stress and see how far the material deforms before it breaks.

The shear strain percentage was significantly higher .. all of the other values were close. Is this a marketing ploy to make it look that much better?

Thank you both for trying to help me out!

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That depends upon what it is you're trying to understand and to compare. Since strain, what ever type, is just - extension length/original length, basically. Whether it is about 1 axis or 2 or 3, it is no different, other than the way you measure the extension.

Exactly... which is why the terms have been purposely mixed.
It is why I answered the way I did too, to make you aware of such - and well done, now you realise, it is not comparing apples with apples - it's subtly different.

Thus you need to decide which one, fits your objective.

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### gonzoSenior Member

They are picking the strain that has a higher value for marketing. If he company is legitimate, they should have a material specification sheet with all the data. I agree we are comparing apples to oranges otherwise.

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### BarrySenior Member

Can you post the data sheets?

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### ScuffSenior Member

I was using the data sheets from each manufacturer'
Is it ok to do that?

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### BarrySenior Member

the data sheets are in the public domain, should not be a problem

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### rxcompositeSenior Member

Strain can be presented as a percentage. A 0.01 strain is 1.0% elongation. Because the material does not behave linearly but rather like a plastic, the shear elongation at break is used. This is the ultimate point where the said material separate from each other.

The shear modulus is a better property if computations are utilized as it is the linear curve riding at the back of the curve. It is used to fit the material into the resin/fiber curve.

Will wait till you post the data sheet to know if we are on the same page.

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### ScuffSenior Member

I'm having trouble uploading the file it's a PDF. It's probably operator error. The product that's using the shear strain is divynicel h80, I was comparing it to Gurit pvc h80. These are the two I was looking at using for hulls and bulkheads.

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### BarrySenior Member

As some readers may not understand modulus, some background follows:

Strain by definition would be the amount of stretch that a material exhibits, and for simplicity let just use tensile strength
Say you apply a tensile force ( 1,000 pounds) to a 10 inches length of material with a 1 inch square cross section and it stretches/strains 1 inch, ie it is now 11 inches long. The strain FOR THAT PARTICULAR LOAD
is 1 inch. But as when testing and wanting to compare different materials you don't want to be looking at the length of each test specimen so designers would just divide the numerator by the denominator to create a
number that would represent a number that can be carried forward easily. So instead of saying the strain is 1 inch per 10 inches, they would say that the strain is .1 inch per inch of material

Strength of Materials designers, engineers etc plot a graph with stress, pounds per square inch on the vertical y axis and strain on the x axis to show how the material stretches or strains as the STRESS goes up, load per unit area.

The slope of the graph determines the Tension Modulus. The rise, stress divided by the run, strain resulting in modulus units of psi. Ie the stiffness of the material. So when comparing two materials, the stiffer the material means that for a given load
if the strain is lower then the modulus (stiffness) is higher.

As the two tables only share MPA as common units I have switched below. The OP is from the US so it was easy to deal with psi in the above

The Divinycell H80 shows a Shear Modulus of between 23 minimum to 27 nominal
The Gurit H80 shows a Shear Modulus of 30 So the Gurit is stiffer in shear

The Divinycell H80 shows a Tensile Modulus of between 85 minimum to 95 nominal
The Gurit H80 shows a Tensile Modulus of 146 So the Gurit is stiffer in Tension

The compressive modulus indicates the same relationship

And Gurit Tensile strength is also higher

So getting back to the OP's original post

The Divinycell H80 Shear Strain shows 30% and the Gurit H80 shows Shear elongation at 19 %. So the Dcell stretches 30% before breaking and the stiffer Gurit stretches 19% before breaking

As both of these terms use % in the table, I would assume that Divinycell is representing the same characteristics ( and they refer to ASTM C273 as the test procedure)

I would suggest that Divinycell should have explained that this is the Shear Elongation at break because you cannot just say a product exhibits 30% shear strain and leave it at that

Last edited: Mar 1, 2020
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### ScuffSenior Member

Thank you for explaining that.

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