Flexural Modulus vs Elastic Modulus - which to use ?

[DonkeyDude]

Many plastics do not have their Elastic Modulus listed but their Flexural Modulus is. (obtained from a bending test as opposed to a tensile test).

Does anyone have any experience in using Flexural Modulus value in place of Elastic Modulus for Material properties. My question really is how well does it compare with reality when used in a stress analysis ? Are there any correlations between the two for at least some materials ?

 

[EdR]

Dude:

While I am not familiar with plastics and their test procedures, etc. I think what you are getting from both a tensile and flexural test is Young's Modulus...i.e. the same property results from both tests....

Hope this helps

Ed.R.

 

[DonkeyDude]

EdR,

You are right about metals and some plastics.

But PTFE, for example has a Flex. Modulus almost 30% higher than Tensile Modulus. (2.7 Gpa vs 1.9 Gpa) Sometimes it is lower too. I wouldn't be concerned about 5 or 10% difference.

I guess the short answer is 'you just have to find it'. Don't you think so?

 

[EdR]

Dude:

If I am correct (as I think you agree) that the result of both tests is Youngs Modulus..(since we don't want to add to the confusion by using flexural and tensile modulus) then it appears that Youngs modulus or Poissons Ratio are dependent on the type of loading...I guess in that case I would plot stress/strain curves for as many values (from both load types and load conditions) as I could and see if I could fit some sort of curve (variable modulus) model to the data....

Lacking data enough to plot curves you have no choice but to guess a value (include type of loads on actual part in the guess)...Where you have 30% differences you can only expect to get gross results from any analysis....If you want to try and get good results from an analysis you are going to have to better define the material behaviour through testing, etc.....

Hope this helps

Ed.R.

 

[DonkeyDude]

EdR:

Where you have a 30% difference, wouldn't it be better to use the Flexural Modulus if the actual loading is also predominantly bending ?

 

[EdR]

Yes....That's what I meant with my () note about including type of loading in your guess....

Ed.R.

P.S. You still should be very cautious about how you use the results of the analysis....They will be good for some things but may be bad for others....

 

[kunc]

You can go to matweb.com and compare the tensile and flexural moduli for several materials. A quick look appeared to show most within 10%. If I have a purely bending application, I'll use flexural properties, including flexural strain.

 

[hacksaw]

the reduction of the modulus from tensile data depends on how you define it. there are 4 definitions in common use(tangent modulus,initial tangent modulus, secant modulus, and chordal modulus...for ductile materials), then you have the elastic moduli determined on the basis of prismatic specimens at resonance, and ultrasonic measurments.

the appropriate measurment is often driven by the material in question (i.e. the ASTM standards)

when it is all said and done, the closer the measurment method fits your application then the higher the confidence in the modulus estimate.

 

[LeonEng]

I have the same doubt as well. The flexural modulus is actually sample's thickness dependent, and the stress/strain is not homogeneous across the thickness in the bending test(change from compressive -> neutral -> tensile across the thickness).While in tensile test for Young's modulus, the stress/strain is homogeneous (tensile)over the sample's cross section.
The definition of the flexural modulus E = F*L*L*L/4*b*h*h*h*Y (Y is deflection at load point, b,h ,L are dimension of sample), can anyone tell how should the flexural modulus incorporate into the general FEM formulation, and what is the effect?

 

[Qshake]

I don't know if this has been addressed already...I apologize if it has been.

The values that you will achieve are in fact different. It is not the loading that makes them different but the material properties themselves. In many disciplines it is acceptable to assume a linear, elastic, isotropic and homogeneous material. In reality very few materials actually are.

During school, particularly undergraduate, where actual testing or lab classes are held, focus is on the tensile and compressive testing of steel or concrete or even aluminum. The result is a narrow focus on a single material property - axial stress/strain and the resulting modulus.

One need only have an introduction to Generalized Hooke's law to understand what it takes to fully describe a material's properties. There are 81 elastic constants! However, as you know, we've reduced them considerably in our simplifications.