Need Help with Yield Strength for FEA

1. Dec 4, 2007

Bumbleflea

Hi, folks,

I'm an aspiring Mechanical Engineer, & as such am very new to the subject of stress, strain, etc. I'm designing an assembly using a hi-performance polymer that has the following characteristics:

Tensile Strength, 73°F 10.1 kpsi (10,100 psi)
Tensile Modulus (E) 340 kpsi (340,000 psi)
Tensile Elongation @ Yield 7.2%
Tensile Elongation @ Break 60-120%
Flexural Strength, 73°F 13.2 kpsi
Flexural Modulus, 73°F 350 kpsi
Tensile Impact Strength 190 ft-lb/in^2
Izod Impact, Notched, 73°F 13 ft-lb/in^2

In order to model my FEA under Cosmo/Solidworks, I need to supply the following information: (Most, I believe, I have derived.)

Elastic(Tensile?) Modulus E=340,000 lb/in^2
Poissons Ratio ν=(E/(2*μ))-1
Shear Modulus μ=??
Thermal Expansion Cooeficient 31 ppm/°F = 3.1e-006
Density .0466 lbs/in^3
Thermal Conductivity ---??
Specific Heat 0.27 BTU/lb-degF
Tensile Strength 10100 lb/in^2
Yield Strength (Flexural?) ??

I'm particularly confused about the Yield Strength. Can it be determined with available information? What about the Shear Modulus? I'm also not sure it's appropriate to enter the "Tensile Modulus" into the field for "Elastic Modulus" as I know the latter is more general than the former. Thanks for any help!

2. Dec 4, 2007

FredGarvin

You are using equations assuming an isotropic material. As long as you are OK with that assumption you are almost there.

Shear Modulus (I am used to G for this): $$\mu=\frac{E}{2(1+\nu)}$$

You are caught a bit in a circular reference though, because you are using shear modulus to calculate Poisson's Ratio already. Do you happen to know the bulk modulus for this material? There are a set of relations that you can use to calculate all of these (plus some others) based on having two already. They are $$E, \nu, \mu, \lambda$$ and $$K$$.

Thermal conductivity is not really something you can calculate without testing. The same goes for yield stress. Depending on what the stress-strain curve looks like, there may not be a distinctive yield point. You need to get that from the manufacturer as well. Tensile modulus and Elastic modulus are the same thing. They are both derived from a tensile test in uniaxial loading.

Last edited: Dec 4, 2007
3. Dec 4, 2007

Bumbleflea

Thanks, Fred, for your reply. Although I'm not certain, I've not yet found any reason to consider the polymer (polyphenylsulfone) anisotropic. And yes, you're right: I don't know either the shear modulus or Poisson's ratio (or the bulk modulus) for this material. I have discovered the set of relations you allude to (on wikipedia.) I'll take your advice & ask the manufacturer for additional information, & I'm glad to have your feedback beforehand.

These are the hurdles facing one aspiring to transcend mere drafting, eh?

-BF

Last edited: Dec 4, 2007
4. Dec 5, 2007

FredGarvin

Asking the manufacturer is really the best way to go. Most times they'll have data sheets that you can download from their web site.

There are plenty of hurdles, so don't fret it. You tackle one at a time. I too came from a drafting background.

Good luck.