Plastic Properties 1

[ashby]

Hi all. I’m currently running a cosmos simulation of some plastic parts. I am very unfamiliar with plastics however and wonder if you could help me. I’ve been given-

Tensile strength at yield
Elongation at yield
Tensile Modulus
Flexural modulus
Izod Impact Strength
Charpy Impact Strength
Hardness Rockwell
Density
Bulk Density

For the purposes of the simulation I must find

Elastic modulus
Poisson’s ratio
Shear modulus
Tensile Strength
Compressive Strength
Yield Strength

I’d really appreciate any help.

Best regards

Colin Brown

 

[patprimmer]

For what plastic and grade

Can the raw materials supplier help

Can you get the CAMPUS data

Plastics are very different in their behaviour than metals.

Most metals are true solids where as plastics are really VERY viscous liquids like glass, lead and tar, so elastic recovery and youngs modulus must be apparent, not actual, as the plastic never fully recovers

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JGalliera]

I found getting material data using these methods:
1) from the software database; 2) from technical publications / journals; 3) online from sites like MatWeb.com, CAMPUSPlastics.com, or the manufacturers website (i.e. duPont or GEplastics); 4) finally you can try to call your supplier or the manufacturer for this information - ask to talk to an engineer or technical person who would have this info. One of these ways will usually yield results. Think about it... the manufacturer has to come up with useful data for the design / engineering community so that its desirable properties for use are known. Someone has generated this data, you just have to locate it.

My other piece of advice is that depending on the analysis you want to perform, you may not need every property. For example, for simple linear stress analysis, you only need the elasticity modulus and Poisson's ratio. Material strengths are used only for comparison, and are not needed to run the problem, except for Yield Strength when doing a non-linear material analysis. Shear modulus can be calculated from elasticity and Poisson's ratio and most programs do this internally. Make sure you know what quantities are needed in the equations and the material models before proceeding with the analysis!

 

[wildcat2]

The task you have at hand is very challenging.
Density depends on the nature of the base resin and of the additives.
Bulk density depends on the size and shape of the pellets. The smaller the pellets the higher the bulk density.
Izod: I assume you are dealing with rubber toughened plastics. It has been my dream to model the izod of HIPS (High Impact Polystyrene). According to the literature Izod will depend on the size of the rubber particles (rubber is not miscible with the base resin matrix), the inter particle distance. To complicate matters bimodal size distributions enhance Izod tremendously. In short it is too complicated to model.

Good luck.

 

[craigj]

Yes this is a tough one. Resins will develop different properties than their data sheets depending processing. Unfortunately we as designers sometimes dont have control over what goes on in the molding shop.
Things that can effect material properties include:
- Drying
- Molding temp, pressure
- Cooling rate
- Regrind use (cring)
- filler
What I am trying to say is that you should view your material data sheet as a "resume" for the material, not accurate data. Well OK you say, this makes it kind of hard to understand if the FEA model is garbage. Unfortunately you have to allow for a bit of garbage because you are going to be very hard pressed to get wonderful material properties in the plastics world unless you test the stuff yourself (which is expensive by the way). There are some exceptions. For example I just looked at some material data on an LNP resin and they have a great online properties. Apprent modulus vs temp, etc...

http://egd.lnp.com

Its hard.. you just have to do things like use one point tesile data to calculate modulus (which is scary) and guess a poissions ratio (most rubbers are virtually incompressible so they are easy, v=0.49, foams are totally compressible...say v=.05). You have to assume that your compression modulus is probably going to be, um...way bigger, but who knows. You can always call the company, but this can be quite a let down when they laugh at you (Ive had it happen).

Im sure that I am rambling so anyway, good luck.

CJ

 

[JoelMyers]

It seems that you have most of the data you need to get a linear structural analysis completed.

Tensile Modulus = Elastic Modulus or Young's Modulus
Tensile strength at yield = Yield Strength
Elongation at yield = Yield Strain

The Poisson's ratio for most plastic is near 0.35, and this value is used in most structural analyses. There is little variance from this number depending on whether it is a semi-crystalline or amorphous polymer. The density of the polymer is only needed if you are running a frequency response analysis (modal analysis). Another tip is to use about 80% of the full Young's Modulus as a safety factor in your analysis. The best data to use is the Secant Modulus for the material, but this requires the stress-strain curve to calculate, which you don't have. The flexural modulus should never be used to design the part as this value takes into account the compressive and tensile nature of the polymer under perfect bending and is artificially high (loss of safety factor).

If you are performing a non-linear structural analysis you will need the stress-strain curves in order to calculate the plastic stress-plastic strain values at about 4 points on the curve and enter these into the cosmos input deck. I am not sure if cosmos will run non-linear analyses as most of my experience is with ABAQUS.

Regards,

Joel Myers