Material properties missing? 1

[jpjamo]

Hi all,

This is a first time in a few years (since about 2nd year uni!) I have to look at materials properties in detail. I have been doing a bunch of research into a few Polypropylene (20% GF and MF) and Nylon 6 (30% GF)materials for a particular application. What I have been finding though is that most of the data sheets available on the web (from MatWeb, IDES, and even the supplier websites themselves), seem to have data missing.

For example yield stress, poissons ratio or tensile strength will be commonly missing.
I am wondering how yield stress can be left out unless the material doesn't have a defined yield point in its stress-strain curve? And if the material doesn't have a defined yield stress how do you approximate it for the purposes of simple FEA or hand calcs? or should I just use the UTS given?
Also if poissons ratio is not given should I just use the 0.3 to 0.5 range and compare results? Or is there somewhere that standard poissons ratios are defined that I should be using?

Thanks in advance!
James

 

[EdStainless]

The poisson's could be all over the place. It will depend on the fill and direction. These are non-isotropic materials.

Usually flexure properties are of more importance in plastics. If they are loaded in tension they will creep, even at low loads.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[plastx]

Material suppliers use datasheets as a sales tool more than a document that supports engineering functions. IDES and MatWeb are only as good as the suppliers test and publish.

That said you can use 0.35 for Poisson's Ratio which is typical of most materials though the range is what you stated of 0.3 - 0.5. You will find that varying the ratio will not greatly affect your results.

Yield stress and tensile are another matter. This will vary more material to material and plays a more important role in your analysis. I would suggest two approaches. First, if you have a specific material and vendor in mind call or email their Tech Support and request the info. The larger suppliers usually have this information (esp. tensile) in some analysis for their top customers but don't/didn't publish it because (a)the values didn't compare well against competition (b)the datasheet was done as a preliminary datasheet before all testing was completed and never updated or (c) the time and money to perform testing isn't valued enough by the supplier. The second approach is to find a similar grade that has the necessary data and use that. The accuracy here is less but if you are doing preliminary analysis this will put you in the ballpark.

 

[jpjamo]

Hiya,

thanks for the responses.

EdStainless - the application I am working on is a temporary support for the building industry and is a metal part replacement. Its predominant loads are in tension - so for my particular case this is more important than flexure. I am curious though why is flexure generally more important for plastics? - Do you mean simply that most applications for plastics involve bending usually?
Also to help with creep (and strength in general) we have been looking at glass filled reinforcing of 20-30% - does this sound reasonable?

Plastx - ok so for data sheets manufactuers mightn't list Yield stress because the application it was intended for might have only been concerned with UTS for example. or it cost too much . Have tried both approaches so far with a sizeable vendor still not coming up with any more numbers (even given a 200 tonne/year volume estimation!) - hence have been looking for close enough grades - although I have seen some fairly different numbers for seemingly similar grades.


Thanks again
James

 

[plastx]

If creep is a concern then glass filled will help, 20-30% is fine if your other key properties are OK with the glass(impact will drop, flex mod will go up, etc). In most cases you can use Tensile Strength at Yield values. Ultimate Tensile Strength can be close with brittle materials but sometimes will give you false results. The test material will have yielded and deformed way beyond the usable stage by the time the material finally breaks in UTS. If you are selling the material you may want to publish a higher value so UTS shows up on the datasheet.

Mike

 

[EdStainless]

Given the low modulus of most plastics flexure is often a limiting issue.
When you injection mold reinforced plastics you get some directionality of properties. this is a function of how much reinforcement, fiber length, and molding practice.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[jpjamo]

MIke - No material selling going on here - just some simple FEA modelling and trying to get reasonably correct material data. I get your point about the brittle materials having a UTS near to Yield Strength.

Ed - thanks for the clarification.

Cheers
James

 

[unclesyd]

Download a product data sheet on one of the products listed and check the bottom of the page for the ways to contact the Vydyne Technical Center at Cantonment, Fl. You should be able to get any information you need. I don’t have any contacts at the present, but if you don’t have any luck please post. This a group with a complete PT lab dedicated to Nylon properties.

http://www.vydyne.com/pages/product_info.asp

 

[jpjamo]

Thanks Unclesyd for the link - they definitely have some interesting stuff. The data sheets they have listed are comprehensive - it seems to be mainly given in DAM as opposed to Conditioned test results which is more what I am interested in.

Cheers
James