Stress analysis of plastic

[elogesh]

Hai,

In our company, a plastic compoent made up of 30-35% glass filled nylon 6.6 failed during endurance testing.

Hence it was decide to carryout stress analysis using Finite element method.To start simple, it was decide to carryout linear static analysis. Later on slowly increase the complexity of the problem.

First, I started collecting the data for this plastic material. The supplier specification mentioned the following properties,


Tensile strength,yield = 95-195 Mpa

compressive,yield strength = 16-152 Mpa

Flexural yield strength = 110-310 Mpa


Poisson's ratio = 0.35

Shear strength = 59-85 Mpa.

The property values are also available in

http://www.matweb.com

I have few questions in this regard

My first question is,I was surprised with low compressive strength.Whether plastic fail in compression?

My second question is what is the failure theory to be followed for the plastics! Someone was referring cauchy stress for plastics.I have not yet beginned search in this regard. I will do google search. If you have any useful information,please share.

My third question is there any text book dealing with Finite element analysis of plastics or else stress analysis and failure theory of plastics.

If so, please share the information.

Definitely Linear analysis mayn't suffice.I have to do non-linear analysis.I am planning to get stress-strain data from our material labaratory.


My fourth question, Plastic doesn't have infinite fatigue life like ferrous metals. Is there any source where I can get some data about S-N curve for this plastic.


Thanks for reading this thread.

Have a nice day...

Regards,
Logesh.E

 

[elogesh]

Hai,

From google search, I come across this book.

Has anyone referred this book.If so, please pass share your comments about this book. I am planning to procure this book.
"Applied Stress Analysis Of Plastics
An Engineering Approach"

by
S.I. Krishnamachari

L.J. Broutman



Regards,
Logesh.E

 

[sean01]

elogesh

With respect to your statement about being surprised about plastic failing in shear - I don't think that you should be surprised - if nothing else murphys law says if it can go wrong it will !! Particularly if it is poorly designed.

Secondly - I always personally direct people to the Nafems website for an introduction to the FEA method

http://www.nafems.org

should get you there. This company can generally supply the information you require from a rudimentary search or phone call.

Thirdly - I personally only use Matweb for either initial investigations or where the material grade hasn't been identified by the design process. If the material grade is known - always go to the material manufacturers web-site - I checked prior to answering and both DuPont and Ticona have S-N curves for their materials.

Fourthly - the way your question reads is a little confusing in regard to the non-linear requirement. Can you re-write this to identify what you require ??!?!?!

Best wishes

Sean

 

[elogesh]

Hai sean,

Thanks for your response.

1) Or else it is subjected to load conditions it is not meant for.....

The plastic component, I was referring rests over a rubber component. During normal loading the plastic component moves up and down along with the rubber. The rubber at the bottom rests over a flat surface.Both the plastic component and rubber part is covered by hollow metallic cylinder.We designed the component meant for normal loading(Zero degree angle)and also some tolerance (2 degree).But during fixture setup for acclerated testing, the angle of application of load was changed to 4 degrees. This made the plastic component restrain against the metallic surface.This made the plastic component fail at 0.852 million cycles during endurance testing,sligtly short of target figure.This is confirmed by rubbing marks on plastic component and metallic part.
We already strengthened the component and also other necessary changes, and the component with design change is already implemented.

2) Thanks for referring Nafems.

3) I got the S-N properties from "fatigue and tribological properties of plastics and elastomers book from plastics design library".I will also try to get any additional information from my DSM supplier

4) The data provided by the

http://www.matweb.com

mentions only single value of E (Modulus of elasticity). Hence I mentioned about getting stress vs strain data for the plastics, there by addressing non-linearity.

Regards,
Logesh.E