Plastic behavior under relaxation followed by cyclic loading
Plastic behavior under relaxation followed by cyclic loading
(OP)
Hi all, I have been browsing around this forum for quite a while now and decided to make my first post as I couldn't find the answer to my question anywhere.
I am currently designing a plastic part that will be subject to stress-relaxation, creep and fatigue, and I am struggling to make a good design/choose the right material as I am not certain of how a plastic will react to this situation.
First of all, the plastic will be compressed by 7% of its length.
The part, still subject to a compression of 7%, will have a compression stress of roughly 10-12 MPa added to it. This stress will sometimes be constant, sometimes cyclic.
With a strain of 7%, I am already passed the yield point on most plastics, so if I add an extra 10-12 MPa I will have an increase of strain until failure ???
If followed by the strain of 7%, I let my part relax until I have a reduction of 20 MPa, how will my part react to an increase of 12MPa, will it be an (mostly) elastic deformation ?
Hope this all makes sense. Thank you very much for your help :)
I am currently designing a plastic part that will be subject to stress-relaxation, creep and fatigue, and I am struggling to make a good design/choose the right material as I am not certain of how a plastic will react to this situation.
First of all, the plastic will be compressed by 7% of its length.
The part, still subject to a compression of 7%, will have a compression stress of roughly 10-12 MPa added to it. This stress will sometimes be constant, sometimes cyclic.
With a strain of 7%, I am already passed the yield point on most plastics, so if I add an extra 10-12 MPa I will have an increase of strain until failure ???
If followed by the strain of 7%, I let my part relax until I have a reduction of 20 MPa, how will my part react to an increase of 12MPa, will it be an (mostly) elastic deformation ?
Hope this all makes sense. Thank you very much for your help :)






RE: Plastic behavior under relaxation followed by cyclic loading
I would look for data for different polymers to see whether your application seems to fall within what plastics can do and contact suppliers who have data. I will look look for books that contain fatigue data. From memory I know that Polypropylene Definitive User's Guide and Databook does. Maybe I can find others.
Having said all of that none of it will tell you for sure whether your application will work or not. You will need to do testing at a lab set up for such testing. The only one I know of is www.norner.no I toured their site and saw such equipment.
Dr. Chris DeArmitt
Plastics consultant to the Fortune 500: www.phantomplastics.com
Webinars on plastics, fillers & impact modification: www.plastictraining.com
RE: Plastic behavior under relaxation followed by cyclic loading
Would you think in this situation fatigue may be an issue ? As we are far into a compression-compression cyclic ?
Is there any reason why compressive data (stength, yield, stress-strain curve, etc) are hard to find ? I would think that a stress-strain curve for plastics in compression and tension are very different, why giving only the tensile data then ?
I could reduce the 10-12 MPa load to a 4-5MPa if required. If we simplify the situation to a strain of 7% followed by a relaxation of 15 MPa and the followed by a short term stress of 5 MPa (no creep considered for the moment), how will my stress-strain curve looks like ? Is there any reference that could be point out to me in this regard ?
Thanks again for your help.
RE: Plastic behavior under relaxation followed by cyclic loading
I had a quick look for fatigue books and this one looks promising.
Fatigue and Tribological Properties of Plastics and Elastomers 2nd Edition - PDL
PDL books are very good and focus on practical aspects.
Dr. Chris DeArmitt
Plastics consultant to the Fortune 500: www.phantomplastics.com
Webinars on plastics, fillers & impact modification: www.plastictraining.com
RE: Plastic behavior under relaxation followed by cyclic loading
I'd expect most other plastics to fail by cracking or crumbling. ... perhaps even developing flaws that will not reveal themselves except under tension or chemical attack.
Yours does not sound like a winning application for plastics.
Mike Halloran
Pembroke Pines, FL, USA
RE: Plastic behavior under relaxation followed by cyclic loading
www.tynevalleyplastics.co.uk
It's ok to soar like an eagle, but weasels don't get sucked into jet engines.
RE: Plastic behavior under relaxation followed by cyclic loading
I will most definitely take a look at these 2 books. I am newish to plastics and I am trying to gather as much information as I can on this domain.
Mike
Yes, you are right. I realised that yesterday after hours of looking at different plastics. Some reinforced Nylon, PC with a strain @ yield could take the 7% compression, but then they couldn't take the 10-12 MPa stress without big strain. I also discovered that with relaxation, the modulus of elasticity decreases over time ? The plastic will have get to an operational temperature of 70ish degrees C, so that won't work. I could go with glass filled to get good creep/relaxation/(fatigue?) resistance, but then I couldn't have a strain of 7%. I'll have to go back to my design and change the geometry so the initial strain will be reduced.
Pud
Softer material would be a good idea, but I forgot to mention that the stress of 10-12MPa can't generate a strain more than the original strain (7%), so a moderate stiffness is required
Thanks all for your feedbacks
If it is of any interest for anyone, I have found answer to my questions on the behaviour of plastic with complex loading (relaxation + creep, etc). There is a nice paper called Experimental study of cyclic visco-elasto-plastic behaviour of a polyamide fibre strap.
Cheers
RE: Plastic behavior under relaxation followed by cyclic loading
www.tynevalleyplastics.co.uk
It's ok to soar like an eagle, but weasels don't get sucked into jet engines.
RE: Plastic behavior under relaxation followed by cyclic loading
Whilst its not good to over stress the material I don't see where fatigue fits in, to my knowledge fatigue cracks only grow under tensile stresses.
That said I am no plastic expert I only normally work with metals.
RE: Plastic behavior under relaxation followed by cyclic loading
Chris DeArmitt PhD
President - Phantom Plastics LLC
Consulting, ideas and training on plastic materials
www.phantomplastics.com
RE: Plastic behavior under relaxation followed by cyclic loading
I would agree that a rod under compression would expand in the transverse direction but I think that any associated stress would be very small indeed, usually in metals if you stay roughly less than 25% of the tensile strength whatever the application your component won't fail in fatigue.
Looking at some examples of rods in compression or tension it appears they use poissions ratio only to obtain the transverse strain but the stress is not calculated, although I believe that the transverse strain is more to do with the shear stresses acting transverse to the applied load.
If I am wrong anybody please shout up
RE: Plastic behavior under relaxation followed by cyclic loading
RE: Plastic behavior under relaxation followed by cyclic loading
My reference to cracks growing was in relation to fatigue, fatigue normally occurs under tensile cyclic loading well below the materials yield.
I searched to try and find fatigue failures under compressive cyclic loading but had no luck.
If you can provide a reference for fatigue failure under compressive stresses or the compression test you mention in your post I would be grateful.
Desertfox
RE: Plastic behavior under relaxation followed by cyclic loading
See this thread where you actually say the same thing as I have.
thread2-204767: Fatigue damage due to compressive load
RE: Plastic behavior under relaxation followed by cyclic loading
I only take issue with one comment you made:
"a rod under compression would expand in the transverse direction but I think that any associated stress would be very small indeed"
Associated stresses are not "very small". They are proportional to Poisson's ratio. Which is significant.
RE: Plastic behavior under relaxation followed by cyclic loading
So the load required to achieve a high tensile stress in the transverse direction would have to be very high, so high that the compressive stress would probably exceed the yield stress of the material before the tensile stresses where high enough to cause failure by cyclic fatigue, this is also stated in the post I provided the link too previously.
The OP asked about a fatigue test and not just about a compression test which you mentioned in your post.
I would agree that in a compression test a force could be applied so high that cracks could appear but then we wouldn't be looking at fatigue.
Cracks can also appear in the material due to the shear stresses acting at 45 degrees to the axial stress again this as nothing to do with fatigue.
RE: Plastic behavior under relaxation followed by cyclic loading
RE: Plastic behavior under relaxation followed by cyclic loading
I looked on this site http://www.toray.jp/plastics/en/products/amilan/te...
if you look at the stress v strain curve (B) it has a linear portion up to a yield point and a modulus, given at poisson's ratio of about 0.4 for Nylon66 ,it would be reasonable to get a similar result for this plastic material in accordance with my earlier example.
If you have a reference to material that counters my standpoint then please post it.