Yield Strength for Plastic 1

[Lee.Conti]

Hi,

I understand from the textbook "Designing with Plastic, Erhard", yield strength is the peak of Stress Strain curve, where the slope is zero. I would like to check if this means before reaching to the top, it is still elastic for plastic? As if compared to metal, the peak of the stress strain is considered Ultimate Strength ... Thank you!

 

[rickfischer51]

First, don't even try to understand polymers by comparing them to metals. These are two entirely different material families with entirely different atomic structure, different bonding, different everything. You will only confuse yourself and suffer irreparable brain damage. Second, a tensile test of any material, but especially of polymers, does not find the yield point. It finds a point were an acceptably small amount of yield may have taken place, and loading beyond that point will result in rapid increase of an unacceptable amount of yield. The tensile test is an idealization. It is imprecise, but still useful.

So, regarding your question. When you have reached the "yield point", it is likely some small amount of yielding will may have taken place. It is probably irrelevant to your situation. Some of it may recover. How much depends on many things. What material? What degree of crystallinity? How was it processed. What is the strain rate? The point where the slope hits zero represents the formation of a stable neck in the specimen. Gross yielding follows, and that is typically not irrelevant.

The yield point derived from the point where the slope goes to zero is a reference point. You may not want to go near it, probably should not go near it. Its useful for comparing materials.

Something to think about. Yield and creep in polymers are the same process. Small load applied for a long time, the shape changes. Big load for a short time, the shape changes. Same process, different time scale. It is a little more complex than that, other factors do weigh in. Small load applied for a long time produces creep, but the long time also may mean plasticizer loss, oxidation, physical aging, etc. will add to it. Big load applied fast (real fast) may result in brittle fracture. And none of this may matter to you depending on what your part is, what its made of, and how its loaded.



Rick Fischer
Principal Engineer
Argonne National Laboratory

 

[Demon3]

Rick is correct. The polymer yields well before that point.

Chris DeArmitt PhD FRSC
President

Plastic materials consultant to the Fortune 100 - As seen on CBS 60 Minutes, BBC & Sky News
Creating New Materials - Problem Solving - Innovation Keynotes - Expert Witness

http://www.phantomplastics.com

 

[Pud]

As above. Purely for comparison.



Politicians like to panic, they need activity. It is their substitute for achievement.

 

[Lee.Conti]

Thanks a lot for the sharing! Especially @rickfischer51.

Honestly, I try to understand polymer from metal and understand that it is different. Polymer shows nonlinear elasticity before yield compared to metal, correct me if it is not true.

I was using 0.2% to define the yield but the polymer material suppliers provide us the yield data is not at 0.2% but at the peak of the curve. "Designing with Plastic, Erhard" explains how it defines the yield...

 

[Demon3]

Plastics flow even at really low force. As a rule of thumb, a plastic part should be designed to experience no more than 1/4 of the nominal yield stress.

https://www.engineeringclicks.com/yield-strength-of-plastics/

Chris DeArmitt PhD FRSC
President

Plastic materials consultant to the Fortune 100 - As seen on CBS 60 Minutes, BBC & Sky News
Creating New Materials - Problem Solving - Innovation Keynotes - Expert Witness

http://www.phantomplastics.com