Calculate eng stress for polymers

[Evila]

So, after doing uniaxial testing, we all use the conventional formula to calculate engineering stress which is: stress= F/A
But my advisor told me that this formula does not work for polymers due to the fact that these are non linear... So he mentioned something like this:
Stress=TF^-1
I havent find any reference or book to verify this , and im not even sure of one of the variables.. Help anyone?

 

[patprimmer]

Calculating stress in plastics by conventional methods does not even go close to working reliably long term in plastics for several reasons, these being:-

1) Thermoplastic creep. Plastics flow under load. The rate they flow at and variations of that rate during a single test depends on several characteristics that vary by polymer and compound. Level of crystallinity and nature of crystal structure and length and shape of the molecule are key factors. Basically without writing a novel, the flow slows a lot as all the molecules pull into parallel formation.

2) Some plastics, especially if reinforced with fibres are quite anisotropic and this needs consideration.

3) Section thickness when moulded can play an important role as it strongly influences crystal structure and orientation of fiberous fillers.

4) Variations from surface layer effect. The surface molecules may well be amorphous, even in a highly crystalline plastic as the surface is cooled rapidly but the core cools slowly as plastics are good insulators, The surface layer molecules may themselves be oriented, creating oriented linear rather than spherical or random orientation laminar crystals.

5) Surface notches, even microscopic ones from any machining process play a much stronger part in failure than they do in metals.

6) Temperatures within a relatively narrow range within the extremes of some ambient temperatures can have a significant role.

7) Rate of application of load also plays a significant part.

Regards
Pat
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[Evila]

Thanks for the response, I understand what you are tellig me, however I cannot find the correct formulation to obtain engineering stress for polymers... Does someone know it?

 

[patprimmer]

I think there is no universally agreed std. Different raw materials companies make their own assumptions and recommendations. Any figure calculated from a formula needs to be given careful consideration before use.

I always referred to isostrainous stress strain curves before making recommendations.

I think Bayer typically recommended only design where you use 10% or lerss of data sheet properties.

Regards
Pat
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[Demon3]

This was explained to me once. From memory it's like this.

For metals you just apply stress until they no longer behave linearly and that point is the yield stress. For polymers they don't behave in a linear manner even at the start of the stress/strain curve (due to creep/flow) so that presents a problem.

To enable one to identify a specific point to take as the end of linear behavior you apply an offset. This means you draw a straight line with the same slope as the initial part of the stress / strain curve I.e. a tangent to the start of your stress / strain curve), then you draw another line parallel to that but offset e.g. 1% strain to the right on the x axis.

The place where your stress / strain curve crosses that second line is taken as your engineering stress.

I am not an expert on this so if someone who is contradicts me go with their explanation.

Chris DeArmitt PhD FRSC CChem

 

[btrueblood]

Well...

How else would one calculate stress than to divide the applied load by the section area of the sample? I.e. Stress = force/area, for a tensile coupon.

There might be a correction for the reduction of area due to Poisson's effect, or due to plastic necking, but usually these are corrections made to the "Engineering Stress" (F/A) to obtain true stress.

Though I may have completely forgotten the difference and have the terminology backwards myself.

 

[Demon3]

I think the issue here is how to define the point on the stress / strain curve you want. For practical purposes they look for the stress at which the stress / strain curve crosses that straight line. In real terms it's meaningless.

Chris DeArmitt PhD FRSC CChem

 

[patprimmer]

The trouble with publishing a useless figure is that one day someone will misuse it then sue YOU for THEIR failure to consider the qualifiers.

Regards
Pat
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