Calculate eng stress for polymers
Calculate eng stress for polymers
(OP)
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?
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?





RE: Calculate eng stress for polymers
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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RE: Calculate eng stress for polymers
RE: Calculate eng stress for polymers
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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RE: Calculate eng stress for polymers
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
RE: Calculate eng stress for polymers
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.
RE: Calculate eng stress for polymers
Chris DeArmitt PhD FRSC CChem
RE: Calculate eng stress for polymers
Regards
Pat
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