# Question on kinetic energy absorption of PET vs PC

I'm feeling a bit confused about something.
As I understand it, the total kinetic energy absorbed by a thickness of material stressed to its breaking point is proportional to the area
under the stress/strain graph describing the energy transfer. This area is roughly proportional to the product of the material's ultimate tensile strength and elongation to break.
Various references (matweb, materials engineering textbooks, etc) on polyethylene terephthalate give it an UTS of around 7000-8500 psi, and an ETB of around 110%-250%, depending on the type and source of PET, and on its method of manufacture, with 140% being most common. These same references give polycarbonate an UTS of around 9500-10500 psi, and an ETB of around 50%-120%, with 100% being most common. Just comparing UTS * ETB suggests PET should absorb as much or more kinetic energy per unit area than PC, but sources commonly give PC's Izod Impact rating 5x-6x as high as PET's.
What am I missing here? Do the stress/strain graphs for these materials' Izod tests just look very, very strange? (Linear for PC, concave for PET, maybe?) Does Izod test something other than what I think it does? I can't find stress/strain graphs for PET or PC for the Izod or Charpy tests.
-- TTK
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Charpy is not a tensile test and measures different things, that is why the test is done. Exactly what charpy measures, or can be used for depends on the material being tested. It is usually a better measure of crack initiation resistance or resistance to brittle fracture, than of energy absorbed during ductile fracture.
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TTK Ciar wrote:

I don't think that the term "Kinetic Energy" has much meaning in the way you have posted the proposition.
Equating the energy absorbtion of material under impact to the energy under a stress-strain curve obtained under non-impact (ie, slower loading) conditions is begging to be wrong in your conclusions.
And, ultimately, this appears to show up in your contradicting conculsions which are still seeking some informatin in the (quasi-static) stress-strain curve that explains dynamic impact failure.
You will find, for example, that high rate loading of polymers gives quite different stress-strain curves than does conventional slow loading, and some impact conditions have a lot to do with bending and shear rather than simple tension.
Then, there is the ductile-brittle transition in steels where one can't tell from the stress strain curve alone what the temperatures of service safe from brittle fracture will be.
Reading about fracture is a good start, and there are quite a few good books on mechanical metallurgy. There are also some excellent books on engineering fracture in the library....
"Failure of Materials in Mechanical Engineering - Analysis, Prediction, Prevention " J A Collins ISBN 0471310190
Jim Buch
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There's always the speed issue with polymers. You have to make sure that you are testing the sample at the sample speed that your application requires. Stress-strain curves can dance around quite a bit as you go faster or slower.
The test data you cited above is most likely done at the same speed for both polymers, but is probably too slow for the real world.
John
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