Puncture Strength Through Multilayer Films

While testing the puncture strength of various films, we have noticed
that we can get different results depending on the side of the
material we test. We use a method similiar to the astm puncture
procedure where a piece of film is held statiorary and a probe is
pulled through the film while an instron tensile testor is used to
measure the force required to puncture the film. The film structure
can vary, but a simplified film can be thought of as a two layer film
with a nylon(polyamide) based layer on one side and a polyethylene
based layer on the other. Obviously these materials have there own
physical characteristics, which can be measured using monolayer films.
But as a bonded whole we get these differing results based on which
side of the film we test. I have some ideas which might be the
solution, but I am not sure.
1. Although the films are thin, 50-125 microns, the side opposite the
probe may be stretched more in the axial directions, which would lead
to thinning and creater weaker strengths. The amount of effect would
be different for each material.
2. The probe may be able to cut into the material prior to the actual
puncture, leaving a weakened structure. The amount of cutting would be
dependant on each material as well.
These solutions may be partly correct, or not at all. We generally
have more complex film structures to complicate things and the results
of going through a stronger material first does not always produce a
stronger puncture nor does it always provide a weaker puncture, it
varies on each structure.
Thanks in advance for any help on this topic.
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The problem is you have a mixed stress field. The side of the film closest to the tup is in compression while the outer layer is in tension. Thus depending up the compressive and tensile properties of the materials you get different results. Typically most materials will fail in tension first. So, if the tensile properties of your two layers are dramatically different, you'll end up with different puncture results depending upon which is placed down.
Larry Effler
Drago wrote:
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Larry Effler

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