Amazing & Strange Kevlar

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fine-tune
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Amazing & Strange Kevlar

Post by fine-tune »

My business has nothing to do with the manufacture of Kevlar. For other reasons, my
partners and I visited a Kevlar factory a few years ago. They gave us spools of Kevlar
thread as samples.

I decided to test the strength of this thread, so I performed some tests. Six cinder
blocks were lifted 4 ft. off the ground, suspended by a single strand. I wrapped a strand
around the frame of a coping saw and cut raw meat. Just imagine what would happen
if you forgot it was Kevlar, and attempted to break this thread with bare hands!

Here is where the story becomes a mystery.

I replaced all the tags on my USB flash drives with a flexible and very durable plastic.
The plastic tags were sanded silky smooth with 1500 grit paper. I decided to attach
the tags using Kevlar thread. After a couple of weeks the thread broke, like it had been
cut with a sharp blade! Obviously, I cannot explain why this happened. Any ideas?
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haklesup
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Re: Amazing & Strange Kevlar

Post by haklesup »

Kevlar has very high tensile strength ( stretching, pulling) but is not quite so strong against sheering forces or abrasion.

From what I have seen on TV of bullet holes in vests, strands around the periphery of the bullet do get cut but strands near the tip of the bullet stretch and stay intact implying that a bullet does sheer some strands as it is stopping.

For example
http://i.dailymail.co.uk/i/pix/2012/12/ ... 06x423.jpg
fine-tune
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Re: Amazing & Strange Kevlar

Post by fine-tune »

I wanted someone besides me to use the word "abrasion."

I've got the usual assortment of Dremel accessories. I also have a more costly flexible shaft
rotary power tool made by Foredom. Foredom power tools are same quality used by dentists.

The edges of any drilled hole can be very sharp. Each hole drilled in the tags was buffed smooth
with a very narrow Foredom cylinder. There simply were no sharp edges to abraid the Kevlar
thread.

I still agree with you, haklesup.

The thread must have been destroyed by some kind of abrasion or sheering force. Exactly how
this happened is still a mystery to me.

That loop attached to the backend of every flash drive is tiny. As a replacement for the Kevlar
thread I used a double wrap of 6 lb. monofilament fishing line. I can pull a flash drive from its
port by the tag. The monofilament shows no signs of damage.

You might ask, "why didn't I call the factory we visited and ask an expert?" Simple answer.
I didn't want to sound stupid.
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Lenp
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Re: Amazing & Strange Kevlar

Post by Lenp »

Maybe this is related to the kevlar failure...

"The layers in the fibers are formed by strong covalent bonds. The sheet-like aggregations readily allow the propagation of cracks. When the fibers bend they fail at very low strain. In other words carbon fibre does not bend much before failing" From http://www.christinedemerchant.com/carb ... stics.html

Side note: I had a concrete pad poured some years ago for an addition, and they used kevlar reinforced concrete. When it cured it looked like a porcupine with all the random fibers sticking out, but they soon broke off from foot traffic.
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)
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