100 lines
6.3 KiB
Plaintext
100 lines
6.3 KiB
Plaintext
Tension testing of four different twines
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By Derek Voll
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EM 307
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4/30/92
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Abstract
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In this experiment I pulled apart cotton, jute, hemp and nylon twine to
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test their ultimate strength. I used a standard tension testing machine
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equipped with a load versus displacement plotter. I could not calculate
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strain and was thereby limited by the lack of theory to back up my
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observations and make descriptive numerical calculations. I did repeat the
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tests to produce an average value of ultimate load for each twine group. I
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used this value to make a rough stress calculation. Nylon is the strongest,
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cotton the weakest and jute and hemp are about equal in strength. I had quite
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a bit of difficulty with the nylon specimens because of their high strength
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but the others worked out all right.
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Introduction
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For my independent project I choose to test the strength of four
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different kinds of twine, cotton, jute, hemp, and nylon. There are many
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factors in choosing the right twine of the job it will be used for, cost,
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temperature to be used at, availability, creep and fatigue characteristics.
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These and other parameters could be analyzed in future studies to find the
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best twine but my project will focus on tensile strength. I think the results
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will be meaningful to someone buying twine and the twine producers. In fact
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the suppliers of the hemp twine were quite interested in my report and would
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like some copies; they would like more scientific information on hemp since
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there is so little scientific investigation or research concerning hemp.
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My procedure was to obtain twines with similar dimensions, pull them
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apart using a standard tension testing machine, collect load versus
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displacement plots for each of the specimens and then compare and analyze the
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data. I used a tension testing machine with a capacity of 1000 pounds which
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had a load versus displacement plotting machine connected to it. By wrapping
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the twine around the round spool three times, I relied on the large friction
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force to hold the twine in place. This force was not large enough for the
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nylon twine and I used the pneumatic grips instead. The pneumatic grips
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provided more friction which I needed to hold the nylon twine in place.
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OBSERVATIONS AND RESULTS
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I would like to start with some general observations of the experiment.
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First, the tests of the cotton, jute and hemp twines proceeded with few
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problems and their failure occured in the middle of the specimen, which is
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desirable in any tensile test since the experimenter can more easily observe
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the fracture area and disregard any stress concentration at the clamp-twine
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connection. however, for my first two samples of nylon I tried to use the
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same clamps that I had used for the other twines but in both cases the twine
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overcame the clampUs friction force before failure but after some stretching
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(the twine was pulled out of the clamps). Therefore, I switched to the
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pneumatic clamps but the nylon still slipped some, wearing the surface of the
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twine and causing stress concentrations. The nylon broke at this worn area
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near the clamps. Stress concentrations are the very tiny notches and
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imperfections in a material that produced a high localized stress. Also, I
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did not have enough nylon so I used the same specimens that I had used in the
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other clamps and one new specimen. The first two specimens broke sooner and
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under less load (see Fig #XX) and this was expected since they had already
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undergone some plastic deformation and recovery. I did not realize how strong
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and difficult to test the nylon would be. I know my procedure and the
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following results for nylon are not accurate but it should be obvious that the
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nylon is definitely the strongest of the four twines. I have graphed each of
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the specimens together in their respective group (Figs XX- XX) to show the
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variances between the individual specimens; nylon has the greatest variance in
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displacement and ultimate load as expected but it should be noted that hemp
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twines show the second largest variance in ultimate load (all hemp specimens
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are from Hungary but the specimens with the lower ultimate load were from a
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different supplier than the other three). From these graphs we see that
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cotton is the weakest and nylon is the strongest. I would like to point out
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that these graphs do not tell the whole story and a better indication of
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strength would be a stress versus strain plot, which was impossible to make
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since stains could not be calculated because we did not have access to an
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extensometer. However, if you look at the sample calculations in the appendix
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you will see that the hemp twine had a slightly thicker cross section and the
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corresponding stress was comparable to that of the jute twine. Even with this
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fundamental calculation we must realize that each twine was probable woven
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differently and all their diameters were slightly different.
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Looking closely at the graphs for the cotton and nylon specimens you will
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see that there are little ridges and drop-offs before fracture; these points
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are where the rope must have been slipping in the grips. The curves reach a
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high point and then drop off suddenly, the high point is the ultimate strength
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point. This high point can be considered the failure point too but I would
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like to point out the sharp rises after this point. These sharp rises occur
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in the jute, cotton and hemp twine and represent the few fibers that did not
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snap at the ultimate strength point. These last fibers stretched a little
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further and then snapped under a lesser load. This is different from the
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characteristic necking and fracturing that we have learned about in class
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where we mainly dealt with metals.
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CONCLUSION
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In conclusion, I have learned more about tensile testing and the
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improvements needed in different applications involving the fundamentals of
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stress, strain and fracture mechanics that we have learned in class. I think
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that with more accurate tests the results I have found would hold up. I
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observed that nylon is indubitably the strongest, cotton the weakest and hemp
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and jute are about even. There was some slipping, variance in cross sections
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and some amount of error attributed to operator inexperience and the overall
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measuring procedure.
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