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Tension testing of four different twines By Derek Voll EM 307 4/30/92

Abstract

   In this experiment I pulled apart cotton, jute, hemp and nylon twine to

test their ultimate strength. I used a standard tension testing machine equipped with a load versus displacement plotter. I could not calculate strain and was thereby limited by the lack of theory to back up my observations and make descriptive numerical calculations. I did repeat the tests to produce an average value of ultimate load for each twine group. I used this value to make a rough stress calculation. Nylon is the strongest, cotton the weakest and jute and hemp are about equal in strength. I had quite a bit of difficulty with the nylon specimens because of their high strength but the others worked out all right.

Introduction

   For my independent project I choose to test the strength of four

different kinds of twine, cotton, jute, hemp, and nylon. There are many factors in choosing the right twine of the job it will be used for, cost, temperature to be used at, availability, creep and fatigue characteristics. These and other parameters could be analyzed in future studies to find the best twine but my project will focus on tensile strength. I think the results will be meaningful to someone buying twine and the twine producers. In fact the suppliers of the hemp twine were quite interested in my report and would like some copies; they would like more scientific information on hemp since there is so little scientific investigation or research concerning hemp.

   
   My procedure was to obtain twines with similar dimensions, pull them

apart using a standard tension testing machine, collect load versus displacement plots for each of the specimens and then compare and analyze the data. I used a tension testing machine with a capacity of 1000 pounds which had a load versus displacement plotting machine connected to it. By wrapping the twine around the round spool three times, I relied on the large friction force to hold the twine in place. This force was not large enough for the nylon twine and I used the pneumatic grips instead. The pneumatic grips provided more friction which I needed to hold the nylon twine in place.

OBSERVATIONS AND RESULTS

   I would like to start with some general observations of the experiment. 

First, the tests of the cotton, jute and hemp twines proceeded with few problems and their failure occured in the middle of the specimen, which is desirable in any tensile test since the experimenter can more easily observe the fracture area and disregard any stress concentration at the clamp-twine connection. however, for my first two samples of nylon I tried to use the same clamps that I had used for the other twines but in both cases the twine overcame the clampUs friction force before failure but after some stretching (the twine was pulled out of the clamps). Therefore, I switched to the pneumatic clamps but the nylon still slipped some, wearing the surface of the twine and causing stress concentrations. The nylon broke at this worn area near the clamps. Stress concentrations are the very tiny notches and imperfections in a material that produced a high localized stress. Also, I did not have enough nylon so I used the same specimens that I had used in the other clamps and one new specimen. The first two specimens broke sooner and under less load (see Fig #XX) and this was expected since they had already undergone some plastic deformation and recovery. I did not realize how strong and difficult to test the nylon would be. I know my procedure and the following results for nylon are not accurate but it should be obvious that the nylon is definitely the strongest of the four twines. I have graphed each of the specimens together in their respective group (Figs XX- XX) to show the variances between the individual specimens; nylon has the greatest variance in displacement and ultimate load as expected but it should be noted that hemp twines show the second largest variance in ultimate load (all hemp specimens are from Hungary but the specimens with the lower ultimate load were from a different supplier than the other three). From these graphs we see that cotton is the weakest and nylon is the strongest. I would like to point out that these graphs do not tell the whole story and a better indication of strength would be a stress versus strain plot, which was impossible to make since stains could not be calculated because we did not have access to an extensometer. However, if you look at the sample calculations in the appendix you will see that the hemp twine had a slightly thicker cross section and the corresponding stress was comparable to that of the jute twine. Even with this fundamental calculation we must realize that each twine was probable woven differently and all their diameters were slightly different.

   
   Looking closely at the graphs for the cotton and nylon specimens you will

see that there are little ridges and drop-offs before fracture; these points are where the rope must have been slipping in the grips. The curves reach a high point and then drop off suddenly, the high point is the ultimate strength point. This high point can be considered the failure point too but I would like to point out the sharp rises after this point. These sharp rises occur in the jute, cotton and hemp twine and represent the few fibers that did not snap at the ultimate strength point. These last fibers stretched a little further and then snapped under a lesser load. This is different from the characteristic necking and fracturing that we have learned about in class where we mainly dealt with metals.

CONCLUSION

   
   In conclusion, I have learned more about tensile testing and the

improvements needed in different applications involving the fundamentals of stress, strain and fracture mechanics that we have learned in class. I think that with more accurate tests the results I have found would hold up. I observed that nylon is indubitably the strongest, cotton the weakest and hemp and jute are about even. There was some slipping, variance in cross sections and some amount of error attributed to operator inexperience and the overall measuring procedure.

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