Tension testing of four different twines
\nBy Derek Voll
\nEM 307
\n4\/30\/92<\/p>\n
Abstract
\n In this experiment I pulled apart cotton, jute, hemp and nylon twine to
\ntest their ultimate strength. I used a standard tension testing machine
\nequipped with a load versus displacement plotter. I could not calculate
\nstrain and was thereby limited by the lack of theory to back up my
\nobservations and make descriptive numerical calculations. I did repeat the
\ntests to produce an average value of ultimate load for each twine group. I
\nused this value to make a rough stress calculation. Nylon is the strongest,
\ncotton the weakest and jute and hemp are about equal in strength. I had quite
\na bit of difficulty with the nylon specimens because of their high strength
\nbut the others worked out all right.<\/p>\n
Introduction
\n For my independent project I choose to test the strength of four
\ndifferent kinds of twine, cotton, jute, hemp, and nylon. There are many
\nfactors in choosing the right twine of the job it will be used for, cost,
\ntemperature to be used at, availability, creep and fatigue characteristics.
\nThese and other parameters could be analyzed in future studies to find the
\nbest twine but my project will focus on tensile strength. I think the results
\nwill be meaningful to someone buying twine and the twine producers. In fact
\nthe suppliers of the hemp twine were quite interested in my report and would
\nlike some copies; they would like more scientific information on hemp since
\nthere is so little scientific investigation or research concerning hemp.<\/p>\n
My procedure was to obtain twines with similar dimensions, pull them
\napart using a standard tension testing machine, collect load versus
\ndisplacement plots for each of the specimens and then compare and analyze the
\ndata. I used a tension testing machine with a capacity of 1000 pounds which
\nhad a load versus displacement plotting machine connected to it. By wrapping
\nthe twine around the round spool three times, I relied on the large friction
\nforce to hold the twine in place. This force was not large enough for the
\nnylon twine and I used the pneumatic grips instead. The pneumatic grips
\nprovided more friction which I needed to hold the nylon twine in place.<\/p>\n
OBSERVATIONS AND RESULTS
\n I would like to start with some general observations of the experiment.
\nFirst, the tests of the cotton, jute and hemp twines proceeded with few
\nproblems and their failure occured in the middle of the specimen, which is
\ndesirable in any tensile test since the experimenter can more easily observe
\nthe fracture area and disregard any stress concentration at the clamp-twine
\nconnection. however, for my first two samples of nylon I tried to use the
\nsame clamps that I had used for the other twines but in both cases the twine
\novercame the clampUs friction force before failure but after some stretching
\n(the twine was pulled out of the clamps). Therefore, I switched to the
\npneumatic clamps but the nylon still slipped some, wearing the surface of the
\ntwine and causing stress concentrations. The nylon broke at this worn area
\nnear the clamps. Stress concentrations are the very tiny notches and
\nimperfections in a material that produced a high localized stress. Also, I
\ndid not have enough nylon so I used the same specimens that I had used in the
\nother clamps and one new specimen. The first two specimens broke sooner and
\nunder less load (see Fig #XX) and this was expected since they had already
\nundergone some plastic deformation and recovery. I did not realize how strong
\nand difficult to test the nylon would be. I know my procedure and the
\nfollowing results for nylon are not accurate but it should be obvious that the
\nnylon is definitely the strongest of the four twines. I have graphed each of
\nthe specimens together in their respective group (Figs XX- XX) to show the
\nvariances between the individual specimens; nylon has the greatest variance in
\ndisplacement and ultimate load as expected but it should be noted that hemp
\ntwines show the second largest variance in ultimate load (all hemp specimens
\nare from Hungary but the specimens with the lower ultimate load were from a
\ndifferent supplier than the other three). From these graphs we see that
\ncotton is the weakest and nylon is the strongest. I would like to point out
\nthat these graphs do not tell the whole story and a better indication of
\nstrength would be a stress versus strain plot, which was impossible to make
\nsince stains could not be calculated because we did not have access to an
\nextensometer. However, if you look at the sample calculations in the appendix
\nyou will see that the hemp twine had a slightly thicker cross section and the
\ncorresponding stress was comparable to that of the jute twine. Even with this
\nfundamental calculation we must realize that each twine was probable woven
\ndifferently and all their diameters were slightly different.<\/p>\n
Looking closely at the graphs for the cotton and nylon specimens you will
\nsee that there are little ridges and drop-offs before fracture; these points
\nare where the rope must have been slipping in the grips. The curves reach a
\nhigh point and then drop off suddenly, the high point is the ultimate strength
\npoint. This high point can be considered the failure point too but I would
\nlike to point out the sharp rises after this point. These sharp rises occur
\nin the jute, cotton and hemp twine and represent the few fibers that did not
\nsnap at the ultimate strength point. These last fibers stretched a little
\nfurther and then snapped under a lesser load. This is different from the
\ncharacteristic necking and fracturing that we have learned about in class
\nwhere we mainly dealt with metals.<\/p>\n
CONCLUSION<\/p>\n
In conclusion, I have learned more about tensile testing and the
\nimprovements needed in different applications involving the fundamentals of
\nstress, strain and fracture mechanics that we have learned in class. I think
\nthat with more accurate tests the results I have found would hold up. I
\nobserved that nylon is indubitably the strongest, cotton the weakest and hemp
\nand jute are about even. There was some slipping, variance in cross sections
\nand some amount of error attributed to operator inexperience and the overall
\nmeasuring procedure.<\/p>\n
