1 Stretch test: Dyneema SK78 Length variations between two equal dyneema ropes 4mm Boudewijn van der Kroon The string concept has 2 Dyneema cables to hold the DOMs. Water currents and drag causes lateral displacement With enough buoyancy
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Presentation Transcript
01
1 Stretch test:
Dyneema SK78
Length variations between two equal dyneema ropes 4mm
Boudewijn van der Kroon<br>
02
The string concept has 2 Dyneema cables to hold the DOM’s.
Water currents and drag causes lateral displacement
With enough buoyancy the string lateral displacement is limited.
Before, during and after the deployment of the strings the dyneema cables will stretch and elongate.
Dyneema is also creep sensitive. depending on Tension Time Temperature 2<br>
03
At around 75 mm length-difference between the two stringcables the DOM has an angle of 10° degrees.
A larger angle is not desirable because of the beacon, the orientation of the PMT’s and the other components
This meant that the length of the cables should be equal within 7,5 x10-5 km
So a maximum difference of 4 mm on 40 meter
We did some tests to see if that’s plausible 3<br>
04
ELASTIC ELONGATION (E.E): Stretch or extension of a rope that is immediately recoverable after releasing the load.
HYSTERESIS: Refers to a recoverable portion of stretch or extension over a period of time after a load is released
PERMANENT EXTENSION (P.E.): That portion of extension which, due to construction deformation and some plastic deformation of the yarn fibers
CREEP (COLD FLOW): Fiber deformation (elongation) due to molecular slippage under a constant, static loading situation. 4<br>
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5<br>
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Test setup:
To measure the difference in elongation between 2 cables of 40 meters.
Roller bearing with low friction The tension in the cables is at least 99% equal.
Laserpointer is mounted between the cables. Small angle-rotation possible Lengthvariations increases by a long arm.
Magnification: 400x 6 40meters<br>
07
The laser is aligned at low cable tension
The tension increased to 500 kg. As a result, the displacement of the laser was a few meters and impossible for accurate and correct measurement results. However, it was clear that there are several centimeters length difference between the cables, as a result of the increase of the tension.
The test is repeated several times, each time with the same observation.
During the tests the cables are becoming permanently longer
It was agreed to pre-stretch the two cabels and re-do the test. This should reduce the differences 7<br>
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8 During the pre-stretching at 500 kg, the cable keeps loosing tension/ getting longer, even after a week. But the effects are tempering down.<br>
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9 Stretch test:
Length of a NON- prestretched cable at different tensions Total elongation: 991 mm from minimal tension(25 kg) til 500 kg about: 200mm per 100 kg<br>
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10 Stretch test:
Length of a stretched cable type A at different tensions Total elongation: 579 mm from minimal tension(25 kg) til 500 kg about: 120mm per 100 kg<br>
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11 Stretch test:
Two stretched cables type B at different tensions Total elongation: 105 mm from 250 kg til 500 kg about: 42mm per 100 kg (for 2 parallel cables 40 meters) Length up to reference point<br>
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After the cables were pre-stretched, the experiments with the laser alignment were repeated.
Tests have been done where the tension is reduced from 500 to 200 kg
Tests have been done where the tension is increased from 200 to 500 kg
At every 50 kg the position of the laserdot is monitored.
There is not a clear pattern to identify the weakest cable. The only thing we can see in the measurements is that at lower tensions, the laserpoint is aimed lower due to sagging of the cables
The maximum deviations are about 200mm. Divided by the magnification of 400x, means that the difference between the two cabels is only 0,5mm for the stretch test from 200 kg to 500 kg tension on the cables. 12<br>
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Buoyancy is around 400 kg (200kg tension coming from the topbuoy and 200 kg from the spheres) on two parallel cables.
40 meter cable E.E* % differences between the cables
Non pre-stetched cable 400 mm 1 % >> 10 mm
Pre-stetched cable type A 240 mm 0.6 % n.y.k.
Pre-stetched cable type B 170 mm 0.43% 1mm
*Note that is only the E.E. The total stretch is higher
Type A= 1 week @ 500kg tension
Type B= Type A + 2 weeks 2 cables parallel @ 500kg tension together 13<br>
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If the pre-stretched cable is keept at a low tension (lower than 200kg) the tension in the cable builds up. Or if the cable is not stored under tension at all, the cable is getting shorter.
This is mostly due to the hysteresis of the dyneema cable 14<br>
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After the cables has been released form tension, there is a noticeble difference in stretch (in just one weekend).
Long pre-stretched after a 1 weekend ~65h
Stretch Stretch 42mm/ 100 kg ~70mm / 100 kg
This seems a hysteresis issue. After just 1 weekend the specs of the cables is lower.
Also the test with the laser showed a slight increase in deviation. But stil around less than 1mm difference from 250 to 500kg. Meaning less than 2 mm lenght difference E.E for 40meters of the string. 15<br>
16
All the stretching test where short term tests, so mainly elastic elongation and it’s differences. Longer term effects like creep are not measured.
The E.E. seems very linair within the range of tension
The differences in E.E. with long prestretched cables are very low. Within the specs, meaning in theorie all the DOM’s would be at an angle less than 10 degree.
-If the cable is pre-stretch for a long time
-If the cable has not been without tension for a long time
Hysteresis is still not 100% clear 16<br>
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Experience with the pre-stretching has increased.
We need pre-stretched cables because the difference between the non stretched cables is to big resulting in to large angle for the DOM’s.
We could look at how long it takes to pre-stretch the cabels to be within the specs.
Maybe there are other methods to speed up the pre-stretching (maybe thermal).
Currently we are doing long term test for longer term effects 17<br>
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18 In the future we would like to test the DM20 kind of Dyneema in stead of SK78.
According to the cable supplier, the E.E. is around the same, but the creep would be far less.<br>
