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      Results

A typical result is shown in its raw data form below and includes output from the load cell in blue and the HE sensors in green and brown, both having units of volts.


1

This data is transformed by a computer program into force with units of kilo Newtons and sample length in metres with respect to time. 

2

One interesting plot of this data is the force length plot


3
This shows how the rope has started to extend after reaching its unstretched length of around 0.75m and rises to a maximum sample length of 1.1m at a peak force of 7.1kN.  The subsequent contraction and cycles reflect the ‘bounces’ seen in the simple force v time plot above.
From this we can calculate energy as is shown

4

The energy within the rope can be seen to rise to a peak of 1.1kJ at maximum sample length, fall back as the rope contracts and then flattens off for a period of time whilst the test mass undergoes ‘free flight’.  A second and third increase in energy can be seen relating to the second and third bounces but the plot hardly shows the subsequent bounces.  (There are check calculations which can be done on the data to compare energy derived from mass * drop height * g’ computation with energy derived from calculations using force nd sample length data which support our confidence in the results; g’ being the acceleration due to gravity as measured during the initial free fall phase.)
One concern we have is quite what this energy information means for an experiment where multiple bounces occur without the rope breaking.  So we developed an approach where by using a heavier test mass and a longer sample length, we would break the rope on the first drop.  A typical plot shows the single force peak.  (The noticeable bounces post rope break are due to the test mass bouncing on the tyre assembly whilst the noise at zero level on the load cell post break is thought to be due to the load cell vibrating on its holder.)

5

We are now progressing with this approach to gain a feel for how much energy it takes to break a rope in a single drop.  The first set of results for ten samples of a 9mm diameter SRT Type B rope tested dry is give below:


BPC 30

Pre OL

g prime

Max Length

Max  Force

Max Energy

Drop No.

m

M/s^2

m

kN

kJ

20

1.46

9.66

1.73

13.6

2.08

21

1.44

9.59

1.71

13.1

1.99

22

1.43

9.61

1.73

14.1

2.19

23

1.45

9.61

1.71

13.6

2.05

24

1.47

9.61

1.71

13.3

1.99

25

1.47

9.56

1.72

13.7

2.10

26

1.46

9.66

1.72

13.7

2.06

27

1.45

9.63

1.74

14.6

2.30

28

1.44

9.61

1.71

13.6

2.08

29

1.44

9.60

1.70

13.2

1.96

 

 

 

 

 

 

mean

1.45

9.61

1.72

13.7

2.08

% SD

0.96

0.32

0.72

3.15

4.84

 
(Pre OL is the overall length of the sample including loops tied with figure of 8 knots supporting the test mass.)  The data shows reasonable consistency and follows a normal distribution.
Work continues to characterise differing overall lengths, test mass weights and diameters of rope.