Cyclic bending of steel wire cables - Summary and Conclusions

 In my tests I created a machine to simulate cyclic bending of steel wire cables under body weight.

Upper picture: loaded with 80 kg

Lower picture: unloaded.

Maximum number of cycles: 10.000.

Cables tested:

• 6 x 19 10 mm cable (Fiber core)
  
• 7 x 19 10 mm cable (steel wire core)
• 7 x 19 12 mm  compressed zip line cable (steel wire core)
  
• 6 x 7 15 mm via ferrata cable (fiber core)
  
• attachment with different torque
  

If I compare my experiments with my experience in practice of the last decades, here are my conclusions. These hypotheses should be verified (or falsified) by further tests.

Conclusion (Hypothesis)

1. The damage can be hidden under a clamp etc.
2. Even small damage should be investigated thoroughly: Open the clamp, bend the cable. Listen to cracking sounds.
3. The number of broken wires does not indicate the whole damage. Also unbroken wires are weakened if cyclic bent.
4.  When the conditions are "bad" (e.g. big bending angle, lot of movement), a few thousand cycles can destroy the cable.
5. Cables with steel wire core last about 5 times longer than fiber core.
6. The diameter seems to play a minor role. 10 mm and 15 mm cable are destroyed by the same number of cycles.
   
7. The torque plays a minor role. It seems that higher torque has a positive effect. 

Recommendation:

 In the picture is an example of a typical bending situation.

 

If load is applied to the cable and realeased we have similar conditions like in our tests (less bending, but more load).
Every participant bends the cable and releases again: Cyclic bending.

Only removing the clamp uncovered the broken wire:

 The growth of the tree causes a larger angle close to the clamp.

Recommendation for Builders 

Prevent big angles  where cyclic loads are expected.

Very bad are situations where one part of the cable is fix, the other is bent. Like a swage that is fixed and the cable moves (zip line).

Cables with steel core should be preferred.

Recommendation for Inspectors

If you see such a situation, open the first clamp which causes the angle and bend the cable. Listen to cracking noises that indicate breaking wires.

 Look carefulle for small damages (displaced wires) close to cable clamps.

Important

It is no problem to move cable clamps as long as there are no broken wires.

Some suppliers/manufacturers say: "Do not move cable clamps. If you do so, you must replace the cable".

In my opinion this is a myth. I could not find any evidence which would support this statement.

Removing or moving a clamp and retighten is no problem and good practice.

Steel wire cables - zip line cable and via ferrata cable

 In my test series with cyclic bending I tested two types of wire cable:

1. compressed 12 mm zip line cable

2. 15 mm via ferrata cable.

Here are the results:

12 mm Zip Line Cable (compressed)

It is comparable with the 10 mm 7x19 cable (wire core). Under similar bending angle they react similar.

 

 

The breaking load after 1000 cycles (33 kN) is about 1/4 of the normal breaking load.

 In the video you see this chart:

 

It shows a principle: For a long time nothing happens, but once a damage occours the detoriation grows rapidly.

15 mm Via ferrata Cable

 This cable was a big surprise. It has a fiber core. And it reacts similar to the 10 mm fiber core cable. After 3000 cycles both cables had similar (severe) damage.

The breaking load after 300 cycles was very low with 13 kN.