Testing steel wire cables: How many bending cycles before they break?

 In ropes courses and Via Ferrata steel wire cables are widely used for life lines and belay cables.

In ropes courses we faced some accidents due to cable failures, in Via Ferrata we are waiting for the first accident.

One typical mechanism for failure is cyclic bending: One part of the cable is fix, the other part moves.

Here is avideo where you can see the basic principle.

Here is an example of a bending situation in a ropes course: Everytime a participant puts weigt to the lifeline it is bent in the thimble.

The angle can be even worse:

 

The wires break first inside the curve.

The damage can be seen if you know where to look and if you look carefully.

Sometimes the damage is underneath the cable clamp and not visible.

I built a machine to simulate the cyclic bending process. I made the angle very steep on one side (145 degrees), and flatter on the other side 110 degrees).

Here is the video where I describe the first test with a 6 x 19 cable, 10 mm diameter, fiber core.

 

Interesting is a closer look at the area below the bail of the clamp: I would say it is a combination of rubbing and bending.

 

 In a second test I took a 7 x 19 cable 10 mm diameter, with a steel core.

Here are tne results:

After 2000 cycles:

 If you inspect, it is important to bend the cable against the curve.

After 3000 cycles:

 

 

 After 6000 cycles: Not very much damage  from the outside visible.

 

But in the inside:

After 7000 cycles I stopped.

 

3 strands were still intact.

The damage was more than 50% of the cable.

Only for checking I inspected the other, lower angle of the cable. It did not look so bad:

But then I tried to bend it.

 Conclusion: If you see a broken wire next to a clamp, open the clamp and look under it. 

If possible, bend the cable against the curve.

It makes sense, to open randomly cable clamps in the operational inspection and look underneath.