Slackline and mechanical fatigue
The aim of this article is to explain the phenomenon of mechanical fatigue wich is frequently approached when talking about slackline. Scientist friends, you will maybe be desapointed as I will not enter into technical details. I will spare you the notions of areas of stress concentration, material grades, heat and surface treatments and other parameters wich relate to the field of engineering and would make it difficult to understand for most of the readers.
Mechanical fatigue, what is this ?
Just so that you understand, I think you all know the principle of the iron wire wich break when we bend it several times at the same location. If you don’t know, just take an iron wire (a paper clip for example) and bend it a first time. Unbend it then ben dit again at the same location and continue like this. After a time, it will break off without having to force more than the other times. This is a fatigue failure.
Notion of cycles
The fatigue appears when a mechanical part (a ratchet or a weblock for example) is stressed repetitively and always in the same configuration. Each stress is called a cycle. In trickline, we can consider that every rebound of the trickliner result in a cycle for the setup elements.
Notion of stress
Stress is the ratio between force and section. If you need to lift a weight of 4kg, You might well use a fishing line Ø0.3mm wich holds 4.5kg. You also can use a wire two times bigger (Ø0.6mm) wich resists 20kg. In either case it will hold. However, even if the weight is the same, the smaller wire will suffer much more than the bigger. It is because the stress on it is more important. Concerning slackline, a weblock with 5mm sideplates will then be two times less stressed than the same weblock with 2.5mm sideplates.
Fatigue modeling
The S-N curve determine for each material the number of cycles that it can endure without breaking relatively to the stress of the cycles.
Fatigue and material
To put it simple, I will use steel and aluminium alloys as examples, wich are the most commons materials in slackline gear. They have totally different behavior each over. We can understand it by analizing their S-N curves.
We notice that steel curve has horizontal asymptote. That means if the stress is low enought, it will never break in fatigue.
However aluminium don’t have endurance limit. It will always exist a number of cycles above wich the metal will break.
So, aluminium or steel ?
When sizing Slack Inov’ products, I based myself on the activity of one of my pro trickliners who use the gear in an extremely intensive way. On a 3 hours session, he enforce at least 4800 cycles on the gear. Regarding his consistency, after a year he is already over 106 cycles of stress ! Thus, for the gear to be safe enought we need it to be able to endure more than 106 cycles, let say 107 to be relax.
- For steel, no problem. We just need to size the product in order to make it work under its endurance limit. The product became then theorically wear-free. Such is the case for Slackimoufle and Slackibloc Jump for example.
- For aluminium it is a bit more complicated . To be able to reach the 107 cycles, the stress has to be excessively low. This implies that the equipment would be such bigger than it will became heavier than the same equipment made from steel…So using aluminium would became useless.
Conclusion
Regarding the results of this study, I decided to use exclusively steel on Slack Inov’ trickline gear. Obviously the products are sized over their endurance limit so there is no risk to break it by mechanical fatigue Aluminium remain possible for longline/highline gear which is subject to much less regular cycles. Provided that the gear is correctly designed in order not to reach the fatigue limit during its lifecycle.
Florent Berthet
Ingénieur Génie mécanique
Concepteur des produits Slack Inov’