How safe are the karabiners used in climbing? ETH Zurich materials scientist Thomas Schambron investigated this question by carrying out loading tests both statically and, as an innovation, dynamically. His conclusion: purely static experiments are not adequate assessments of the complex forces acting on the equipment.
Every climber knows the dilemma: so as to tackle the wall carrying as little extra weight as necessary, he puts the lightest aluminium karabiners he can find into his rucksack, knowing full well that heavier steel hooks would really be safer. The “ideal” climbing karabiner should be as light, strong and easy to operate as possible but must not open unintentionally.
Up to now there is still no karabiner that satisfies all these requirements. This is why different kinds of karabiner are used in climbing; this involves each karabiner type being a compromise between weight, strength, safety and ease of operation. However: although karabiners with no safety lock are simpler to operate, they are also more prone to opening unintentionally. Karabiners with a safety lock prevent unintended opening but are more awkward to operate.
Static versus dynamic strength
Thomas Schambron, an ETH Zurich graduate materials scientist currently employed by BlueScope Steel in Australia, says, “Climbing karabiners are an essential part of the safety chain in mountaineering. They form the link between anchor points on the rock wall and the climbing rope. If the climber falls, they are exposed to dynamic loads in the order of magnitude of 5 to 15 kilonewtons (kN), equal to between 0.5 and 1.5 tons.” Under the leadership of Peter J. Uggowitzer, Professor at the Department of Materials Science of ETH Zurich, Thomas Schambron carried out a series of tests with various karabiners to check their strength; the results were recently published in the Journal “Sports Engineering”.
The international EN standard for climbing karabiners requires a minimum breaking load of 20 kN. However, this value only has to be proved in a static tensile test. The load is applied significantly more slowly in such a test than would be the case in the event of a fall, where it would be a question of dynamic forces. There has still not been any research on the relationship between static and dynamic strength. Thomas Schambron has now compared these values for the first time and has also studied the effect of mechanical wear on karabiner strength. Schambron thinks that, “This question has significant importance in view of the rapidly growing popularity of climbing as a sport in recent years.”
Making the test standard more stringent?
Specifically, the materials scientist tested two different models of karabiner – representing the many commercially available variants. In the test models, he simulated wear of the kind that can occur in practice, e.g. rope abrasion, which reduces the cross-section of the hook. The test also included taking the karabiners to breaking point statically and dynamically. In this process Schambron carried out the static test according to the EN standard, whereas for the dynamic tests he used the drop test rig of a major rope manufacturer.
The dynamic tests led to markedly smaller breakage loads than the static tests; the difference amounted to between 25 and 50 percent. Thomas Schambron comments that, “The size of this result was completely unexpected and was something we had never seen before.” The scientist concludes that “Consideration should be given to carrying out routine dynamic tests when developing new karabiners and it is something which possibly ought to be prescribed in the EN standard.” He says that this is because a mere static tensile test significantly overestimates the strength of climbing karabiners, which is the decisive factor in practice.
Caution when the gate is open
The analysis of wear also revealed astonishing results: slight to moderate damage to the karabiner, such as nicking or abrasion, did not cause any significant reduction in the breakage load. The endangered cross-section is situated in the gate or at the point of transition from the lower radius to the bridge; however, in normal use, there is scarcely any damage at either of these positions. Thomas Schambron says reassuringly, “This means that old or slightly damaged karabiners can continue to be used.”
The static and dynamic tests yielded similar breakage loads with the karabiner open, but marked differences with it closed. The strength with the gate open is significantly less than with the gate closed. This is why karabiners with the gate open can break with even a relatively small loading. This can occur if the karabiner is loaded at exactly the moment when it strikes against a rock. This can cause the gate to open for a brief moment due to its inertia.
Thus the biggest danger of karabiner failure, objectively speaking, lies in the unintentional opening of a gate. This is why Schambron recommends the use of karabiners with a steel wire gate; these are lighter than conventional gates, which is why they remain closed in the event of impact. Although even wire gates can be forced open by small rock spurs if the bolt is in an unfavourable position, vibration does not cause them to open, neither does the karabiner striking the rock. Even greater security can be achieved through redundancy, for example by having several hooks at hazardous points or by using screw karabiners. Caution is also needed on glaciers or when climbing on granite, where the climber can encounter fine sand. These particles make karabiners wear faster, which is why frequently used karabiners such as those used to make changes of direction in climbing parks and climbing halls should be made of steel and operators should inspect them regularly for wear and check that the gate is functioning correctly.
The ABC of climbing karabiners
Modern climbing karabiners are made of aluminium alloys. Steel karabiners are stronger but heavier and therefore not common, although they are used for example in mountain rescue. The strength of karabiners is stated in kilonewtons (kN); one kilonewton corresponds approximately to a weight of 100 kilograms. The strength for three types of load must be stated on each karabiner: loading in the longitudinal direction, in the transverse direction and in the longitudinal direction with the gate open.
The standard Type B (basic) karabiner has a minimum breaking force of 20 kN in the longitudinal direction, 7 kN in the transverse direction and 7 kN with the gate open. Forces larger than 7 kN can occur if a standard karabiner is loaded with the gate open or transversely during a fall. This means that the karabiner may break even though it complies with the requirements of the standard. It is advisable to use karabiners with a minimum breaking force of 10 kN with the gate open. Standard karabiners are also available with a safety lock, but the version without a safety lock is more common.
- Schambron T & Uggowitzer PJ. Effects of wear on static and dynamic failure loads of aluminium-based alloy climbing karabiners. Sports Eng, 13 November 2008
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