Vertical caving terminology and methods > General hardware
The strength of carabiners, maillons, ropes, slings and other SRT gear is specified in several different ways. Most PPE equipment will have its minimum breaking strength (MBS) specified, commonly called its breaking strain or strength. The equipment can be expected to hold at least that load, before it breaks. Often, it will be able to hold slightly more, sometimes a lot more (it is quite common for carabiners rated at 22kN to actually break at closer to 30 kN during testing). Individual items might be slightly stronger than others due to natural variances and imperfections in the material, but even the weakest one should not break at loads lower than the MBS. With most manufacturers, this is actually a statistical confidence of at least 3σ, meaning that 99.7% of items will not break at loads lower than the MBS. This means that out of every 333 devices tested, only one should fail below the MBS.
According to European EN PPE standards and international UIAA standards, several samples must be tested, often several representative samples from each batch of products, and none of the sample products that are tested are allowed to break at a lower amount than the MBS, when tested according to the standards. The standards may require ropes or metal pins of a certain diameter to be used when pulling on the product during the test, depending on what product is being tested. These may not match what will actually be used to connect to the sproduct in practice, so the results in practice may be different. Other national or international PPE testing requirements may allow a tiny percentage to fail lower than the MBS, but this is not allowed by any of the major international PPE testing standards.
However, after being subjected to loads significantly lower than the MBS, the equipment may be permanently deformed and no longer function correctly. For example, carabiners or maillons may no longer open and close correctly. As a general rule, no equipment should be intentionally subjected to loads above 20% (a fifth) of their MBS. This may be called their working load limit (WLL) or safe working load (SWL), and the ratio of the working load limit to the MBS, is called a safety factor (SF). At that load, they are expected not to become deformed, and should continue to function correctly. The actual safety factor might be different with some products, sometimes 25% (a quarter), 17% (a sixth) or 10% (a tenth) of their MBS, and manufacturers do not often state what factor they use.
With some equipment, particularly maillons, it is common to specify the working load limit in kilograms, rather than the MBS, and the safety factor is assumed to be 20%, so a maillon with a 400 kg WLL is assumed to have an MBS of approximately 2 tonnes. The ratings are typically specified in kilonewtons (kN), where 1 kN is the force experienced when holding a 101.9 kg load on Earth. For the sake of simplicity, think of it as 1kN being approximately 100 kg, and 10 kN being approximately 1 tonne (1000 kg). 22 kN is therefore approximately 2.2 tonnes.
The ratings are normally only for equipment that is being used correctly, such as the gate being closed on a carabiner, and the load only pulling in the correct direction. However, with carabiners, there will be separate MBS ratings for a incorrect usage as well. With some devices that can legitimately be used in numerous different configurations, such as rigging plates used for climbing (not caving), the MBS will typically apply to the weakest configuration. With ropes, the MBS is almost always specified for a rope without any knots in it. With some devices, such as belay devices and ascenders, the MBS is largely meaningless, since the devices may intentionally slip down the rope, or unintentionally damage the rope, at loads much lower than they would break at. As a result, these devices might only supply a working load limit.
It is worth noting that if a metal product (such as a carabiner) is loaded at nearly its MBS, then unloaded, and loaded again, it might break at lower than its stated MBS. The more times it is loaded at very high loads, the weaker it becomes. An aluminium carabiner with its MBS rated as 22 kN, might actually break at 30 kN if loaded just once. However, if it is loaded several times at 20 kN, it might eventually break at 20 kN, rather than 22kN or 30 kN. This effect is known material fatigue.
As shown in real testing with aluminium, when a carabiner is loaded to 85% of its MBS about 250 times, its strength will probably have reduced enough to snap. At 90% its full strength, only a couple of cycles could be enough to break it. The number of times it can be loaded increases dramatically as the loading level decreases, so that if it is loaded to 50% of its MBS, it takes about 3'000 cycles before it snaps. With loading up to 30% of its MBS (as much as it will normally encounter in a severe climbing fall), it takes over 10'000 cycles before it snaps. This number increases exponentially for lower loads, and with loads as low as those seen during SRT, the number of cycles should be into the millions (enough that no testing results are known because it would take too long to test). This effect continues for any loading level, and even the tiniest loads can cause fatigue effects, given enough cycles.
With steel, there is normally no fatigue effect at all for 40-50% of the MBS, and it can endure infinite loading cycles below those levels. Above the 40-50% fatigue threshold, steel products show exponential growth in the number of cycles for lower loads, but with a much higher endurance than aluminium. By 90% of the MBS, they can normally cope with a few hundred cycles, and by 70%, they can cope with 10'000 cycles or more. By 60%, they can already tolerate millions of cycles. Stainless steel follows the same pattern, but the fatigue limit is lower (the MBS is typically higher). Cyclic loads, fatigue and deformation are why it is important for carabiners and other metal devices to be rated at much higher loads than they will actually experience, and with the loads experienced during SRT, fatigue should not normally be the reason for retiring them, as they will have been retired for other reasons long before fatigue becomes an issue. However, aluminium carabiners used very frequently to catch high load falls, such as those used at indoor climbing facilities, will need to be periodically replaced.
All of these ratings are only valid for items that are new. Over time, abrasion, corrosion and material fatigue will reduce the strength.
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