A suggested basis for the interpretation of the NCA rope tests

Introduction

The safety of a rope when in use is affected by a great many factors, including strength, stretch, flexibility, water absorption and lots of other things. However, the single most important factor is the strength of the rope - if it breaks when in use, you'll probably die: if it doesn't, you probably won't. Thus, to a very good approximation, the thing that matters in trying to determine if a rope is safe or not is to know how strong it is. This is most easily represented by knowing what load the rope will support before breaking.

Rope safety

Rope strength is obviously affected by a great many factors - including age, cleaning, use, contacts with agents such as acids or alkalis, exposure to sunlight and storage temperature and there are indications that rust seriously affects rope strength, beyond its visible impact. The European Personal Equipment directive under which all the performance standards for ropes of the types used by cavers have been issued, requires that manufacturers give an indication of the safe life of the rope, but it is obvious to all that a new rope that has been stored unused for some years in ideal conditions will be stronger than a younger rope which has been subject to arduous usage or poor maintenance. Clearly ropes do not get stronger with age, even if they are unused, and in order for them to be adequately strong for a useful period after their date of manufacture they must be made with excess strength to that actually required for their application. The only way to reliably tell how strong a rope is after a period of use and/or storage is to test a representative sample of it.

The reason for drop tests

There are several reasons why a drop test is used rather than a slowly increasing load as would more normally be used for ultimate tensile strength (breaking strain) tests (of either knotted or unknotted rope).

Fall factors

The purpose of a drop test is to put a very high load on a rope very quickly. Because the weight is falling, it puts a much higher load on a rope than if it was simply hanging static on the end of it. The actual peak load is related to the speed at which the load is moving when it is stopped by the rope, and the distance over which it is brought to a halt. Since all ropes are elastic to a greater or lesser degree, the distance over which the weight comes to a stop is dependent on the length of the rope which is being tested. A long, elastic rope will stretch a great deal and bring the weight to a stop over a long distance. This will minimise the maximum load on the rope and is the reason why climbing ropes are stretchy - they need to arrest falls without breaking and without applying excessive forces to the climber.

Caving ropes are much less stretchy, since prussiking on an elastic rope is much less efficient. Consequently, however, they have to withstand much higher peak forces in the case of any given fall than would a climbing rope. Obviously a longer rope has more elastic stretch in it than a short length of the same rope. At first sight you might think that this might seriously affect the results of the drop test since the peak force of the falling weight will be reduced for the longer rope.

However, the force applied by the falling weight is, over even fairly long distances, related directly to how far it has fallen. The weight is being accelerated by gravity, and the further it falls, the faster it is going when the rope starts dragging it to a halt. This cancels out the effect of any extra stretch of the rope caused by it being longer, and to a good approximation, the results of the drop test are independent of the length of rope being used.

What does matter is how far the weight falls in relation to the length of the rope. A weight which is dropped over a distance of half the length of the rope will apply a lot less force than a weight dropped over the full length. Therefore the tests are described in terms of the Fall Factor, which is the ratio of the length of the sample to the distance than the weight is dropped. A test done where the weight is dropped for half the length of the rope is said to be Fall Factor 0.5. Where one is dropped the full length of the rope is said to be Fall Factor 1, and one where the weight is dropped twice the length of the rope is said to be Fall Factor 2.

When applied to a cave pitch, it can be seen that a Fall Factor 1 can be seen to be equivalent to a fall from level with the belay, whereas a Fall Factor 2 is one where the caver is vertically above the belay with the rope taut - this is the worst fall that can occur in a caving situation. In practice, the strength of the human body and the manufacturers' conditions of use only allow for a maximum Fall Factor of 1.

The NCA tests and the European Standard tests

The purpose of the manufacturers tests is to check that the rope as manufactured is suitable for use, and as such every new rope is expected to survive the series of tests without breaking. In contrast the purpose of the NCA test is to discover how much of the initial surplus strength (if any) remains after usage and storage . Both of the series of tests accept that the only way to determine whether a rope is strong enough for Single Rope Technique (SRT) usage is to carry out a drop test using a falling weight.

The actual breaking strain can obviously only be determined with the best accuracy if the rope breaks during the test series although a rope which breaks in the later stages of the series may still have adequate strength.. The test for A type ropes in prEN1891 is five unity Fall Factor drops of a 100Kg mass. The 1988 Speleo rope trials established that the peak tension in the rope on the first drop is only around 80% of that in the later drops due to the tightening of the knots during the first drop. Since all ropes which broke during NCA tests have shown melting at places on the sheath the breaking of the rope must be considered to be due to both a physical and thermal stress. Thus the NCA test where the increasing Fall Factor after the second drop ensures that the knots are being tightened at each drop gives a truer relationship to the condition of arresting a fall than the repetition of the same Fall Factor in prEN1891.

PrEN1891 also allows for a less severe test for SRT ropes of 9mm diameter or less on the assumption that they are used for descent only. The NCA tests makes no distinction between 9mm and thicker ropes since cavers use all sizes of rope for ascent as well as descent.

Relationship between the NCA tests and the worst case in practice

The relationship between NCA tests and the actual situation of a fall onto the SRT rope is that in use there will be only one severe fall onto any knot or knots. The NCA tests have traditionally required two drops to be held before the rope could be considered safe for use. A further safety factor is produced by the much greater flexibility of the caver in his caving harness in relation to a steel weight on a large maillion. This greater flexibility may produce serious damage in the caver, but does reduce the strain on the rope. The use of resisting the second drop as the indicator of suitability for continued use gives a considerable margin of safety for incidental damage during the fall caused by rock or descent device, BUT NOT FOR FALL FACTORS GREATER THAN ONE.

In addition, the NCA advises that the rope be retired while it is still above the minimum safety level if it fails to survive the third drop (at 1.3 Fall Factor). This is because the step nature of the test makes it impossible to tell whether the rope is at the maximum or minimum value within the range for one result. This means that it is not possible to tell whether the rope still has a large amount of strength in reserve before it reduces to being at the minimum level for safety, or if it will make the change in the next couple of times it is used.

The prediction of strength loss between tests

If the amount of annual usage remains constant, then a comparison between the excess strength already lost and the excess strength remaining gives an indication of the years remaining of use in relation to the years that the rope has been used. Because of the step nature of the test results, and because of the impossibility of maintaining an absolutely true record of usage of any individual rope, division of the estimated remaining years by two is advised. It can be expected that following this advice would result in the rope being tested 3 times (shortened by 7.5 metres) during its useful life.

And finally.....

Everybody knows that a chain is only as strong as its weakest link, and the same is true of a length of rope. The NCA tests (or any other, for that matter) only have any validity if the rope sample used is representative of the worst section of the rope. Unfortunate it may be, but this does mean that is some cases the rope will have to cut in half so a sample can be taken from its centre!

13 October 1998

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