A Degrading Life

Care and maintenance of your metallic equipment

By the British Mountaineering Council

Life moves fast these days. With the increasing popularity of climbing, manufacturers are taking every advantage of the latest advances in materials and technology to make exciting new gear available each year. But one thing never changes, climbing has been and always will be a risk sport, and all the latest equipment in the world will only increase the margin of safety at which you operate.

Risk can never be eliminated, so it's absolutely vital to understand the limitations of equipment, and recognize the situations that may cause it to become damaged or even fail.

But how do you know if a certain piece of equipment is damaged? Should you throw a rope away after washing it in detergent? What's a stress fracture? Just one of the important subjects is degradation of metallic equipment, so read on if even if you only own a single carabiner.

Equipment degradation
Every time you use your gear it is under attack from a variety of sources - UV light, dirt, dust, water, salt - really, the surprise is not that climbing gear sometimes fails, but that it doesn't more often! Here are some of the more common forms of metallic degradation that occur, and some of the ways in which you can combat them:

Whilst the maximum failure load is a concept central to all engineering applications and their designs, another aspect of equal concern is fatigue. Fatigue is well recognized by professional engineers in nearly all materials, but especially in metallic alloys since it affects the life span of a product.

If a load is applied that is lower than the stipulated failure load of a particular piece of equipment, but is done so repeatedly, then the piece may eventually fail. This process is known as fatigue failure. Each load application is known as a cycle, and the less the load, the more cycles are required to cause a failure (if the load is small enough then a failure will never happen) However in reality for climbing equipment, the load level over a sequence of cycles is not constant, and a sudden big load equaling the failure load will cause the piece to fail.

The importance of fatigue effects implies the need to keep the amount of load to a reasonable level- well below the failure load- and to avoid sudden large loads, thus prolonging the fatigue life of the item. The design of a piece of equipment should take fatigue into account in the following way:

"The usual loads expected should be such that many thousands of cycles are required before fatigue failure, and this should be beyond the expected usable lifetime of the product".

However, if through exceptionally heavy or abnormal use too many cycles have been accumulated and failure is near, then cracks in the highest stressed areas will be forming and growing. Hence, a close examination with a magnifying glass on well-used equipment will be very worthwhile. In climbing terms, this means that reasonable use should attempt to avoid any chance of fatigue failure by limiting the number and/or severity of the load cycles. Examples of use that go against this are excessive torque, or repeated hitting on rocks. Both scenarios will shorten the useful life of the equipment involved.

Wear is a phenomenon that occurs when metallic surfaces are rubbed by other surfaces under pressure, and local shear stresses arise. Because of these, particles on the surface are eroded, and the physical volume of material locally decreases. The higher the contact pressure and the softer the material, the greater the rate of wear will be. Sharp surface features (like edges and corners) will also erode quicker than smoother parts, because the stresses in those features are higher under a rubbing force. For climbing equipment, any contacting surfaces are liable to wear since most alloys in common use are relatively soft. The most usual high-pressure situation encountered in practice is that of ropes passing through carabiners, rappelling and belay devices etc. Grooves can appear in these items of metallic equipment, and since a noticeable groove represents a new surface geometry with some material removed, the performance of these items will alter over time. This has the further implication that the failure load and other design parameters of the item are changed, and a safe course of action is to retire the piece of equipment once such a groove has become noticeable.

Stress degradation
Any piece of equipment that operates under applied loads becomes subject to the effects of the stresses that result from those loads. Thus, any structure or component used in an engineering application becomes stressed during the normal course of their intended operation, and metallic climbing equipment is no exception to this.

For instance, when a climber falls, the rope takes the climbers weight along with the harness, carabiners, slings and protection placed - all become stressed to a degree. The manufacturers design their equipment to withstand these stresses, but in conjunction with wear & tear and time, continued stressing sometimes leads to a failure.

The presence of cracks in any structure that is designed to carry load is potentially dangerous, but obviously so where metallic climbing equipment is involved. Cracks can arise for several reasons:

During manufacture or heat-treatment of the equipment, commonly during welding processes - these are invariably detected at an early stage, or not critical to the intended performance of the equipment.

During a sudden overload when the failure load is exceeded and the item breaks - this is effectively the sudden initiation and catastrophic growth of a crack through the weakest part of the equipment.

Due to corrosion - see previous section.

By fatigue - as previously explained. Such a crack starts at a microscopically small size, growing as the fatigue life progresses until it is big enough to be noticed by the naked eye. In climbing equipment, this usually means that the fatigue life is nearly at an end, and that a failure is imminent.

This is the reason for recommending regular inspections of equipment both with the naked eye and with a magnifying glass, and when such a crack is detected to retire the item immediately. Continued use of a cracked item will almost certainly lead rapidly to sudden failure - a very dangerous situation.

Another relevant consideration when considering cracks is the influence of temperature. If the temperature is low enough, the brittleness of a material can increase significantly and any small cracks are liable to sudden and catastrophic breaking, like shattering glass. At higher temperatures, materials exhibit a more ductile characteristic making sudden cracking much less likely. For the alloys used in climbing equipment, the transition temperature between this brittle and ductile behavior occurs somewhere in the range -58°F (-50°C) to 122°F (+50°C).

The important factor for climbing equipment is that at cold extremes, it is more liable to brittle fracture, and prolonged use in these conditions increases the likelihood of this occurring (e.g. during an expedition). Although a lot of care is taken to consider this during design and material choice, it is prudent to check regularly for cracks in equipment that is used at cold temperatures for extended periods of time.


Further information is available from www.thebmc.co.uk


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