Executive Summary
In recent years, multiple forms of climbing anchor corrosion has posed a problem for professional and recreational climbers worldwide. In an effort to combat this safety concern, this design project intends to create a safer, more reliable, and more predictable climbing anchor for implementation in numerous types of outdoor environments. This goal is accomplished by crafting climbing anchors out of a stronger titanium alloy coated with a paint and secured by a modified epoxy acrylic adhesive, which operates in conjunction with the implementation of piezoelectric materials which allow crews to electronically monitor the cracking/corrosion status of climbing anchors out in the environment. This solution provides not only enhanced safety and increased longevity, but even in the case where various types of corrosion may occur to the enhanced anchors, the design allows for the prevention of failure before it occurs with constant monitoring of the structural integrity of the anchors.
Motivation
A primary goal of improving the materials used in rock climbing anchors is to improve climber safety. A study examining over a decade of rock climbing accidents in Colorado found, that 2.5% of climbing accidents were due to anchor failure [1]. UIAA reports suggests that this number may be substantially higher in marine environments. While this is a small percentage of overall climbing accidents, anchor failures frequently lead to tragic accidents. Increases in safety not only make the sport more enjoyable for current climber but could also make the sport more appealing to those who currently do not participate due to safety concerns.
Background
Rock climbing as a sport and pastime originated from Victorian mountaineering in the 19th century. Anchors were made by wrapping ropes around rocks and shoving them in cracks in rock faces or securing ropes on natural features like trees. In traditional rock climbing, the climber ascends with a rope and places anchoring gear into the rock as they go, and removes the anchoring gear afterward. In the early years, the primary gear used for anchoring was the piton, a metal spike with a loop to feed a rope through or clip a carabiner into, that was driven into the rock using a hammer. Pitons are mainly used simply to secure the rope in place, like a checkpoint, in case of a fall, but they have also been used as artificial hand and foot holds to aid in an ascent. The latter is considered aid climbing and is less preferable as it puts much more force on the peg consistently and increases the possible deformation of the rock.
The earliest recorded use of metal anchors secured in rock was 327 B.C.E by Alexander the Great. He drove iron tent pegs into frozen rock and used them to secure flaxen lines to ascend Sogdian Rock with his army [2]. There are very few other recorded uses of these metal pegs until the 19th century, when mountaineering became more popular and transitioned into modern rock climbing. One of the better recorded early uses of these metal pegs was in the United States in 1875 by George Anderson, on Half Dome at Yosemite. He forged eye-bolts out of iron, similar to the one shown below, Figure 1, but they were too soft to hammer directly into the Yosemite granite, so he hand drilled holes and inserted wooden pins, into which he then hammered the iron bolts [3].
Figure 1
The method of forging used to make these iron pegs would strengthen the iron by deforming the grains in the metal and aligning them with the shape of the final product. The strength of iron pitons was enough for European limestone, where the piton would deform into the crack as it was driven in. As climbing started in the United States, the granite in Yosemite was found to be too hard for forged iron, so it was alloyed with carbon to form steel. Modern pitons are made of chromium-molybdenum alloy steel, which is even stronger and harder in order to prevent failure [4]. This alloying afforded higher strength, but less deformation, so the insertion of pitons into cracks had to be much more precise. This lead to innovation in the shape and design of pitons, as shown in the variety below in Figure 2 [5].
Figure 2
Despite the definition of traditional rock climbing, old pitons placed during the late 1800s and early 1900s, which were made of soft iron and steel, could not be removed without damaging them and they can still be found in many rock faces in Europe.
The conservationist ethics of “clean climbing†to avoid permanent damage to the rock prevailed in the 1970’s, and caused pitons to be replaced by easily removable, friction-driven protection such as spring-loaded camming devices, hexentrics, and nuts. Even with these advancements, permanent bolts are used occasionally in traditional climbing where removable protection is extremely difficult or unsafe to place. In contrast to traditional climbing, sport climbing uses many pre-placed bolts throughout the route for climbers to clip their rope into as they ascend, allowing for more physically strenuous routes. Pictured figure 3 below is a view of a standard climbing anchor setup utilizing permanent bolts [6].
Figure 3
Bolted climbing anchors include a bolt that is fixed into a predrilled hole in a rock face, and a hanger for attaching a ring or carabiner to secure the climbing rope to the rock. Bolts and hangers, like pitons, likely started with soft carbon steels and progressed to stronger and stronger steel alloys. A major functional difference between bolts and pitons that affects material selection is the permanence: pitons are supposed to be removed after ascent, and bolts are supposed to stay in the rock as long as possible until they fail. Currently, the standard lifetime of an anchor is 50 years [7]. This warrants the use of more chemically inert metals like stainless steel and titanium, and chemical coatings to prevent weathering and rust. This difference also requires a change in design from pitons to bolts to make bolt placement more durable and secure since they rarely ever need to be removed.
A wide variety of bolt system designs have been invented over the years, and some are better than others. Most have a simple rod shape with a bolt at the end, securing a radial hanger to the wall. Others are more like a hairpin or U shape, which does not require a hanger. The primary two methods of securing the bolt into the rock are friction and chemical bonding with adhesives. The types of bolts that use friction are sleeve bolts, wedge bolts, compression/expansion bolts, and nail drives, all of which use some sort of sleeve over the bolt length that presses into the hole as the bolt is tightened. The longer and thicker a bolt is, the better it will be at holding large loads; ¼†bolts are popularly known to be dangerous because they are too thin and break easily. Externally threaded bolts are also known for failing more easily. Apart from some bad models, many mechanical bolts are great in hard rock, but can fail in soft and medium rock. Resin or glue-in bolts are becoming more common because they provide a strong chemical bond between the rock and the bolt, and are especially strong in softer, more porous rock [8]. Examples of good and bad bolts are shown in figure 4 below, with the four furthest to the left all being glue-ins [9].
Figure 4
Since steel rusts when exposed to air, it is often coated in some way to prevent rust. Cars are coated in paint, but this does not last long enough for climbing bolts. Some bolts are coated in a thin layer of zinc (zinc flake), which oxidizes instead of the iron in the steel, but this type of zinc coating is too thin and can easily be scratched off. Galvanizing steel takes zinc coating one step further by submerging the steel in a bath of molten zinc, forming a thick, strong protective layer. Even with this much thicker layer, decades of exposure to air, moisture, and salt or acid rain will eventually eat through to the steel underneath. The next best material which is used in most bolts nowadays is stainless steel, which is low carbon steel alloyed with chromium, nickel, and molybdenum. Common grades of stainless steel used for bolts are AISI 304, 316, and 904L. In coastal areas, even stainless steel can fail, for reasons which will be further explained in the next section. In these cases, titanium can be used to provide to most corrosion resistance possible while retaining nearly the same tensile strength as steel. Titanium bolts are only available as glue-ins; an example is shown Figure 5 below. Each advancement from plain carbon steel makes bolts more expensive, but it also makes them last longer [10].
Figure 5
Adhesives used for glue-in bolts are almost all two-part curing systems. They vary in strength as shown in Figure 6 below, reproduced from , but a large factor in the strength of the bonding between the anchor and the rock is the cleanliness of the drilled hole before the adhesive is added. Once the hole is drilled, it must be blown and brushed to ensure all loose dust and rock is removed for good contact.
Figure 6
Present Weaknesses in Climbing Anchors
Modern climbing anchors are prone to failure when there is a presence of Stress Corrosion Cracking (SCC). This has been recently confirmed as the cause of a number of recent bolt and climbing anchor failures. Some high profile incidents have begun to bring awareness of bolt corrosion to the rock-climbing community [12].
Stress Corrosion Cracking
The UIAA published a report in December of 2015 elaborately detailing the unignorable high number of SCC issues in climbing anchors being experienced around the world, and declaring their own proposals for resolving these problems. The following information details key findings from that report, and ideas on what it implies for climbing anchor innovation. SCC is probable to occur at various, non-predictable times after installation – in some cases it has been a few months, and in other cases it can be years down the road. The failures are experienced in loads as low as solely body weight in stainless steel anchors, even with 316 grade stainless steel [12].
It is hard to evaluate the risk of SCC due to a lack of visibility – SCC degradation is often not visible. SCC is also not easy to predict, because it depends on a complex set of different factors such as: low humidity, high acidity and temperature, magnesium rich rock, and unwashed rock due to low rainfall. SCC degradation can be seen with even the smallest differences in microclimate while other bolts on the same climb remain unaffected. SCC is most typically associated with seaside climbing, but it is also seen to occur inland, either deposited there by sea breezes or occuring in the rock itself. The following tables detail the environmental and anchor characteristics that lead to SCC [12].
Figure 7: Table outlining the Environmental Characteristics of Stress Corrosion Cracking [12]
Figure 8: Table outlining the Anchor Characteristics of Stress Corrosion Cracking [12]
The UIAA has recently put out alerts informing climbers that bolts in these areas may last significantly less than the expected 20 year lifetime. The UIAA standard states a minimum of 22 KN of force for fixed anchors, but studies have shown that in marine environments up to 20 percent would fail with a force of just 5 KN [12].
In response to these failures, the UIAA has worked on coming up with some solutions for climbers, anchor manufacturers, and climbing organizations alike. To begin with, for areas where stress corrosion cracking is common, the UIAA recommends using only grade 2 Titanium for the bolts. Even for outdoor locations where SCC has never been documented, the organization suggests using a 316L grade or better stainless steel alloy to aid in ensuring that the climbing anchors will not fail, especially at the bolt. Additionally, they suggest utilizing a calibrated torque wrench to fasten the nut for the purpose of avoiding plastic deformation of material of the climbing anchor and to keep “axial†stress at reasonable levels. These materials suggestions alongside scheduled inspections of bolts and climbing equipment are the most up-to-date recommendations provided by the International Climbing and Mountaineering Federation. This relatively small number of courses of action suggests a great deal of room for improvement of the safety and reliability of climbing anchors, and most especially for the bolts involved in climbing anchors [12].
One of the most important points brought to light by the UIAA report is that more or less the number one factor prohibiting the use of strictly SCC and corrosion resistant anchors is the cost and corresponding availability. Many bolts, when put into place, remain there for years on end, so individual bolters are reluctant to spend their own money on bolts that last years down the road just so climbers that did not pay for them might be safer in the future. Therefore, a resultant economic necessity might be that more of a majority of the climbing population needs to start paying for permanent anchors. However, on the other hand, this issue naturally lends itself to a materials solution, one that could utilize a both safer and cheaper composite of materials in order to ensure security while simultaneously not being unreasonably expensive such that those anchors produced of this new composite might never be able to be brought to the competitive financial market [12].
According to the 2015 report, the UIAA plans to do testing for SCC by taking climbing anchors to extreme environments such as coastal Thailand and doing tests on them there in order to develop a standard for the corrosion resistance of bolts. These extreme conditions, as well as various other time-accelerated tests, will be utilized to determine whether given anchors are capable of a fifty year or more SCC-free lifetime, which appears to be the desired target for how long climbing anchors should last [12].
Galvanic Corrosion
In addition to SCC, galvanic corrosion is cited by the UIAA as the second leading cause of bolt failure. Galvanic corrosion is caused by two different metals in contact, such as a stainless steel hanger on a carbon-steel bolt. A difference in electric potentials causes a small current to flow to between the metals to create equilibrium. A dissolved electrolyte in a either rain or from monist marine environments as described above. Over time, this corrosion can also weaken the physical and mechanical properties of the anchor that climbers rely on for their safety [7].
Design Proposal
Potential Materials
Since Stress Corrosion Cracking and Galvanic Corrosion are two of the biggest problems plaguing climbing anchors currently, one of the largest considerations for remedying these problems is determining a corrosion-resistant material or combination of materials that can withstand the outdoor elements that ultimately are responsible for these problems. As such, it is necessary to do a survey and analysis of potential materials to replace the consistently problematic and cheap stainless steel that has posed such a great threat to the climbing community. The ultimate goals of our product are safety, reliability, and predictability of performance of our product for the climber utilizing it. These goals are achieved through the various following components.
One of the more or less “obvious†selections for a “better†alternative material is Titanium, and more precisely, titanium alloys. There most common of these alloys is Ti6-4, which would be used most likely in its form which contains either solely β or both α and β phases such that its properties of strong corrosion resistance might be harnessed. However, perhaps the better-fitting titanium alloy for the job is Ti 3 Al 2.5, also known as a grade 12 titanium alloy. This alloy is superb because it has incredibly high corrosion resistance, while also exhibiting many of the characteristics of an aforementioned common climbing anchor metal: stainless steel. This alloy also has the best weldability among all titanium alloys, meaning that the anchors created from it could be very precisely fashioned to fit the environments in which they would be utilized, all while being stronger and safer than the standard stainless steel [13]. In conjunction with the selection of this grade 12 titanium alloy, to further prevent corrosion and possibility of failure, we would coat our product with an organic coating, more specifically a paint. This paint will provide a polymeric film barrier that protects the surface and structural integrity of the titanium alloy underneath, all while using its pigments to infuse corrosion inhibitive chemicals at the titanium-alloy/coating interface. The corrosion-resistant titanium alloy working in tandem with the polymeric coating will go above and beyond in ensuring a reliable and safe climbing anchor for climbers over many years of use, and will be created to live up to the UIAA’s desired 50+ years SCC-free lifespan [14].
The Ashby plot below shows the density of Ti 6-4 and Ti Grade 12 in comparison to commercial titanium, a variety of stainless steels, galvanized steel, and wrought iron with respect to Young’s modulus. Titanium is nearly half as dense as steel, and also has half the Young’s modulus.
Figure 9: Ashby Plot evaluating materials based off of Density and Young’s Modulus
In consideration of an adhesive material to better prevent the SCC, focus should be set on a modified epoxy acrylate. Due to its lack of pull out resistance, meaning its capability to resist extrusion from the rock, it is not a full substitute to previous used epoxys on glue-in bolts [11]. However, the adhesive itself has a very low shrinkage rate meaning it will prevent failures in the large diameter holes. Also, this material is very water resistant, meaning it is good for damp areas, as well as areas that are already known for causing failures by Stress corrosion [11].
Piezoelectric
In addition to reducing corrosion of climbing anchors, piezoelectrics could help climbers know when corrosion has occurred. Piezoelectric materials, such as piezoelectric ceramics and polymers, are able to convert mechanical energy into electrical energy. This presents an interesting opportunity to use the mechanical energy present in the climbing bolt when it is in use to power a sensor that could allow rock climber and climbing supervisors to learn about the state of their course. A piezoelectric element embedded in the bolt could provide information about the bolt under load and a piezoelectric element embedded in the nut could help understand if the bolt is loosening. By measuring the impedance of piezoelectric immediately after installation and using it as a baseline, the structural health and integrity of the bolt can be examined over time by making continuous measurements[15]. Piezoelectrics have begun to be experimentally used in other applications where monitoring structural health is critically important including bridges [16] and underground rock roadways [17] and show a promising potential in their application to knowing when the structural integrity of a climbing bolt has been compromised. Further, the combination of piezoelectrics with passive RFID tags allows for the remote inspection of the structural health of the climbing bolts. In the future, aerial or perhaps even autonomous systems could automate the inspection process [18].
Conclusion
Even though climbing anchors have been in use since around 300 B.C.E, there is still a long way for them to come in terms of safety and reliability. A combination of a stronger, more corrosion-resistant titanium alloy with a coating, a modified epoxy acrylic adhesive to secure the anchor, and piezoelectric technology to monitor the health of climbing anchors out in the field all make great leaps towards the idealist goal of completely safe and trustworthy climbing equipment. Post-prototyping, the greatest challenge for the group will be having this product received by the wide scale climbing community, such that this level of safety becomes an industry standard and expectation.
References
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