Abstract

This project explores the feasibility of creating a lightweight carabiner using composite materials. A set of detailed requirements for a carabiner have been defined based on international safety standards, geometrical criteria and environmental resistance criteria. Material and manufacturing options have been studied and related to the carabiner application. Toughness, damage tolerance, damage detection and wear were determined to be problematic aspects of composite materials for lightweight carabiner design. Literature research was used to find methods of improving composite toughness. Two design options were proposed and one of these designs was described in detail. The proposed design makes use of injection moulded, short carbon fibre/PEEK composite, specifically, Victrex 90HMF40. A CAD model and a rapid prototyped model of the proposed design were created. A carabiner made from composite materials is likely to be up to 40% lighter than a conventional aluminium carabiner. Producing composite carabiners is likely to be more expensive than aluminium carabiner production - particularly if hand lay-up is used. Limited testing and finite element analysis were carried out as part of the early stages of design optimisation. Further analysis and development is required before carabiner production can begin.

Introduction

Almost all carabiners currently used for rock climbing are made from aluminium alloy. For situations where weight is not an important factor, such as fixed anchors, steel carabiners are occasionally used for their improved wear resistance and greater tensile strength.

Aluminium alloys have successfully been replaced by composites in aerospace, cycling and marine products with wide ranging benefits [4]. In all of these sectors there is a similar drive to reduce weight, and by moving to composites a significant weight reduction has resulted in many proven applications. In part, the aim of this project is to determine if a carabiner made from composite materials is likely to be significantly lighter than current aluminium alloy carabiners - without sacrificing safety. In addition, the performance of available materials and manufacturing processes will be investigated to determine the feasibility of creating a composite carabiner. Finally, a number of design options will be suggested and one will be examined in more detail. First, the context of the project will be discussed in terms of what carabiners are currently available and how they are used - this will help establish a detailed set of requirements.

In rock climbing, carabiners are used to connect a climber's safety rope to anchors. In the event of a fall the climber will be caught by the rope as it becomes taught and pulls against the anchor. Carabiners often come in pairs referred to as quickdraws - a quickdraw is two carabiners attached together via a nylon sling. One end of the quickdraw is clipped into the anchor; the other is clipped into the rope (figure 2).

rock climbing quickdraw usagerock climbing carabiner usage

Figure 2: A climber uses carabiners to protect himself in the event of a fall. Reproduced from [5].

The requirements of a carabiner can be put into three general categories: loading, environmental and geometrical. Minimum load requirements for climbing connectors have been established by international and European standards [6, 7]. For current aluminium alloy based carabiners the static tests described in the EN and UIAA standards have proven to be sufficient to judge their safety. However, composites have low impact strength relative to aluminium alloys and composite failure mechanisms can be highly rate dependant [8, 9]. In this case, a static test is likely to be insufficient to determine the ability of a composite carabiner to withstand the sudden impact of a climbing fall. A more appropriate test might involve reproducing a 'worst case scenario' fall under controlled conditions. This kind of test will approximate the force history and the force distributions (of the rope-carabiner and carabiner-anchor interfaces) of a real fall.

During normal use a carabiner is likely to be dropped onto hard surfaces, aluminium alloy carabiners often sustain minor surface indentations and scratches from these impacts and continue to hold falls without failure. The drop height could vary from less than 1 m to hundreds of metres, although a carabiner is unlikely to be reused after a drop of that distance. A composite carabiner might incur defects or delamination from these kinds of impacts. This internal damage would not necessarily be visible [10] to the naked eye and other forms of non-destructive testing (NDT), such as ultrasound, would be required.

en standard for connectors

Figure 3: UIAA standards for the minimum strength of a carabiner, the relevant rows for a standard carabiner are highlighted [7].

Environmental considerations are important because moisture adsorption and temperature variation can have significant effects on the strength of composites [8]. Carabiners can be exposed to harsh temperature ranges; from -40°C at the summit of Everest to +80°C or higher due to frictional heating whilst abseiling. In addition, it is pertinent to know the effects of a change in environmental pH, UV light and general chemical resistance properties of the carabiner material.

There are geometrical and other design requirements on a carabiner due to the existing framework of interactions between items of rock climbing protective equipment. Some of these are set out in the EU and UIAA standards (figure 4).

en standard for connectorsen standard for connectors

Figure 4: An extract of the design requirements set out in UIAA standards [7].

The EN standard [6] defines in more detail the required clearances and other geometrical properties that enable carabiners to function correctly when used with climbing ropes; some aspects are not defined strictly and instead come as recommendations. For example it is recommended that carabiners are designed such that when loaded, most of the force is taken by the spine (figure 5). Carabiners are inevitably weaker on the gated side due to stress concentrations where the gate connects to the body; biasing the load towards the spine reduces the stress on the weaker side. In addition it means that there is less bending - that is, the spine is essentially held in pure tension, thus failure occurs close to its pure tensile strength (this discussed further in the FEA and testing section later).

en standard for connectors

Figure 5: The EN standard for carabiners recommends a design that relocates the rope to reduce bending and transmits most of the force through the spine [6].

There is also no strict standard for the shape of the surface that the rope slides over, this is an important characteristic - if the radius of curvature is too small then there is a danger of cutting the rope. The UIAA standard recommends the radius of curvature is at least 4.5 mm with a contact angle of not less than 120°. Despite these recommendations the standards are not exhaustive. Carabiners must also be compatible with anchors and belay devices [11] and the standards do not include guidance for this. Generally most carabiners are compatible with belay devices and anchors, exceptions include some carabiners that have a large cross sectional diameter and do not fit through the hole in some pitons [12]. Another aspect of the carabiner is that it must be easily usable and easily manipulated by hand so that a climber can clip his rope whilst in a demanding situation. Table 1 contains a summary of the design considerations mentioned in the text above.

Table 1: A summary of important factors affecting carabiner design.

Loading

Environmental

Geometrical

Major axis

Minor axis

Open gate

Impact

Residual strength

Damage detection

Defect tolerance

Fatigue

Wear

Temperature range

Friction heating

Moisture

Chemical resistance

Corrosion and degradation

UV light resistance

Rope contact surface

Rope diameter

Open gate clearance

Rope relocation

Anchor constraints

Knot compatible

Double rope compatible

Rope clipping

Human handling

Existing products

It is important to consider existing carabiners in order to determine a reasonable goal for the weight of a composite carabiner. Carabiner manufacturers have employed a combination of methods to reduce the weight of their products, tweaking alloying ingredients, re-designing to remove material where it is not needed and simply reducing the overall size. In 1995 Black Diamond popularised the wiregate carabiner with their Hotwire [13], previously carabiner gates were made from the same material as the main body, in a wiregate carabiner the aluminium gate is replaced with a smaller diameter steel 'wire' (figure 6). This gave a weight saving of around 6g compared to previous carabiners.

carabinercarabiner

Figure 6: Comparing a solidgate to a wiregate carabiner, using a wiregate reduces weight by around 6g. Images from [14].

The wiregate was initially met with some scepticism; however it has since become the norm for top-end carabiners. Furthermore, wiregates appear to be superior in two unexpected ways - when climbing in icy conditions a wiregate is less likely to become frozen closed. Also, in rare circumstances during a fall it is possible that a solidgate carabiner could knock itself open if the spine collides forcefully against the rock surface. This would momentarily reduce the strength to that of the open-gate rating - which is usually less than a third of the closed-gate rating. However, in practice carabiner failure is very rare and no thorough tests have been completed to determine if this phenomenon has been responsible for carabiner failures in the field.

Since the inception of the wiregate, the next major weight reduction came from using I-beam designs. Several companies have produced smaller carabiners as a weight reduction method; but this can have a negative affect on usability. The EN standard for carabiners defines a minimum gate opening distance (15 mm), however this criterion alone is not sufficient to judge how the size of a carabiner affects its usability, particularly as most carabiners comfortably exceed this value and yet still have varying degrees of usability. Table 2 and figure 7 show the lightest carabiners currently available. The carabiners are divided into two categories; 'full size' and 'reduced size'. It is difficult to strictly define criteria to make this distinction - it should be noted that carabiners in each category are by no means all of identical size, for example the Black Diamond Oz is significantly smaller than the DMM Phantom. The lightest full size carabiners make use of I-beam designs.

Table 2: Comparing various lightweight carabiners [15, 16].

Reduced Size

Weight g

Full size

Weight g

DMM Phantom

26

Wild Country Helium

33

Wild Country Xenon Lite

29

Wild Country Xenon

36

Camp Nano

23

DMM Spectre

33

Black Diamond Oz

30

 

 

 

carabinercarabinercarabinercarabinercarabiner

Figure 7: A selection of the lightest carabiners currently available. Left to right: DMM Phantom, Wild Country Xenon Lite, Camp Nano , Black Diamond Oz, DMM Spectre [15, 16].

Design criteria

As a legal requirement, all carabiners intended for climbing and sold in the EU must adhere to the EN standard, furthermore, these standards are generally considered a minimum and most carabiners exceed them. In order to gain general acceptance amongst climbers it is very important to pass these standards. Detailed design criteria are defined below, these are divided into two categories - strict requirements and design guidelines. The requirements consist of quantifiable values for load ratings and dimensions (based on the standards), and any other quantifiable properties (such as operating temperatures). The design guidelines are harder to quantify, more like design advice, which is still very important (e.g. does the carabiner tend to relocate the rope to transmit most of the force through the spine?). For conciseness the requirements which are based on the standards are defined only roughly here, these are marked with an asterisk*, for stricter definitions of these requirements please see EN 12275 [6]. The final part of this section describes additional testing carried out by Black Diamond that goes beyond the requirements of the standards.

Requirements

For bolt, piton and belay device compatibility the cross section of the carabiner at any point must fit inside a 13 mm diameter circle.

The gate opening clearance should be at least 15 mm when the gate is fully open*.

The gate should be spring loaded so that it closes automatically and requires a force of at least 5N to open*.

The gate should not be obstructed when two 11 mm ropes are clipped inside the carabiner and are resting on the short or long arm*.

When force of 800 N is applied in longitudinal tension, the carabiner must not plastically deform such that it can no longer be opened by hand and the gate must remain operational (able to open by hand) whilst the force is applied*.

The carabiner must not break when a force of 20 kN is applied longitudinally.*

The carabiner must not break when a force of 7 kN is applied transversely.*

The carabiner must not break when a force of 7 kN is applied longitudinally with the gate open.*

All of the above requirements must be satisfied in operating temperatures from -40 to +60°C.

Design guidelines

It is expected that a composite carabiner will cost more than one made from aluminium, nevertheless, the carabiner should be economically viable to manufacture and sell.

The carabiner material should be sufficiently tough to withstand the impact of a climbing fall.

The carabiner material should have sufficiently low friction against a sliding rope that frictional heating does not bring the carabiner out of its operating temperature range.

The carabiner material should not adsorb excessive moisture such that is adversely affects its operation or strength.

The carabiner should be resistant to salt water, mild acids and alkalis, UV radiation, and have good resistance to chemical corrosion and degradation.

There should be sufficient space to tie a clove hitch [18], using an 11 mm rope, on the long arm of the carabiner without restricting the gate movement.

The arms of the carabiner should be at an acute angle to the spine such that the rope tends to slide towards the spine, reducing bending.*

The radius of curvature of the surface that the rope slides over is recommended to be at least 4.5 mm over an angle of 120°.*

The hook or other gate binding mechanism on the inside of the nose of the carabiner should not unduly snag on the rope when it is clipped or unclipped.

The carabiner should be easy to manipulate with one hand, have a smooth rope-clipping action and be easy to clip with either the left or right hand (figure 9).

carabiner clippingcarabiner clipping

Figure 9: It is important that a carabiner can be clipped easily with either hand; the ease of clipping is largely determined by the length and angle of the long arm, the stiffness of the gate spring and shape of the gate.

Additional Testing

Although standards have been developed to ensure the safety of carabiners, these are generally considered to be minimum requirements. For example Black diamond, a major outdoor equipment manufacturer, carries out testing well beyond the requirements of the standards during carabiner development as they feel that additional qualification is required. These tests are relevant as an example of the diligence required when producing safety equipment, they are particularly significant for this project because they include tests that closely emulate real climbing scenarios. Composites generally have poor impact resistance - this means that these 'real life' tests are very appropriate because they occur at much greater loading rates compared to the static tests in the standards. Black Diamond carry out drop testing (figure 10) and cyclic testing in various scenarios - the specific details of these tests are withheld at their request.

drop tower testing

Figure 10: An example of a drop tower used to test climbing equipment. Reproduced from [19].

Next: Materials