Overview of carabiner strength study
I became a climber back in engineering college. As I learned “‘the ropes” I began to realize (as I think all beginner climbers do) that our sport has quite a few unanswered questions regarding the technical limitations of gear; questions like,
How much does rope wear reduce the strength of a carabiner?
How strong are old slings and webbings in comparison to new ones?
How does the load printed on the spine of a carabiner actually relate to the breaking strength of that carabiner?
With a little research and testing I was able to answer these questions! In turn those answers gave me a renewed faith and raised caution for the gear in which I was trusting my life. The following is the first of a 3-part series recounting the scientific process I went through in conducting my tests.
Background research on climbing gear testing
Before testing I conducted a bit of research regarding how gear testing is currently performed in the industry. The goal of this research was to get a better grasp of how to adapt my university’s wealth of resources to conduct tests on campus. I asked these questions:
What percentage of the total number of carabiners manufactured does a company test?
Information regarding the exact percentage is not published; however, the number is small and always shrinking. Currently in the carabiner manufacturing industry, companies strive to test as few carabiners as possible . The idea behind this is that as a company grows along with increased demand for their product, it becomes more expensive to expand their facility to test the same percentage of gear they did before the company growth.
To successfully produce a safe product given this required decrease in testing, companies like Black Diamond test samples of the gear they sell with increasingly faster and smarter methods. Their testing method follows BS EN 362:2004, which specifies the requirements and test methods of all gear pieces that aid in fall protection .
What methods do climbing gear companies use to test climbing gear?
Climbing gear tests range in cost and setup based on the type of load condition a piece of gear might experience during use in the field. For carabiners, load conditions important to consider are crosswise, along the axis with closed gate, and along the axis with open gate.
These three load conditions are tested in a simple Single Pull To Failure test (SPTF)  using an Instron Tensile Test Machine, which is basically a 8-foot tall mechanically driven piston that powerfully (and very accurately) controls the slow speed with which it pulls on a fixed object. The resultant strengths from three of these tests are printed on the side of the carabiner’s spine in units of kilonewtons (kN). One kilonewton is equivalent to approximately 225 pounds. A carabiner is strongest in tension loaded along the spine with the gate closed.
To test a carabiner to failure, a carabiner is taken at random from the manufacturing line and placed in a special fixture. The fixture is clamped into place in an Instron Test Machine and pulled at a fixed rate until the carabiner breaks. The maximum load seen by the carabiner before initial failure is factored and a reduced load printed on the spine of the carabiner. It should be noted that every carabiner produced by companies like Black Diamond goes through an automated inline proof test to one half of the strength of the closed gate rating to ensure product quality before exiting the factory .
For testing textile gear, namely ropes and slings, a slightly different method is used than the Instron on carabiners. While many tests are conducted, one such test specified by UIAA involves shock loading rope by securing one end in a fixture, running the rope through a hole (see Figure 2) to simulate the shape of a carabiner and attaching the other end to a free falling weight.
The number of factor 1.76 falls that the rope can withstand before failure is then published and characteristic to that rope. Generally a thicker rope withstands more falls than a thinner one, and the radius of curvature of the hole that the rope falls over has a significant effect on the number of falls until sheath failure.
The setup for securing and testing ropes for this test (known as the UIAA Drop Test) requires quite a large, expensive setup in order to simulate the dynamic loading of a rope supporting a falling climber . Therefore, I had to leave this kind of testing out of my study.
From the beginning of this independent study it was understood that test procedure would be limited to loading specimens to failure with the Instron; thus, any literature review on Drop Testing and Inline Proof Testing was done simply for the sake of an advance in personal knowledge.
Up next in Part 2 and 3: test procedure, results, and conclusions drawn from testing aluminum carabiners!