Should You Retire a Dropped Carabiner?

Engineering students play Mythbusters with dropped carabiners



I have often heard other climbers and even teachers at outdoor programs say something like this:



Dropping a carabiner onto a hard surface can cause it to “microfracture” - weakening it so much as to make it unsafe. If you drop a carabiner, even from your waist and only once, you should retire it.



Microfracture is indeed a real thing, but you might need a PhD to understand it. A search for metal microfracture on Google returns scientific journal papers on metals and composite materials, as well as a SuperTopo thread with a debate on whether or not microfracture is real. (Have trouble sleeping at night? Simulating Micro-Fracture in Metal-Matrix Composites, a NASA technical report, should put you to sleep quickly).



A microfracture is a fracture invisible to the naked eye. The fracture could cause a stress concentration and propagate under load, reducing the strength of the material. Metal, or any material for that matter, is imperfect by nature and has defects in its crystal structure, including small cracks [1]. The practical question is not whether an aluminum carabiner microfractures when it sustains an impact, but if that impact actually makes a difference in its strength.



In 2007, when I was a mechanical engineering student at the University of Colorado, friends and fellow climbers Ross Callison, Justin O’Brien, Paul Ginzburg, and I decided to investigate that question. We dropped 30 carabiners from heights of 21, 40, and 109 feet onto concrete, filmed the impact on high-speed video, and tested their ultimate strength.



The results? There was no difference in breaking strength between brand new carabiners and ones that had been dropped, even from 110 feet. While this should not be treated as a license to recklessly abuse your gear, it seems like you shouldn’t worry about a dropping a carabiner from your waist. Read on to learn more about our testing.



[Editor's note: Strength testing was done using Instron instrument pictured below. See videos of carabiner drops and strength testing from the study at the end of this article.]



Background



Carabiners are made from 7075-T6 Aluminum, which has the following mechanical properties [2]:

Elastic Modulus: 71.7 GPa

Yield Strength: 462 MPa

Ultimate Strength: 524 MPa

Traditionally, carabiners were made by a process called cold forging. In cold forging, a metal is plastically deformed [irreversibly changed in shape by force, Ed.] below its recrystallization temperature [temperature at which internal crystalline structures begins to change form, Ed.] and undergoes strain hardening, making it harder and stronger but reducing ductility [ability to be altered by strain without fracturing, Ed.]. Many modern carabiners are hot forged. In hot forging, a metal is plastically deformed above its recrystallization temperature so that it does not strain harden [3]. Because the ductility of the metal is preserved, hot forging allows for more complex carabiner shapes.





Test Setup



We chose high-rise dorms as our testing site. Our drop heights were determined by who would let us into their room to drop something out of the window – residents of the 3rd, 5th, and 13th stories turned out to be the most amenable. (We did actually get permission from the University to do this, by the way).



Our drop heights were 21, 40, and 109 feet, respectively. The impact surface was 6-inch thick concrete, with a compressive strength of 3500 psi. We tested two types of carabiners: cold forged and hot forged versions of the same model. All carabiners were brand new. We dropped five of each type from each height. We dropped each carabiner only once. We tied a streamer on the end of each carabiner to control the location of the impact – it always landed on the rounded end on the side where the gate opens. We tested 40 total carabiners – five hot forged and cold forged from each height, plus control samples that had not been dropped. We filmed the impact with an Olympus iSpeed High Speed Video Camera at 1500 frames per second.



The carabiners were pulled to failure in an Instron tensile test machine. We followed the UIAA 121/EN 12275 carabiner testing standards and pulled them apart with 12 mm diameter pins [4]. We used a load rate of 50 mm/minute. The upper Instron fixture was always started at the same height. The carabiner was always loaded upright with the gate facing left.



Test site:









Impact Surface:











Streamer with impact location:









Instron Setup:











Results and Discussion



The average breaking strength plus/minus one standard deviation is shown in the table below. There was no meaningful difference between the control sample and the ones that were dropped.









The breaking strength data came from the force-elongation curve recorded in LabView and exported to Excel. Two sample force-elongation curves are shown below. You can see where the gate engages with the carabiner hook, where the linear elastic region ends and plastic deformation begins, and where failure occurs. Plastic deformation is permanent. If the load is released before plastic deformation occurs, the carabiner will return to its original length.









Thirty-seven of forty samples failed at the hook and three failed at the pin. The failure location is a perfect example of a concomitant experimental variable—one that is measured but uncontrolled. Since one of the control samples failed at the pin and the data do not show that breaking strength was affected by drop height, it seems that the failure location is a concomitant variable and not an indication that the impact weakened the carabiner. The manufacturer confirmed that this variation is normal even in testing brand new carabiners and is not associated with the impact location.



Failure at hook:









Failure at pin:











After being dropped, the spring at the base of the gate on some of the carabiners popped out of place, making the gate non-functional. Specifically, this happened on three of the five hot forged samples dropped from 40 feet, and on four of the five hot forged samples dropped from 109 feet. This was not observed on any of the cold forged carabiners. We were able to push the gate closed on these carabiners before testing them.



High-Speed Film Observations



Due to the camera only being able to record in a very small frame window, not every drop was caught on camera. We saved 15 clips and, from some of them, estimated the carabiner’s velocity just before impact by playing the video back frame by frame. The carabiner did not fall in exactly the same spot each time, meaning it was to hard compare it to the ruler in the background. Additionally, there was some wind at the testing site, so these are very rough estimates. The speeds are shown in the table below.









The theoretical impact velocity (assuming no air resistance), can be calculated from this equation:



v= 2gh,



where v is impact velocity, g = acceleration due to gravity, and h = drop height [5]. The carabiners should fall a little slower with air resistance, which matches up with the drops from 40 and 109 feet. There is clearly some error in our velocity estimates from 21 feet.



Filming the drops revealed a fascinating insight: the carabiner gate opened on impact. The strength of carabiners with the gate open ranges from 7 - 10 kN – forces that you could potentially generate in a severe lead fall. While most lead falls are in the 2 - 5 kN range [6], that doesn’t leave much margin for safety. Obviously, it’s best if your carabiner gate stays closed. We did not film any wire gate carabiners, but presumably the lighter gate is less likely to open in a fall. It would be interesting to film it and see if that is in fact true. It is scary to think about taking a lead fall and having a carabiner on your protection or rope impact the rock, causing the gate to open. Searching the "Accidents in North American Mountaineering" journals brings up a handful of open-gate failure cases [7, 8], but it’s hard to determine if they are due to impact alone or to the rock wedging the gate open.



Limitations and Conclusion



Any test has limitations, and it is important to be aware of them. First, our data says nothing about repeated impacts. We dropped each carabiner only once. In a very unscientific test in the lab, we dropped various heavy objects onto one carabiner until it was visibly deformed. It broke at about 11 kN, which is too weak for climbing use. This shows that you can actually damage carabiners. Never climb on a visibly deformed carabiner!



We did not test heights above 109 feet. The results of this test cannot necessarily be extended to locking, wire gate, or steel carabiners, as differences in geometry and material could produce different results. We also did not test other metal equipment like cams and stoppers. Some rock types, such as granite, have a higher compressive strength than the 3500 psi concrete that we used, which could result in greater impact forces.



Though a sample size of five is better than a sample size of one, a larger sample would provide more certainty that the results were not random. Our hot forged sample data closely matched the manufacturer’s average, but our cold forged sample data was about 3.5 kN higher than their average. This was possibly due to a calibration error with the tensile testing machine when we were loading the samples. However, the results still seem relevant – one drop did not change the ultimate strength of the carabiners.



Climbing is inherently dangerous, and there is no reason to add more risk by using unsafe equipment. Based on our testing, it seems like you shouldn’t worry about a dropping a carabiner from your waist, as I was once told. However, if you are ever in doubt about the integrity of a carabiner, and especially if there is visible damage, just retire it.



Impact Videos



Calibratation Drop:











Carabiner Drop 7:











Carabiner Drop 8:











Carabiner Drop 11:











Carabiner Drop 12:











Carabiner Drop 13:











Testing Videos





Failure 2 - Hook











Failure 4 - Pin:











Failure 7 - Hook:











Failure 11 - Hook:











Failure 16 - Hook:











References



Jim Margolis has worked as a field instructor at NOLS since 2010 in the rock climbing, mountaineering, winter, and backpacking programs. Prior to NOLS, he was an Outward Bound instructor in Montana and Colorado. He graduated from the University of Colorado with a masters in mechanical engineering in May 2008. He is known for eating ridiculous amounts of food and imitating comedian Mitch Hedberg.





The opinions of contributing writers may not be the the same as the opinions of OSI.