Moderator: Tim White
ek wrote:Don't canyoneers drop aluminum 'biners long distances onto rock frequently as part of some pull-down techniques, and then reuse them?
ek wrote:I've been taught that if you drop an aluminum (alloy) carabiner down a pit, and it falls a significant distance, hitting a rock floor, that you should retire it. That it can sustain invisible damage this way, and potentially fail later when it's in use. But is that really true, or is it a myth?
Don't canyoneers drop aluminum 'biners long distances onto rock frequently as part of some pull-down techniques, and then reuse them? They seem to have no trail of dead bodies. And yet I've known climbers and cavers who will retire a carabiner that was dropped 20'.
Do we really have to retire our carabiners after dropping them...or can we just inspect them, make sure they're not cracked and that the gate still works, and continue using them?
Marduke wrote:The problem isn't that a dropped biner forms a crack then has a lower residual strength when tested immediately, but rather that a significant drop could produce crack initiation. If the biner is then put back in service where it is cyclically loaded, the initial damage will propagate in what is called fatigue crack growth. Each load cycle grows the crack and lowers the residual strength. Eventually the crack will reach a critical size in which the biner will fail, most likely via ductile tearing.
Exposure to mineral rich aqueous environments (cave mud and sweat) will accelerate crack growth.
I have seen a croll which failed in exactly this manner, starting from a non-visible (in service) corrosion pit at the edge of the rivet hole. The owner never knew of a problem until his foot ascender gave way and broke into two pieces. The final step was a ductile failure. You could see the beach marks on the failure surface from every use session leading up to the final failure. Every time he put his gear away in a humid bag filled with sweaty gear, an oxide layer would form making marker bands.
Keep in mind that when unloaded, any cracks are most likely closed and not visible. They will open under load, when it is difficult to inspect (especially without actual inspection equipment or training).
For what it's worth, I am a Damage Tolerance Engineer for NASA.
ek wrote:The question that comes to my mind is, could a drop significant enough to produce crack initiation nonetheless not produce visible damage?
Stridergdm wrote:I think the key though is the initial paragraph. No one doubts how it can occur. I think it goes back to as you say, "...significant drop could produce crack initiation." The question is how likely is that could. No one seems to have solid numbers on that. Is it 50% of the time? 5%? .05%? etc.
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