Sunday, 25 October 2009
The carabiner to the left was sent into DMM for testing - I am not quite sure of its history, but the size of the groove that had been worn into the top bar is pretty impressive. DMM see bolt damage to carabiners fairly regularly, but this is exceptional.
The groove had eaten through 27% of the thickness of the bar and the volume of metal that had disappered was even greater.
I fully expected this carabiner to break under 10kN, but it actually acheived 26kN - greater than its rating when new.
It is difficult to explain this especially considering that a much smaller nick in a similar carabiner (see Mamba post below) caused a serious loss in strength. I don't want this post to encourage people to use carabiners in such a bad state, but rather use it as a pointer that there are a lot of variables involved in how and when kit breaks - some obvious and some that obviouslly are not.
DMM customers have asked in the past "how much wear and tear before I should reire equipment" - hopefully the last two posts show that this is a difficult question to answer.
It should be stressed that this carabiner should have been retired a long time before it reached this state.
The second image shows a 30 year old HMS from the days when DMM was called Clog and employed about 10 people. It had a rope groove that had eaten away 6% of the thickness of the bar. It made 24KN.