Thursday, 8 October 2009

Old Carabiners + Tapes - Breaking Strengths

Sports climbing carabiners get a hard life.

They are constantly subjected to short hard falls, clipped into sharp or badly aligned bolt hangers and ground against the rock. The tapes also get a hard life as they get grabbed with chalky hands and suffer harsh abrasion, especially as the run over the carabiners.

DMM recently received a couple of old Mamba quickdraws back that were over 10 years old and which the owner wanted tensile testing.

The quickdraws looked in reasonable condition - there was no bad visible fraying or fading on the quickdraw tape, although the ends had some mild fluffing where the tape had been caught between the rock and the carabiner. The carabiners functioned well and with a little lubrication would have been perfect. The metal was free from corrosion, although there were some sharp bolt marks on on of the straight gate carabiners.

The first Mamba we loaded up included the straight gate carabiner with 3 bolt scars on the inside apex. The tensile test started - at about 8kN it was obvious that something was going to pop early because you could hear the tape fibres starting to snap. This does not normally happen to around 18-20kN normally. The real surprise was when the straight gate carabiner suddenly blew out at just 13.6kN - almost half of its rated strength.

On examination it was obvious that the one of the bolt scars had created a stress multiplier that had then opened out the scar to create a full blown fracture.

The second Mamba blew out the tape at 15.5kN - about 60% of its strength when new. Once again it was obvious it was going to break early because the micro damage to the fibres could be heard starting around 10kN.

In a real world situation both of these Mambas would still have done their job - but they would be far less tolerant of stupidity or misuse i.e. connecting a short sling to the Mamba to hold you in position why you work a move and then slipping/jolting back onto it - this creates massive forces that can break new carabiners let alone half strength ones.

The slings blew early because they were old and some minor damage accelerated quickly to cause a complete failure. The carabiner blowing out early was surprising and probably worthy of further experiments because although the bolt scars were noticeable they did not seem excessive.

There is more information on climbing equipment at Rope Rescue Techniques below the main text and informatio on choosing carabiners on the Rock Climbing Outside pages - again below the main text.

No comments: