Hydrogen embrittlement occurs as a result of exposure to hydrogen, for example, in the manufacture or processing of high-strength steels and titanium and aluminium alloys. It reduces the ductility and load-bearing capacity of affected materials, which can cause cracking or failure at below normal yield stresses.
Hydrogen embrittlement takes place when recently-produced, nascent hydrogen atoms diffuse into the metal. When load is applied, the hydrogen atoms migrate towards areas of high stress, and the resulting pressure causes sub-microscopic cracking. These cracks themselves become areas of high stress, and so the process continues and the cracks spread through the material, which can ultimately result in failure.
In November 2014, two large bolts fractured at the 47-storey, 737ft Leadenhall Building, popularly known as ‘The Cheesegrater’ because of its distinctive slanting profile, created to respect a protected sight line of St Paul’s Cathedral. A small part of one of the bolts fell from the fifth floor to the ground, but the area was not publicly accessible due to ongoing construction works and nobody was injured.
A third bolt then fractured but was captured by tethering which had been installed following the first two failures.
Investigations undertaken by contractor Laing O’Rourke and structural engineers Arup confirmed that the problem was limited to certain bolts. Tests concluded that the bolts had failed due to hydrogen embrittlement. Arup also confirmed that there was no adverse effect on the structural integrity of the building. However, a programme was undertaken to replace a number of bolts a precautionary measure.
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