The problem was resolved by increasing the tightening torque
applied to the bolt so that a higher proportion of its yield
strength was used.
The strength of the fastener was not being utilised, reducing
the preload that the fastener was capable of providing. A tightening
torque of 300 Nm was subsequently specified that resulted in
a preload of 119000 N that was sufficient to sustain the forces
applied to the joint.
View another example of a bolt failure
Failure of a M16 flanged fastener of strength grade 12.9. This
fastener failed as a result of fatigue failure. Following a
detailed investigation of the assembly, the torque specification
was too low. The torque specified was 180 Nm that resulted in
a preload of 71000 N. This preload was insufficient to prevent
the forces applied to the bolt opening the joint causing high
alternating stresses to be sustained by the bolt.