Failure analysis of collared stud

Fractographic studies indicated that the collared studs of an aeroengine have failed by over load. Both the studs got fractured in the threaded region in identical fashion. One of the studs had fractured in the compressor side, while the other had fractured in the turbine side. It appears most probable that the high unlocking torque during dis-assembly was responsible for the overload failure. The deformation of the threads and/or bending of the studs are likely to take place in applications such as the present one and hence high unlocking torque is unavoidable. Therefore, the possibility of threads getting damaged and/or studs getting sheared off during dis-assembly always exists. It is, therefore, recommended that the studs be used only once. After dis-assembly, the old studs have to be replaced by new ones