Evaluation of Fractured “Stress Proof” Coupling Bolts in a Generator Application

Sixteen coupling bolts that were used to fasten a coupler to the shaft of a generator failed during an operational test. The bolts were found broken in the aftermath of the test, and the coupling itself had been displaced in the axial direction on the shaft during the run. All of the failed bolts displayed uniform shear fracture surfaces and prominent topography, with no evidence to support extensive fatigue initiation or propagation. Evaluations via scanning electron microscope and energy dispersive spectroscopy, metallography, dimensional analysis, mechanical testing, and chemical analysis were performed to determine the failure mechanism. The investigative methodology, mode of failure, and potential root causes of failure are discussed. Opportunities for future design improvements to prevent future failures of this kind are presented.

Keywords: failure analysis, coupling bolt, metallurgical, mechanical, shear fracture, testing