Failure Analysis on Fracture of Fixed Screws of a Diesel Generator

The fixed screws of a diesel generator of a nuclear power unit fractured. The fracture causes of the fixed screws were analyzed by macro observation, hardness test, tensile test, metallographic examination, scanning electron microscopy analysis, energy spectrum analysis and so on. The results show that the depth of the screws into the through-hole was not enough, which increased the stress level of the stress concentration point. And there were more folding defects on the surface of the screw thread. The cracks initiated and propagated at the folding defect in the stress concentration area under the action of alternating stress, which eventually led to the fracture of the screws.