A root cause investigation was performed on a failed coupling end of a 6-inch bore hydraulic cylinder of a 30 ton automotive forming press.
A visual examination of the shaft showed surface polishing along one edge at the bearing surface, which is most likely due to non-axial loading during service. The fracture appearance is consistent with cyclic axial and bending fatigue, where the fracture initiated at diametrically opposite locations in the notch radius. Beach marks are evident on both fatigue fracture faces. The fatigue fracture areas are not uniform in size, with the larger of the fatigue fracture faces opposite the surface polishing on the bearing face.
Select features on the larger fracture face were evaluated using high magnification imaging in a SEM outfitted with an energy dispersive X-ray spectrometer (EDS) for elemental analysis. SEM images of the fracture initiation area found machining marks. The fracture surface exhibited considerable mechanical damage due to its presence during continued service. The final overload fracture surface contains primarily brittle fracture (cleavage) features, with limited ductile features.
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