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After the coupling end of a 6-inch bore hydraulic cylinder of a 30 ton automotive forming press had failed – we were asked to investigate.
The failure analysis of the hydraulic cylinder shaft was accomplished by performing the following tasks:
• Visual Examination
• Scanning Electron Microscopic (SEM) Examination
• Metallographic Examination
• Hardness Testing
Visual Examination
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.
SEM Examination
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.
A longitudinal metallographic section was taken from the large fatigue fracture initiation site and evaluated for internal material defects, fracture characteristics and micro-hardness. A macrophotograph of the etched metallographic specimen, showed an approximate 0.062 inch deep case at the cylinder outside surface.
The case was not present in the notch. Evaluations of the specimen in the as-polished condition found good internal cleanliness and a 0.001 inch thick chromium plate layer at the outside surface. Another showed an as-polished and etched views of the notch radius at the fracture initiation, where a secondary crack is present in the notch radius containing machine tool marks. Higher magnification views in the lapped area of the notch radius near the fracture initiation revealed additional fine scale surface cracks along with a thin plastically deformed surface layer. Microstructural evaluations indicate the case observed at the cylinder outside surface consist of ferrite plus fine carbides, with a core microstructure consisting of pearlite plus ferrite.
Hardness Testing
The microhardness readings taken at the case, 98 Rb (which is less than 20 Rc), and at the fracture initiation site, 23 Rc, were significantly lower than the specified hardness of 50 to 54 Rc. The notch was also apparently machined and lapped after all cylinder surface plating and thermal treatments were complete, exposing the normalized core microstructure to cyclic loading. Considering the brittle nature of the final fast fracture surface, it appears this microstructure does not posses sufficient fracture toughness for the service environment.
Summary
All of the data obtained was consistent with cyclic axial and bending fatigue failure. The initiation of the larger fatigue fracture face being opposite the polished bearing face indicates the cylinder was subjected to non-axial loading in service. Non-axial loading is suspected of inducing a bending moment that concentrated tensile stresses in the notch radius during each load cycle. Surface machining marks in the notch radius further concentrated these stresses, resulting in numerous fatigue crack initiations.
An improved surface finish and a more generous radius in the notch are expected to improve the fatigue life by reducing stress concentration factors. It is believed improvements in the surface microstructure and hardness will eliminate the potential for premature failure in this design.
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O’Donnell Consulting Engineers Performs Failure Analysis Services for Clients in Industries including Petrochemical, Manufacturing and Automotive.Â
