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Fractography Fracture failure of a med-carbon steel screw thread

Techniques: SE/BEI imaging, EDX spectroscopy, optical metallography, micro-hardness test


Summary: The failed part is a fastening through stub of an alternator subjected to tensile and vibration loads. Visual inspection identified the regions of crack initiation (labelled ‘A’ in the upper-left image), crack propagation (B-D-E) and the final catastrophic fracture (C). SEM observation (the lower image) indicated that crack initiation was caused by repeated shear deformation occurring in the surface layer. The crack propagation regions featured low-cycle fatigue fracture (the upper-right image). Metallographic observation and micro-hardness test showed ferrite/pearlite microstructure and hardness values ~HV250. 

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