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In this study, the analysis and characterization of the failure that occurred during the torsional fatigue test of the flange yoke unit part of the cardan shaft used in heavy commercial vehicles were carried out. In order to determine the root cause of the failure according to the production, structure, property and performance relationship in materials science, chemical analysis of the steel raw material used in the production of flange yoke, hardness measurement, grain size analysis, decarburization measurement, inclusion analysis and raw material characterization by scanning electron microscopy (SEM) were carried out. As a result of the studies, it was determined that the values obtained for 41Cr4 steel raw material were conformant according to the TS EN ISO 683-2 standard, but in the SEM examination and EDS analysis, elongated MnS inclusions in the microstructure and cracks were detected at the interface of the metal matrix and MnS inclusions. In this context, as a result of the characterization of the flange yoke unit part, it was determined that the MnS ratio was 2-4 µm x 898-1.181 µm according to the ASTM E45 standard, at the same level as the raw material and equivalent to the A-type value of 3.0-3.5. It was determined that MnS inclusion formations in the subsurface region formed crack propagation zones. [ABSTRACT FROM AUTHOR]