Fracture failure of the multi-throw crankshaft in a sport utility vehicle.

• Failure analysis was performed to find out the failure cause of the crankshaft. • Evidences indicated the mechanism of the premature failure was fatigue failure. • Fatigue fracture initiation was induced by the dense inclusions. A sport utility vehicle crankshaft was premature failure after a short service life of about 3 years. The fracture took place at the transition arc between the crankpin and the journal. The fracture surface has an angle of 45° inclination with the axial direction of the crankshaft, which is in accordance with the maximum primary stress plane caused by the torsion. A fractographic study indicates that the fracture surface exhibits a brittle fracture morphology reflective of a low toughness of the material. Macro beach marks and micro striations indicate that the failure mechanism of the crankshaft is fatigue failure. Metallographic analysis shows that flake pearlite and reticular ferrite are the characteristics of the microstructure near the fracture surface, which may lead to the lower plasticity of the material. Reticular ferrite is induced by the heterogeneous inclusion with a chemical structure of CaO-MnS complex. The weaker ferrite destroys the continuous matrix material and reduces the plasticity of the material, resulting in a brittle fracture. Collected evidence reveal that a series of manganese sulfide inclusions are distributed near the crack initiation. According to the quantification analysis of inclusion distribution, the density of inclusions is great enough to act as the initiation of cracks. Moreover, finite element analysis indicates the stress concentration induced by the geometry structure is less than the inclusion. Given all of that, the failed crankshaft is supposed to be initiated by the harmful inclusions. Therefore, it is recommended that rigorous testing of the inclusions and improving the heat treatment quality of crankshafts are effective ways to solve the problem. [ABSTRACT FROM AUTHOR]