Failure Analysis of a Truck Diesel Engine Crankshaft Made from Spheroidal Cast Iron
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CASE HISTORY—PEER-REVIEWED
Failure Analysis of a Truck Diesel Engine Crankshaft Made from Spheroidal Cast Iron Xiao-lei Xu • Zhi-wei Yu
Submitted: 29 December 2010 / in revised form: 4 March 2011 / Published online: 26 March 2011 Ó ASM International 2011
Abstract A diesel engine crankshaft fractured in service after 13,656 km of operation. The fracture took place on the sixth, the fifth, and the fourth crankpins and the fracture surfaces have a 45° inclination with respect to the axial of crankshaft. The cracks of the sixth and the fifth crankpin are across the oil hole and a complete fracture took place at the sixth crankpin which bore the maximum torque load. On the fourth crankpin, crack is only through the thin wall side of oil hole. The results indicate that fatigue fracture is the dominant failure mechanism of the crankshaft. It was observed that the fatigue cracks in the crankpins initiated at machining dents present on the wall of oil hole. The appearance of the machining dents on the wall of oil hole suggests improper machining and these dents supplied the stress concentration site that was mainly responsible for the fatigue fracture of crankshaft. Keywords Crankshaft Fatigue fracture Machining dent Oil hole Failure analysis
Matrix microstructure is specified as 80–90% pearlite and 10–20% ferrite with spheroidal graphite distributed in the matrix. The crankpin surface is required to be inductionquenched and the depth of the induction-hardening case is specified as 1.00–2.50 mm. The mechanical properties values are, respectively, specified as ultimate strength C748 MPa, yield strength C548 MPa, elongation C4%, and the hardness values of core and surface of crankpin as HB 260–300 and HRC 45–55. This case history describes the failure analysis that was conducted to determine the cause of the premature breakage.
Experimental Methods The microstructure in various regions was observed by scanning electron microscopy (SEM). The mechanical properties of the crankshaft material and the hardness of core and surface of crankpin were evaluated. The fractured surfaces were observed visually and using SEM to study the failure modes and mechanism.
Introduction The premature breakage of a truck diesel engine crankshaft was reported from a diesel engine plant. The crankshaft had been in service for 13,656 km of operation before failure. The crankshaft is made from spheroidal cast iron.
X. Xu (&) Z. Yu Electromechanics and Material Engineering College, Dalian Maritime University, Dalian 116026, People’s Republic of China e-mail: [email protected] Z. Yu e-mail: [email protected]
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Observation and Examination Results Fractography The failed crankshaft is shown in Fig. 1. It was found that cracking took place at the sixth, the fifth, and the fourth crankpin. The cracks of the sixth and the fifth crankpin are across the oil hole (Fig. 2a, b) and the complete fracture took place at the sixth crankpin. Cracking of the fourth crankpin occurred at the thin wall side of oil hole (Fig. 2c). The c
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