Failure of diesel-engine crankshafts

Abstract

A failure investigation has been conducted on diesel engine crankshafts used in tractors and made from 0.45 % carbon steel. Premature failure was reported in the web region of the crankshaft. The crankshafts were forged, normalized and partly induction hardened. The investigation included determination of chemical composition, microstructural examination, evaluation of tensile properties and charpy toughness as well as hardness determination. The fracture toughness was estimated from the charpy energy data. The failure zones in various crankshafts were examined using the scanning electron microscope (SEM) and the micro mechanism of failure in the crankshafts was studied. Fractographic studies indicated fatigue as the dominant mechanism of failure of crankshafts. Attempts were made to estimate the stress level required for fatigue initiation from the crankpin-web fillet region. Further, fractographic methods were used to estimate the stress required for fatigue propagation. Subsequently, the failure time was estimated and this was correlated with the observed failure times in different crankshafts. The studies indicated that fatigue initiation from the crankpin-web fillet region necessitated a stress level of about 175 MPa. To avoid recurrence of failure, machining and final grinding has to be done carefully to prevent formation of discontinuities or crack-like defects in the fillet region and induction hardening of the fillet is desirable. Also, the fillet radius needs to be increased.