Fatigue Failure Analysis of Diesel Engine Connecting Rod 2018-28-0067

Connecting rod of a high performance reciprocating internal combustion engine is one of the critical components exhibiting complex motion. This is subjected to both compressive load due to combustion force as well as tensile load due to inertia of the moving components. These loadings are cyclic in nature and the component is highly prone to fatigue failure if not deigned or manufactured carefully. Therefore connecting rods are designed and manufactured with high degree of precision for infinite loading cycle. But failures in connecting rod is often reported which is associated to either fatigue, bending, bearing failure or assembly faults. This study deals with one of such failure of connecting rod reported during fatigue testing. Failures occurred at around 1 million fatigue loading cycle as against target life of 5 million cycles. The present study represents the investigations done for engine connecting rod and with a view to identify the root cause of failure. Factors affecting failure including structural design, type of material and dynamic loads were assessed using standard failure analysis method. This included Visual observation, metallurgical testing, magnetic particle testing, fractography analysis by stereo and scanning electron microscopy, residual stress analysis, dimensional inspection, chemical analysis, Brinell hardness testing, tensile testing, inclusion analysis, microstructure analysis and grain flow analysis. Results of this investigation suggest that the connecting rod has failed due to improper machining/drilling of oil hole and chamfer at the small end of connecting rod which had generated rough tool marks at inner diameter, serration marks and material chip off. This induced harmful tensile residual stresses which resulted in early fatigue failure.