Micro-cracks on electro-discharge machined surface and the fatigue failure of a diesel engine injector

Abstract Spraying oil occurred abruptly during a duration test of diesel engine injector for around 26 h and a crack was found on the outer periphery of injector. SEM observation indicates that multiple origins and high cyclic fatigue fracture is the dominant failure mechanism of the injector. Fracture initiated on the electro-discharge machining (EDM) wall of the transverse oil hole of the injector and propagated under cyclic load (alternative oil pressure). Metallurgical examination indicates that a lot of radial micro-cracks are presented within the remelted layer on the EMD wall of the transverse oil hole. In this paper the reasons for this failure were discussed by fracture analysis and metallographic examination results and the stress analysis of the oil holes under the operation oil pressure of 160 MPa. The discharge energy in EMD process should be controlled to reduce the crack density and depth within the remelted layer induced by EDM process and to avoid multiple fatigue-crack initiation. Chamfer probably the orifice of the transverse oil hole intersecting with the wall of the center oil hole to decrease stress concentration degree at the orifice of transverse hole.