Fatigue Failure of Electronic Unit Pumps Used in Truck Diesel Engine

Abstract

Several electronic unit pumps (EUPs) used in the truck diesel engine failed during the trial tests. All the failures occurred at the same location, the fifth thread root on the outer diameter of oil-outlet of EUP. Two typical failed EUPs were analyzed to assess failure causes. Fractographic investigation shows that the fatigue fracture was the dominant failure mechanism of two EUPs. Crack origins of the failed EUPs initiated from the fillet root of the step in the internal hole of oil-outlet and propagated toward the fifth thread root on the outer diameter of oil-outlet. Over-small radius of the root fillet caused a high notch effect, and the presence of porous structure and network nitrides on the nitrided internal hole surface created a “metallurgical stress concentration.” Under the action of the great contact force between the plug and the side wall of internal hole of oil-outlet resulting from the press-fitting the plug, the crack initiated easily at the fillet root. The fillet root of step in the internal hole was almost in the same level with the fifth thread root on the outer diameter of oil-outlet. The fifth thread on the outer diameter of oil-outlet was just corresponding to the first engaged thread inside the nut of oil-pipe matched with the threaded portion of the outer diameter of oil-outlet. This resulted in a maximum axial tensile load on the fifth thread due to pre-tightening the nut of oil-pipe. The crack propagated rapidly toward the fifth thread on the outer diameter under the great axial tensile load to lead to premature fatigue failure of EUP.