Fatigue failure of an intermediate transition block in fuel-injection pump fork assembly of a truck diesel engine

A truck diesel engine fuel-injection pump fork assembly failed after service of 50703 km. Fuel-injection pump fork assembly was constituted of the fork at the side of air-compressor pump, the intermediate transition block and the fork at the side of high-pressure oil-pump. General inspection in site indicated that the intermediate transition block fractured into four pieces. Fractographic analysis indicated that the block cracked due to fatigue. Four cracks occurred at respective U-shaped groove on the block and originated from the electro-discharge machined (EDMed) fillet root of U-shaped groove, where high stress concentration was caused by the geometry design. A remelted layer of about 5-20 μm was observed on the EDMed U-shaped groove surface. A lot of micro-cracks were presented within the remelted layer, which acted as stress raisers and led to a considerable reduction in fatigue strength of material. The cracks and surface imperfections associated with the remelted layer provided sites for fatigue crack initiation and subsequence propagation into the substrate material, which ultimately caused fatigue fracture of block. A finite element model of the block predicted that the highest stress concentration areas coincided with the first fracture zones and the maximum stress gradient direction was in agreement with the crack initiating direction.