Fatigue Failure of Electronic Unit Pumps Used in Truck Diesel Engine
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TECHNICAL ARTICLE—PEER-REVIEWED
Fatigue Failure of Electronic Unit Pumps Used in Truck Diesel Engine Zhi-wei Yu . Xiao-lei Xu . Haixuan Yu
Submitted: 13 June 2016 / in revised form: 31 July 2016 / Published online: 6 September 2016 Ó ASM International 2016
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 oiloutlet. 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
Z. Yu X. Xu (&) Department of Materials Science and Engineering, Key Laboratory of Ship-Machinery Maintenance & Manufacture Ministry of Communication, Dalian Maritime University, Dalian 116026, People’s Republic of China e-mail: [email protected]; [email protected] H. Yu Center of Heat Treating Excellence, Worcester Polytechnic Institute, Worcester, MA 01609, USA
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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. Keywords Electronic unit pump Fatigue fracture Defective design Defective microstructure
Background The injector system is the ‘‘heart’’ of the diesel engine of truck, and the electronic unit pump (EUP) is a device to use for producing injection pressure of injector. Each EUP is assembled on the corresponding cylinder, respectively, for a 4-cylinder or 6-cylinder engine. The EUPs are droved by the camshaft in engine. Several EUPs used in the truck diesel engine with 4 cylinders fractured or cracked during the trial tests. Inspection in site found that all the failures occurred at the fifth thread root on the outer diameter of oil-outlet of EUP. Two typical EUPs were received for analysis of failure causes (Fig. 1). It was reported that EUP 1 and EUP 2 failed after 582 and 18 h of operation, respectively. The EUPs w
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