Fatigue failure of high-pressure oil-pipes of truck diesel engine

Abstract

Abstract Fifteen cracked high-pressure oil-pipes of truck diesel engine were received for failure analysis. All the fifteen oil-pipes cracked on the concave side at the bend locations of oil-pipes without on the straight sections. The cracks were almost along the longitudinal of the oil-pipe but a slight deflection. The fatigue fracture was the failure mode of all the oil-pipes. The internal surface of oil-pipe on concave side of the bending location subjected to the maximum tangential stress due to the internal oil-pressure. The fatigue cracks initiated from the internal surface and propagated toward the external circle surface of oil-pipe under the alternating tangential stress resulting from the alternative oil-pressure inside pipe during service. The intense longitudinal micro-cutting marks were found on the internal surface of oil-pipe resulting from high-speed impact of hard and sharp grits in the process of grit-blasting internal surface of oil-pipe. Such micro-cutting marks acted as small notches and enhanced the stress concentration at the bottom of marks with ultimately resulting in crack initiation. The operating stress may become just high enough to make the micro-cutting marks on internal surface of oil-pipe at the bending location more critical. The stress intensification caused by the micro-cutting marks may exceed the fatigue limit of the pipe material.