1992-02-01

Combined Finite Element and Metallurgical Analyses of Piston Ring Groove Insert Retention Failures 920088

The performance of a large diesel engine piston has been investigated to determine a mechanism for the failures encountered during engine research testing. During engine testing, it was found that the pistons were failing to retain the ring groove insert and fracture of the top land above the insert was observed.
The finite element analysis was performed on the piston to ascertain the combined thermal and mechanical stresses on the piston and its ring groove insert. Finite element models were employed to study the effects of a crack growing in the Alfin bond between the ring groove insert and the aluminum alloy of the piston. The data showed that as a crack in the bond between the ring groove insert and the aluminum alloy of the piston grows, the stresses in the bond area drastically increase. When total failure of the back bond was modelled, the stresses at the back of the insert, adjacent to the upper back radius, were shown to exceed the fatigue strength of the piston alloy at the temperature of operation.
A metallurgical analysis was employed on the failed pistons in order to validate the failure theories drawn from the finite element analysis results. These data provided insight into the fact that, once total failure of the back bond had occurred, a low cycle fatigue crack grew from the top of the ring groove insert through the aluminum alloy to the top of the piston, causing fracture of the top land. The metallurgical analysis confirmed the results of the finite element analysis and showed that the performance of the piston was critically dependent of the integrity of the Alfin bond.

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