Fatigue Strength of Combustion Chamber Walls of Marine Diesel Engine under Service Condition

Abstract

For estimation of the fatigue life of the combustion chamber walls, a study has to be made on the influence of various stress fluctuation on the fatigue strength of material.
From this point of view, taking the cooling side of combustion chamber walls as the object of study, the fatigue tests on Cr-Mo cast steel were carried out through four types fluctuating stress wave patterns set on the basis of the measurement results of load on ship under service condition and stress analysis.
The following results have been obtained:
1) It has been confirmed that the fatigue life under superimposed stress wave pattern decreases even when superimposed stress amplitude is below the fatigue limit of the material.
2) The fatigue life Nt by various fluctuating stress wave patterns is nearly within a range of 3 Ne - 1/3 Ne of the estimated life Ne by the modified Miner rule. This means that rough life estimation of the combustion chamber walls can be made by this method.
3) The influence of the stress change by cleaning work of turbo chargers and the engine starting/stopping in a harbor, the fatigue strength of cooling side of the combustion chamber walls is small. This can be neglected.
4) It has been clarified that there exists a linear relationship between number of superimposed stress cycle and fatigue life in log-log scale. Based on this result, we proposed an experimental equation on the influence of number of the superimposed stress cycle to the superimposed fatigue strength of material.