Stress and fatigue analysis of a bow-door arm

Abstract

Bow-door systems are designed according to the class rules, of which the IACS UR S8 rules serve as a base. Even though bow-door arms, with the purpose of carrying the bow-doors in an open position, are subjected to some dynamic loads, they are dimensioned according to static loads. On one ferry, a large crack arose in one of the bow-door arms. If the arm had cracked off completely, the bow-door would probably have been lost at sea, as the arm was part of the locking arrangement.

This analysis has been carried out in order to analyse whether arms, dimensioned according to a static load, should be allowed to be a part of the locking arrangements of bow-doors. The analysis was carried out with field measurements, in the form of strain and acceleration measurements on an arm during the different load cases it is subjected to. The measured results were applied on a finite element model of the arm in order to find the stress and stress ranges everywhere in the arm during the different load cases. With this information, areas subjected to large stress ranges were identified for which fatigue calculations were carried out.

None of the load cases proved to give rise to any serious fatigue, except for the bow propellers, which, in certain situations, excite the arms into resonance cycling. The resonance gives rise to a great number of stress cycles with amplitudes large enough to cause fatigue failure.