Magnetic non-destructive monitoring of a ship's propeller blade after long-term operation

Abstract

This study examines the potential of magnetic non-destructive techniques for monitoring surface damage to ship propeller blades made of stainless steel. The propeller considered in this investigation operated for 35 years in the Danube river. Sheet and vortex cavitations, as well as random mechanical impacts on the trailing and leading edges, were detected as the major mechanisms of damage to the blade. Non-destructive monitoring of this damage was based on the Barkhausen noise and Helmholtz feritscope techniques. The blades investigated here had a composite structure in which the ferrite stainless body was mixed with the deposition of austenite on the trailing and especially the leading edges. Damage to the ferritic phase was mostly associated with sheet cavitation and a decreasing magnitude of Barkhausen noise as a result of repetitive plastic deformation. Conversely, vortex cavitation and random impacts to the edges in the austenite regions were associated with increasing Barkhausen noise due to strain-induced martensite transformation, a result that was also confirmed by data obtained from a Helmholtz feritscope. Information about plastic deformation and the fraction of strain-induced transformation was also confirmed by X-ray and electron backscatter diffraction techniques.