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Cavitation erosion fracture mechanisms and their detection in ship rudders

Armakolas, I. (2018). Cavitation erosion fracture mechanisms and their detection in ship rudders. (Unpublished Doctoral thesis, City, University of London)


The phenomenon of cavitation is of great importance when ship propellers and rudders are considered, as it can often be the cause of vibrations, noise, reduced efficiency and even erosion in some instances. The underlying fracture mechanisms of erosion, however, have not been fully understood yet.

As such, this study aims to expand our knowledge regarding the fracture mechanisms of common shipbuilding alloys and explore whether cavitation erosion can be monitored, by using the relevant quantitative and qualitative data. As such, an experimental test rig was built, based on the induction of cavitation by ultrasonic means, in order for a series of tests, including mass loss and acoustic emission measurements as well as microscopic observations to be conducted. Due to the interest of BAE Systems, a number of protective coatings were also examined under an analogous context.

Specimens were initially exposed to ultrasonically induced cavitation under identical experimental conditions. Mass loss was periodically measured thus materials were categorized in that respect while the positive effect of cathodic protection on the resulting erosion was confirmed. Examination through optical and scanning electron microscopes was also conducted thus the fracture mechanisms and macroscopic characteristics of cavitation erosion were identified, for each of the examined materials. Results showed that, erosion initiates through plastic deformation (orange peeling) before proceeding into ductile and brittle, due to work hardening, fracture, whereas the extent and crack propagation characteristics of each phase, depend on the material’s mechanical properties and crystalline structure.

Acoustic emissions were also examined, with the aim of, characterizing the materials and potentially be utilized for erosion monitoring. Upon the successful establishment of acoustic thresholds for cavitation erosion, in the case of small specimens, a small model rudder was also examined under an analogous context, although in that instance, cavitation localization was also considered, through a triangulation source location technique. In that instance, cavitation induced erosion, was effectively monitored and characterized both in terms of intensity and location. A model rudder twice as large as the small one was also examined in order for any possible scale effects to be identified. Cavitation induced erosion, was again effectively monitored, both in terms of intensity and location, although results indicated that the method should be optimized, with respect to the parameter of size.

As such, the future researcher could further promote the evolvement of the aforementioned ship rudder monitoring system, by means of optimizing the analytical procedures in order to overcome any possible scale effects, further adapting the characteristics of the system to match the size of the objects to be monitored and eventually lead to the full – scale application of the system. The conduction of sea trials would also be of great benefit and importance towards the direction of forming a solid cavitation erosion monitoring system.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments: Doctoral Theses
School of Science & Technology > School of Science & Technology Doctoral Theses
School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
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