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Title: Theory and numerical modelling of dynamic ice-structure interaction for ice-induced vibrations
Author: Ji, Xu
ISNI:       0000 0004 7972 2857
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2019
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This PhD work is done as a trilogy. In the first stage, a dynamic single degree-of-freedom ice-structure interaction model is developed based on a novel physical mechanism combination between self-excited vibration and forced vibration. Van der Pol equation, together with ice stress-strain rate curve and ice-velocity failure length are coupled to model the internal fluctuating nature of ice force in conjunction with the relative velocity caused by the structure as an external effect. Three basic modes of response were reproduced, such as intermittent crushing, frequency lock-in and continuous crushing. The results are in good match with experimental data at different ice velocities and different structural stiffnesses. Ice force frequency lock-in phenomenon during ice-induced vibrations (IIV) is also observed. In the second stage, analysis on physical mechanism of ice-structure interaction is presented based on feedback mechanism and energy mechanism, respectively. Internal effect and external effect from ice and structure were both explained in the feedback branch. Based on reproduced results, energy exchanges at different configurations are computed from the energy conservation using the first law of thermodynamics. A conclusion on the predominant type of vibration when the ice velocity increases during the interaction process is forced, self-excited and forced in each three modes of responses. Ice force variations also shows that there is more impulse energy during the lock-in range. Moreover, IIV demonstrates an analogy of friction-induced self-excited vibration. The similarity between stress-strain curve and Stribeck curve shows that static and kinetic friction force variations are attributed to ice force characteristic, and can be used to explain the lower effective pressure magnitude during continuous crushing than the peak pressure during intermittent crushing. In the third stage, a two-dimensional non-simultaneous ice failure model is developed. The concept of multiple ice failure zones is proposed to fulfil non-simultaneous crushing characteristics. The size of ice failure zone is assumed to become smaller with increasing ice velocity, which increases the occurrence of non-simultaneous ice failures. Similarly, the decreasing size of ice failure zone as velocity increases is explained as the reason of different ice failure modes shifting from large-area ductile bending to small-area brittle crushing. In addition, an analysis of the ice indentation experiments indicates that the mean and minimum effective pressure have an approximately linear relationship with ice velocity, which testified the assumption on variations of ice failure zone in the model. The simulation results from a series of 134 demonstration cases show that the model is capable of predicting results at different ice velocities, structural widths and ice thicknesses. To sum up, this Van der Pol based model is more powerful than the others in kind by far because of its accurate results, wide applicability and novel physical mechanism behind. Thus, the numerical models produced as part of this research can be helpful in ice failure analysis and in the design of ice-resistant structures.
Supervisor: Oterkus, Erkan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral