Theoretical Study of Stick-Slip Behaviour in Ice-Structure Interaction

Abstract

Structures built in lakes and rivers or at sea are under huge forces induced by ice. These cause friction at the ice-structure interface that may pose a threat to the structural integrity of the construction. For the detailed design and realisation of structures in such regions an understanding is required of this friction process. The friction process at the ice-structure interface is driven by a phenomenon called the stick-slip phenomenon, where during stick mode the structure and the ice move simultaneously with the same velocity, and during slip mode the ice and structure interface slide over one another. This phenomenon has been observed and researched extensively, yet the corresponding static and kinetic friction coefficients reported show a wide range and the relation between the involved parameters and the friction coefficients is inconclusive. The most common theory used in describing general friction processes is Coulomb's Law of Friction. This thesis aims to determine whether and when ice-structure interaction can or cannot be described by Coulomb's Law of Friction in particular cases.

For this graduation project, first an analysis of several test set-ups was carried out to determine which set-up was best suited for experiments with stick-slip behaviour. The chosen test set-up consists of a one-dimensional rotating conveyor belt made from sandpaper on which an ice sample would be placed. This sample would be held in place by four springs attached to the back and front of the sample. This would allow the ice sample to move over the rotating belt, but remain relatively in its place. This would enable to the ice to displace in one direction, the direction of the rotating conveyor belt. This test set-up would be compact and it would be easy to switch the belt or ice sample if they had deteriorated too much. However, the use of a 1D conveyor belt prohibits construction materials such as steel or concrete to be used. Using sandpaper with similar roughness as concrete or steel would serve as a substitute to overcome this problem.

Ultimately, it proved to be impossible to realise the set-up and the focus of this thesis switched towards modelling the two-dimensional stick-slip behaviour based on data from previous research. For this a three degree of freedom (3DOF) model was created. As the forces exerted by the springs on the mass act under an angle that is depending on the position of the mass four different approaches were tested to see how this angle could best be incorporated. Furthermore, the friction force acting on the ice-structure interface was modelled using the friction coefficients found by previous research, and using two different approaches to find out whether the friction force should be modelled using the relative sliding velocity at the interface or not. It proved that the angle between the springs’ original position and it’s instantaneous position should be updated every step during slip mode, and that the friction force should be modelled using the relative sliding velocity. Both are cause for higher computational time. When validating this model against the experimental data it was found that friction coefficients were overestimated substantially for the 3DOF model. Using this the 3DOF model was updated and it was found that it now matches with the experimental data.

After analysing the output of the model, it was found that the kinetic friction coefficients are now in the range of 0.05-0.11 for the used parameters (mass 0.67 kg – 2.03 kg, slab velocity 0.15 m/s – 0.82 m/s, stiffness 20.17 N/m – 68.22 N/m). Also, the friction coefficients show a linear dependency with mass, and an slight inverse linear relationship with the slab velocity. Furthermore, with a low stiffness varying friction coefficients were found when varying the slab velocity, but when increasing the stiffness the friction coefficients converge and this variation decreases. Although further improvements can be made to create a better model, in general it is concluded that under the made assumptions, the 3DOF model describes the two-dimensional behaviour well, and shows how research into friction can be carried out while still using construction materials in an experimental set-up.