Finite Element Simulations of Punch Tests on Ice Rubble with the Modified Cam Clay Model - Comparison of Full Scale and Model Scale Experiments
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Ice ridges are features created by the relative motion between ice sheets, andappear as a pile up of broken ice or ice blocks both above (sail) and belowthe water surface (keel). First-year ice ridges are important in the designof offshore and marine structures, but the knowledge about the ice ridgeproperties are still limited. Model scale experiments on artificially producedice ridges have been developed, leaving the question of how to scale theproperties correctly. This thesis concerns the scaling of the properties of the unconsolidated layerof the keel (the ice rubble) from model scale to full scale, as a part of thedetermination of ice ridge properties for design. Two types of punch testshave been simulated numerically in order to study the difference between therequired material parameters:1. Four full scale punch tests performed in the Gulf of Bothnia in 19992. Two model scale punch tests performed at the Hamburg Ship ModelBasin (HSVA) in 2001 The punch tests involve loading the ice ridge with a loading plate whilemeasuring the resistance from the ridge. The experiments were modelledin the Finite Element software Abaqus, version 6.12, using explicit analysis.Both cases were modelled using the Coupled Eulerian-Lagrangian (CEL)approach, which copes with the challenge of large displacements of the icerubble as well as the interface between the puncher and the ridge. The icerubble was described using the Modified cam clay material model. Thismaterial model was originally developed for clays and has been used due toits relatively simple formulation and because it allows for volumetric changes.The numerical simulations successfully managed to fit the experimental resultsuntil the maximum force was reached. After the peak force, the iceridges in the experiments failed in shear. The global failure caused a largeload drop which was not captured in the numerical simulations since theModified cam clay model does not include a failure criterion.The depth measurements of the keel turned out to be vital for the numericaldetermination of the keel resistance, underlining the importance of properfield measurements. The loading of the full scale ice ridges was more complexthan in the model scale tests, and a more advanced numerical model isprobably necessary to properly simulate the entire time history. The material parameters which influenced the numerical results the most were the stress ratio M and the initial pressure p0. The stress ratio wasabout the same for model scale and full scale simulations (1.8-1.85), whichis expected since the parameter is dimensionless and dependent upon thecritical friction angle. The initial pressure was much larger for the full scaleice ridges, leaving a scale factor of around 20-40. On the contrary,the length scaling between the two cases were in the order of 10, meaningthat the model scale ice ridges are much weaker than the full scale ridges.The large initial pressure required in the full scale simulations resulted insmaller plastic strains than what was the case for model scale.