Time-dependant behaviour of ramming paste used in hall-héroult cell : characterization and constitutive law

Authors: Orangi, Sakineh
Advisor: Fafard, MarioDarvishi Alamdari, Houshang
Abstract: One of the components of the carbonaceous lining of a Hall-Héroult cell is ramming paste. During preheating and start-up periods of the Hall-Héroult cell from room temperature to 1000 °C, the paste is baked under varying loads and it experiences several phenomena such as chemical and mechanical ones. Creep/relaxation is one of the mechanical phenomena that the paste experiences during baking. Few papers have been published on the modelling of creep/relaxation behaviour of the ramming paste. In order to propose a constitutive model, it was necessary to have some insight into its creep behaviour. To this end, creep tests were carried out on the paste which had been baked at different temperatures ranging from 200 to 1000 °C. Based on the new observations obtained from these tests, a rheological model was proposed for the creep behaviour of the paste. Using the rheological model, a three-dimensional constitutive law was devised that represents nonlinear creep/relaxation behaviour of the materials sensitive to hydrostatic pressure. A novel dissipation potential was postulated which includes the effect of the hydrostatic pressure in addition to shear. The devised model was developed for the creep case under constant stress. In order to obtain the creep strain, it was necessary to obtain analytical, when they existed, and numerical solutions from a system of nonlinear differential equations. For numerical implementation, discretization was performed using implicit Euler scheme and nonlinearity was treated using Newton-Raphson iterative process. To this end, some codes were developed in Matlab software. The model parameters were identified using axial and radial strains obtained from uniaxial creep tests on the paste baked at 250 °C and tested at room temperature. Furthermore, the model parameters were identified using the axial strain obtained from uniaxial creep tests on the paste baked at 350, 560 and 1000 °C as well as tested at 300, 300 °C and room temperature, respectively. Optimization of the parameters was performed employing pattern search algorithm available in the Matlab software. Comparison of the model with experimental results showed that the model well represents the nonlinear viscoelastic-viscoplastic behaviour of the paste baked at different temperatures.
Document Type: Thèse de doctorat
Issue Date: 2014
Open Access Date: 20 April 2018
Permalink: http://hdl.handle.net/20.500.11794/25205
Grantor: Université Laval
Collection:Thèses et mémoires

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