Constitutive equations of earthenware material during firing processes and determination of their function forms

Abstract

We propose a set of constitutive equations of earthenware material subjected to the firing process and appropriate ways of determining their function forms and relevant material parameters from a series of respective experiments. The firing process under a specific heat curves is generally divided into three different phases; “thermal dilatation phase”, “sintering phase” and “thermal contraction phase”. These non-mechanical deformations are assumed not to be coincident, and the mechanical ones are assumed to be represented by viscoplastic constitutive model. Then, the key issue must be how to determine sintering strains by calibrating the employed function form of the densification rate with the data obtained from the stairway thermal cycle (STC) tests. Also, the presentations of the dependencies of the elastic and creep properties on both temperature and density are of particular importance to accurately predict the overall deformation of earthware. All the relevant material parameters as well as coefficients of thermal expansion and contraction are identified with the data obtained from respective mechanical experiments conducted in testing equipments for thermo-mechanical analysis (TMA) under several levels of termperature. The method of differential evolution, which is one of the metaheuristic optimization techniques, is employed to identify the creep parameters, since the mechanical responses to several levels of loading during firing processes involve all the non-mechanical and mechanical deformation of specimens simultaneously. The calculation results with the proposed constitutive equations are compared with the original experimental data to demonstrate the applicability for practical use of their function forms determined by the proposed strategy.

Keywords

• earthware material
• sintering
• firing
• densification
• viscoplasticity
• differential evolution