Fabien Nazaret

Abstract

Durability of ceramics tools for superplastic forming

Fabien Nazaret - 2 December 2005

This study deals with the characterization and modelling of the thermo-mechanical behaviour of a refractory castable reinforced by a 1.5% volume fraction of short metallic fibres (FRRC). Main objectives are to provide constitutive laws that allow to size industrial parts by a numerical approach. The first aim of the study is to attain a better knowledge of this thermomechanical behaviour when the FRRC is subjected to different mechanical loadings at various testing and/or firing temperatures. The second objective is to take into account and to develop modelling approaches to enable the numerical sizing. A database was constituted from experimental tensile, bending and compression test results. Tests have been performed in a wide temperature range both for the FRRC fired at 500°C and at 900°C. According to the testing and firing temperatures, results have highlighted large evolutions both for microcracking processes in the pre-peak domain and for the post-peak softening behaviour. Moreover, the discrepancies of the Young modulus and of the maximum strength measured from bending and tensile test results were explained by considering the mechanical characteristics of the FRRC. Considering the mechanical behaviour, the scale effect sensitivity was studied too. Three points bending tests have been performed on five different sizes of unnotched beams. A so-called "two transition law" has been proposed to model the experimental FRRC size effect. This law is based on the deterministic theory and on the statistic theory. Concerning the damage behaviour modelling, two models were considered. The first one deals with a smeared-crack model regularized by a fracture energy approach. The second one deals with the non-local model of Mazars in a differential formulation. The scale effect predicted by these two models was studied and compared with experimental data. Difficulties due to parameter identification were particularly underlined. In a last step, models were used to simulate the behaviour of a sample characterized by a complex geometry. They allow correct predictions concerning the location of crack initiation sites, their order of appearance and the maximum load levels. When considering parameters that have been identified from the tensile behaviour, this loading case has shown the large influence of the softening behaviour characteristics. Thus, for complex parts, the FRRC softening capacity allows stress field redistribution. As a consequence, it has been shown that FRRC parts are able to sustain larger load levels compared to materials that do not exhibit any softening properties.

Last modified: 02/22/2007 11:42 AM