Identification of asymmetric constitutive laws at high temperature based on Digital Image Correlation

Abstract

The mechanical characterization of ceramics can be very challenging. Depending on their composition and fabrication process, ceramic materials may exhibit at room temperature different types of mechanical behaviours, ranging from linear elastic to quasi-brittle, like rocks, concrete or plasters. At elevated temperature, they may present a non-linear behaviour, due for instance to the presence of a vitreous phase. However, estimating the evolution of their behaviour from room temperature to elevated temperature is a challenging topic, especially when there is an asymmetry between tension and compression. A methodology based on Digital Image Correlation is presented in this paper where four point bending tests are analysed for temperatures ranging from 25 • C to 900 • C. For a ceramic mainly made of aluminium titanate, the evolution of an asymmetric constitutive law is identified with a strong link with microstructural observations using SEM images.

New advances in the laboratory characterization of refractories: testing and modelling

“…In zones 2 and 3, the texture and microstructure of the sample were similar to the ones of the sample tested alone. C(A;F) 6 and liquid phase were observed as expected. The sharp change in composition between zone 2 and zone 1 indicates that the interaction between the molten oxides and the sample is differed from zone 2.…”

“…Full-field measurement techniques are more frequently used in the mechanical field to analyse the mechanical behaviour of a material and to study the real test conditions and their effects (i.e., boundary conditions) [4][5][6]. In classical DIC (often denoted as "local"), the picture is discretized in a small zone of interest, which is registered in the reference configuration.…”

“…The shrinkage due to reactive sintering is higher than the thermal expansion; -1380-1450°C: melting of the oxides pellets and reactive impregnation into the porous space of the material. Nonuniform swelling of the sample due to precipitation of the expansive phases C(AF) 6 and C(AF) 2 ; -1450°C-RT: slight increase of the diameter at the beginning of cooling due to the solidification of the liquid phase.…”

“…C(A;F) 6 precipitated where the liquid phase amount was small in zone 3 and where the reactivity of the liquid has decreased in zone 2. According to the calculation from the volumetric lattice parameter of the involved oxide C(AF) 6 , precipitation leads to a macroscopic expansion of 3% when precipitation of C(AF) 2 expands the volume by approximately 24%. This explains the difference in diameter change between zone 1 and the rest of the sample.…”

“…This explains the difference in diameter change between zone 1 and the rest of the sample. The intermediate zone 2 corresponds to the junction between zone 1, with a high content of C(AF) 2 and large expansion, and zone 3, with a high content of C(AF) 6 and lower expansion. The gradient of diameter in zone 2 can be explained by elastic strain induced by the difference between zone 1 and zone 3.…”