Optimisation des méthodes optiques de mesure de champs de déformation pour la caractérisation du comportement à rupture des réfractaires : Application aux matériaux à base de magnésie

Magnesia-spinel and magnesia-hercynite bricks destined for thermal shock applications in cement rotary kilns often show an enhanced crack propagation resistance due to an engineered microstructure design. In these materials, microcrack networks resulting from the thermal expansion mismatch between magnesia matrix and spinel/hercynite aggregates promote the activation of energy dissipating mechanisms within the so-called Fracture Process Zone (FPZ) during loading. In this research, the fracture behaviour of magnesia-based model materials was investigated by coupling a refined Digital Image Correlation method (2P-DIC) with the Wedge Splitting Test (WST). The coupling of these advanced characterisation methods has proven to be very effective in measuring important fracture parameters accurately and in highlighting characteristic fracture mechanisms, such as crack-branching. The investigation of microstructure-property relationships underlined the impact of thermally induced microcracks on the thermomechanical behaviour of magnesia-spinel and magnesia-hercynite materials. Despite the rather similar elastic and dilatometric properties of spinel and hercynite single constituents, peculiar microcracking patterns were observed, especially in magnesia-hercynite. In fact, extensive diffusion between magnesia and hercynite during sintering led to the formation of spinel solid solutions around hercynite aggregates. As a result of thermal expansion mismatch with magnesia, these solid solutions contributed to creating numerous fine microcracks confined within the diffusion zone. Initially present within the microstructure, microcrack networks promote an increase of the specific fracture energy during WST experiments. Moreover, the analysis of strain fields measured by 2P-DIC revealed extensive crack branching for magnesia-hercynite materials. In essence, 2P-DIC and WST measurements showed that microcrack networks promoted the development of the FPZ, which in turn induced higher fracture energies. In a refined R-curve approach, effective fracture energies were calculated using crack lengths measured by 2P-DIC, which helped establish strong links between FPZ development and an enhanced crack propagation resistance. The tendencies observed at room temperature during WST experiments were confirmed during thermal cycling experiments using a novel thermal shock device.Les briques de magnésie-spinelle et magnésie-hercynite sont fréquemment utilisées dans les fours rotatifs de cimenterie pour leur tenue aux chocs thermiques venant d’une résistance accrue à la propagation de fissures conférée par leur microstructure. Les réseaux de microfissures résultant du différentiel de dilatation thermique entre la matrice magnésienne et les agrégats de spinelle/hercynite favorisent l'activation de mécanismes dissipatifs d’énergie au sein de la zone d’élaboration de la rupture (FPZ) lors d’un chargement mécanique. Dans cette étude, le comportement à la rupture de matériaux modèles à base de magnésie a été étudié en couplant une méthode de corrélation d'images numériques (2P-DIC) adaptée aux problématiques de fissuration avec l’essai de « Wedge Splitting » (WST). Le couplage judicieux de ces méthodes innovantes a fourni des mesures précises des propriétés de rupture et a mis en évidence des mécanismes de rupture caractéristiques, tels que la bifurcation de fissures. L’étude des relations propriété-microstructure a démontré l’impact des microfissures introduites volontairement au sein du matériau sur le comportement thermomécanique de la magnésie-spinelle et la magnésie-hercynite. Malgré des propriétés élastiques et dilatométriques assez similaires entre les inclusions de spinelle et d’hercynite, des mécanismes de microfissuration différents ont été observés, en particulier pour la magnésie-hercynite. En effet, la diffusion entre la magnésie et l’hercynite pendant le frittage a conduit à la formation de solutions solides de spinelles autour des agrégats d'hercynite. Ces solutions solides ont contribué à leur tour à la création de nombreuses microfissures fines par l’effet d’un différentiel de dilatation thermique avec la magnésie. Initialement présents dans la microstructure, les réseaux de microfissures favorisent l’augmentation de l'énergie de rupture des matériaux modèles pendant le WST. De plus, l’analyse des champs de déformation mesurée par 2P-DIC a révélé d’importantes bifurcation de la fissure principale pour les matériaux magnésie-hercynite. Le couplage de la 2P-DIC avec l’essai du WST a démontré que les réseaux de microfissures favorisaient le développement de la FPZ, qui induisait à son tour une augmentation notable de l’énergie de rupture. Dans une approche affinée des courbes-R, des énergies de rupture effectives ont été calculées à l'aide des longueurs de fissure mesurées par 2P-DIC. Cela a permis d'établir des liens étroits entre le développement de la FPZ et une résistance accrue à la propagation de la fissure. Les tendances observées à température ambiante lors des essais de WST ont été confirmées à l’aide d’essais de cyclage dans un nouveau banc de chocs thermiques

Analysis of process zone development in refractory materials by a refined Digital Image Correlation Method

National audienc

Fracture process analysis in Magnesia-Hercynite refractory materials by combining an enhanced Digital Image Correlation method with Wedge Splitting Test

International audienceMagnesia-Hercynite bricks destined for thermal shock applications in cement rotary kilns often show an enhanced crack propagation resistance due to an engineered microstructure design. In these materials, microcrack networks, resulting from the thermal expansion mismatch between magnesia matrix and Hercynite aggregates, promote the activation of energy dissipating mechanisms within the so-called Fracture Process Zone (FPZ) during loading. In this research, the fracture behaviour of a Magnesia-Hercynite material has been investigated by coupling an enhanced Digital Image Correlation method (2P-DIC) with the Wedge Splitting Test (WST). The coupling of these advanced characterisation methods is very effective in measuring important fracture parameters accurately and in highlighting characteristic fracture mechanisms, such as crack-branching. A refined R-curve approach is proposed with effective fracture energy calculations based on 2P-DIC measurements. The results demonstrate interesting correlations between FPZ development and an enhanced crack propagation resistance

Investigation of microstructure-property relantionships of magnesia-hercynite refractory composites by a refined digital image correlation technique

International audiencendustrial magnesia-spinel bricks destined for thermal shock applications often show more flexibility and improved crack growth resistance. Components from the spinel structure group are usually added to promote microcracking coming from thermal expansion mismatch. This leads to the development of toughening mechanisms that are very effective in improving the crack propagation resistance.Magnesia-hercynite composites were investigated in order to highlight their fracture process, with regard to their microstructure, by using Digital Image Correlation (DIC). The direct measurement of displacement fields between digital images of the reference state and the deformed one has provided valuable information on material deformation during loading. The aim of this work was to investigate the fracture behaviour of refractories through the coupling of the Wedge Splitting Test (WST) and DIC. By using a refined DIC process transformation taking into account a discontinuity of displacement, called 2P-DIC, a more effective characterisation of the fracture behaviour was achieved

Investigation of different discrete modeling strategies to mimic microstructural aspects that influence the fracture energy of refractory materials

The mismatch between the coefficient of thermal expansion of the constituents within refractory ceramics could advantageously be used to tune the fracturing behavior by inducing numerous microcracks within the microstructure. The Wedge Splitting Test (WST) is thus commonly used to characterize such different fracturing behaviors. The present study aims to model the different fracture behaviors of refractory ceramics by proposing a Discrete Element Method (DEM) approach to reproduce fracture energy variation and crack branching during WSTs.Two model ceramics are used as references: a highly brittle pure Magnesia and a quasi-brittle Magnesia Hercynite. By using the proposed DEM approach for local strength randomization, a wide range of fracture behaviors is simulated and compared to the reference materials. Moreover, the crack branching obtained from these simulations was qualitatively compared to the experimental observations by Digital Image Correlation (DIC). Finally, a discrete/continuous hybrid model (DEM/FVM) was proposed to optimize the WST simulations

Detection of cracks in refractory materials by an enhanced digital image correlation technique

International audienceThis paper is devoted to the study of the fracture behaviour of two industrialrefractory materials thanks to the development of a new technique of digitalimage correlation (DIC). DIC, already known as a helpful and effective tool for the measurement of displacement and deformation fields in materials, has been adapted to take into account displacement discontinuities as cracks. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex, while each subset can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening) during the loading with a higher spatial resolution than the one obtained by standard DIC. After having presented the principle of the new technique, its metrological performances are assessed from synthetic images and the choice of crack detection criterion is discussed. The capacity of this new technique is shown through a comparative study with standard DIC. Its application is led on magnesia-spinel refractory materials, specifically to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. We show that refractories with aggregates of iron aluminate spinel present a fracture mechanism with crack branching and can dissipate more energy thanks to a longer crack network

Improvement of Digital Image Correlation for the analysis of the fracture behaviour of Refractories

Comptes rendusInternational audienc

ATHORNA: an innovative device for monitoring thermal shock behaviour of refractories combining various in-situ advanced measurements

International audienc

Advanced measurements for in-situ THermo-mechanical monitORing of large sample uNder thermal grAdient (ATHORNA)

International audienc