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Optimisation of optical methods for strain field measurements dedicated to the characterisation of the fracture behaviour of refractories : Application to magnesia based materials

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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.
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... An acoustic emission event is the result of the propagation of an elastic wave coming from a localised source within the sample. Each event is counted, which finally lead to present the results as accumulative numbers of events versus temperature (Khlifi 2019). ...
... Hence, lowering the Weibull modulus (m) promotes non-linear mechanical behaviour by increasing the occurrence of diffuse microcracks. This fracturing process is quite similar to the phenomena that explain the quasi-brittle behaviour of certain refractory materials (Khlifi 2019). In addition to the influence on non-linear mechanical behaviour, the Weibull modulus also has a strong influence on the ultimate apparent tensile strength (as shown in Fig. IV-7 (a)). ...
... In this regard, the Two Parts-DIC (2P-DIC) method (Dupré et al. 2017) was used to detect and visualise the crack propagations during WST. Before presenting the experimental results, which are taken from (Khlifi 2019), the crack length notion should be introduced. According to Khlifi et al. (Khlifi 2019), thanks to the calculated strain field by 2P-DIC, the crack length can be evaluated in two ways, as shown in Fig. V-4: ...
Thesis
Cette thèse s'inscrit dans le cadre du projet Européen ATHOR (Advanced THermomechanical Multiscale Modelling of Refractory Linings). Les matériaux réfractaires sont des céramiques hétérogènes, résistantes à des températures élevées pour lesquelles, dans de nombreux cas, des microfissures préexistantes au sein de la microstructure jouent un rôle clé dans la résistance aux chocs thermiques. La Méthode des Eléments Discrets (MED) est aujourd’hui considérée comme un outil numérique majeur qui peut contribuer, à l'avenir, à concevoir des microstructures plus performantes. Ainsi, cette thèse est dédiée à la simulation numérique de matériaux réfractaires intégrant leur microstructure, leurs hétérogénéités, ainsi que la présence de fissures, et leur influence sur le comportement mécanique macroscopique. Ces travaux ont par ailleurs été réalisés dans le cadre d'un partenariat avec la société "ITASCA consultants". Au sein de l’outil Particle Flow Code (PFC), utilisé comme plateforme numérique MED, le model de contact Flat Joint Model (FJM) a été choisi car celui-ci permet de modéliser des microstructures de grains imbriqués, analogues à celles des matériaux réfractaires. Afin de développer des modèles numériques permettant de décrire les relations entre la microstructure et le comportement thermomécanique macroscopique, il est essentiel de mettre en oeuvre une approche multi-échelles, micro à macro, précise pour chacune des propriétés physiques clés intervenant dans la tenue aux chocs thermiques, en commençant par les propriétés d’élasticité. Dans cet objectif, la MED n'étant pas, à ce stade, aussi robuste que la Méthode des Eléments Finis (MEF), une approche d'homogénéisation périodique est proposée pour des milieux continus. Cette approche a ici été validée en comparant les résultats obtenus à des matériaux modèles, des modèles analytiques et MEF. A l’échelle méso-scopique, un modèle MED utilisant une approche statistique pour imiter l'influence mécanique des microfissures préexistantes est mis en oeuvre. Cette approche a également été validée par des données mécaniques expérimentales. Par la suite, afin de vérifier l'applicabilité du modèle MED proposé, des simulations de Wedge Splitting Test (WST) sont proposées pour étudier le processus de fissuration et le comparer qualitativement aux résultats expérimentaux obtenus en DIC. Finalement, un modèle hybride discret/continu (MED/MVF) est proposé pour optimiser ces simulations de WST et réduire les temps de calculs. Ces résultats clés ouvrent de nouvelles voies très intéressantes d'utilisation de la MED pour prédire le comportement thermomécanique de matériaux hétérogènes contenant de nombreuses microfissures pouvant se propager simultanément.
... More information on thermal stresses in refractory linings could be found in the cited publications. [98][99][100] In an industrial vessel, such as steel ladle, preheating is very important, as it closes joints between bricks, as well as it increases temperature of the lining and of the steel shell (allows lining to expand), leading to a reduction of temperature gradient during tapping (and thus decreases tensile stresses generated due to hot thermal shock). 98 Shape of the bricks and appropriate preheating procedure are especially important in the type A restriction (joints closure). ...
... It is worth to mention that some experiences regarding crack monitoring with DIC methods have already been acquired at the IRCER laboratory (located in Limoges, France) during the PhD studies of Younes Belrhiti and Imad Khlifi. 99,122 However, until the beginning of this present work, these measurements were performed in IRCER lab only at ambient temperature and therefore, one of the goals for this PhD study was to perform mechanical tests with strain field monitoring by optical methods at high-temperature. This part of the work has been performed using the Brazilian test but the developed methodology and suggested improvements are useful also in other high-temperature tests coupled with the DIC method. ...
... Materials selected for this investigation originate from the PhD study of Imad Khlifi (performed in frames of the FIRE network) who worked on magnesia-based refractories with spinel aggregates. 99 The key point of their microstructure consists on introduction of magnesium aluminate spinel (MgAl2O4) and/or iron aluminate spinel (FeAl2O4) aggregates, being well-known for improving both fracture and thermal shock resistances of magnesia-based refractories. ...
Thesis
Les réfractaires d'alumine spinelle sont connus pour leurs bonnes performances en tant que revêtement de poche d'acier. Une première partie de l’étude s’est focalisée sur le comportement thermomécanique de briques industrielles d'alumine spinelle et a permis de mettre en évidence le développement de microfissures (au cours du refroidissant en dessous de 650 °C), une transition ductile fragile (aux environs de 1100 °C) et des changements de teneurs en spinelle et de stoechiométrie étroitement liés au traitement thermique appliqué. Une deuxième partie de l’étude concerne des bétons modèles d'alumine spinelle, et a permis de révéler l’impact de différents types d’agrégats, de différents types d'alumines réactives et de différents ciments. En sus des investigations portant sur les propriétés thermomécaniques des matériaux, une partie importante de cette thèse a également été consacrée à l'amélioration de deux techniques expérimentales. La première concerne des mesures de champs de déformations par corrélation d'images numériques (DIC) lors d’essais mécaniques à haute température (1200 °C). Afin d’optimiser la précision de mesures dans ces conditions expérimentales difficiles, une optimisation a spécifiquement porté sur des mouchetis résistant à haute température et sur l’application de prétraitements d'images avant les mesures DIC proprement dites. La deuxième concerne un nouveau banc de choc thermique (dispositif ATHORNA) qui est en cours de développement. Les résultats expérimentaux, obtenus sur ce banc par des méthodes optiques, ont pour la première fois pu être comparés aux résultats de modélisés par éléments finis. En outre, une étude comparant les résultats des tests d'ATHORNA et des essais de wedge splitting, réalisée sur des matériaux modèles de spinelle de magnésie, a permis de démontrer qu'ATHORNA pouvait également être utilisé pour comparer la résistance aux chocs thermiques de réfractaires.
... The linings shall perform different functions, including protection of the industrial equipment, safety of personnel, confinement of the solid, liquid and gases in the containers and a contribution to limit heat loses [1]. The service temperatures of the vessels may reach 1650ºC for the steel ladles and 1450 ºC for the cement kilns [1,2]. Besides the high temperatures, other actions may influence the thermomechanical behaviour of the linings, such as mechanical stresses, abrasion, creep, thermal shock and corrosion. ...
... Besides the high temperatures, other actions may influence the thermomechanical behaviour of the linings, such as mechanical stresses, abrasion, creep, thermal shock and corrosion. The working lining of these vessels are usually built with dry-joint masonry [2][3][4][5][6][7]. ...
... (1) kn = ab dn b-1 + c (2) Preprint version, Reference: Oliveira R, Rodrigues JP, Pereira JM, Lourenço PB, Marschall HU (2021), Thermomechanical behaviour of refractory dry-stacked masonry walls under uniaxial compression. Engineering Structures, 240, 112361. ...
Article
Vessels used in industrial high temperature processes of steel and cement production are protected by refractory linings built with dry-stacked masonry, which plays a crucial role on the overall behaviour of the vessels. This paper presents the results of an experimental and numerical research on dry-joint refractory masonry subjected to uniaxial compression. Its main purpose is to fully characterize the masonry walls at different temperatures and different loading conditions. Several aspects that may influence the behaviour of these walls have been tested, namely the loadbearing capacity, the behaviour under cyclic loading and the restrained thermal elongation. The experimental results allowed to identify the effects of the stress concentrations caused by brick’s height imperfections in the mechanical behaviour of the bricks and in the loadbearing capacity of the specimens, the evolution of the wall’s Young’s modulus with the load application, the developed crack patterns and the mechanical behaviour of the samples at ambient and high temperatures. Numerical models were also developed to simulate the behaviour of the walls under different testing conditions and a good agreement with the experimental results was obtained. The concrete damaged plasticity model, using a micro-modelling approach, proved to be suitable for representing the behaviour of these walls at ambient and high temperatures.
... In this work, the fracture behaviors of the castables were characterized using WST and AE, which were frequently adopted [47,50]. Fig. 8 depicts the vertical force-displacement curves for castables treated at 110 and 1600 • C. As for 110 • C, the 3 d specimens had higher fracture forces and displacements than the 1 d specimens, indicating that more hydrates would enhance the bonding strength [35]. ...
Article
The present work investigated the pore structure evolution and fracture behavior of calcium aluminate cement (CAC)-bonded alumina-spinel castables treated at 110 and 1600 °C after curing at 25 °C for 1 day and 3 days. The pore structure and fracture behavior were characterized by mercury intrusion combined with micro-CT scanning and a wedge splitting test coupled with acoustic emission, respectively. The results showed that the hydration degree was enhanced by extending the curing time, and more hydrates were conducive to the generation of complex pore structures after drying. In comparison, the heat treatment at 1600 °C resulted in a substantial reduction in nano-sized pores, the 3 d sample obtained developed and complex micro-sized pores than 1 d sample. Therefore, specimens cured for 3 d had better hot modulus of rupture (HMOR) and thermal shock resistance compared to those cured for 1 d, which could be attributed to the induced tortuous microcrack propagation within the matrix and along the aggregate-matrix interfaces.
... The working lining of steel ladles and the lining of rotary cement kilns are usually composed by refractory dry joints masonries [3 -6]. Due to the extreme service conditions, these layers must not only withstand the high temperatures, up to 1650ºC in the case of steel ladles and up to 1450ºC in the case of cement kilns, but other types of actions that directly influence its thermomechanical behaviour for which premature failure should be avoided: mechanical stresses, cracking, abrasion, creep, thermal shock and corrosion [2,7]. ...
Article
Industrial vessels used in high temperature processes of steel and cement production are protected by refractory linings built with mortarless joints. These dry joints, formed by stacked bricks have a crucial importance on the mechanical behaviour of the lining. The stiffness and consequently the stresses generated by the thermal elongation are reduced due to the joints. The present article presents the results of experimental and numerical tests on the thermomechanical behaviour of these joints. The compressive strength of the brick was assessed at ambient and high temperatures and a statistical analysis of the distribution of the bricks’ shape imperfections was carried out. Several studies have been carried out on the normal behaviour of the joints, such as: classical joint closure test; bed joint closing action in a masonry wallet measured with a DIC; effects of brick’s height imperfections on its loadbearing capacity; effects of brick’s height imperfections on the wall’s behaviour at ambient and high temperatures and a comparison between the bed and head joints behaviour. To characterize the joint’s tangential behaviour at high temperatures a novel device was developed and presented.
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Full-text available
This paper is devoted to the study of the fracture behaviour of two industrial refractory materials thanks to the development of a new technique of digital image 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.
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