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Mechanics of refractory materials at high-temperature for advanced industrial technologies

Final Report Summary - HOTBRICKS (Mechanics of refractory materials at high-temperature for advanced industrial technologies)

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1. Description of the HOTBRICKS project objectives

Rock-like materials are of great engineering interest, because they are employed for many industrial purposes and their high melting temperature and the great thermal and chemical stability make them ideal for high-temperature applications, as for instance to work molten steel and iron in steel mills. The project addresses the development of an accurate constitutive modelling of rock-like materials with a specific consideration of thermal effects and the objective is the design of a new class of elements, to be employed for technologies involving high-temperature.
The HOTBRICKS consortium involves two research teams: one working at the University of Trento (UNITN) specialized in the Mechanics of Solids and Fracture and the Vesuvius group (VESUVIUS), a global leader in the supply of specialized refractory products and packaged technology solutions to the steel making, foundry and glass industries). The consortium has been set up to attain the following main research targets:
- Constitutive modelling of rock-like materials at high temperature;
- Modelling of failure and fracture of thermal shock resistant elements;
- Design and experimental validation of innovative pieces involved in the handling of molten materials.
The two partners have collaborated through secondment of Early Stage, Experienced and More Experienced Researchers and recruitment of research fellows. In particular, the research targets have been achieved through the implementation of the following Work Packages:
WP1: Microstructural and Mechanical characterization of refractory materials for high-temperature applications;
WP2: Thermo-mechanical modelling of refractory materials;
WP3: Thermoplastic material instabilities;
WP4: Computer implementation and simulation of processes involved in high-temperature technology;
WP5: Production and final testing of mechanical systems involved in high-temperature technology.

2. Description of the work performed in the HOTBRICKS project and of the main results achieved

The detailed characterization of refractory materials, both from a microscopic and a macroscopic point of view, is the key issue to a correct modelling of the materials to be employed in the steel industry, related to safety enhancement, pollution reduction and improving of the economics of process. Microstructural analysis of the materials has been guided by UNITN in order to provide a detailed starting point for the mechanical modelling. Testing protocols for experiments have been performed at room temperature and at high temperature by VESUVIUS with the purpose to set-up an effective mechanical characterization of refractories. Experiments at room temperature have been developed to characterize elastic and inelastic parameters of refractory material. Experiments at high temperature have been performed by exploiting an original facility developed by VESUVIUS for testing ceramic samples under uniaxial loading at 1500 °C.
The development of the mechanical modelling of elastoplastic constitutive behaviour tailored for rock-like materials at high temperature has been dealt by UNITN. This is a fundamental step in the accurate description of the mechanical behavior of refractories working at metal casting temperatures. An elastic model has been first developed to describe the thermal effects in the stress/strain behaviour of refractory materials within the elastic range. An elastoplastic constitutive model, based on a smooth yield surface and simple hardening laws, has been developed for brittle-cohesive materials and tested for numerical implementation via F.E.M. This model considers a new class of isotropic hardening laws, which can be given both an incremental and a corresponding finite form, describing a smooth transition from linear elastic to plastic behaviour, incorporating linear and nonlinear hardening.
Then, a thermodynamically-consistent constitutive framework has been introduced for rate-independent, small-strain, thermo-elasto-plasticity. In this regard, a thermoplastic model with an account for elasto-plastic coupling has been formulated for isothermal and adiabatic conditions, in terms of incremental constitutive law. This constitutive description features the possibility of non-associated flow for the inelastic deformation and non-linear evolution for hardening parameters. The occurrence of thermal softening has been related to the failure of ellipticity for the tangent constitutive operator and to shear band nucleation in thermoplastic materials. It has been found that thermomechanical couplings play an important role in these phenomena. Two conditions, namely, the singularity of the isothermal and the adiabatic acoustic tensors have been found to define the ellipticity condition, at which failure the emergence of shear band is predicted. Discontinuities in the temperature rate and in the spatial derivatives of the heat flux are shown to be essential in this analysis.
The developed constitutive models, capable to describe complex effects (such as coupling and thermo-plasticity) have been implemented as external subroutines to be used with Finite Elements commercial codes, in order to overcome the lack of advanced thermo-mechanical models. The constitutive parameters governing the thermo-visco-plastic behaviour have been calibrated on the experimental results in order to effectively predict the refractories mechanical behaviour.
New devices for the steel industry have been designed showing an increase in the safety factor in operating conditions.

3. Final results and their potential impact

The final results of the HOTBRICKS project are:
- the definition of a testing protocol and the microscopic and mechanical characterization of refractory materials at room and high-temperature;
- the development of a thermo-visco-plastic model based on the obtained experimental results;
- the analysis of thermoplastic instabilities and the related failure mechanisms;
- the implementation of the developed constitutive modelling, the model parameters calibration based on experimental data, and the simulation of thermal shocks;
- the design of optimized refractory components and their experimental testing.
As overall result, the strong interaction between the two partners have provided new technologically oriented numerical design methods for high-temperature applications. The developed tools are currently exploited to improve the products manufactured by VESUVIUS, with an increase in the safety factors and reduction of material waste, energy and environmental pollution. Moreover, results from the HOTBRICKS project enable VESUVIUS to implement appropriate processes and technology to be applied more sustainably, cost-effectively and faster. The research outcome has the potential to enhance the economic performance and competitiveness of industry by producing a team of highly qualified professionals, equipped with the breadth and depth of technical competencies and practical skills to make an immediate impact in the field, and fulfill the skill gap often identified by the industrial sector.