Invited Speakers


Prof. Mikhail Itskov

Prof. Mikhail Itskov

Department of Continuum Mechanics, RWTH Aachen University, Germany
Speech Title: Mechanics and Thermodynamics of Strain-Induced Crystallization

Abstract: Strain-induced crystallization is a very interesting and important phenomenon appearing in elastomers as for example natural rubbers subjected to large stretches. In this case, polymer molecules come very close to each other and begin to form crystallites oriented mostly in the direction of stretch. These crystallites can retard or even stop crack propagation in highly loaded elastomeric structures as for instance truck tires and thus improve their durability and lifetime. So far, the strain-induced crystallization has mostly been investigated by using X-ray diffraction, which is a very expensive and hardly accessible procedure for a classical mechanical lab. Moreover, the X-ray diffraction is not able to measure calorimetric effects always accompanying the strain-induced crystallization. These effects results from the phase transition during crystallite nucleation, growth and melting. In this regard, the infrared thermography combined with mechanical measurements appears to be very suitable. Indeed, by means of such a procedure the heat production and absorption as well as the crystallization degree during cyclic loading can be detected and characterized in natural rubbers. Based on the energy balance the intrinsic dissipation due to viscosity and stress softening can thus be evaluated under cyclic loading. Energy contributions to the hysteresis loop converted into heat and stored in the material can further be separated. In the present contribution, we also present a physically based constitutive model for filled natural rubbers coupled with infrared thermography based calorimetry to study the strain-induced crystallization. The kinetics of phase transition outside thermodynamic equilibrium is also discussed and underlying mechanisms of nonequilibrium strain-induced crystallization are interpreted. To capture multiaxiality of strain-induced crystallinity, we apply the analytical network-averaging concept proposed in our earlier papers. Predictions of the so-developed model of the strain-induced crystallization demonstrate good agreement with various experimental data.

Keywords: Strain-induced crystallization, calorimetric effects, physically based modeling, natural rubbers.



Prof. Miklos Zrinyi

Prof. Miklos Zrinyi

Department of Biophysics and Radiation Biology, Semmelweis University, Hungary
Speech Title: Mechanical Behavior of Fibrous Materials

Abstract: Fibrous materials are becoming critical technological applications due to their high mechanical performances and low mass. When the fibrous texture is subjected unidirectional strain along one of the axis, the deformations are inherently nonlinear the corresponding stress depend on the underlying material properties. It is widely accepted that the mechanical behavior depends on the strength and the toughness of single fibres as well as on their geometrical arrangement. Little is known on the deformation mechanism and rigidity of spun fabrics despite of the fact that strength and load bearing capacity of these materials are important factors for several technological and biomedical applications. Unidirectional strain-controlled experiments on fibrous electrospun networks during elongation have been studied. The aim of the project to determine and classify the essential mechanical and structural parameters that control the elasticity of biological- and artificial fibrous tissues. Experimental technique combined with the modern statistical theories provide significant potential for the characterization of fiber texture during deformation and suggests reliable mechanical models. The study aims were also to understand and characterize the damage formation in weak electrospun fibers subjected to an external force. We have found a characteristic (typical) load - elongation dependence for the 2D electrospun fiber mats. At the beginning of the applied load, the force has an approximate linear relationship in the low strain regime. At some higher elongation, the slope of this dependence is continuously decreasing and the stress reaches a maximum. Then, increasing the extension, the force declines. Detailed analysis of the shape of the loading curves were carried out by enlarging parts of the loading curves. We have found at small scale that the loading curves show tooth-like profile. These experiments reviled that the unusual loading curve is the result of continuous stiffness reduction caused by damage formation due to either rupture of adhesive fiber-fiber bond fracture or fiber degradation. It is important to know how microscopic failure processes gives rise to macroscopic deformation. Two basically different theoretical approaches have been introduced recently. The Fiber Bund Model (FBM) and the Sacrificial Bond and Hidden Length (SBHL) model. The FBM model takes into account the fiber breaking during deformation, the SBHL model describes the effect of fiber slipping and uncoiling. Both approaches result in similar sequential force drops during elongation, however the FBM model predicts stiffness reduction, while the SBHL model predicts toughening during elongation. Our working hypotheses is based on the assumption, that in real weak fibrous texture, both fiber sliding and unfolding as well as fiber splitting occur during deformation. In this proposal an effort is made to interpret of the unusual mechanical properties of weak fiber texture on the basis of these theoretical models. Numerical simulation based on FBM and SBHL model provide a better understanding of mechanism of deformation. The reported experimental technique has significant potential for the characterization of fiber texture and suggests a further numerical simulations and development of probabilistic models for the load bearing behavior of electrospun fibers.

REFERENCES:
E.Sipos,T.Kaneko, M.Zrinyi: SCIENTIFIC REPORTS 2816 6 (2019)
E.Sipos, M.Zrinyi: JOURNAL OF MOLECULAR LIQUIDS 329 115459 (2021)



Dr. Enrico Zacchei

Dr. Enrico Zacchei

Itecons – Institute for Research and Technological Development in Construction, Energy, Environment and Sustainability, Portugal
Speech Title: Multifactorial and Multiphases Models for Chloride Ingress in RC Structures under Unsaturated Conditions

Abstract: The attack of chloride ions is one of the most important factors affecting the durability of reinforced concrete (RC) structures. The ingress of chloride ions into concrete is usually studied by assuming constant diffusivity and constant surface chloride concentration. However, these two approximations could estimate in an unsatisfactory way the chloride concentration in structures thus the lifetime assessment. Several factors influence the chloride concentration and ingress mechanisms in the convection area. In this presentation, a new time-dependent multifactorial and multiphase model that accounts some effects on chloride surface concentrations in the convection zone is proposed. Several values have been collected to identify the position and the chloride concentration in the section between the diffusion and convection zones. Diffusivity, which is the key parameter of the mechanical diffusion, accounts the water/cement ratio, chloride binding, temperature, concrete age, humidity, concrete deformation, and damage. The surface chloride model considers environment humidity, temperature, irregularities of the concrete surface, and convection area of concrete. Advanced numerical solutions have been carried out to consider space and time dependencies in the model. Results show that the error function-based solutions could underestimate the chloride concentration C for periods < 10 years and for concrete depths > 4.0 cm.

Keywords: Chloride diffusion, superficial chloride, non-constant diffusivity, convection area, concrete irregularities.




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