The aim of the paper was to investigate the helical rolling parameters (a number of passes) for the microstructural modification and the low-temperature impact toughness improvement of the 09Mn2Si High Strength Low-Alloyed (HSLA) steel. In order to achieve this purpose, work spent to crack initiation and propagation was analyzed and compared with patterns of fracture surfaces. The microstructure and impact toughness values were presented in the temperature range from +20 to –70°C. Also, the fracture mechanisms in individual regions on the fracture surfaces were discussed. In addition, a methodology for computer simulation of the process was developed and implemented within the framework of the excitable cellular automata method and its integration with the kinetic theory of fracture. Finally, a theoretical analysis of the effect of grain shapes and orientations on the strain response patterns of a certain meso-volume simulating the material after the helical rolling was carried out.

A new hybrid discrete-continuum cellular automata approach is proposed to simulate the process of new phase/grain nucleation and growth. The method couples classical thermomechanics and the logics of cellular automata switching. Within the framework of the hybrid discrete-continuum cellular automata method, the space occupied by the simulated specimen is represented as a cellular automaton—a set of ordered active elements. Every element imitates an immovable region of space related to a part of material being characterized by the certain numerical parameters. The proposed approach enables calculating the magnitude of the local force moments and simulating dissipation of torsion energy leading to the formation of new defect structures. To illustrate the capacity of the proposed hybrid discrete-continuum cellular automata approach, the numerical simulations of thermally activated recrystallization of pure titanium near crack faces were conducted. The 3D cellular automaton simulated the microstructure evolution of the V-notched specimen region that imitated the crack tip vicinity at high homologous temperatures. Calculation of heat expansion with simultaneous thermal stresses accumulation and microrotation initiation was incorporated in the simulations permitting thereby to evaluate the local entropy and to monitor the evolution of crystal defects from initiation to storage. Perspectives of the proposed algorithms for simulations of the mechanical behavior of materials experiencing thermally induced twining or phase transformations are discussed.

By means of computer simulation, deformation behavior of 09Mn2Si structural steel subjected to helical rolling was studied. Two different kinds of grain structure orientation were simulated. Calculations were aimed at constructing patterns of spatial distribution of (i) elastic energy, (ii) torsion energy, and (iii) components of local force moments with the use of excitable cellular automata method. It has been shown that preferential pattern of grain structure orientation gives rise to a high level of microrotations, which, in turn, leads to decreasing the stress level. Comparison of two studied structures under uniaxial cyclic tension has shown that the preferential pattern of grain structure orientation along the loading axis provides higher resistance to defect nucleation compared to a transversely oriented one.

The structure and mechanical properties of the 09Mn2Si high-strength low-alloyed steel after the five-stage helical rolling (HR) were studied. It was revealed that the fine-grained structure had been formed in the surface layer ≈ 1 mm deep as a result of severe plastic strains. In the lower layers, the "lamellar" structure had been formed, which consisted of thin elongated ferrite grains oriented in the HR direction. It was shown that the five-stage HR resulted in the increase in the steel fatigue life by more than 3.5 times under cyclic tension. The highest values of the number of cycles before failure were obtained for the samples cut from the bar core. It was demonstrated that the degree of the elastic energy dissipation in the steel samples under loading directly depended on the area of the grain boundaries as well as on the grain shapes. The fine-grained structure possessed the maximum value of the average torsional energy among all the studied samples, which caused the local material structure transformation and the decrease in the elastic energy level. This improved the crack resistance under the cyclic mechanical loading. The effect of the accumulation of the rotational strain modes at the grain boundaries was discovered, which caused the local structure transformation at the boundary zones. In the fine-grained structure, the formation of grain conglomerates was observed, which increased the values of the specific modulus of the moment of force. This could be mutually compensated due to the small sizes of grains. At the same time, the coarse-grained structures were characterized by the presence of the small number of grains with a high level of the moments of forces at their boundaries. They could result in trans-crystalline cracking.

A modification of a multiscale hybrid discrete-continual approach of excitable cellular automata is developed.The new version of the method is completed by taking into account porosity and nanocrystalline structure of a material and the algorithms of calculation of local force moments and angular velocities of microrotations.The excitable cellular automata method was used to carry out numerical experiment (NE) for heating of continuous and nanoporous specimens consisting of nanocrystalline TiAlC coatings.The numerical experiments have shown that nanoporosity allows to substantially reducing the rate of collective crystallization.Nanoporosity slowed down propagation of the heat front in specimens.This fact can play both positive and negative roles in deposition of coating and its further use.On the one hand, by slowing the heat front propagation one can significantly reduce the level of thermal stresses in deeper layers of the material.On the other hand, such deceleration in case of the high value of the coefficient of thermal expansion can give rise to the formation of large gradients of thermal stress, which initiate nucleation and rapid growth of the main crack.

The paper presents a next stage in the development of excitable cellular automata method which allows one to model energy flow propagation in a loaded solid under external mechanical and thermal-dynamic loads. At this stage the method is proposed for assessing the probability of crack initiation near geometric stress concentrators. Patterns of various mechanical characteristics distribution on the specimen surfaces are given, as well as the time dependence graphs of the average values of these parameters in the vicinity of notch tip. Some aspects of the notch shape effect over the deformation patterns as well as prospects of the proposed approach development are discussed.

A Hybrid discrete-continuum Cellular Automata (HCA) approach based on coupling classical thermomechanics and logics of CA-switching to simulate new phase generation and grain growth is proposed. On the basis of the HCA, numerical experiments on thermal-activated recrystallization of pure titanium in the vicinity of crack edges are performed. The 3D cellular automaton is aimed at simulating the behavior of a V-notched specimen region that imitates the crack tip vicinity. Numerical experiments provide calculating heat expansion in the material under study through taking into account thermal stress accumulation and microrotation initiation. The latter gives rise to the generation of new defects and increases local entropy.

The paper deals with the impact deformation and fracture behaviour of commercial plain carbon pipe steel 17Mn1Si The explicit account of the internal grain structure, temperature and geometry of the notch have been made in theoretical physical mesomechanics formulation aiming at in depth understanding of the role of strain energy factors in dynamic fracture. Theoretical method of excitable cellular automata and laboratory impact bending tests followed by fractographic analysis were paired with time frequency analysis of acoustic emission accompanying local deformation and fracture processes. It was shown that formulation of the crack opening criterion under dynamic loading conditions should explicitly account for rotation energy accumulation and incorporate the microscopic temporal and spatial details of defect generation from internal (grain) boundaries. A fairly good agreement has been found between the strain energy characteristics obtained from mechanical loading data and independently measured acoustic emission signal being distinguished in terms of consumed and released energy. The impact toughness almost linearly decreased with temperature, which was consistent with fractographic observations. At the stage of crack initiation, when the energy dissipation processes at the internal structure elements significantly affect the initiation of dynamic fracture, the acoustic emission energy reduced in proportion to the expended mechanical energy, which considerably decreased with temperature. The vital role of the energy release at interface/grain boundaries and its decreased significance with decreasing temperature was demonstrated both in numeric simulations and in dynamic experiments.

The multiscale hybrid discrete-continuous approach of excitable cellular automata was applied to account for the porosity and nanocrystalline structure when taking an idealized quasi-elastic thermal barrier coating as an object under simulation. The developed technique includes a set of algorithms for calculating the local moments of forces and angular velocities of micro-rotations occurring in a heterogeneous medium with regard to energy dissipation. It was found that at thermal cycling of polycrystalline system high level microstresses occurs resulting from the thermal expansion anisotropy of grains. The stress level was shown to decrease with increasing fraction of rotational modes. This tendency takes place over the entire depth of the studied specimen and during the entire thermal loading time. The simulation results indicate on the existence of an optimal porosity level when the structure exhibits the highest relaxation capacity.

The fracture energy of 12Cr1MoV steel specimen with V-shaped, U-shaped and I-shaped notches under impact loading was measured and analysed. The results were described using three common approaches to ductile-brittle fracture: force-based, energy-based and strain-based. Within the stage-wise approach of physical mesomechanics of materials, the rate of increase/decrease of load at the stage of initiation and propagation of a macroscopic defect was evaluated, providing a good correlation with the work of fracture. The excitable cellular automata technique was applied to simulate the deformational behaviour of the specimens with different shape of notches. It was demonstrated that in the case of the blunted notch, the maximum impact toughness is facilitated by a more uniform distribution of the load along the notch, which hinders brittle fracture at lower testing temperature. For the specimen with the sharp I-notch, the bands of localised shear are oriented normally to the loading axis, inhibiting macroscopic localisation of strain and crack propagation. For this reason, the impact toughness of the specimen with the I-notch appeared to be higher than that of the V-notched one. Using the fractographic analysis and the size of shear lips as a quantitative fracture parameter, a physical-mechanistic scheme of fracture was suggested for the case of enhanced localised plasticity near the stress riser.

In the study a Hybrid discrete-continuum Cellular Automata approach (HCA) based on coupling classical thermomechanics and logics of CA-switching to simulate new phase generation and grain growth is proposed. On the basis of the HCA the numerical experiments on thermal-activated recrystallization of pure titanium in the vicinity of crack edges were conducted. In doing so the 3D cellular automaton simulates the behavior of the V-notched specimen region that imitates the crack tip vicinity. Numerical experiments are aimed at calculating heat expansion in the material under study through taking into account thermal stresses accumulation and microrotation initiation. The latter gives rise to generation of new defects and increasing the local entropy

The paper proposes a discrete-continual method of excitable cellular automata for simulating the stress-strain state at crack tips and in notches with account of lattice curvature and plastic distortion through ion motion from lattice sites to interstices. The proposed nonlinear method allows one to determine the crack type and the character of fracture, to predict the possibility of dynamic rotations and structural turbulence, and to describe the processes of nonlinear wave structural transformations in strain localization bands involved in microporosity and tearing mode cracking.

The structure and impact toughness of the pipeline 17Mn1Si steel have been studied. The main attention was paid to the analysis of various conditions of stress concentration under dynamic loadings. The process of strain localization with increasing stress state stiffness at the tip of the concentrator with decreasing testing temperature was investigated. Impact loading diagrams for specimens with various stress concentrator shapes were registered and analyzed.

The effect of the microstructure parameters as well as the testing temperature on the impact toughness of 17Mn1Si steel has been studied. The interrelation between the testing temperature during impact bending and deformation and fracture energy consumption has been studied. The relation between fracture energy characteristics at high-rate impact bending and the microstructure of the steel has been established. It is shown that the decrease of the testing temperature leads to almost linear reduction of the impact toughness.

The paper deals with a theoretical and experimental study of the relationship between the microstructural parameters, mechanical properties, and impact deformation and fracture of steels using the example of 17Mn1Si pipe steel. A model for the behavior of a polycrystalline grain conglomerate under impact loading at different temperatures was proposed within a cellular automata framework. It was shown that the intensity of dissipation processes explicitly depends on temperature and these processes play an important role in stress relaxation at the boundaries of structural elements. The Experimental study reveals the relationship between pendulum impact test temperature and the deformation/fracture energy of the steel. The impact toughness was shown to decrease almost linearly with the decreasing test temperature, which agrees with the fractographic analysis data confirming the increase in the fraction of brittle fracture in this case. It was shown with the aid of the proposed model and numerical simulations that the use of the excitable cellular automata method and an explicit account of test temperature through the possibility of energy release at internal interfaces help to explain the experimentally observed features of impact failure at different temperatures.

Impact loading curves and fracture energy of the notched 12Cr1MoV ductile steel specimens are analysed. The qualitative description and quantitative parameters are obtained for major stages of ductile and brittle fracture depending on the shape of the notch and the stress stiffness ahead. It was shown that a zone with enhanced plasticity forms in the vicinity of V-, U- and I- shaped notches at 20 degrees C testing temperature, giving rise to ductile fracture. The stress stiffness at the notch tip increased with testing temperature reduced to -40 degrees C. We demonstrated that the size of shear lips on a fracture surface is a quantitative characteristic of fracture. Using this approach, which is close by nature to non-linear fracture mechanics, together with the quantitative description of fracture surfaces, a physical-mechanical scheme of the specimen fracture was suggested for the case of enhanced and localized (constrained) plasticity near the stress concentrator tip.

The energy of 12Cr1MoV steel specimen fracture with V-, U- and I-shaped notches under impact loading was measured and analyzed. The results were described using common energy-based approach to ductile-brittle fracture. Within the stage-wise approach of physical mesomechanics of materials, the rate of increase/decrease of load at the stage of initiation and propagation of a macroscopic defect was evaluated, providing a good correlation with the work of fracture. The excitable cellular automata technique was applied to simulate the deformational behavior of the specimens with different shape of notches. It was demonstrated that in the case of the blunted notch, the maximum impact toughness is facilitated by a more uniform distribution of the load along the notch, which hinders brittle fracture at lower testing temperature. For the specimen with the sharp I-notch, the bands of localized shear are oriented normally to the loading axis, inhibiting macrolocalization of strain and crack propagation. For this reason, the impact toughness of the specimen with the I-notch appeared to be higher than that of the V- notched one.

A hybrid discrete-continuum Excitable Cellular Automata (ECA) approach is proposed to study the vector character of the mass and energy transfer flows in a solid under high-rate loading. Based on the torsion energy calculations, the model of generation and accumulation of defect structures is proposed. Numerical simulations of uniaxial shock loading of specimens with three typical notch shapes are performed to validate the proposed approach. It is shown that stress relaxation occurs most effectively when the modulation of various components of the force moment takes place at various scales. This offers the possibility to reduce the stress concentration substantially by tailoring the materials microstructure.

The computational approach for modeling the deformation behavior and fracture of materials with stress concentrators is proposed on the basis of the cellular automata technique. Numerical simulations performed on miniaturized specimens under dynamic loading are accompanied by laboratory tensile testing under quasi‐static conditions applied to the specimens of a ductile structural steel with three typical notch shapes. Using in situ acoustic emission paired with digital image correlation techniques, it is shown that the notch shape exerts a very strong influence on the development of localized plastic flow and crack initiation ahead of the notch tip. It is concluded that the stress release occurs most effectively when the modulation of various components of the force moment takes place on mesoscopic scale.