Plasticity Zone Research Articles

Determination of Dynamic Stresses and Displacements under the Action of an Impact Load on a Two-Layer Structure during the Indentation Process

Introduction. Numerous researchers of the reliability of building structures pay attention to hardness, an important characteristic of the structural material. It is determined by indentation — pressing the tip of the tool into the surface. The advantages of dynamic indentation methods and the distribution of stress intensity on the surface and inside the sample are investigated. However, the condition of layered materials on impact has been poorly studied. The objective of the presented work is to consider indentation for a two-layer sample and determine the sensitivity of the top layer to the strength of the substrate. This will allow us to identify significant characteristics of the strength properties of homogeneous and heterogeneous structures.Materials and Methods. An elastoplastic model of material behavior and a shock indentation scheme were used, which took into account the masses of the indenter and the striker coupled by linear springs. The surface of the indenter was conical, the opening angle was 120°. The impact was simulated in the MATLAB system. Finite element model in Ansys APDL was used to verify the data and analyze the results of the experiment. Traditional models of elasticity theory were used for calculations. The behavior of the material in the zone of plastic deformation was described using the options of multilinear isotropic hardening and the von Mises plasticity criterion.Results. The results of comparing three versions of varying the level of yield strength in the bottom layer are presented: when the yield strength in the bottom layer is half as high as the top one, equal to it, and twice as high. Displacements at different observation points for samples with a top layer of 2 mm and 1 mm were analyzed. In the first case, under horizontal shear, the displacement indices inside the sample did not change if the yield strength level was twice lower or higher than in the top one. If these indicators were equal, the difference became noticeable. In the second case (layer 1 mm), the difference in displacement was visible at all observation points. Thus, it can be reasonably concluded that a structure with a smaller top layer is more sensitive to impact. In the course of the research, it became known that vibrations associated with the transition to the plasticity zone occurred in the 2 mm zone, and elastic damping vibrations occurred below this zone. We solved the classification problem for the top layer of the material with changing characteristics of the base. The indicator for comparison was the Brinell hardness (HB) in the range of 200–600. The results were processed using a neural network and visualized in the form of graphs. The accuracy of its calculations was 98%.Discussion and Conclusion. To determine the strength properties of homogeneous structures, it is sufficient to characterize the speed of displacement inside the sample. For an inhomogeneous structure, additional parameters should be introduced — displacements on the surface and inside the sample at fixed observation points. An integrated approach to determining the strength properties of an inhomogeneous structure improves the accuracy of calculations, and the use of neural networks increases their speed.

Achieving excellent uniform tensile ductility and strength in dislocation-cell-structured high-entropy alloys

Body-centered-cubic (BCC) high-entropy alloys (HEAs) encounter significant challenges in obtaining a high uniform tensile ductility (UTD). A dense dislocation-cell (DC) structure is produced in a heterogeneously grained HEA under tensile deformation, resulting from the anchored dislocation motion by grain interior elemental segregation. This fluctuation in elemental concentration is facilitated by thermomechanical processing. The activation of multiple-slip mechanisms, prompted by strain incompatibility among grains of varying sizes, significantly propels this process forward. This novel DC structure simultaneously increased the UTD (by 349.1 %) and yield strength (σ0.2, by 29.0 %) for a stable BCC HEA. Specifically, the single-phase alloy achieved a record-high UTD of 7.5 % and an σ0.2 of > 1,200 MPa, outperforming the counterparts of all the single-phase BCC HEAs. We employed a combination of transmission electron microscopy, in-situ scanning electron microscopy tensile testing coupled with an electron backscatter diffraction technology to investigate underlying strengthening mechanisms and identified that the serious stress concentration as a result of prevalent planar slip caused premature failure and localized strain of common BCC HEAs. At the initial stage of deformation, the DC structure promoted the activation of multiple slip systems and facilitated the extension of a plastic flow across the sample volume, effectively weakening stress concentration and premature failure. The extended plasticity zone and intensified dislocation interactions contributed to the increased UTD and σ0.2. These findings offer valuable inspiration for tailoring alloy properties via microstructure strategies and promoting their adoption in advanced manufacturing.

Acid-assisted subcritical blunt-tip crack propagation in carbonate rocks

AbstractSubcritical crack propagation in stressed carbonate rocks in a chemically reactive environment is a fundamental mechanism underlying many geomechanical processes frequently encountered in the engineering of geo-energy, including unconventional shale gas, geothermal energy, carbon sequestration and utilization. How a macroscopic Mode I crack propagates driven by a reactive fluid pressurizing on the crack surfaces with acidic agents diffusing into the rock matrix remains an open question. Here, the carbonate rock is modeled as an elasto-viscoplastic material with the mineral mass removal process affecting the rock properties in both elastic and plastic domains. A blunt-tip crack is considered to avoid any geometrically induced singularity problem and to allow a numerical analysis on the evolution of the chemical field being linked to the micro-cracking activities in front of the crack tip, affecting the delivery of acid. The model is capable of reproducing an archetypal three-region behavior of subcritical crack growth in a reactive environment. The crack propagation exhibits a prominent acceleration in Region III due to a two-way mutually enhancing feedback between mineral dissolution and the degradation process, which is most pronounced in front of the crack tip. With the consideration of initial imperfections in the rock, the macroscopic crack propagation is further accelerated with a secondary acceleration arising due to self-organization of micro-bands inside the chemically enabled plasticity zone.

Моделирование упругопластического разрушения компактного образца

The strength of a compact specimen under normal fracture (fracture mode I) is studied within the framework of the Neuber-Novozhilov approach. A model of an ideal elasto-plastic material with limiting relative elongation is chosen as the model of the deformable solid. This class of materials includes low-alloy steels that are used in structures operating at temperatures below the cold brittleness threshold. The crack propagation criterion is formulated using the modified Leonov-Panasyuk-Dugdale model, which uses an additional parameter, i.e., the diameter of the plasticity zone (the width of the prefracture zone). In the case of stress field singularity occurring in the vicinity of the crack tip, a two-parameter (coupled) criterion for quasi-brittle fracture is developed for type I cracks in an elastoplastic material. The coupled fracture criterion includes the deformation criterion attributed to the crack tip and the force criterion attributed to the model crack tip. The lengths of the original and model cracks differ by the length of the prefracture zone. Diagrams of the quasi-brittle fracture of the specimen under plane strain and plane stress conditions are constructed. The constitutive equations of the analytical model are analyzed in terms of the characteristic linear dimension of the material structure. Simple formulas applicable to verification calculations of the critical fracture load and pre-fracture zone length for quasi-brittle and quasi-ductile fractures are obtained. The parameters in the presented model of quasi-brittle fracture are analyzed. It is proposed to select the parameters of the model according to the approximation of the uniaxial tension diagram and the critical stress intensity factor.

SIMULATION OF ELASTOPLASTIC FRACTURE OF A CENTER CRACKED PLATE

The strength of a square plate with a central crack at normal separation was studied within the framework of the Neuber–Novozhilov approach using a modified Leonov–Panasyuk–Dugdale model using an additional parameter, the diameter of the plasticity zone (width of the pre-fracture zone). As a model of a deformable solid body, a model of an ideal elastic-plastic material with a limiting relative elongation was chosen. This class of materials includes, for example, low-alloy steels used in structures operating at temperatures below the cold brittleness threshold. In the presence of a singular feature in the stress field in the vicinity of the crack tip, it is proposed to use a two-parameter discrete integral strength criterion. The deformation fracture criterion is formulated at the tip of a real crack, and the force criterion for normal stresses, taking into account averaging, is formulated at the tip of a model crack. The lengths of real and model cracks differ by the length of the pre-fracture zone. The constitutive equations of the analytical model are analyzed in detail depending on the characteristic linear dimension of the material structure. Simple formulas suitable for verification calculations for the critical breaking load and the length of the pre-fracture zone are obtained. Numerical modeling of the propagation of plasticity zones in square plates under quasi-static loading has been performed. In the numerical model, the updated Lagrangian formulation of the equations of mechanics of a deformable solid body is used, which is most preferable for modeling the deformation of bodies made of an elastic-plastic material at large deformation. The plastic zone in the vicinity of the crack tip is obtained by the finite element method. The results of analytical and numerical prediction of plate fracture under plane deformation are compared. It is shown that the results of numerical experiments are in good agreement with the results of calculations using the analytical model of fracture of materials with a structure under normal separation. Diagrams of quasi-brittle and quasi-ductile fracture of a structured plate are constructed.

Whole genome sequencing characteristics of Chlamydia psittaci caprine AMK-16 strain, a promising killed whole cell veterinary vaccine candidate against chlamydia infection.

Chlamydia psittaci is a primary zoonotic pathogen with a broad host range causing severe respiratory and reproductive system infection in animals and humans. To reduce the global burden of C. psittaci-associated diseases on animal welfare and health and to control the pathogen spread in husbandry, effective vaccines based on promising vaccine candidate(s) are required. Recently, the caprine C. psittaci AMK-16 strain (AMK-16) demonstrated a high level of protection (up to 80-100%) in outbred mice and pregnant rabbits immunized with these formaldehyde-inactivated bacteria against experimental chlamydial wild-type infection. This study investigated the molecular characteristics of AMK-16 by whole-genome sequencing followed by molecular typing, phylogenetic analysis and detection of main immunodominant protein(s) eliciting the immune response in mouse model. Similarly to other C. psittaci, AMK-16 harbored an extrachromosomal plasmid. The whole-genome phylogenetic analysis proved that AMK-16 strain belonging to ST28 clustered with only C. psittaci but not with Chlamydia abortus strains. However, AMK-16 possessed the insert which resulted from the recombination event as the additional single chromosome region of a 23,100 bp size with higher homology to C. abortus (98.38-99.94%) rather than to C. psittaci (92.06-92.55%). At least six of 16 CDSs were absent in AMK-16 plasticity zone and 41 CDSs in other loci compared with the reference C. psittaci 6BC strain. Two SNPs identified in the AMK-16 ompA sequence resulted in MOMP polymorphism followed by the formation of a novel genotype/subtype including three other C. psittaci strains else. AMK-16 MOMP provided marked specific cellular and humoral immune response in 100% of mice immunized with the inactivated AMK-16 bacteria. Both DnaK and GrpE encoded by the recombination region genes were less immunoreactive, inducing only a negligible T-cell murine immune response, while homologous antibodies could be detected in 50% and 30% of immunized mice, respectively. Thus, AMK-16 could be a promising vaccine candidate for the development of a killed whole cell vaccine against chlamydiosis in livestock.

Chlamydia suis displays high transformation capacity with complete cloning vector integration into the chromosomal rrn-nqrF plasticity zone.

Chlamydia, comprising several human and zoonotic pathogens, is a genus of the conserved bacterial phylum Chlamydiota. Their obligate intracellular niche serves as a barrier for natural genetic exchange via horizontal gene transfer (HGT), and further limits the development and application of genetic tools. To date, the only example for recent inter-phylum HGT among the Chlamydiota is tetracycline resistance in the potentially zoonotic species Chlamydia suis, a close phylogenetic relative of human C. trachomatis, which causes bacterial sexually transmitted infections and ocular trachoma. Tetracycline resistance in porcine C. suis strains has been described worldwide and is always part of a genomic island dividing invasin (inv), located within a chromosomal region between the rRNA operon (rrn) and the nqrF reductase. Here, we aimed to expand the still modest number of available genetic manipulation systems for Chlamydia by generating allele-replacement and integration vectors for C. suis. These vectors comprised homologous C. suis sequences of the chromosomal region of interest, an E. coli origin of replication (ori) and selection markers but lacked the native chlamydial plasmids or its ori. We first recovered allele-replacement mutants using a vector that targets the tryptophan (trp) operon of C. suis. The vector was further successfully maintained as a free plasmid in C. trachomatis without allele replacement, suggesting complex plasmid dynamics in the absence of a chlamydial ori. Moreover, we showed that the hypervariable rrn-nqrF intergenic region of C. suis is highly susceptible to transformation, resulting in complete vector integration upstream of nqrF without interruption of the targeted inv gene.IMPORTANCEThe obligate intracellular Chlamydia genus contains many pathogens with a negative impact on global health and economy. Despite recent progress, there is still a lack of genetic tools limiting our understanding of these complex bacteria. This study provides new insights into genetic manipulation of Chlamydia with the opportunistic porcine pathogen Chlamydia suis, the only chlamydial species naturally harboring an antibiotic resistance gene, originally obtained by horizontal gene transfer. C. suis is transmissible to humans, posing a potential public health concern. We report that C. suis can take up vectors that lack the native plasmid, a requirement for most chlamydial transformation systems described to date. Additionally, we show that C. trachomatis, the most common cause for bacterial sexually transmitted infections and infectious blindness worldwide, can be transformed with C. suis vectors. Finally, the chromosomal region that harbors the resistance gene of C. suis is highly susceptible to complete vector integration.

Influence of Microstructure on Tensile Properties and Fatigue Crack Propagation Behavior for Lath Martensitic Steel

This paper addresses the role of multilevel microstructures on the fatigue crack propagation behavior and the tensile properties of lath martensite with different substructure sizes. Microstructure characterization of the alloy was carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron back-scattering diffraction (EBSD), and optical microscopy (OM). Based on the classic Hall–Petch relationship, the results of tensile tests showed that martensitic block is the effective control unit of yield strength. Furthermore, the plasticity of lath martensite is not sensitive to grain size. The tensile deformation mechanisms were also discussed. Fatigue crack propagation tests revealed that the coarse grain has a higher crack propagation threshold and lower crack propagation rate than the fine grain in lath martensitic steel. The change in the plasticity zone ahead of the crack tip leads to the transitional behavior of the fatigue crack propagation rate. When plasticity zone sizes are equal to the block size, the fatigue crack propagation reverts to a stable propagation stage. Finally, an empirical model was established to predict the fatigue crack propagation rate of the stable propagation stage based on the tensile properties of the lath martensitic steel.

Damage of CRTS III type ballastless track-32 m simply supported girder structural system under long-term deformation effects

Until now, the CRTS (China’s Railway Track System) III type ballastless track-girder system has been gradually used in high-speed railway engineering. However, the effect of long-term deformation of prestressed box girders on the upper track structure is often overlooked. Given the strict requirements for track smoothness in high-speed railways, an in-depth study of the long-term deformation of the CRTS III type ballastless track-32 m simply supported girder system (girder-track system) is particularly important. In this study, a refined finite element model was established for a three-span CRTS III type ballastless track- simply supported box girder system. Based on the aid of the CEB-FIP 90 creep model and the UMAT (User-Material) subroutine, the long-term deformation calculations were investigated. Furthermore, an in-depth analysis of stress and damage of the CRTS III type ballastless track under the long-term deformation of the box girder was conducted by combining concrete damaged plasticity (CDP) and cohesive zone model (CZM) constitutive models. The numerical simulated results indicated that after four years of operation, the creep camber of the box girder in the girder-track system would reach to 3.41 mm. The maximum tensile and compressive longitudinal stresses of the base plate would reach to 0.537 MPa and 11.983 MPa, respectively. Meanwhile, the maximum tensile and compressive longitudinal stresses of the self-compacting concrete layer would reach to 0.958 MPa and 2.701 MPa, respectively. In the box girder mid-span, significant damage could be observed in the edge of the two base plates near the mid-span, with a maximum compressive damage rate of 0.164 and the maximum tensile damage rate of 0.304. The research of this study has achieved the prediction of long-term deformation in the girder-track system, providing an important basis for the reliability assessment and life prediction of the girder-track system.

Analysis of crack closure and wake of plasticity with the distributed dislocation technique

A new method for the analysis of plasticity-induced closure of finite cracks is developed based on the distributed dislocation technique. The method is applied to analyse closure and opening processes for an embedded fatigue crack propagating under constant amplitude cyclic loading. The obtained solution and presented results can serve as a benchmark for numerical procedures utilising the same yield-strip methodology. Comparison with past numerical studies reveals similar trends, though there are some differences in the crack tip opening values, particularly for low and negative R-ratios. The latter may indicate the need for a better approach to the discretisation of the contact, direct and reverse plasticity zones in the numerical procedures which utilise the same methodology. The developed method is general, and it can be adapted to evaluate the crack tip opening stress for other crack geometries and loading conditions.

Three-dimensional laser scanning and numerical investigation on the influence of freeze-thaw and hang-wall mining on the instability of an open-pit slope

The instability of the open-pit slope and associated disasters of complex orebodies such as hanging-wall mining are the key problems to be solved urgently in the development of western resources. In this work, taking the hanging-wall mining in the open-pit mine of Hejing iron mine, for example, the disaster mechanism influenced by the coupling freeze-thaw and hanging-wall mining is systematically studied by 3D laser scanning and numerical simulation. Firstly, the rock mass structure information such as dip, dip angle, spacing, and equivalent trace length characteristics was obtained using 3D intelligent recognition technology. Then, numerical simulation is employed to reveal the influence of freeze-thaw and excavation sequences on the overall stability of the open-pit slope. The stress, displacement, plasticity zone, and maximum shear strain patterns are revealed in detail. The results show that the excavation engineering will lead to frequent increase and unloading of the internal stress of the rock mass, and the gradual increase of the goaf area will cause great damage to the rock mass. The slope failure mode is strongly impacted by freeze-thaw weathering and orebody excavation.

Whole Genome Sequencing and Comparative Genomic Analysis of Chlamydia gallinacea Field Strains Isolated from Poultry in Poland.

Chlamydia gallinacea is an intracellular bacterium belonging to the Chlamydiaceae family. Poultry is considered to be the major reservoir of this agent, which has worldwide distribution and a particularly consistent worldwide occurrence in chicken flocks. The bacterium has been linked to respiratory disease in humans but without definitive confirmation; nevertheless, while it has not been proved to be the cause of human respiratory disease, a recent report from Italy verified its bird-to-human transmission. This aspect being significant for public health, more research is needed to gain insight into the infection biology of C. gallinacea. In this study, the genomes of eleven novel C. gallinacea field strains from different regions of Poland were analyzed comparatively. It was confirmed that C. gallinacea strains are closely related, with at least 99.46% sequence identity. They possess a conservative genome structure involving the plasticity zone with a complete cytotoxin, the type three secretion system, inclusion membrane proteins, polymorphic membrane proteins, hctA and hctB histone-like proteins, and the chlamydial protease-like activating factor exoenzyme, as well as plasmids. Genetic diversity seems to be restricted. However, some genetic loci, such as ompA and multi-locus sequence typing target genes, are diverse enough to enable high-resolution genotyping and epidemiological tracing.

Genomic analysis of 61 Chlamydia psittaci strains reveals extensive divergence associated with host preference

BackgroundChlamydia (C.) psittaci, the causative agent of avian chlamydiosis and human psittacosis, is a genetically heterogeneous species. Its broad host range includes parrots and many other birds, but occasionally also humans (via zoonotic transmission), ruminants, horses, swine and rodents. To assess whether there are genetic markers associated with host tropism we comparatively analyzed whole-genome sequences of 61 C. psittaci strains, 47 of which carrying a 7.6-kbp plasmid.ResultsFollowing clean-up, reassembly and polishing of poorly assembled genomes from public databases, phylogenetic analyses using C. psittaci whole-genome sequence alignment revealed four major clades within this species. Clade 1 represents the most recent lineage comprising 40/61 strains and contains 9/10 of the psittacine strains, including type strain 6BC, and 10/13 of human isolates. Strains from different non-psittacine hosts clustered in Clades 2– 4. We found that clade membership correlates with typing schemes based on SNP types, ompA genotypes, multilocus sequence types as well as plasticity zone (PZ) structure and host preference. Genome analysis also revealed that i) sequence variation in the major outer membrane porin MOMP can result in 3D structural changes of immunogenic domains, ii) past host change of Clade 3 and 4 strains could be associated with loss of MAC/perforin in the PZ, rather than the large cytotoxin, iii) the distinct phylogeny of atypical strains (Clades 3 and 4) is also reflected in their repertoire of inclusion proteins (Inc family) and polymorphic membrane proteins (Pmps).ConclusionsOur study identified a number of genomic features that can be correlated with the phylogeny and host preference of C. psittaci strains. Our data show that intra-species genomic divergence is associated with past host change and includes deletions in the plasticity zone, structural variations in immunogenic domains and distinct repertoires of virulence factors.

Mesoscale modelling of concrete damage in FRP-concrete debonding failure

It is challenging to simulate fibre-reinforced polymer (FRP)-concrete debonding failure in detail because concrete damage usually occurs with a thin layer of mortar debonding attached to the FRP. This study applied a single-lap shear test with a digital image correlation technique to externally bonded FRP–strengthened concrete elements. An improved random walk algorithm method generated a realistic mesostructure of concrete. The two-dimensional mesoscale finite element model for FRP–strengthened concrete was established. The cohesive interface elements were inserted along all mortar–aggregate interfaces. Concrete damaged plasticity and cohesive zone model were applied to simulate the nonlinear properties of mortar and mortar–aggerate interface, respectively. The experimental and numerical load–displacement curves, strain/stress distribution, and local bond–slip curves were obtained and compared. A numerical parametric study investigated the effects of the aggregate, mortar, and adhesive on debonding failure. The main conclusions are: (1) the initiation and propagation of mortar damage and mortar–aggregate interface cracking in the FRP debonding process were directly conducted by the mesoscale model; (2) the numerical curves showed considerable oscillation, possibly due to the stress concentration near the aggregate boundary when the crack transmitted the aggregate, and the stiffness decreased after cracking was initiated at the mortar–aggregate interface; (3) concrete damage may extend to a depth of 25 mm, indicating that the concrete damage is greater than a thin layer of mortar debonding observed in the test; (4) applying polygonal aggregates, increasing aggregate size and surface mortar thickness, and reducing the adhesive layer thickness improves bond performance.

A framework to simulate the crack initiation and propagation in very-high-cycle fatigue of an additively manufactured AlSi10Mg alloy

Currently, no systematic approach exists to simulate the crack initiation and propagation process in very-high-cycle fatigue including the microstructure sensitivity. A combined crystal plasticity and cohesive zone model based computational framework is developed to simulate the defect-induced short crack growth of an additively manufactured AlSi10Mg alloy. The crystal plasticity formulation is used to model the anisotropic deformation in the grains. The accumulated plastic shear strain obtained by the crystal plasticity is introduced into the cohesive zone model as the crack initiation damage evolution criteria. The total damage is suggested to be divided into static damage, fatigue initiation damage and fatigue propagation damage. The framework can control the crack growth rate and the proportion of crack initiation damage in the total damage. Besides, an acceleration strategy is proposed to accelerate the computational efficiency for the very-high-cycle fatigue (VHCF). It provides an equivalence of loading cycles and the computational efficiency and accuracy are controlled by the acceleration factor. The internal defect induced short crack initiation and propagation of VHCF are investigated experimentally and simulated by the proposed framework. Experimental observations of tortuous crack paths and fine grain regions are reasonably well captured by the simulation.

Effect of thermal degradation on mechanical properties of cast Al-Si alloys for cylinder heads

The effect of thermal degradation on mechanical properties of cast Al-Si alloys for cylinder heads is investigated. Thermal degradation has a significant deteriorating effect on the tensile strength caused by the coarsening and coalescence of the θ phase (Al2Cu) due to precipitation reaction. The dominant factor for degradation at 250 ℃ is exposure temperature, while exposure time has a greater effect at 200 ℃. Thermal degradation increases the long crack growth rate significantly due to the increase in plasticity scale ahead of the crack tip. The effective stress intensity factor range (ΔKeff) is modified for thermally exposed alloys by adding the plasticity zone size to the crack length and considering the reduction in crack resistance from the precipitates.

Аналитическая модель квазихрупкого разрушения пластины с трещиной

The paper considers a rectangular plate with the edge crack of mode I of normal separation from the elastoplastic material with the ultimate strain. This class of materials includes, for example, the low-alloy steels used in structures operating at temperatures below the cold brittleness threshold. The strength of the plate was studied within the framework of the Neuber --- Novozhilov approach. The crack propagation criterion was formulated using the modified Leonov --- Panasyuk --- Dugdale model using an additional parameter, i.e., the plasticity zone diameter (pre-fracture zone width). Under conditions of small-scale yielding in the presence of the stress field singular feature in the vicinity of the crack tip, the two-parameter (dual) criterion for quasi-brittle fracture was formulated for mode I cracks in the elastoplastic material. The fracture dual criterion included deformation criterion at the crack tip, as well as the force criterion at the fictitious crack tip. The lengths of the original and fictitious cracks were differing by the length of the pre-fracture zone. Diagrams of the plate quasi-brittle fracture under conditions of plane deformation and plane stress were constructed. The parameters included in the proposed quasi-brittle fracture model were analyzed. It was proposed to select model parameters according to the approximation (σ--ε)-diagram of uniaxial tension and the KIc critical stress intensity factor

Regularities of plastic deformation development in the crack tip vicinity

When determining the service life of cyclically loaded metal structures at the stage of fatigue crack development, an important role is played by the accuracy of assessing the plastic deformation zone size at the crack tip formed by the tensile overload. Within this zone, after reducing the load to the initial level, the fatigue crack development slows down. Despite the large number of studies in this area, none of the proposed dependencies is currently fully confirmed by experimental results in determining the plasticity zone size. The aim of the work is to assess the size of the plastic deformation zone formed by a single tensile overload. In the work of the finite element method for central-notch specimens and compact tension specimens, an analysis of the stress-strain state in the crack top vicinity is performed. As a calculation, a flat and three-dimensional model was used. Calculations are performed in the Ansys 14.0. Data on the mechanical properties of steels were used: St20, VSt3sp, 09G2S, 15G2SF. With increasing load, the plastic zone size in front of the crack tip was estimated, in the process of reduction, the distribution and length of the fields of residual compressive stresses and the size of the cyclic plastic zone were recorded, the length of which was also determined experimentally using strain gauges with a base of 0.5 mm. To assess the plastic zone size formed by a single tensile overload, a dependence is proposed that takes into account the patterns of plastic deformation development in the vicinity of the fatigue crack tip, and the results of the calculation agree well with the experimental data given in the literature. The obtained expression makes it possible to predict with greater reliability the development of fatigue cracks at variable load amplitudes.

A study of macroscopic plastic strain localization during creep of polycrystalline aluminum

This paper studies the deformation behavior of commercially pure aluminum 1050A during creep tests at room temperature. The plastic flow parameters at the stage of steady-state creep at different stress levels are determined. The kinetics of localized plasticity zones in the creep mode is studied using the digital image correlation method. The stress dependence of the spatial period of plastic strain localization is established.

Development of irreversible deformation and localized shear bands under horizontal compression of the sedimentary layer

Within the framework of the elastoplastic model, the development of irreversible deformation during horizontal compression of a sedimentary layer lying on a rigid foundation is considered. The influence of strength parameters of the medium and friction in the base on the configuration of plasticity zones and localization of deformation has been studied. Analytical estimates are made of the depths at which irreversible deformation develops, as well as their changes as a result of an increase in horizontal stresses. With the help of numerical simulation, the features of the origin and development of localized shear bands are shown. Schemes are constructed illustrating the nature of the development of localization bands depending on the properties of the medium.