Atomistic modeling of the dislocation dynamics and evaluation of static yield stress

Static strength characteristics of structural materials are of great importance for the analysis of the materials behaviour under mechanical loadings. Mechanical characteristics of structural materials such as elastic limit, strength limit, ultimate tensile strength, plasticity are, unlike elastic moduli, very sensitive to the presence of impurities and defects of crystal structure. Direct atomistic modeling of the static mechanical strength characteristics of real materials is an extremely difficult task since the typical time scales available for the direct modeling in the frames of classical molecular dynamics do not exceed a hundred of nanoseconds. This means that the direct atomistic modeling of the material deformation can be done for the regimes with rather high strain rates at which the yield stress and other mechanical strength characteristics are controlled by microscopic mechanisms different from those at low (quasi-static) strain rates. In essence, the plastic properties of structural materials are determined by the dynamics of the extended defects of crystal structure (edge and screw dislocations) and by interactions between them and with the other defects in the crystal. In the present work we propose a method that is capable to model the dynamics of edge dislocations in the fcc and hcp materials at dynamic deformations and to estimate the material static yield stress in the states of interest in the frames of the atomistic approach. The method is based on the numerical characterization of the stress relaxation processes in specially generated samples containing solitary edge dislocations

Atomistic modeling of the dislocation dynamics and evaluation of static yield stress

Static strength characteristics of structural materials are of great importance for the analysis of the materials behaviour under mechanical loadings. Mechanical characteristics of structural materials such as elastic limit, strength limit, ultimate tensile strength, plasticity are, unlike elastic moduli, very sensitive to the presence of impurities and defects of crystal structure. Direct atomistic modeling of the static mechanical strength characteristics of real materials is an extremely difficult task since the typical time scales available for the direct modeling in the frames of classical molecular dynamics do not exceed a hundred of nanoseconds. This means that the direct atomistic modeling of the material deformation can be done for the regimes with rather high strain rates at which the yield stress and other mechanical strength characteristics are controlled by microscopic mechanisms different from those at low (quasi-static) strain rates. In essence, the plastic properties of structural materials are determined by the dynamics of the extended defects of crystal structure (edge and screw dislocations) and by interactions between them and with the other defects in the crystal. In the present work we propose a method that is capable to model the dynamics of edge dislocations in the fcc and hcp materials at dynamic deformations and to estimate the material static yield stress in the states of interest in the frames of the atomistic approach. The method is based on the numerical characterization of the stress relaxation processes in specially generated samples containing solitary edge dislocations

ВАРИАНТЫ ПРЕБЫВАНИЯ МАГНИТНЫХ ИНОРОДНЫХ ТЕЛ В ЖЕЛУДОЧНО-КИШЕЧНОМ ТРАКТЕ У ДЕТЕЙ

Magnetic foreign body in the digestive tract in children may have different ways to stay and outcomes. The authors present clinical examples of location, surgical removal of the magnetic bodies and the described case of complications as a result of multiple magnetic foreign bodies in the gastrointestinal tract. Developed clinical management of patients with suspected presence of magnetic foreign bodies in different sections of the gastrointestinal tract.Представлены клинические примеры нахождения и оперативного удаления магнитных объектов, описаны случаи развития осложнений в результате множественного попадания магнитных инородных тел в желудочно-кишечный тракт. Выработана тактика ведения пациентов с подозрением на наличие у них магнитных инородных тел в различных отделах желудочно-кишечного тракта

Клинические случаи острого аппендицита у новорожденных

Aim. The goal of this study is to describe the clinical observations of acute appendicitis in two newborn infants. This study was a retrospective analysis performed using data obtained from medical records. Two preterm infants had a gestational age of 33 weeks, a threat of pregnancy termination, rapid childbirth in one case, and a cesarean section in another. At birth, their body weights were low, 2340 and 2420 g, respectively. The condition of the babies was evaluated on the Apgar scale, and both scored 7/8 points. We studied the data of clinical, laboratory, and instrumental study data and surgical intervention protocols. In the presented newborns, neonatal jaundice, respiratory failure of the III degree, and hypoxic perinatal damage to the central nervous system occurred from birth. On the first childs twelfth day and the second childs ninth day, they experienced a clinically acute inflammatory process in the abdominal cavity, confirmed by inflammation markers (high white blood cell counts and levels of C-reactive protein). According to the sonography of the abdominal organs, the absence of intestinal motility in the right abdominal cavity, the presence of intestinal wall pneumatosis, signs of conglomerate formation from the intestinal loops were revealed. The surgical interventions performed were a laparoscopy and a conversion to laparotomy. In both cases, inflammatory bowel changes corresponded to the course of necrotic enterocolitis, diagnosed with gangrenous-perforated appendicitis and purulent-fibrinous peritonitis. In the section, the mucous appendix was not changed. Conclusion. The aggravated premorbid background in premature infants predisposes them to necrotic enterocolitis, which can occur in children of this age with isolated perforation of the appendix. The course of necrotic enterocolitis is complicated by the formation of purulent-fibrinous peritonitis due to the destruction of the appendix with minimal changes in its walls.Цель. Описать клинические наблюдения острого аппендицита у детей периода новорожденности. Ретроспективный анализ медицинских карт: 2 недоношенных младенца, возраст гестации 33 недели, угроза прерывания беременности, стремительные роды в одном случае и кесарево сечение в другом. При рождении масса тела низкая 2340 и 2420 г соответственно. Состояние малышей оценивалось по шкале Апгар 7/8 баллов в обоих случаях. Изучали данные клинических, лабораторных и инструментальных исследований, протоколы выполненных оперативных вмешательств. У представленных новорожденных с рождения протекала неонатальная желтуха, дыхательная недостаточность III степени, гипоксическое перинатальное поражение центральной нервной системы. На 12-е сутки у первого и 9-е сутки у второго ребенка возникла клиника острого воспалительного процесса в брюшной полости, подтвержденная маркерами воспаления (высокие показатели лейкоцитов и уровней С-реактивного белка). По данным сонографии органов брюшной полости выявлено отсутствие перистальтики кишечника в правых отделах брюшной полости, наличие пневматоза кишечной стенки, признаки формирования конгломерата из петель кишечника. Оперативное вмешательство: лапароскопия, конверсия на лапаротомию. В обоих случаях воспалительные изменения кишечника соответствовали течению некротического энтероколита, диагностирован гангренозно-перфоративный аппендицит и гнойно-фибринозный перитонит. На разрезе слизистая аппендикса не изменена. Заключение. Отягощенный преморбидный фон у недоношенных новорожденных предрасполагает возникновению некротического энтероколита, который у детей данного возраста может протекать с изолированной перфорацией червеобразного отростка. Течение некротического энтероколита осложняется формированием гнойно-фибринозного перитонита за счет деструкции аппендикса при минимальных изменениях в его стенках

MD modeling of screw dislocation influence upon initiation and mechanism of BCC-HCP polymorphous transition in iron

The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcc-hcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuffe) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron

MD modeling of screw dislocation influence upon initiation and mechanism of BCC-HCP polymorphous transition in iron

The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcc-hcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuffe) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron

MD modeling of screw dislocation influence upon initiation and mechanism of BCC-HCP polymorphous transition in iron

The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcc-hcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuffe) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron