Complex Loading Processes Research Articles

Cdt1 Self-associates via the Winged-Helix Domain of the Central Region during the Licensing Reaction, Which Is Inhibited by Geminin.

The initiation of DNA replication is tightly controlled by the licensing system that loads replicative DNA helicases onto replication origins to form pre-replicative complexes (pre-RCs) once per cell cycle. Cdc10-dependent transcript 1 (Cdt1) plays an essential role in the licensing reaction by recruiting mini-chromosome maintenance (MCM) complexes, which are eukaryotic replicative DNA helicases, to their origins via direct protein-protein interactions. Cdt1 interacts with other pre-RC components, the origin recognition complex, and the cell division cycle 6 (Cdc6) protein; however, the molecular mechanism by which Cdt1 functions in the MCM complex loading process has not been fully elucidated. Here, we analyzed the protein-protein interactions of recombinant Cdt1 and observed that Cdt1 self-associates via the central region of the molecule, which is inhibited by the endogenous licensing inhibitor, geminin. Mutation of two β-strands of the winged-helix domain in the central region of Cdt1 attenuated its self-association but could still interact with other pre-RC components and DNA similarly to wild-type Cdt1. Moreover, the Cdt1 mutant showed decreased licensing activity in Xenopus egg extracts. Together, these results suggest that the self-association of Cdt1 is crucial for licensing.

Application of a Three-Level Elastoviscoplastic Model for Describing Complex Loading Processes

It is known that the meso- and microstructures of metals determine the physical, mechanical and operational properties of their final products. Scientific and technological progress of recent decades has given impetus to the elaboration and use of models capable of describing the evolving structure of materials. The most promising are multilevel models that include internal variables and are based on physical theories of elastoplasticity (elastoviscoplasticity). This paper presents the structure and basic relationships of a three-level (macro-, meso-1 and meso-2 levels) elastoviscoplastic model. The developed model operates on such internal variables as dislocation densities on slip systems, barriers on split dislocations and sources of edge dislocations. The model describes the mechanisms of production, annihilation, formation of barriers and sources of dislocations. The law of hardening directly takes into account the densities of dislocations and barriers. The mechanism of inelastic deformation is the gliding of edge dislocations along slip systems. Special emphasis is placed on the influence of split dislocations (prone to forming hard Lomer–Cottrell and Hirth barriers) on the deformation of the material. The model is used to describe the behavior of an elastoviscoplastic polycrystalline aggregate with an FCC lattice. Geometric nonlinearity is taken into account by utilizing decomposition of the crystallite motion into quasi-rigid and deformation components. For this purpose, a rigid moving coordinate system for the crystal lattice is introduced. Examples of the application of the model for analyzing the simple and complex deformation mechanisms of materials with different stacking fault energies and, consequently, with different tendencies toward the decomposition of dislocations and barrier formation are given.

A Homogeneous Anisotropic Hardening Model in Plane Stress State for Sheet Metal under Nonlinear Loading Paths.

In sheet metal forming, the material is usually subjected to a complex nonlinear loading process, and the anisotropic hardening behavior of the material must be considered in order to accurately predict the deformation of the sheet. In recent years, the homogeneous anisotropic hardening (HAH) model has been applied in the simulation of sheet metal forming. However, the existing HAH model is established in the second-order stress deviator space, which makes the calculation complicated and costly, especially for a plane stress problem such as sheet metal forming. In an attempt to reduce the computational cost, an HAH model in plane stress state is proposed, and called the HAH-2d model in this paper. In the HAH-2d model, both the stress vector and microstructure vector contain only three in-plane components, so the calculation is significantly simplified. The characteristics of the model under typical nonlinear loading paths are analyzed. Additionally, the feasibility of the model is verified by the stress-strain responses of DP780 and EDDQ steel sheets under different two-step uniaxial tension tests. The results show that the HAH-2d model can reasonably reflect the Bauschinger effect and the permanent softening effect in reverse loading, and the latent hardening effect in cross loading, while the predictive accuracy for cross-loading softening remains to be improved. In the future, the HAH-2d model can be further modified to describe more anisotropic hardening behaviors and applied to numerical simulations.

Deformation and seepage properties of raw coal during the simultaneous loading and unloading of axial stress and confining pressure

The mining of coal resources causes the stress on the front and both sides of the working face to be in a complex loading and unloading process, and the gas contained in the coal body migrates, which can cause gasdynamic disasters in severe cases. Therefore, this study designed a three-dimensional stress path based on the in situ stress distribution characteristics of the No. 2461 working face and conducted the experiment on the deformation and seepage characteristics of raw coal under simultaneous unloading (loading) axial stress and loading (unloading) confining pressure. It is considered that in the process of loading axial stress, the volumetric strain increases gradually with the increase in effective stress, and the permeability decreases exponentially with the increase in effective stress. In the process of simultaneous unloading axial stress and loading confining pressure, the volumetric strain of coal sample increases with the increase in effective stress, and the permeability decreases exponentially with the increase in effective stress. However, the processes of simultaneous loading axial stress and unloading confining pressure are opposite, and the variation curves of the two do not overlap, indicating that the coal sample undergoes elastic-plastic deformation after cyclic loading, the permeability cannot be restored to the initial state, and the volumetric strain further expands. The permeability damage rate and maximum permeability damage rate are used to evaluate the recovery degree and reduction range of coal samples' permeability. The volume expansion rate and maximum volume expansion rate are used to evaluate the expansion degree and increase range of coal samples' volume.

Experimental and calculation investigations of deformation and fracture processes of various types of steels during impact shear tests

The paper proposes the experimental technique for investigating the deformation and fracture of the materials under dynamic shear. The authors have tested the specimens with two shear zones. A series of numerical calculations are performed using the finite element method. The variation of the shear zone length in such specimens allows one to change the strain rate in these zones at a constant loading rate. The specimens are made of the following steels: steel 20, Hardox 400 high strength steel, and ARMOX 500T armored steel. The tests are made on the instrumented drop-weight impact testing machine with a high-speed registration system. The obtained results make it possible to assess the influence of many factors (strain rate, specimen geometry, material, and direction of the specimen cutting) on the deformation and fracture of materials under dynamic shear conditions. It is employed to specify the parameters of the material models for the numerical simulation of complex loading processes, as well as to develop the experimental method for the mechanical characteristics determination considering fracture during dynamic shear.

A comparative study of three forms of an uncoupled damage model as fracture judgment for thin-walled metal sheets

Ductile fracture always occurs in plastic forming processes of thin-walled metal sheets. Damage models, both coupled and uncoupled ones, have been extensively developed to predict fracture of the sheets. Among these, uncoupled damage models, owing to their simple form are widely used. While for an uncoupled damage model, there exist three forms for its fracture prediction: (1) damage threshold (DT), which is related to the damage accumulation; (2) fracture strain (FS), which is always a function of stress triaxiality and Lode parameter; (3) fracture forming limit diagram (FFLD), which is always related to the principal strains. However, for an uncoupled damage model, its three forms (DT, FS and FFLD) for fracture prediction have never been compared before, and thus scholars have always been confused about how to choose the right form of the three in different loading processes. To clarify this confusion, two popular uncoupled damage models of the MMC4 and DF2016 are embedded into an anisotropic yield function first. Then the above-mentioned three forms of the two models are separately applied for fracture prediction in shear test, dog-bone specimen tensile test, hole specimen tensile test, notched specimen tensile test, bulging test in proportional loadings, and a complex non-proportional loading process of spin forming, respectively. Finally, the predicted results with these three forms are systematically compared and a new explanation for their applicability under different stress states is displayed.

Tunnel failure mechanism during loading and unloading processes through physical model testing and DEM simulation

Geo-materials may present varying mechanical properties under different stress paths, especially for tunnel excavation, which is typically characterized by the decreased radial stress and increased axial stress during the complex loading and unloading process. This study carried out a comparative analysis between the loading and unloading model testing, which was then combined with PFC2D simulation, aiming to reveal the fracture propagation pattern, microscopic stress and force chain distribution of the rock mass surrounding the tunnel. Comparisons of extents and development of tensile strain between loading and unloading testing results were made. The overall stability, the integrity of rock mass, and the failure pattern transition under loading and unloading processes were systematically examined. In addition, for the two unloading cases with different vertical stresses imposed, the failure patterns were both identified as the collapse of the V − shaped extruded sidewall, due to the coupling of the shear failure and the vertical tensile failure in the sidewall wedge.

Biaxial Experiments and Numerical Analysis on Stress-State-Dependent Damage and Failure Behavior of the Anisotropic Aluminum Alloy EN AW-2017A

Many experiments indicated the remarkable dependence of the strength and failure behavior of anisotropic ductile metals on the loading direction and on the stress state. These influences have to be taken into account in accurate material models and in the numerical simulation of complex loading processes predicting the safety and lifetime of aerospace structures. Therefore, the present paper discusses the effect of loading direction and stress state on the damage and failure behavior of the anisotropic aluminum alloy EN AW-2017A. Experiments and corresponding numerical analysis with the newly developed, biaxially loaded X0 specimen have been performed and the influence of different load ratios is examined. The formation of strain fields in critical parts of the X0 specimen is monitored by digital image correlation. Different failure modes are visualized by scanning electron microscopy of fracture surfaces. Stress states are predicted by finite element calculations and they are used to explain damage and fracture processes at the micro-level. The experimental–numerical analysis shows that the loading direction and the stress state remarkably affect the evolution of the width and orientation of localized strain fields as well as the formation of damage processes and fracture modes. As a consequence, characterization of anisotropic metals is highly recommended to be based on an enhanced experimental program with biaxial tests including different load ratios and loading directions.

The effect of replacing non-analytic trajectories with break points on smooth paths to the complexity of deformation and loading processes of materials

This article is devoted to an experimental study of the effect of rounding off corner points of two-link strain trajectories on complex loading processes during elastoplastic deformation of materials. Replacing corner points in their vicinity with local sections of circles allows a nonanalytic trajectory to be replaced with a smooth trajectory. Experimental studies were performed on thin-walled tubular specimens of the low-carbon steel St3 on an SN-EVM automated testing system. The loading programs for tubular specimens were set in the Ilyushin's deviatoric strain space. The rounding of the corner point of a two-link strain trajectory with an angle of 90° between the branches by arcs of circles with curvatures of 200, 400, as well as the rounding of the corner point of a two-link strain trajectory with an angle of 135° between the branches by arcs with curvatures of 400, 800 are considered. The experimental data characterizing the vector and scalar properties of the material are presented. The experimental data show that the effect of complex loading on the relationship between stresses and strains in a curved section is not immediately apparent. In the curved section, the magnitude of the stress vector modulus first increases, and then decreases with the formation of stress dives. The minimum point of the stress dive is located on the next straight branch of the strain trajectory. In the curvilinear section, the angle of delay increases, and in the next straight branch it decreases, and with the increase of the strain it tends to be zero. The rate of decrease of the angle of delay depends little on the differences in the geometry of the previous history of strain trajectory. In the second straight branch, the experimental results for a smooth and original two-link strain trajectories become little distinguishable from each other. Thus, replacing the original non-analytical strain trajectory to a smooth trajectory affects the complexity of the process of deformation and loading of the materials only in the vicinity of the corner point. This circumstance can be taken into account when numerically modeling the processes of elastoplastic deformation of materials and integrating the defining relations, replacing nonanalytic trajectories with smooth ones. This can be taken into account in the numerical calculation of elastic-plastic deformation and integration of constitutive relations, replacing non-analytical strain trajectories by smooth ones.

On behaviour of plasticity functionals at complex loading

For plasticity with scleronome constraints at isothermal processes the functionals which described of loading process do not depend on time and are found for some classes of complex loading processes. Functionals in constitutive relations have to be depend from history of loading. These aspects on the base of experimental and theoretical investigations using of Ilyushin’s approach of plastic processes are considered in this issue.

Bioinspired, multifunctional dual-mode pressure sensors as electronic skin for decoding complex loading processes and human motions

Mimicry of the somatosensory system in the human skin via electronic devices exhibits broad applications in intelligent robotics and wearable electronics. Here, we report a novel biomimetic flexible dual-mode pressure sensor that is based on the interlocked piezoelectric and piezoresistive films with pyramid microstructures. The sensitivity of the sensor is significantly enhanced because of the induced larger stain variation along the thickness direction of the former piezoelectric film and increased contact area in the latter piezoresistive film. The synergistic effect of the piezoelectric and piezoresistive responses to stimuli also allows the dual-mode sensor to detect over broad pressure and frequency ranges. The analysis of these signals can deconvolute multiple aspects of the complex stimuli loading processes, including their loading direction, rate, magnitude, and duration. As a proof-of-concept demonstration, the dual-mode pressure sensor is successfully integrated with manipulators and human bodies to decode the complex and delicate picking processes and human motions, respectively. When combined with the other sensing modalities, the multifunctional dual-mode pressure sensor delivers new application opportunities in intelligent soft robotics and human-machine interfaces. • A biomimetic dual-mode pressure sensor that is based on the interlocked piezoelectric and piezoresistive films. • The synergistic effect allows the dual-mode sensor to exhibit a high sensitivity over a broad pressure and frequency range. • The dual-mode sensor deconvolutes the loading rate, magnitude, and direction of the complex loading process. • The application of the dual-mode sensor for human-machine interfaces demonstrates its capability to control the manipulator.

Applied Questions of Il’yushin Theory of Elastoplastic Processes

The experimental results of the processes of complex loading along helical strain trajectories are used to find out that the response to the helical strain trajectory following the simple loading after exhaustion of some trace takes a certain shape of the limit mode, that is, there is a correspondence between the deformation trajectory geometry and the form of response. A new variant of constitutive equations for describing complex loading processes with strain trajectories of arbitrary geometry and dimension is considered. The vector constitutive equations and the system of differential equations for the four angles from the Frenet decomposition are obtained. It is proved that the stress vector is represented in the form of sum of three terms: rapidly decaying plastic traces of elastic states, instantaneous responses to the deformation process, and irreversible stresses accumulated along the deformation trajectory. A new method for mathematical modeling of five-dimensional processes of complex loading is constructed and tested on two- and three-dimensional processes.

DESIGN, OPTIMIZATION, AND STATISTICAL EVALUATION OF ORAL RECONSTITUTED SUSPENSION OF CLARITHROMYCIN USING ION-EXCHANGE RESINS

Objectives: The objective of the present study is taste masking of bitter clarithromycin using Indion 204, Indion 234, and Tulsion 335 as ion-exchange resins, which forms insoluble complexes, inhibiting the drug release in saliva as ion-exchange resins are cross-linked polymers, water-insoluble that contains salt-forming groups in repeating positions on the polymer chain. Drugs that are bitter and cationic get adsorbed onto weak cationic exchange resins of carboxylic acid functionality such as Indion 204, Indion 234, and Tulsion 335 to form non-bitter complexes.
 Methods: The drug-resin complex loading process was optimized for the resin content, activation, swelling time, stirring time, influence of pH, and temperature for maximum drug loading and the formed complex was evaluated by differential scanning calorimetry (DSC) to confirm complex formation. The drug-resin complex was also characterized by their micromeritic and rheological properties. These complexes were used to prepare oral reconstituted suspensions and the taste was evaluated. The formulation was evaluated for various parameters such as sedimentation volume, pH, redispersibility, viscosity, drug content, and in vitro drug release.
 Results: Acid-activated resins comprising Indion 204, Indion 234, and Tulsion 335 with the drug:resin ratio of 1:2, stirred in a solution of pH 7–8 at 70° for 6 h had a maximum drug loading and masked the bitter taste of clarithromycin. DSC of the drug-resin complex (DRC) revealed that there was interaction leading to complex formation. The drug-resin complex was formulated into suspension formulations (S1-S9) and evaluated. Various parameters were found to be within permissible limits. Formulations S3, S6, and S9 containing 1:2 ratios of the drug-resin complex of Indion 204, Indion 234, and Tulsion 335 revealed maximum taste masking. This was further confirmed by treatment of taste evaluation scores obtained from the volunteers by ANOVA, Dunnett’s multiple comparison test, and Tukey’s multiple comparison test. All the three optimized formulations had a significant difference of p<0.001 when compared to control S10. S6 formulation was widely accepted.
 Conclusion: Ion-exchange complexation could efficiently mask the bitter taste of clarithromycin and achieve palatable taste suitable for pediatric use.

The supported landslide slopes’ operation simulation

The article describes the landslide processes formation processes modeling in free and reinforced slopes. Unlike many well-known approaches, the proposed approach does not introduce simplifying hypotheses. As, for example, hypotheses about the slip lines form or about the slope conventionally “solid” areas form. The slope stability problem is considered in the general formulation of a three-dimensional or two-dimensional (plane strain) plasticity theory. The limiting equilibrium approach and the continuation methods by the loading parameter are used. The resolving equations are constructed in the form of a finite element method (FEM) in displacements. It is shown how, in addition to physical, to take into account geometrical and constructive nonlinearity.The small fragment limiting state hypotheses can be assigned arbitrary. In the general formulation, a complex loading process is assumed — an external load can grow arbitrarily. In this case, the growth load linear nature is used within one external loading branch. Inside the branch, a “cycle in a cycle” process is organized in small increments of load. The process continues either until the limit state of the considered computational domain is reached, or until the load and the transition growth next branch end to the next one.

The General Mathematical Theory of Plasticity and the Il’yushin Postulates of Macroscopic Definability and Isotropy

The physical laws characterizing the relation between stresses and strains are considered and analyzed in the general modern theory of elastoplastic deformations and in its postulates of macroscopic definability and isotropy for initially isotropic continuous media. The fundamentals of this theory in continuum mechanics were developed by A.A. Il’yushin in the mid-twentieth century. His theory of small elastoplastic deformations under simple loading became a generalization of Hencky’s deformation theory of flow, whereas his theory of elastoplastic processes which are close to simple loading became a generalization of the Saint-Venant–Mises flow theory to the case of hardening media. In these theories, the concepts of simple arid complex loading processes arid the concept of directing form change tensors are introduced; the Bridgman law of volume elastic change and the universal Roche–Eichinger laws of a single hardening curve under simple loading are adopted; and the Odquist hardening for plastic deformations is generalized to the case of elastoplastic hardening media for the processes of almost simple loading without consideration of a specific history of deformations for the trajectories with small arid mean curvatures. In this paper we discuss the possibility of using the isotropy postulate to estimate the effect of forming parameters in the stress-strain state appeared due to the strain-induced anisotropy during the change of the internal structures of materials. We also discuss the possibility of representing the second-rank symmetric stress and strain tensors in the form of vectors in the linear coordinate six-dimensional Euclidean space. An identity principle is proposed for tensors and vectors.

On Some Properties of Metals Under Complex Loading

Drucker’s Associated Law of yielding and gradient plasticity cannot always adequately reflect the processes of stress-strain state in deformable continua. Aim of this issue explains these effects via theory of complex loading processes by Ilyushin.

ИССЛЕДОВАНИЕ ИНВАРИАНТНОСТИ J-ИНТЕГРАЛА ПРИ СЛОЖНОМ СДВИГЕ

The problem of determining the stress-strain state of an elastic-plastic beam with an edge crack under conditions of a complex shear is considered. A quasi-static external loading is given, which leads to complex loading processes at each point of the beam. Deformations are considered small. A numerical algorithm for solving the problem based on the application of the finite element method and the differential-nonlinear version of the plasticity theory taking into account microdeformation (Novozhilov, Kadashevich, Chernyakov) is proposed. The no-uniformity of plastic deformation is approximately taken into account by representing the plastic deformation tensor as a sum of elementary plastic deformations, each of which has its own yield surface and a system of internal micro-elastic forces. Mathematically, the boundary value problem is formulated as a nonlinear boundary value problem and the Cauchy problem with respect to the loading parameter. To solve the Cauchy problem, a step-by-step method that reduces the solution of the original problem to a sequence of solutions on time layers is used. At each step in time, using the iterative method, the nonlinear boundary value problem is reduced to a sequence of quasilinear problems that are solved by the finite element method. The quadrangular isoparametric finite element is chosen as the basis for the finite element method. To calculate the stiffness matrices of finite elements, an algorithm is used that makes it possible to reduce the number of multiple integrals to be calculated in the defining relations of the theory of plasticity taking into account microdeformations, up to the dimensionality of the loading trajectory. To calculate the J-integral, a direct method is used, which is based on its direct calculation on the basis of finite element method. When carrying out the calculations, St45 steel is chosen as the material of the beam, for which, when describing elastic-plastic deformation within the framework of the theory of plasticity taking into account microdeformations, it is permissible to use constants as universal material functions. Under various loading schemes, zones of plasticity in the section of the beam are constructed. The influence of the loading history on form and size of plastic zones and on the J-integral is investigated. It is shown that under the accepted loading schemes, within the admissible accuracy, the J-integral is invariant under both proportional and complex loading, and also that its magnitude depends on the loading history.

The deformation induced martensitic transformation and mechanical behavior of quenching and partitioning steels under complex loading process

The quenching and partitioning (QP) steels have received much attention due to its high strength and good ductility, which results from deformation induced martensitic transformation (DIMT). However, due to the stress state, temperature and strain rate dependent DIMT behavior, the DIMT related flow behavior of QP steels might be more complex than other advanced high strength steels (AHSS) without martensitic transformation. During forming process of such materials (non-linear loading process), effects of DIMT behavior and DIMT related flow behavior on springback prediction are still unknown. In this paper, we focused on the martensitic transformation behavior of QP steels under complex loading (linear loading and cyclic loading). The X-ray diffraction and digital image correlation techniques were performed to monitor the retained austenite evolution and strain history of the specimen, respectively. Then, flow behaviors of QP sheets steels subjected to complex loading are analyzed and compared with DP980 sheets steels (without DIMT). Finally, modified Yoshida and Uemori model calibrated by two different cyclic loading tests, i.e., tension-compression test and cyclic shear test, are adopted in this study to investigate the stress state dependent DIMT effect on springback prediction. The results show, for QP steels, the DIMT is stress state dependent. Accordingly, compared with DP steels, its flow behavior exhibits remarkable stress state sensitivity. Due to these special DIMT behavior and flow behavior of QP steels, its springback prediction should be treated differently compared with DP steels.

Numerical Simulation of Spall Behavior of Metal Under Strong Impact Loading

When the metal material undergo complex loading processes, such as strong impact loading, unloading, backward drawing and reloading, they are likely to spall in the reverse tension stage. In this paper, the following four items are considered in the modeling of material spallation:①the damage variable in the spallation modeling adopts the concept of porosity; ②the equation of state containing hole material is calculated by the solid component in the damage unit;③when the porosity of the unit reaches the critical porosity, it is considered that the pores are interpenetrated with each other to form a macro crack, and the material is fractured as a whole; ④When the macro fracture unit is compressed again, if the average compression ratio of the unit reaches the closed critical value, it is considered that the unit is compressed into holed hole. In the existing the two dimension Lagrange fluid dynamics program, we have added the hole fracture growth, polymerization and collapse effect of spallation processing function, and verify the model and program code through a numerical example. The damage evolution analysis of the unit near a single measuring point shows that the platform area in the velocity curve corresponds to the damage fracture zone in the metal. It is considered that the unit of the location has undergo the physical process of unload-stretching-damage-fracture-free flight- recompression.

A model of one-surface cyclic plasticity with Lemaitre damage criterion for plastic instability prediction in the incremental forming process

The precise prediction of plastic deformation instabilities during the incremental forming process is quite complex because of the complex stress state and extremely large plastic strain. Until now, most elasto-plastic constitutive models are only suitable within a small strain range and describe the isotropic behavior. However, for extremely large plastic deformation under complex loading processes such as an incremental forming process, these models are ineffective and ill-conceived. This paper presents an elasto-plastic constitutive model which combines a mixed isotropic-kinematic hardening model with the Lemaitre damage model. This model can predict the Bauschinger effect, transient behavior, permanent softening and fractures over a large plastic strain range. By using explicit integration schemes, an integration algorithm of numerical implementation is proposed using simple expressions. A cyclic loading test comparison between experiments and numerical analysis is carried out to validate these models. The model is also applied to FE analysis of incremental forming. The deformation and the prediction of thickness are consistent with experimental results and thus verify the accuracy of the model.