Elastic-plastic Deformation Research Articles

Ultrafast visualization of incipient plasticity in dynamically compressed matter

Plasticity is ubiquitous and plays a critical role in material deformation and damage; it inherently involves the atomistic length scale and picosecond time scale. A fundamental understanding of the elastic-plastic deformation transition, in particular, incipient plasticity, has been a grand challenge in high-pressure and high-strain-rate environments, impeded largely by experimental limitations on spatial and temporal resolution. Here, we report femtosecond MeV electron diffraction measurements visualizing the three-dimensional (3D) response of single-crystal aluminum to the ultrafast laser-induced compression. We capture lattice transitioning from a purely elastic to a plastically relaxed state within 5 ps, after reaching an elastic limit of ~25 GPa. Our results allow the direct determination of dislocation nucleation and transport that constitute the underlying defect kinetics of incipient plasticity. Large-scale molecular dynamics simulations show good agreement with the experiment and provide an atomic-level description of the dislocation-mediated plasticity.

Investigation of the Nature of Damage to Welded Flexible Compensating Elements Made of Austenitic Stainless Steels

Welded flexible compensating elements made of austenitic stainless steels, such as metal hoses and bellows expansion joints, operate in a complex stress state; therefore, almost all corrosion damage of these products occurs in a stressed state. In addition, in welded joints, defects are possible associated with elastic-plastic deformations and stresses arising in the manufacturing process. Paper considers the defects of welded joints of multilayer bellows expansion joints made of stainless steels AISI 304 and AISI 316 types. The influence of the parameters of the argon tungsten electrode welding mode, with or without filler wire, on the mechanical characteristics of welded joints was studied. Mechanical tests of samples of welded joints were carried out on a tensile testing machine. The optimal range of variation of the parameters of the welding mode has been established, in which the plastic characteristics of the welded joints have maximum values. A metallographic study of the microstructure of welded joints of metal hoses and bellows expansion joints was carried out. The influence of the content of nonmetallic inclusions in the material of products on the conditions of melting and crystallization of the weld pool, contributing to the appearance of pores and hot cracks in the metal of the weld and the heat affected zone, has been established. To reduce the influence of the content of non-metallic inclusions in the weld metal and the heat-affected zone on the conditions of melting and crystallization of the weld pool, it is recommended to carry out preliminary heat treatment of the austenitic steel strip.

A CDM-like constitutive law for predicting degradation of strength and ductility of steel subjected to cyclic loading

This paper presents a continuum-damage-model-like constitutive law embedding cohesive cracks with plasticity-induced damage to represent the degradation of strength and ductility of steel under cyclic loading. The proposed constitutive law accommodates an arbitrary hyperelasticity-based plastic model with the use of the deformation gradient multiplicatively decomposed into separation-induced, elastic and plastic parts, and incorporated with an additional isotropic hardening rule endowed with a memory surface. While the elastic–plastic deformation along with isotropic and kinematic hardening is represented by a Hencky-type model, the separation-induced deformation gradient due to material separation is employed to embed an arbitrary cohesive traction separation law (CTSL) into the hyperelasticity-based plastic model. Also, a plasticity-induced damage variable is added to the selected CTSL to degrade the critical energy release rate. In addition, a new material constant is introduced to adjust the ratio between the critical values of cohesive traction and material separation width in the CTSL. On the other hand, the additional hardening rule depending on memory surface is appended in conjunction with the conventional isotropic and kinematic hardening rules to reflect the difference in plastic deformation with various ranges of cyclic loading. Several experiments are conducted to identify the material parameters and verify the validity of the proposed model. By reference to the experimental results, the capability of our proposed constitutive law is demonstrated in predicting the degradation of tensile strength and breaking elongation of a steel after cyclic loading.

Comparative Study on the Anchor Performance of Wooden Tension Anchor Bolts and Tension–Compression Composite Anchor Bolts in Earthen Sites

ABSTRACT The full-length bond wood tensile anchor system (WTS) conventionally used in earthen sites is primarily broken by the debonding of the bolt–slurry interface. To solve this limitation, material performance and pull-out tests were performed for a new type of wood tension–compression composite anchor system (WTCS) and WTS, and the strain of the bolt–slurry and the slurry–soil interface were monitored. A comparison of test results indicated the following: The average pull-out force of the WTCS is approximately 23–51% higher than that of the WTS. The WTCS has the strong ductility characteristics of small elastic and large plastic deformation. As the load increases, the strain at the bolt–slurry interface changes from a negative index of low strain to a high-strain “S” curve at the end along the bond length. The peak value of slurry–soil interface strain exhibits a transition from the front part of bond length to the middle and back parts of bond length. The mechanical operating mechanism between bolt and slurry is divided into three simplified progressive structural force models, namely “1,” “T,” and “Y.” Finally, the calculation formulas of the average pull-out force of the WTCS are provided.

Nano-scratch testing on thin diamond-like carbon coatings for microactuators: friction, wear and elastic-plastic deformation

Abstract In micromechanical systems the mechanical and tribological requirements for the materials in use are getting rapidly higher. The aim is to find out the fundamental frictional and wear behavior of thin coatings by using the hitherto knowledge in mechanical characterization of coatings and selective experiments. Nanoindentation and linear oscillating microwear experiments have been performed on micro-actuator components and on thin diamond-like carbon coatings, using diamond tips and loads in the range of some millinewtons. The wear depth and friction coefficients were determined as a function of load and number of wear cycles. New methods of data analysis will be discussed, which allow a separation of the different contributions of wear due to material loss, and elastic and plastic deformation. The friction coefficient shows deviations from Amonton’s law because of the dependence of the contact area on the normal load.

Wear Estimation of DLC Films Based on Energy-Dissipation Analysis: A Molecular Dynamics Study.

This study employs the energy-dissipation method to analyze the tribological behaviors of diamond-like carbon (DLC) films through molecular dynamics simulation. It is found that at small load and sliding velocity, the variation trend of average friction force is only dependent on the number of interface bonds (or contact area). However, at large load and sliding velocity, the friction mechanism is not only related to the number of interface bonds but also related to the presence of the transfer layer. The elastic–plastic deformation mainly occurs in the early sliding stage, and a part of the stored elastic potential energy is dissipated by plastic potential energy or internal frictional heat. After the sliding stabilization, over 95% of the total frictional energy is dissipated by thermal conduction, and the rest is mostly dissipated by wear. The increase in load, velocity, and temperature cause more frictional energy dissipated by elastic–plastic deformation, atomic motion, and elastic deformation instead of thermal conduction, respectively. Finally, the wear rate obtained in this work is the same order of magnitude as the experiment. Generally, this work provides an effective atomic-scale method to comprehensively analyze the microscopic wear mechanism of materials.

Effect of loading path on grain misorientation and geometrically necessary dislocation density in polycrystalline aluminum under reciprocating shear

Solid phase processing (SPP) is a promising alloy fabrication technique to produce fine and homogeneous grain structures for high-performance alloys. However, there is very limited modeling capability to understand and predict the grain refinement during SPP. In this work, the crystal plasticity theory was used to study elastic–plastic deformation in polycrystalline aluminums under large shear deformation. Two approaches, kernel averaged misorientation (KAM) and grain reference orientation deviation (GROD), were used to assess the grain misorientations. The geometrically necessary dislocation (GND) density was computed with the plastic strain rate. The deformation simulations were carried out under two loading conditions to investigate the effect of loading paths on the evolutions of grain misorientation and GND density. The results show that the regions with high misorientation and GND density first appear near grain boundaries. These regions then extend toward interior grains. The loading path affects plastic deformation accumulation and recovery, hence dislocation evolution and misorientation. Both two- and three-dimensional simulations showed that the spatial and temporal evolutions of GROD, KAM, and GND density are closely correlated, which indicates they all can be used as criteria of grain refinement or recrystallization, the essential information for grain refinement simulations.

Experimental study and analysis on seismic performance of special-shaped steel reinforced concrete column frame

Using a prototype special-shaped steel reinforced concrete (SRC) column frame from a subway project, a low-cycle reversed loading test on 1/10 scale, 2-bay, 2-story special-shaped SRC column frame was carried out. The load-bearing capacity, stiffness and degradation, ductility, hysteresis and energy consumption characteristics were analyzed. A finite element model (FEM) of the frame was formulated to analyze the stress and the deformation behavior. The analysis and experimental results were compared to one another. The results show that the hysteresis loop of the frame is well defined and the elastic–plastic deformation capacity is ideal for seismic collapse resistance. The numerical simulation results are in good agreement with the experimental test results. The second story of the special-shaped SRC column frame exhibited the most damage in the loading process, therefore it should be further evaluated and optimized.

Determining the Stages of Deformation and Destruction of Composite Materials in a Static Tensile Test by Acoustic Emission.

Composite materials are used in many industries. They are construction materials that are being used more and more often, which makes it necessary to accurately identify the process of their destruction. Recent decades have resulted in an intensive increase in diagnostic tests of structures and mechanical elements. Non-destructive testing (NDT) represents a group of test methods (surface and volumetric) that provide information about the properties of the tested element without changing its structure. The method of acoustic emission (AE) is also being used more frequently. Thanks to the ability to detect and locate signal sources, as well as to perform tests during operation, it is a method that is increasingly used in industry. In this article, the acoustic emission was used to analyze the changes occurring in composite materials. Obtained parameters helped to determine the signals originating from fibre delamination, fibre cracking, etc., as well as the starting point of these changes and the stress values at which these changes occurred. The analysis of acoustic emission signals recorded during the tests helped to determine the values of amplitudes characteristic for the destruction mechanisms of considered composite materials. Signals with an amplitude in the range of 30–41 dB may indicate elastic–plastic deformation of the matrix. Signals with an amplitude in the range of 42–50 dB indicate matrix cracks with the accompanying phenomenon of fibre delamination. Signals with amplitudes greater than 50 dB indicate fibre breakage. Based on the test results, the permissible stress was determined; when exceeded, the mechanisms of damage to the structure of composite materials accumulate. This stress limit for the tested material is 70 MPa. The use of the acoustic emission method in mechanical tests may contribute to a greater knowledge of composite materials used as a construction material, as well as determine the stresses allowable for a given structure.

Analysis of tangential contact damping mechanism by direct observation using X-ray computed tomography

The damping of metal contact surfaces considerably influences the dynamic characteristics of machines. However, the contact damping mechanism has not yet been sufficiently clarified because it is difficult to directly observe metal–metal contact surfaces. In this study, the deformation of the metal surface asperity caused by a tangential load was analyzed using a combination of X-ray computed tomography and finite element method (FEM) simulations. The FEM simulation considering the elastic–plastic deformation and slip reproduced the experimental results well. The contribution of the elastic–plastic deformation and slip of asperities to contact damping was discussed using the developed simulation.

Analysis of indentation size effect behaviour in nonlinear optical LT-TTZS single crystal

The slow solvent evaporation approach was used to effectively generate a high-quality L-threonine doped tris thiourea zinc sulphate crystal. Single crystal x-ray analysis was performed to understand the crystalline nature of the grown substance. The mechanical properties of the as grown single crystal was investigated and discussed through the measurements done by a microhardness tester with Vicker’s indenter at room temperature. The physical properties such as microhardness, fracture toughness, brittleness index and yield strength due to the reflection of applied force were investigated and discussed. The experimental results were reasoned according to theoretical models such as Meyer's Law, Proportional Sample Resistance (PSR), Elastic/Plastic Deformation (EPD) model, and Hays–Kendall (HK) approach. The best model for the explanation of RISE behaviour through load-independent microhardness of the samples was accepted to be Hays–Kendall (HK)approachbased on the supportedresults obtained. Finally, the mechanical characteristics of L-threonine doped tris thiourea zinc sulphate crystals were explored in depth in this paper, along with plausible justifications.

Thermo elasto-plastic contact analyzis for high temperature applications

This study presents the development of a thermo-mechanical simulation model for an elastic-plastic contact problem between a half cylinder and a plane plate. The set of equations was solved using direct coupling method by ANSYS mechanical. The results obtained from the present numerical model of the structural contact without heat transfer are compared with those of analytical, experimental and other numerical models. Then, the contact problem was solved using a coupled thermo-mechanical model. Computational results showed significant effects of thermal consideration in the elastic-plastic contact problem. Large deformations of structure due to high temperature are predicted using the thermo-mechanical model with elastic-plastic deformations. This model is useful to predict deformations on the structural components due to contact at high temperature situation.

Stress-strain state of surface layer under reversible surface plastic deformation of machine parts

The article considers an approach to increasing the stress state in the elastic-plastic deformation zone during surface plastic deformation. The technical idea of solving the problem is based on the complication of the kinematics of a two-radius roller. To achieve this goal, the finite element method based on the ANSYS computer program was used, which makes it possible to determine the values of temporary, residual stresses and strains for different schemes of reversible surface plastic deformation with a two-radius roller. It has been established that the greatest influence on the stress-strain state in the deformation zone is exerted by the scheme of reverse rotation of a two-radius roller located at an angle of 45 degrees relative to the direction of movement, and the least effect is when hardening with a two-radius roller, the axis of rotation of which coincides with the direction of movement.

Determination of Constitutive Parameters of Crystal Plasticity using Micro-pillar Testing with Pure Aluminum

The uniaxial compression test using the micrometer-sized specimen is employed to evaluate elastic-plastic deformation behavior of the selective single crystal from polycrystalline materials. Through reflecting these properties into the crystal plasticity finite element method (CPFEM), elastic-plastic analyses of the polycrystalline materials can be performed based on the single crystalline properties of real materials. In this study, the micro-pillar compression tests were conducted in the some single crystals of recrystallized polycrystalline pure aluminum. A method identifying CRSS, work hardening parameters and anisotropic elastic constants was proposed. The predicted stress-strain curves of single crystals obtained by CPFEM agree well with the original experimental data.

Impact of fatigue loading on the surface of polyurethane treated in argon and acetylene plasma

Soft plasma-treated polymers have good prospects in the creation of biomedical products. Their application is associated with cyclic mechanical loads. The practical applicability of such coatings is unclear without investigating the effect of deformation on the structure of modified surfaces. Elastic polyurethane (synthetic two-phase polymer) was used in this work. Its surface was preliminarily cleaned from low-molecular fractions by argon plasma and then treated by plasma chemical decomposition of acetylene in argon plasma for 30...120 s. The structural and mechanical inhomogeneities of the obtained carbon-containing nanocoatings depend on the treatment time and were caused by the composition of the initial substrate. Uniaxial fatigue loading leads to the formation of transverse folds on the coatings in the unloaded state (a result of elastic-plastic deformation of the coating); under tension these folds are open and form local nanocracks. One of the variants of plasma modification did not give the formation of cracks; it is associated with the areas with a pronounced stiffness contrast of this coating.

ЧИСЛЕННОЕ МОДЕЛИРОВАНИЕ УСТАЛОСТНОЙ ДОЛГОВЕЧНОСТИ ДЕТАЛЕЙ АВИАЦИОННЫХ ГТД ПРИ ЦИКЛИЧЕСКОМ ДВУХЧАСТОТНОМ НАГРУЖЕНИИ

The problem of assessing the resource characteristics of responsible engineering facilities, taking into account the peculiarities of operational loading modes, is discussed. The processes of fatigue durability of polycrystalline structural alloys under cyclic two-frequency loading (with the combined action of mechanisms of low and multi-cycle fatigue) are considered. A mathematical model describing the processes of cyclic elastoplastic deformation and accumulation of fatigue damage in structural alloys under multiaxial disproportionate modes of combined thermomechanical loading has been developed from the modern positions of damaged medium mechanics and fracture mechanics. The model consists of three interrelated parts: relations determining the cyclic elastic-plastic behavior of the material, taking into account the dependence on the destruction process; evolutionary equations describing the kinetics of fatigue damage accumulation; criteria for the strength of the damaged material. The variant of the defining elastic-plasticity relations is based on the idea of the yield surface and the principle of gradiency of the velocity vector of plastic deformations to the yield surface at the loading point. This version of the equations of state reflects the main effects of cyclic elastic-plastic deformation of the material for arbitrary folded loading trajectories. The variant of kinetic equations of fatigue damage accumulation is based on the introduction of a scalar damage parameter, is based on energy principles and takes into account the main effects of formation, growth and fusion of microdefects under arbitrary complex cyclic loading regimes. A joint form of the evolutionary equation of fatigue damage accumulation in the areas of low- and multi-cycle fatigue is proposed. As a criterion of the strength of the damaged material, the condition of reaching the critical value of the damage value is used. The results of numerical simulation of fatigue durability of a compact sample (model of the disk deflector of the high pressure turbine of an aviation gas turbine engine) are presented with cyclic two-frequency loading. The results of the comparison of the calculated and experimental data showed that the proposed model qualitatively and with the accuracy necessary for practical calculations quantitatively describes the fatigue life of structural elements under cyclic two-frequency loading.

Finite element modeling of the processes of elastic-plastic deformation of reclamation objects of the agro-industrial complex

Annotation. To study the processes of nonlinear deformation of reclamation objects and engineering systems of the agro-industrial complex, taking into account the plastic stage of the used structural material, a finite element model was created based on a volumetric prismatic discretization element with quadrangular bases. The plastic stage of deformation of the applied structural material of the object is taken into account on the basis of the provisions of the deformation theory of plasticity. The plasticity matrix at the (j + 1)-th stage of sequential loading was compiled as a result of applying the operation of differentiating the stress tensor components accumulated over the j previous stages of sequential loading with respect to the strain tensor components. The stiffness matrix and the column of nodal forces of a prismatic discretization element with quadrangular bases at the (j + 1)-th stage of loading were obtained by minimizing the Lagrange functional. Numerical experiments on the calculation of a tubular outlet proved the adequacy of the developed finite element model and the accuracy of calculating the required strength parameters of the studied reclamation objects and other engineering systems of the agro-industrial complex, sufficient for engineering practice.

АЛГОРИТМ КОНСЕРВАТИВНОГО СГЛАЖИВАНИЯ РАЗРЫВНЫХ ВОЛН НАПРЯЖЕНИЙ В МКЭ

Three-dimensional geometrically and physically nonlinear problems of nonstationary deformation of structures are considered. The defining system of equations is formulated in Lagrange variables. The equation of motion is derived from the balance of virtual work capacities. The elastic-plastic deformation of structural materials is described by the relations of the flow theory with isotropic hardening. The solution of the problem is based on the moment scheme of the finite element method. The discretization of the problem by spatial variables is carried out by eight nodal finite elements with multilinear functions of the approximation form of the displacement velocity. Time integration is performed according to an explicit finite-difference scheme of the “cross” type, which does not have the monotonicity property. Due to the dispersion in the vicinity of the gap, it generates non-physical oscillations, which significantly limits the scope of its applicability. In this paper, to suppress high-frequency oscillations, it is proposed to use an algorithm for conservative smoothing of a numerical solution with a space-time monotony analyzer. Based on it, software modules have been developed for the “Dynamics-3” computing complex. Verification of the developed technique and its software implementation was carried out by solving a one-dimensional problem in a three-dimensional formulation of the compression wave passing through a fixed elastic layer. For comparison, the results of numerical solution of the problem according to the “cross” scheme without smoothing, with linear viscosity, with conservative smoothing using spatial and spatiotemporal monotonicity analyzers are obtained. The problem of penetration of an elastic cylinder into a round steel plate is solved in a three-dimensional formulation. It is shown that the developed technique not only suppresses high-frequency oscillations, but also prevents the occurrence of zero-energy modes. So, in the second problem, without using the procedure of conservative smoothing of the numerical solution, the final elements of the plate in the collision zone are significantly distorted, which leads to an early interruption of the calculation.

Experimental study of elastic-plastic deformation of a cylindrical shell made of steel 45

Relevance. The use of shells as thin-walled structures for various industries is very diverse. Spherical shells are widely used in the aircraft industry, circular cylindrical shells are used in the oil and gas industry, and more than 30 analytical forms of shells have been used in the construction industry. All elements of shell structures are undoubtedly subject to various strength calculations. Also, a separate role in the study is occupied by the experimental part, which confirms or refutes the calculated equations, this indicates the undoubted perspective and relevance of modeling the loading of shell structures. The aim of the study is to assess the condition of a thin-walled shell under three-parameter loading by tensile, compression and torsion forces. Methods. Experimental studies on the deformation of shells carried out in laboratory conditions on prototypes made of steel 45 GOST 1050-2013 are presented, and the loading conditions of the sample are modeled, similar to the possible conditions for applying loads to existing structures. Results. Experimental diagrams of deformation of the sample material in various planes are given, the development of deformation of the shell after applying joint efforts of stretching and torsion, as well as compression and torsion to the destruction of the material is estimated. For the relevance of the conducted experiment, a real design has been selected, which, when certain conditions are created, can experience appropriate experimental loads.

ПЕРЕМЕЩЕНИЯ В СТЕРЖНЕВЫХ СИСТЕМАХ ЗА ПРЕДЕЛОМ УПРУГОСТИ

One of the provisions of the method for calculating building structures by the limiting state is the satisfaction of the operational requirements in relation to the displacements of their elements under load. An urgent problem is their determination at the stage of elastic-plastic deformation of the material. A technique is proposed for its solution for the case of the Prandtl diagram. The core system is a two-span statically indeterminate beam. Its limiting state in terms of bearing capacity, as well as an intermediate stage of deformation, are considered. The introduction of the reduced moment makes it possible to extend the Mohr-Maxwell formula beyond the limit of linear elasticity. The use of classical physical models of solid mechanics leads to the solution of the problem in an analytical form. Experiments were carried out to test the theoretical results obtained by the proposed method. In one of them, a two-span beam with a span of 50 cm and a cross section of 3×0.42 cm made of duralumin was tested. The mechanical characteristics of the material were previously obtained. Displacements were measured with the help of indicators. The maximum load (428 N) in each of the spans of the beam was 80 % of the limit value. In this case, the beam had elastic-plastic regions. The experiment revealed the dependence of displacements on the load, acceptable for the form of deformation: a linear graph with Hooke's law and a curve in the presence of plastic deformations. Deviations from theoretical values were no more than 3.3 %.