Critical Deformation Research Articles

On deformation criteria of a special limit state of eccentrically compressed elements of RC building frames

At accidental impacts caused by the sudden removal of a load-bearing element of the building frame, the stress-strain state of the sections of eccentrically compressed structural elements such as columns, pylons, braces and belts of trusses, etc. becomes more disadvantageous than their stress-strain state at the stage of normal operation. The factors that lead to such a situation are sharp increase in the eccentricities of loads attaching, slenderness of cross sections, corrosion, initial curvature and mechanical damages. In these cases, buckling scenario for the exhaustion of their bearing capacity is a possible scenario. The paper proposes an approach to the obtaining of deformation criteria for assessing the special limiting state of eccentrically compressed bar elements of reinforced concrete frames. Such an approach is based on the use of a combination of nonlinear deformation analysis and nonlinear stability analysis. Paper shows that the limiting values of deformations for the considered reinforced concrete element can be determined by the criteria of strength or buckling, depending on the ratio of the forces acting in the sections of the structural element.

Crystallographic and Experimental Disproof of Pyramidal < c+a > Slip in Magnesium

The activation of non-basal pyramidal slip has been perceived as key to enhance the ductility of magnesium and its alloys. However, there has never been convincing evidence to show the physical existence of dislocations and their involvement in deformation has been a core issue in magnesium research. In the present work, the impossibility of slip is analyzed based on fundamental concepts of dislocation and atomic interactions. The atomic configurations and crystallographic features in association with dislocations are unambiguously revealed for the first time, demonstrating that any possible dislocation core structures would involve too many atoms on multiple lattice planes and are physically impossible. Experiments of magnesium single crystal compression along its c-axis were conducted at temperatures from 20°C to 500°C and the results showed no evidence of the involvement of dislocations in any form as a mechanism of deformation during either plastic flow or fracture. von Mises criterion for compatible deformation, which drives the pursuit of pyramidal slip, is critically discussed.

Effect of Rolling Deformation on Corrosion Behaviour of AISI 304L in 3% NaCl Solution

It is well known that the mechanical resistance of austenitic stainless steels can be increased considerably by cold rolling process.¶ The cold rolling effect on corrosion resistance of AISI 304L stainless steel in 3% Sodium Chloride solution was investigated by potentiodynamic polarisation and by Scanning Electronic Microscopy (SEM). The pitting corrosion in this environment is related to the rate of cold deformation. The cold rolling induces important changes in the microstructure and involves phase transformation (γ→a'). The AISI 304L developes martensitic structure after 16% cold working. The potentiodynamic results show a moderate variation of the passivity zone, a remarkable decrease in the pitting potential and a free potential. The results also show an increase in the current density. However, it seems that the critical deformation rate appears to start at approximately 50% of the rolling deformation where the passivation current is minimal. After the polarisation tests, metastable pits are observed using SEM and the most probable initiation causes are discussed

Estimation of Damage Rotor Shaft of Turbogenerators with a Diode Brushless Excitation System

The damage to the rotor shafts of high-power turbo generators is estimated as a result of the resonant in-teraction between the generator and the exciter caused by the operation of the automatic excitation regulator (AER) by mathematical modeling. The analysis covers turbo generators with a capacity of 300 to 1000 MW with a diode brushless excitation system (DBES). The simulated circuit includes a turbo generator with a block transformer, a power transmission line, an exciter in the form of a reversed synchronous generator with a rotat-ing rectifier diode converter unit, a static thyristor converter, and a sub-exciter in the form of a synchronous machine with permanent magnets. When modeling the electrical part, an approach is used from the positions of its own coordinates, which ensures maximum methodological consistency of the models of the listed devices and allows directly reproducing resonant phenomena at torsional vibration frequencies with the determination of instantaneous values of currents, voltages and electromagnetic moments of the turbo generator and exciter. The mechanical system is presented taking into account the exciter and sub-exciter as a seven-mass system. AER is introduced into the mathematical model by means of transfer functions with corresponding coefficients and time constants. The control system of the thyristor converter is represented in the model by a generator of line-arly increasing signals, a body for comparing these signals with the signal from the AER and a control pulse generator. To assess the damage rate, the deformation criterion for soft and hard loads in the zone of low-cycle fatigue and the force criterion in the zone of multi-cycle fatigue were used. A comparative quantitative assess-ment of the damage in the neck of the G-E shaft line in the resonant interaction between the exciter and the generator with different automatic excitation control systems is given. The influence of attenuation of electro-magnetic transients and damping of torsional vibrations on the damage values is analyzed. The results ob-tained can be used to analyze the functioning and determine the settings of the AER.

Criterion for elastoplastic failure of bimetallic plate with edge crack of transverse shear at the boundary of the materials joint

The initiation of a transverse shear edge crack in elastoplastic materials with the limiting strain is considered. The crack propagation criterion is formulated using a modified Leonov-Panasyuk-Dugdale model using the additional parameter - the width of the plasticity zone. The coupled quasi-brittle fracture criterion for mode II cracks in an elastoplastic material is formulated under conditions of small-scale yield in the presence of a singular feature of the stress field in the vicinity of the crack tip. The coupled fracture criterion includes the deformation criterion, which is formulated at the crack tip, as well as the force criterion, which is formulated at the model crack tip. The lengths of the initial and model cracks differ by the length of the prefracture zone. The sequential analysis of the possibility of applying the proposed fracture criterion in determining the critical loads for solids containing edge cracks of transverse shear at the interface of different media is performed. Quasi-brittle fracture diagrams are constructed for a composite plate with an edge crack under plane strain and plane stress conditions. he analysis of the parameters included in the proposed model of quasi-brittle fracture is carried out. The model parameters are proposed to be selected by approximation of simple shear diagram and critical stress intensity factor. The critical loads were found numerically for the quasi-ductile and ductile fracture types. The finite element method is used to solve the problem of drawing out a reinforcing layer from a metal composite under quasi-static loading. The process of propagation of plastic zones in the vicinity of the crack tip is described consistently. It is shown that the shapes of the constructed plastic zones differ significantly from the well-known classical concepts.

ЖИВУЧЕСТЬ РАМНО-СТЕРЖНЕВОГО ЖЕЛЕЗОБЕТОННОГО КАРКАСА ЗДАНИЯ В ЗАПРЕДЕЛЬНЫХ СОСТОЯНИЯХ

A methodology and an algorithm for calculating the survivability parameters of a long-term deformable reinforced concrete building frame in extreme states are presented. Analytical dependencies for determining the value of the creep measure are taken in accordance with the use of approximate dependencies from the recommendations of the NIIZHB. On this basis, a method is proposed for determining the cross-section bending stiffness of the frame elements. The deformation criterion of a special limiting state is formulated taking into account the nonequilibrium processes of prolonged deformation of the structural system elements. The numerical analysis results of the long-term deformable reinforced concrete frame survivability potential with a sudden removal of the one structural element, taking into account the long-term deformation prehistory of the considered building frame under an operating load, are presented. The exposure of the structural system survivability from the its loading moment to its transformation into a kinematically variable system has been determined.

Operational control of rib pillar stability

In room-and-pillar mineral mining, rib pillars should support overlying rock mass for the specified time limit of production. Therefore, one of the mining safety components is monitoring of the behavior of rib pillars in the course of time. For the conditions of the room-andpillar method of mining, the authors propose a monitoring procedure for rib pillar deformation based on operational measurements of horizontal convergence in stopes. The theoretical and experimental research proves that transverse deformation of rib pillars is an informative parameter suitable for generalized assessment of pillar failure. The obtained ranges of critical transverse deformation rates (50–100 mm/m/yr) in rib pillars can tentatively be used as an indicator of the critical stability of load-bearing structures in room-and-pillar mining. In-situ determination of the integral transverse deformation rates in rib pillars is based on the ratio of the measured horizontal convergence in stopes to the width of pillars. Implementation of the proposed approach in the Upper Kama potash salt mines has proved its applicability to identification of rock mass areas where intense deformation is expected. The comparison of the monitoring data of the transverse deformation rates and their critical values determined makes it possible to predict service life of rib pillars, which is very important in terms of safety of mining operations. This study was supported by the Russian Science Foundation, Grant No. 19-77-30008.

Fluctuating nonlinear spring theory: Strength, deformability, and toughness of biological nanoparticles from theoretical reconstruction of force-deformation spectra

We developed the Fluctuating Nonlinear Spring (FNS) model to describe the dynamics of mechanical deformation of biological particles, such as virus capsids. The theory interprets the force-deformation spectra in terms of the “Hertzian stiffness” (non-linear regime of a particle's small-amplitude deformations), elastic constant (large-amplitude elastic deformations), and force range in which the particle's fracture occurs. The FNS theory enables one to quantify the particles’ elasticity (Young's moduli for Hertzian and bending deformations), and the limits of their strength (critical forces, fracture toughness) and deformability (critical deformations) as well as the probability distributions of these properties, and to calculate the free energy changes for the particle's Hertzian, elastic, and plastic deformations, and eventual fracture. We applied the FNS theory to describe the protein capsids of bacteriophage P22, Human Adenovirus, and Herpes Simplex virus characterized by deformations before fracture that did not exceed 10–19% of their size. These nanoshells are soft (~1–10-GPa elastic modulus), with low ~50–480-kPa toughness – a regime of material behavior that is not well understood, and with the strength increasing while toughness decreases with their size. The particles’ fracture is stochastic, with the average values of critical forces, critical deformations, and fracture toughness comparable with their standard deviations. The FNS theory predicts 0.7-MJ/mol free energy for P22 capsid maturation, and it could be extended to describe uniaxial deformation of cylindrical microtubules and ellipsoidal cellular organelles.

APPLICATION OF THE GROUND LASER SCANNING RESULTS IN MONITORING OF ADJACENT ROCK MASSES OF DEEP CAREERS

The method of laser scanning, used in combination with the traditional mine surveying, is one of the most effective and safest ways to carry out mine surveying and instrumental observations on deforming areas of open mining objects. The article presents the results of ground-based laser scanning of the Kharanutsky coal pit and a comparative analysis to determine the deformations and displacements of the sides for different periods, which was performed in the RISCAN PRO environment. Digital elevation models (DEM) were built, distinguishing observation cycles by color. As a result of comparison, the places of manifestation of deformations and displacements were revealed. Based on the results of geomonitoring, it was concluded that critical displacements and deformations were not detected, despite the places where small rockslides were manifested, and further systematic observations were recommended. The results of laser scanning are confirmed by the data of satellite observations on the control points and the survey of the section sides

Features of the kinetics of cyclic elastoplastic deformation diagrams at dwells in cycles and superimposition of variable stresses on them

The goal of the study is determination of the regularities of changes in cyclic strains and related deformation diagrams attributed to the existence of time dwells in the loading modes and imposition of additional variable stresses on them. Analysis of the obtained experimental data on the kinetics of cyclic elastoplastic deformation diagrams and their parameters revealed that in contrast to regular cyclic loading (equal in stresses), additional deformations of static and dynamic creep are developed. The results of the studys are especially relevant for assessing the cyclic strength of unique extremely loaded objects of technology, including nuclear power equipment, units of aviation and space systems, etc. The experiments were carried out on the samples of austenitic stainless steel under low-cycle loading and high temperatures of testing. Static and dynamic creep deformations arising under those loading conditions promote an increase in the range of cyclic plastic strain in each loading cycle and also stimulate an increase in the range of elastoplastic strain due to active cyclic deformation. At the same time the existence of dwells on extrema of stresses in cycles without imposition of additional variable stresses on them most strongly affects the growth of plastic strain ranges in cycles. Imposition of additional variable stresses on dwells also results in the development of creep strains, but their growth turns out to be somewhat less than in the presence of dwells without stresses imposed. The diagrams of cyclic deformation obtained in the experiments are approximated by power dependences, their kinetics being described in terms of the number of loading cycles using corresponding temperature-time functions. At the same time, it is shown that increase in the cyclic plastic deformation for cycles with dwells and imposition of additional variable stresses on them decreases low cycle fatigue life compared to regular loading without dwells at the same stress amplitudes, moreover, the higher the values of static and dynamic creep, the greater decrease in low-cycle fatigue life. This conclusion results from experimental data and analysis of conditions of damage accumulation for the considered forms of the loading cycle using the deformation criterion of reaching the limit state leading to fracture.

Analysis of Deformation and Prediction of Cracks in the Cogging Process for Die Steel at Elevated Temperatures.

In this paper, an analysis of a three-dimensional state of strain and stress in the case of the hot cogging process of X32CrMoV12-28 die steel with the application of the finite element method is presented. The results of the investigations connected with the simulation of the kinematics of metal flow and thermal phenomena are presented, accompanied by prognosing the formation of ductile fractures in the course of the hot cogging process conducted with the application of three different shape tools and of a proposed deformation criterion of the loss of cohesion. The applied anvils were found to be highly effective in the aspects of distribution of effective strains and stresses, absence of tensile stresses in the axial zones of a forging, and also of a significant thermal stability in the internal layers of a deformed material. The developed course of changes in the deformation of the damage factor in the case of forging in the investigated anvils renders it possible to predict the situation and the phase of deformation in which the loss of cohesion by a deformed material will occur. The comparison between the predicted and the experimental results showed a good agreement.

Optimization of Transformer Winding Deformation Assessment Criterion Considering Insulation Aging and Moisture Content

Frequency response analysis is widely used to diagnose transformer winding deformation faults due to its high sensitivity, strong anti-interference capability, and equipment portability, but the results of frequency response analysis can be affected by insulation aging and moisture in the transformer, leading to errors in the diagnosis of winding deformation faults. Currently, there is no effective method to prevent such errors. This paper focuses on optimizing the criterion for diagnosing winding deformations when insulation aging and moisture are present. First, the winding frequency response curves of oil-paper insulation were determined by combining insulation aging and moisture tests of the oil-paper insulation with frequency response simulations of the transformer winding. Next, the winding deformation criterion predicting the likelihood and extent of errors diagnosing transformer winding deformations due to the insulation aging and moisture content is discussed. Finally, the corresponding criterion optimization method is proposed. The corresponding results show that insulation aging and moisture can lead to errors when using the correlation coefficient R criterion to diagnose the transformer winding deformations. Moreover, the possibility of winding deformation errors caused by the change of insulation state can be reduced by introducing the corresponding auxiliary criterion through comparing the capacitance change rate based on the frequency response method and that based on the dielectric spectrum method.

Study of Effects of Stress Concentration and Variation of Mechanical Properties

In this paper, we compare two approaches to the determination of stress-strain states (SSS) and the strength analysis of a thin plate with a circular hole concentrator under static and cyclic loading by solving the elastic problem (Kirsch problem) and applying the deformation criteria developed at the Mechanical Engineering Research Institute for deformation and fracture. In the new approach, the characteristics of the main mechanical properties, the values ​​of the nominal stresses, and the strain field in the zones of stress concentration (a total of four calculated cases) were varied for the first time. Studies showed that elastoplastic stresses and strains, which differ significantly from those obtained as a result of solving the elastic problem, arise even under the initial loading at the points of the cross section near the concentrator. Determined by deformation criteria, the results of the SSS analysis and strength analysis of a thin plate with a circular hole significantly differed from those obtained by the traditional elastic solution. This applies to the initial loading both in the maximum concentration zone (on the hole contour) and at other points of the dangerous cross section under cyclic loading. Using the above method, the cyclic kinetics of stresses and strains in the concentration zones (for a specified plate section) for any loading cycle can be calculated, while cyclic durability can be more accurately calculated taking into account the dispersion of the basic mechanical properties of the structural material.

Parametric study of nonlinear buckling capacity of short cylinders with Hemispherical heads under hydrostatic pressure

This study investigates the buckling behavior of short cylindrical shells with hemi-spherical heads subjected to hydrostatic pressure. It is assumed that the length of the cylindrical part is smaller than or equal to its diameter while its material may be dif-ferent from that of hemispherical heads. Finite element analysis was used to seek out the effect of geometric parameters such as thickness, length, and volume of the tank on the ultimate buckling load. Results indicate that the buckling load is directly pro-portional with the thickness and inversely proportional with the volume of the vessel. A close examination of the buckling modes reveals that under uniform hydrostatic pressure, the cylindrical part undergoes the most critical deformation compared with its hemispherical heads. This behavior was observed for the two loading cases of (a), a hydrostatic pressure applied to the whole structure and (b), the hydrostatic pressure was only applied to the cylindrical part of the vessel.

Effect of forming conditions on frictional characteristic in cold bulk forming process of copper alloy

Effects of working speed (v) of die and lubricant on frictional characteristics of C5191 copper alloy during cold forming process are studied. The friction on contact surface is mainly caused by slight ploughing in small deformation and tearing after adhesion in large deformation, which translates at a critical deformation (deformation threshold (DT)). DT increases with increasing v. Lubricants have little effect on DT at 0.15 and 15 mm/s, but have a significant effect at 1.5 mm/s. Lubricating effect of oil and Molykote is equivalent at low v, especially before the DT, but has notable difference at high v. However, the ploughing action and tearing area by adhesive friction at high speed are smaller than those at low speed.

Suppression of the annealing-induced hardening effect in ultrafine-grained Al at low temperatures

The influence of decreased temperatures of mechanical tension on the effect of annealing-induced hardening (AIH) in ultrafine-grained (UFG) aluminum of commercial purity is revealed and discussed for the first time. The UFG structure was formed by high-pressure torsion processing. The dependences of changes in yield strength, tensile strength and ductility, caused by annealing, on the deformation test temperature were experimentally obtained. The critical deformation temperature was revealed at which the AIH effect is completely suppressed. A theoretical explanation of this phenomenon is suggested.

Impact dynamics for gravity-driven motion of a particle

Introductory mechanics courses use the bouncing ball model to familiarize students with the principles of binary inelastic collisions. Nonetheless, in undergraduate courses, the modeling of binary systems typically disregards the time of contact and the effects of gravity, which yields to a constant coefficient of restitution (COR) and, as a consequence, prevents students from elucidating the real dependence of COR with impact speed. In this work, we proposed a simple experimental setup to investigate the impact dynamics of a spring-mass body that bounces on a rigid plate as well as a theoretical framework that captures the velocity dependence of the COR for low-impact speeds. Our analytical expression for the COR highlights the role-played by gravity, impact speed and collision time on the collision dynamics. Our results suggest that the inclusion of gravity force allows an adequate estimation of the critical impact speed and maximum deformation distance, crucial parameters to differentiate between repulsive and attractive interaction regimes. Our experimental setup enables the clarification of several key concepts of mechanics while it is easy to be performed in most undergraduate laboratories.

Prediction of Ultimate Strain for Rectangular Reinforced Concrete Columns Confined with Fiber Reinforced Polymers under Cyclic Axial Compression.

This paper investigates the crucial design parameters for the prediction of the ultimate axial compressive deformation of reinforced concrete columns externally confined with fiber reinforced polymer (FRP) materials. Numerous test results of available columns with a square and rectangular section under cyclic axial loading were gathered in an advanced database. Herein, the database is enriched with necessary design parameters in order to address the unique tensile strain field variation of the FRP jacket. Since there is a lack of consequent recording of the FRP strain field in existing experiments, three dimensional pseudodynamic finite element analyses results from several characteristic cases of tested columns are utilized to address this gap. Therefore, a hybrid experimental–analytical database is formed, including several critical FRP strains, steel strains and deformations. A modified model is proposed to predict the ultimate axial strain for reinforced concrete columns externally confined with FRP materials. The proposed model aims to address indirectly the effects of the internal steel cage, concrete section shape and of their interaction with the external FRP jacket on the critical tensile strain of the FRP jacket at failure of the column. The predictive performance of the model over the available tests of (reinforced concrete) RC columns under cyclic compression is remarkably improved when compared against the performance of other existing models. It provides predictions with average ratio (AR) of 0.96 and average absolute error (AAE) of 36.5% and therefore may contribute to safer seismic resistant redesign.

Deformation of an Isotropic Plate with Periodic System of Curvilinear Holes and Plasticity Bands

By the method of singular integral equations, we solve a plane elastoplastic problem of fracture mechanics for an isotropic plane with an infinite series of curvilinear holes and plasticity bands at their tips. We study the influence of the shapes of smooth holes and the radii of rounding of the contours at their tips on the opening displacements and lengths of the plasticity bands. The solutions of the corresponding problems for semiinfinite bilateral rounded notches whose tips serve as the origins of plasticity bands are obtained by the limit transition in the case where the relative distance between the holes tends to zero. On the basis of the deformation criterion of fracture and solutions of the periodic elastoplastic problem, we approximately determine the strength of rectangular specimens with bilateral U-shaped notches.

Dynamic Recovery and Recrystallization Behaviors of C71500 Copper-Nickel Alloy Under Hot Deformation

The isothermal compression test was employed to study the influence of deformation temperature and strain rate on the hot deformation behavior of C71500 copper-nickel alloy. According to the characteristics of dynamic recovery (DRV) and dynamic recrystallization (DRX), the Zener–Hollomon parameter lnZ is used as an important parameter to measure the influence of temperature and strain rate on the hot deformation behavior. The critical deformation conditions of DRX are determined by its different values. By analyzing the critical change of work hardening rate, the relationship among critical stress, peak stress and saturation stress transition point is obtained, and the dynamic mathematical model of the whole deformation process is established, which can predict the volume fraction of DRV and DRX well. The DRX nucleation and growth mechanism of C71500 copper-nickel alloy was determined by metallographic analysis. In the current work, the thermodynamic parameters are introduced to determine the critical conditions of the transformation of the deformation state, the transformation process of the hot deformation structure is predicted by the dynamic model, the nucleation and growth mechanism is analyzed, which could provide a complete set of important references for the DRV-based hot deformation alloy, and provide a theoretical basis for the control of the structure state after hot processing.