Residual Stress-strain State Research Articles

Determination of Residual Stresses in 3D-Printed Polymer Parts.

This paper presents the results of an investigation of the possibility of the reliable determination of the residual stress-strain state in polymers and composites using a combination of bridge curvature, optical scanning, and finite element methods. A three-factor experiment was conducted to determine the strength of printed PLA plastic products. The effect of the residual stresses on the strength of the printed products was evaluated. By comparing the values of the same strength stresses, a relationship between the nature of the stresses and the strength of the samples was found. A tendency of the negative influence of tensile stresses and the opposite strengthening effect of compressive stresses was obvious, so at the same values of tensile strength, the value of residual stress of 42.9 MPa is lower than that of the fibre compression at the value of 88.9 MPa. The proposed new methods of the residual stress determination allow obtaining a complete picture of the stressed state of the material in the investigated areas of the products. This may be necessary in confirming the calculated models of the residual stress-strain state, clarifying the strength criteria and assessing the quality of the selected technological modes of manufacturing the products.

On the features of inhomogeneous residual stress identification using the digital speckle interferometry and the hole-drilling method

A phenomenological approach to the actual problem of determining the inhomogeneous residual stress-strain state in the components of high-tech engineering systems at the stages of their design and operation is presented. The approach is based on physical and mechanical methods of measuring displacements. Current physical models describe the physical regularities of the residual states attributed to changes in the structure by the interaction of defects and dislocations in the field of micro- and meso-stresses. At the same time, there are the problems of the transition to the macrolevel, the construction of multilevel models, and the conversion of these models in engineering practice. In the framework of phenomenological approaches, in the general case, the solution of this problem requires the solution of three-dimensional inverse problems of thermoelasticity. A well-known mechanical method for determining a uniform field of residual elastic stresses recommended by ASTM E837 is described. The method proposed earlier by one of the authors for determining an inhomogeneous (in the plane) field of residual elastic stresses is discussed. A method of the three-dimensional inhomogeneous residual elastic stress-strain state determination based on the experimental determination of the displacement vector components by the method of step-by-step point hole-drilling and data of digital speckle interferometry and digital image correlation is developed. The constitutive relations for the components of the displacement vector are written in the form of Volterra integral operators. The basic operator functions are the functions of four variables, i.e., the coordinates of the cylindrical system (r, θ, z) associated with the hole, and the hole depth h. A method for verification of the basic functions is presented. The problem is reduced to the determination of three displacement functions of three variables: hole radius r, h, and z. Numerical simulation of basic functions is carried out. The obtained results are consistent with the known experimental data and calculated values of the deformation on the surface depending on the depth of the hole according to the ASTM E837 Standard.

Micromechanical Modeling for Predicting Residual Stress–Strain State around Nodules in Ductile Cast Irons

In this paper, a micromechanical model was developed to predict the residual stress–strain state that is generated around nodules of a ferritic ductile cast iron during solidification. A finite element analysis was performed on a reference volume element of the material to analyze the local strain development, having modeled both matrix and nodule as deformable bodies in contact. The behavior of the nodule was assumed linear–elastic because of the low stresses to which it is subjected during cooling. On the other hand, elasto-plastic viscous behavior was considered for the matrix, considering both the primary and secondary creep regimes. To make up for the lack of information on the physical–thermomechanical properties of the constituents, the available literature data were integrated with the results obtained from the CALPHAD methodology applied to both cast iron and the steel that constitutes its matrix. The micromechanical model was validated by comparing the resulting residual strains with experimental data available in the literature for a ferritic ductile cast iron. Then, it was used for analyzing the correlation between the solidification history and the mechanical response of cast iron in terms of the uniaxial stress–strain curve.

Analysis of prestressing in old full-scale concrete members

The effective prestressing force is a key parameter that affects the performance of prestressed concrete members (PCM). In fact, the accurate determination of the residual prestressing force is crucial to assess existing PCM since it influences load-carrying capacities, in-service structural properties and remaining service life. The aging of structures, with limited functional capacity, implies a major problem that must be addressed without delay. Progress in the promotion of innovative solutions to extend the service life of structures depends on: (a) the availability of reliable indirect methods, since old PCM were usually no instrumented during casting; (b) the formulation of adequate diagnostics of the residual stress–strain state of structures; and (c) the availability of highly refined predictive tools. The implementation of these elements in the management and maintenance systems of PCM will serve to support decision making associated with structural assessment and to gather information to establish criteria for the design of more resilient PCM.Among the destructive methods available to empirically assess the actual condition of PCM, it can be found: crack initiation, crack re-opening and tendon cutting. As destructive methods inevitably cause structural damage and are not suitable for application to in-service structures, non-destructive methods or with conditions that only require aesthetic restitution (e.g. exposed tendon deformation, hole-drilling and saw-cuts) appear as an alternative of increasing interest.As a first step to obtain feed-back from several experimental tests carried out on unmonitored full-scale PCM over the years, this paper examines different techniques and methods used to evaluate the residual prestressing force, and also presents the deviations and uncertainties regarding predictions of short- and long-term prestress losses from main code models used in design. It is concluded that it is interesting to follow a non-destructive indirect testing methodology to be able to keep in service and monitor the current structures. This would make it possible to draw up a maintenance plan based on the condition of the PCM.

Investigation of mechanical properties of composite materials reinforced with carbon fibers

Theoretical and experimental methods for obtaining and investigating effective thermomechanical characteristics - residual stresses and deformation in panels made of nanomodified materials with asymmetrical reinforcement scheme have been developed in this paper. The study of the residual stress-strain state of structural elements made of carbon plastic using the values of thermoelastic characteristics of composite monolayers identified on the basis of the developed methods meade it possible to reveal the possibility of reducing the residual stress-strain state in structures with asymmetric reinforcement schemes.

The nature of change in the residual stress-strain state of dovetail lock joints, taking into account nonlinear strains

A general algorithm for calculating residual stresses based on the method of successive loadings is presented. The process of formation of residual stresses and strains in dovetail lock joints during its loading by centrifugal forces and subsequent unloading is simulated. The problem is solved in non-linear statement taking into account the elasticity, plasticity and creep strains. The fields of residual stresses and deformations in the grooves of the disk are obtained. The numerical solution of the problem was performed by the finite element method in the ANSYS software package. The influence of the opening angle on the nature of plastic strain zoning is analyzed. The opening angle was changed in the range from 45 to 70. It was established that with the development of creep strain, there is a change in the zone of nonlinear deformations and the nature of the distribution of residual stresses in the zone of the concentrator. The dependences of the value of maximum residual stresses on the opening angle , as well as their variation over time, are obtained.

Numerical and Analytical Modeling of Permanent Deformations in Panels Made of Nanomodified Carbon Fiber Reinforced Plastic with Asymmetric Packing

In this paper, a mathematical model of a multilayer panel made of nanomodified carbon fiber reinforced plastic with asymmetric packing is proposed. The introduction of nanosized particles into the composition of the composite or its components (fiber or binder) allows not only to increase its physical and mechanical properties, but also to improve the picture of the residual stress-strain state. The paper investigates the effect of nanomodification of carbon fiber reinforced plastic on the residual stress-strain state after molding using numerical and analytical methods. Numerous results of computational experiments have been obtained. The results of numerical and analytical modeling are compared with experimental data. Conclusions are drawn about the possibility of reducing the residual stress-strain state in structures with asymmetric reinforcement schemes when using a matrix containing carbon nanoparticles. A mathematical model of a multilayer panel made of nano-modified carbon fiber with asymmetric packing has been built. Investigation of the residual stress-strain state of structural elements made of carbon fiber reinforced plastic made it possible to reveal the possibility of reducing the residual stress-strain state and leash in structures with asymmetric reinforcement schemes when using a matrix containing carbon nanoparticles.

Численное исследование термомеханических процессов при короткоимпульсном лазерном облучении полупространства.

Лазерное облучение металлических поверхностей интенсивными тепловыми источниками используется для генерации коротких зондирующих импульсов, которые распространяются внутрь тонких образцов и позволяют оценивать структуру и механические свойства последних в рамках классического акустического подхода. При кратковременном облучении поверхности конструкции источником энергии высокой плотности возникают большие тепловые напряжения и остаточные деформации. В данной статье численно исследуется осесимметричная задача о термомеханической нагрузке полупространства. При этом учитывается влияние объемных и неупругих характеристик отдельных фаз на остаточное напряженно деформированное состояние полупространства. Постановка задачи включает соотношение Коши, уравнение движения, уравнение теплопроводности, исходные условия, тепловые и механические граничные условия. Термомеханическое поведение изотропного материала описывается унифицированной моделью течения Боднера–Партома. Задача решается с помощью конечно-элементной методики. Численная реализация задачи проводится с помощью пошагового интегрирования по времени. Уравнения движения интегрируются методом Ньюмарка, а уравнение теплопроводности — неявным методом первого порядка. С помощью методики численного решения осесимметричной динамической задачи для полупространства при термомеханической нагрузке и модели течения описано остаточное напряженно-деформированное состояние. Установлено, что микроструктурные превращения, учитываемые через термотрансформационную деформацию, и зависимость неупругих характеристик материала от фазового состава существенно уменьшают остаточные неупругие деформации и способствуют появлению сжимающих напряжений. Получена трехзонная область формирования поля остаточных напряжений.

Calculation of residual stress-strain state of deposited steel sheet plates

The Paton Welding Journal, 2021, №05. International Scientific-Technical and Production Journal «The Paton Welding Journal» «The Paton Welding Journal» has been published monthly since 2000 in English, ISSN 0957-798X. «The Paton Welding Journal» is a cover-to-cover English translation of the «Avtomaticheskaya Svarka» (Automatic Welding) journal. The «Avtomaticheskaya Svarka» journal has been published monthly since 1948 in Russian, ISSN 005-111X.

Obtaining of Carbon Fibers Based Composite Materials and Study of Their Mechanical Properties

In this work, theoretical and experimental methods have been developed for obtaining and studying effective thermomechanical characteristics - residual stresses and deformations in panels made of nanomodified materials with an asymmetric reinforcement scheme. The study of the residual stress-strain state of structural elements made of carbon fiber reinforced plastic using the values ​​of thermoelastic characteristics of composite monolayers identified on the basis of the developed methods made it possible to reveal the possibility of reducing the residual stress-strain state in structures with asymmetric reinforcement schemes.

Исследование динамических эффектов при микромасштабном импульсном нагружении

Рассматривается осесимметричная задача облучения тепловым импульсом цилиндрического тела. Для описания нелинейного поведения материала используется модель Боднера-Партома, обобщенная на случай многофазового состава материала. Сущность обобщения заключается в использовании правила смесей для определения параметров модели, отвечающих за предел текучести и временное сопротивление. Рассмотренная модель позволяет более точно оценить остаточное напряженно-деформированное состояние, которое при дальнейшей эксплуатационной нагрузке цилиндрических деталей существенно влияет на временное сопротивление элементов конструкций. Задача решается численно методом пошагового неявного интегрирования по времени, итерационным методом и методом конечных элементов. На каждом временном шаге реализуется двойной итерационный процесс. Первый – внутренний – связан с интегрированием системы нелинейных уравнений течения, второй – внешний – с решением уравнений движения и теплопроводности. Расчеты проводятся для конечно-элементной сетки, сгущенной в области облучения, для более точного моделирования термомеханического поведения материала. Параметры сетки выбираются по критерию практической сходимости решений. На основе численного моделирования исследуется напряженно деформированное состояние неупругого материала при учете зависимости параметров модели от фазового состава материала. Основной результат: установлены качественные и количественные эффекты влияния фазового состава на неупругие характеристики; изменение растягивающих остаточных напряжений на сжимающие. Полученные результаты могут использоваться в инженерных расчетах на прочность элементов конструкций, а также параметров технологий поверхностного укрепления.

Чисельне моделювання термо-напружено-деформованого стану елементів у процесі 3D друку

The paper is devoted to the assessment of the thermo-stress-strain state of elements of structures obtained by 3D printing using FDM (Fusing Deposition Modeling) technology. Three stages of solving this problem are considered: (1) - mathematical formulation of the problem, that includes universal balance relations, constitutive equations of mechanical behavior of the material and is based on the model of growing bodies; (2) the technique of finite-element solution with increasing mesh due to the addition of new elements; (3) - study of a specific problem of growing plate-like element by polymer PLA (polylactide) with temperature-dependent physical properties. Options of deposed layers of different thickness are considered. The residual stress-strain state of the body has a two-scale structure, which reflects the change of state characteristics in the scale of body size, as well as the thickness of the layers. As the thickness of the layers increases, the average values of the residual stresses decrease, but the amplitude of the stress fluctuations along the thickness of the layer slightly increase.

Stress-strain state during recovery objects with corrosion defects by welding

The urgency of the problems connected with periodical diagnostics and repair of the linear section of main pipelines and the expediency of the use of repair technologies for restoration of the required wall thickness are proved. The experimental part of the work was the welding of a corrosion defect on the outer surface of the pipe by successive surfacing of the “filament” rollers in the environment of CO2 (MAG) – metal active gas and manual arc (MMA) – manual metal arc method in three layers, with a contour seam. The results of the residual stress-strain state obtained experimentally by brewing the defect of corrosive origin are presented, as well as the modeling of the stress-strain state (NDS) by the finite element method (ITU), which is formed at the same time on the section of a tube of 17G1C steel with a diameter of 1420 × 16 mm. As a defect model, a hollow oval shape with dimensions of 100 × 60 mm and a depth of 6 mm in the central part was made. The analysis and comparison of stress distribution, plastic deformations and displacements for these methods were made. The main factors influencing the VAT are established. It is shown that the VAT for different welding methods has the same distribution pattern, but differs in size. An assumption is made about the interconnection of displacements of the sample in the formation of residual stresses in the free state and, in the presence of rigid ligaments. The appropriateness of using the MAG method of repairing the thinned section of the main pipeline in terms of the formation of residual VAT rates has been confirmed. The effect is related to the formation of an area with increased values of residual tensile stress after the overlap of the last seam of each layer due to the effect of transverse and longitudinal shrinkage of the metal through the interaction of the last roller in the layer with the side wall of the defect.

INVESTIGATION OF PERMANENT STRAINS IN NANOMODIFIED COMPOSITES AFTER MOLDING AT ELEVATED TEMPERATURES

This work investigates the effect of carbon nanomodification on the residual stress-strain state (SSS) after molding. One of the ways to reduce residual stresses and deformities is nanomodification. The main objective was to determine the degree of influence of the nanomodification parameters on the residual SSS. Within the framework of this study, 4 slabs were made. Two slabs are made of a conventional binder with laying [010/9010] and [010/4510] and two slabs of a modified binding material with the same layer structure. For the fabricated plates, deflections were measured on each of the four sides, during which residual strains were obtained in the panels of nanomodified carbon fiber for the considered layings with and without modified binding material. To analyze the residual stress-strain state, a numerical and analytical calculation was performed. The numerical calculation was carried out by means of the finite element method for the case when the slab is fixed at the point of the geometric center, with no power load, and the temperature load is a difference of 100 °C. An analytical calculation was carried out for the case when the slab is free from fastening and external power load, and the temperature load is a difference of 100°C. During the study, variants of the physicomechanical properties of the monolayer were obtained using the Digimat software and the Mori-Tanaka averaging method. The results obtained by analytical and numerical methods have a good correlation between each other, and in the course of comparison with the experiment, a method for calculating the characteristics of the monolayer that was closest to the experimental result was determined. On the basis of the obtained results, conclusions were made on the possibility of reducing residual SSS and deformation in structures with asymmetric reinforcement schemes using a matrix containing carbon nanoparticles.

Analysis of non-uniform residual stress state in plates with in-plane and transverse heterogeneities

In the present paper, we describe a model of a prestressed inhomogeneous body and provide the variational and weak statements. On its basis, we study a problem of mixed vibrations of a non-uniform plate with inclusions and a sandwich-structured three-ply plate under the conditions of the residual stress-strain state. We build the numerical finite-element solution on the basis of the weak problem statement and analyze the effect of the prestress states on its frequency response functions. In addition, we identify the prestress level in the middle layer of the composite plate by the data on frequency characteristics measurements.

The role of local plasticity in the redistribution of stress fields caused by in-service fatigue overloads

The aim of this paper is to analyze the effect of diverse cyclic loading conditions on the residual stress field in cold-drawn pearlitic steel wires. To this end, on one hand, a numerical simulation by the finite element method of the manufacturing process (cold drawing) of a prestressing steel wire was carried out for revealing the residual stress state induced by the manufacture technique. On the other hand, a numerical simulation was performed of the in-service fatigue loading applied to a cold drawn wire (including the manufacturing-induced residual stress-strain state). According to the obtained results, the plastic strain generated during fatigue loading causes a reduction of the initial residual stress at the surface and a redistribution of the residual stress field.

Modeling of the Stressed and Phase States of Titanium-Nickelide Plates Under Thermomechanical Loading

On the basis of the proposed model, we study the phase and stressed states of titanium-nickelide plates under the conditions of bending and heating-and-cooling by using the methods of continuum mechanics and thermodynamics of nonequilibrium processes with an aim to get a quantitative description of the behavior of bodies made of shape memory materials. We perform calculations for various combinations of mechanical loads and temperatures. It is shown that, under the conditions of phase transformations in the plate, we observe the appearance of a distribution of martensite with variable thickness, which leads to the formation of an inhomogeneous residual stress-strain state.

DEVELOPMENT AND VERIFICATION OF COLD BENDING OFFSET NUMERICAL MODEL

For profiling of the pipeline route at the crossed land relief the elastic bend of pipes and curve inserts (branches) which in most cases are made cold flexible pipes are used. When calculating on durability and stability of the section of the pipeline with cold bending offsets (CBO), it is necessary to take into consideration the level of residual stresses and deformations in a branch wall. For the analysis of the residual stress-strain state (SSS) of CBO on stream, in this article the CBO generalized mathematical (numerical) model is developed and also verification on compliance to empirical formulas and the pilot studies is carried out. The developed CBO numerical model allows to define characteristics of the SSS depending on many parameters, such as diameter, pipe wall thick- ness, mechanical characteristics of steel, tap curvature value and a bending angle. Analytical dependences which allow to carry out primary assess- ment of the residual SSS of CBO without application of the program environment of model operation are also presented.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ ПРЕДВАРИТЕЛЬНОГО СОСТОЯНИЯ НА МЕХАНИЧЕСКИЕ СВОЙСТВА ВЯЗКОУПРУГИХ ТЕЛ

We present a new general statement of the problem on steady-state oscillations of a non-uniform viscoelastic body, taking into account the residual stress-strain state. To derive it, we use the theory of complex modules and the correspondence principle which makes it possible to write down the problem for viscoelastic bodies in the same form as the corresponding elasticity problems by replacing the elastic characteristics with the complex functions of vibration frequency. To describe the viscoelastic behavior, we employ a three-parameter model of the standard viscoelastic body consisting of instantaneous and long-term modules, as well as relaxation time. On the basis of the statement proposed, we consider the model problems on vibrations of inhomogeneous viscoelastic rods and pipes taking into account the presence of both pre-stress and residual strain. Within the problem for the viscoelastic rod, we analyze the longitudinal oscillations and consider the pre-stress and residual strain as functions of the longitudinal coordinate. Regarding the problem for the viscoelastic pipe, we consider the plane stress-strain state and assume that the oscillations are excited by the radial load; the mechanical properties of the cylinder and the residual stress-strain state depend on the radial coordinate. The numerical solutions in both cases are based on the shooting method. For both models problems under study, we present and observe the results of the comparative analysis of the effect of pre-stress and residual deformations on the amplitude-frequency characteristics (AFC), based on the computational experiments. We also provide the analysis of such an influence of each factor on the most sensitive frequencies values. The study conducted shows that for both rod and pipe the residual strain fields have a significantly greater effect on the AFC than the pre-stress fields do. This conclusion correlates with the results obtained previously for elastic bodies.

Vibrations of inhomogeneous piezoelectric bodies in conditions of residual stress–strain state

• The linearized statement for inhomogeneous piezoelectric prestressed body is derived. • The general variational principles and weak formulations of the original problem are described. • Particular boundary problems for rods and thin disk are investigated. • A formula for frequency change as a function of residual stress level is obtained. • An effect of residual stress on frequency response functions for a disk is estimated. In the article, we present the general linearized statement of the boundary problem on vibrations of inhomogeneous piezoelectric body under residual stress–strain state. We have derived the weak statement of the problem for the test functions satisfying the essential boundary conditions, formulated the general variational principle for a prestressed piezoelectric body and proposed several options for the potential energy representation. On the basis of the principles proposed, we have formulated and investigated a number of particular boundary problems on steady-state vibrations of inhomogeneous piezoelectric prestressed rods and thin prestressed disk polarized in the direction of thickness. The analysis of the residual stress levels on frequency response function for the bodies considered is provided.