Search

The effectiveness of the previously proposed improved approach for determining the non-proportional cyclic hardening coefficient in predicting the maximum level of strain hardening and durability of metallic materials was tested. The approach is based on the correlation between static and cyclic strain hardening of metallic materials, takes into account the amplitude of cyclic deformation, and does not require fatigue experiments under non-proportional loading. The calculated and experimental values of this coefficient were compared for structural materials with different cyclic and physical properties. For the 27 analyzed materials, the maximum level of strain hardening was predicted using the obtained calculated coefficient, and a good agreement with experimental data was demonstrated. Using the strain criterion for assessing durability, which includes the calculated non-proportional cyclic hardening coefficient, the durability for circular cyclic trajectories of non-proportional deformation was predicted on the basis of the basic uniaxial fatigue diagram. Satisfactory results of durability prediction (in comparison with the experiment) were obtained for materials with FCC metal lattice structure. For materials with BCC structure, the agreement between the calculated and experimental data was somewhat worse. It is shown that for this type of materials, the use of an alternative method for determining the non-proportional cyclic hardening coefficient can improve the results of durability prediction. [ABSTRACT FROM AUTHOR]

The deformation of a straight pipeline segment under stepwise cyclic loading by internal pressure is investigated. The tests were carried out on a pneumohydraulic bench, allowing one to determine the strength and durability characteristics of full-scale pressure vessels and pipeline elements under conditions close to operational ones. The pipe element was loaded with stress control. The asymmetric loading regime was a sequence of three types of cycles at a constant stress amplitude with a step change in the mean stress after stabilizing the strain rate. The load level corresponded to the lowcycle fatigue region with distinctive elastoplastic strain of the material. The experiment aimed to obtain diagrams of ratcheting (cyclic creep) of the pipe under the action of a biaxial proportional load from internal pressure (pulsating cycle) depending on the change in the mean stress of the cycle. It is shown that for a pipe element, similar to laboratory specimens, an increase in the load level (compared to that applied at the previous stage) enhances the effect of ratcheting in the circular direction. In contrast, a return to the initial lower level eliminates this effect. Unlike laboratory specimens, the accumulation of ratcheting strains in the pipe element during the first 20 cycles is uneven. A steady-state rate of strain accumulation is observed after reaching 40 loading cycles. An increase in the accumulated plastic strains of the ratcheting in the circular direction with an increase in loading cycles significantly affects the initial distribution of strains in the axial direction. [ABSTRACT FROM AUTHOR]

Deformation of rectilinear elements of Du 200 pipeline made of 20 steel under cyclic loading with internal pressure has been studied experimentally. Tests were carried out on a pneumohydraulic bench under conditions close to plane stress state, room temperature, and cycle asymmetry under soft loading mode (stress control). Circumferential and axial strains were recorded at three points of a diametrical section located in the middle of the element under study. Two tubular elements were tested at different values of maximum and amplitude stresses based on 100 cycles of loading. Deformation curves (cyclic creep) of tubes in a circular direction have been obtained, the character of which is significantly non-uniform in comparison with deformation of laboratory specimens. It is shown that even a slight increase in maximum (by 4%) and amplitude (by 8%) stresses significantly (up to 50%) increases the cyclic creep strains in the circular direction. Such growth of deformations can be explained by a peculiarity of 20 steel, the deformation diagram of which in the elastoplastic transition area contains a yield point, which determines intensive deformation of the material. Fixed plastic deformations in the axial direction are more than an order smaller than circular ones, and with increasing loading cycles, they change insignificantly and coincide for the two studied tubular elements. [ABSTRACT FROM AUTHOR]

The research on determination of sensitivity of metal materials to non-proportional cyclic loading and the accompanying effects is analyzed. The indirect methods for determining the coefficient of non-proportional cyclic hardening of materials based on simple experiments are considered. A comparative analysis of methods for predicting non-proportional cyclic hardening for out-of-phase with a 90°displacement of biaxial cyclic loading with strain control is performed. It is shown that methods that use only the parameters of the stress–strain curve of materials are less effective, since they do not take into account the effect of cyclic loading amplitude on non-proportional hardening, which is quite significant for metals and alloys with face-centered cubic microstructure. Analysis of experimental data obtained at cyclic non-proportional deformation of stainless and carbon steels, aluminum and titanium alloys showed that the coefficient of non-proportional cyclic hardening varies depending on the loading amplitude. For most of the studied materials with face-centered cubic structure, there is a tendency to increase this hardening coefficient with increasing amplitude of cyclic loading and to its stability or slight decrease for metals with volume-centered cubic structure. Based on the behavior of the vast majority of materials, a logarithmic functional dependence is chosen to take into account the effect of the strain amplitude on the non-proportional cyclic hardening. A modification of the key parameter for determining mentioned hardening is proposed, taking into account both the characteristics of the stress–strain curve and the dimensionless value of the cyclic strain amplitude. It is shown that its use allows one to obtain a better correlation of experimental data with the approximation dependence proposed earlier by the author. The results of determining the coefficients of non-proportional cyclic hardening due to the improved approach meet the requirements of engineering use. [ABSTRACT FROM AUTHOR]

In this paper, a method is proposed for specifying the parameters of the exponential hereditary function kernels of the endochronic theory of plasticity to describe the ratcheting (cyclic creep) effect of metallic materials under stress-controlled complex non-proportional loading. The method involves the dependence of the difference of plastic moduli on the ratcheting rate on the steady-state portion of the cyclic stress-strain curve. The plastic moduli are determined at the points where the maximum stresses act in different half-cycles of asymmetric loading. It is believed that the greater the difference of plastic moduli towards the mean stress in the cycle, the greater the strain increment in each cycle of loading. The statements of our previously proposed approach were used to determine the rate of plastic strain accumulation at the steady-state stage of deformation under biaxial loading. This approach, based on the data of uniaxial experiments under cyclic asymmetric loading in tension-compression and reversed torsion with the known value of the cycle nonproportionality parameter, is developed to analyze the cyclic loading paths with equal mean and amplitude von Mises stress values. With some simplifications, the expression is proposed to determine the parameters of the exponential kernel of the hereditary function depending on the cyclic path geometry and the known ratcheting rate for the basic loading path. Similar values for the exponential kernel parameters of the hereditary function are obtained in terms of the difference of plastic moduli by using a simpler bilinear model of elastoplastic deformation. The obtained values of the hereditary function parameters were used to simulate the ratcheting effect under uniaxial and biaxial cyclic loading. The loading programs and data of experiments were taken from the literature. The results of simulation have shown that the parameters of the constitutive equations for cyclic plasticity obtained with this method allow one to describe satisfactorily the kinetics of the stress-strain state of metallic materials subjected to biaxial non-proportional loading under cyclic creep. [ABSTRACT FROM AUTHOR]

An approach to the determination of steady ratcheting strain rate under biaxial loading depending on the loading cycle geometry is proposed. Based on the data analysis, the microstructural investigation results, as well as the similarity detected between the microstructural formations after deformation at the stress- and strain-controlled modes of loading, it is proposed to use the cycle nonproportionality parameter to predict the ratcheting rate. It is shown that the geometric cycle parameters and basic experiments allow one to adequately predict the ratcheting rate for complex cyclic paths. [ABSTRACT FROM AUTHOR]

A version of the endochronic theory of plasticity is developed to describe the stress-strain state under stress-controlled, asymmetric biaxial non-proportional cyclic loading. In order to solve the obtained non-linear system of differential equations, the numerical procedure is proposed and the relevant computational program is developed. The model validity is verified by satisfactory agreement between the calculated and experimental data. [ABSTRACT FROM AUTHOR]

The paper addresses the mechanical behavior of carbon steel 20 in asymmetric loading under the conditions of plane stress state that occurs in a thin-walled specimen in tension-compression and alternating torsion. The asymmetric cyclic loading of specimens was carried out in a stress-controlled mode along three paths with a frequency of 0.1 Hz with a base up to 1200 cycles. Cyclic stress-strain diagrams have been constructed for the specimens under tension-compression, alternating torsion, and proportional loading. The curves of von Mises equivalent ratcheting strains have been plotted. The highest ratcheting rate is shown to take place in the tension-compression loading case. [ABSTRACT FROM AUTHOR]

We present results of a numerical simulation of the effect of cyclic creep (ratcheting) with different programs of stress-controlled uniaxial asymmetrical loading of two metallic materials with different cyclic properties. An improvement of the kinematic hardening rule in the cyclic plasticity model on the basis of defining equations of the endochronic theory of plasticity is proposed. It has been shown that the proposed model allows one to describe with satisfactory accuracy the kinetics of the stress-strain state of specimens under low-cycle asymmetrical loading on the basis of the minimum number of basic experiments. [ABSTRACT FROM AUTHOR]

We review the currently available approaches, applicable codes and standards for assessment of defects in trunk pipelines and discuss advantages and disadvantages of the National pipeline regulations. The paper outlines the main principles of the newly introduced normative document that regulates the assessment of strength and remaining life of pipelines containing service defects. [ABSTRACT FROM AUTHOR]

We propose a procedure for the numerical analysis of the stress-strain state and criteria of its admissible state in the course of repairs of the air crossings of gas mains with regard for possible defects of the pipe metal. Using the “3D PipeMaster” software complex, we compute different versions and present recommendations concerning the optimal location of pipe-laying machines with an aim of realization of the possibility of lifting the pipe to a height required for repair works with guaranteed validity of the prescribed conditions of strength. The values of tractive forces in the pipe-laying machines are determined. The optimal sites of fixing the pipeline in the transverse direction are indicated and the corresponding forces acting in the cables used to prevent the transverse motions of the pipeline are calculated. [ABSTRACT FROM AUTHOR]

Experimental data on low-cycle fatigue of 304 steel and a VT9 titanium alloy in deformation by complex loading histories, which are the sequence of blocks of cycles of different shapes in the space of total strains, cited in the literature, are analyzed to develop adequate models for life calculations. The four damage accumulation rules and the low-cycle fatigue deformation criterion were used as basic approaches. Life prediction models were compared. It is shown that the application of a modified nonlinear damage accumulation rule can improve life prediction results, with better outcomes obtained for the programs involving nonproportional cycles. [ABSTRACT FROM AUTHOR]

Using the findings of analysis of deformation curves for metallic materials under static and cyclic loading, an approximate method is put forward for the determination of maximum strain hardening levels in the case of non-proportional low-cycle loading with strain monitoring. Based on the correlation between strain hardening data obtained from the static and proportional and nonproportional cyclic deformation curves, an approximate analytical relationship is built up which allows for predicting maximum strain hardening levels under nonproportional low-cycle loading. [ABSTRACT FROM AUTHOR]

The data on cyclic deformation of materials with different cyclic properties are analyzed. The relationship between the level of additional strain hardening and the mechanical characteristics of a material is established. The proposed models take into account the influence of the form of stressed state on strain hardening and durability under conditions of nonproportional cyclic deformation. The models are constructed on the basis of the analysis of the experimental data on cyclic deformation of 304 stainless steel. The prediction of strain hardening and durability is performed with separate analysis of the influence of shape of the cycle and the form of stressed state. To take into account the influence of shape of the cycle, we use the coefficient of disproportionality of the cycle and the parameter of sensitivity to disproportionality. To take into account the form of stressed state, we use the coefficient of the form of stressed state and the corresponding parameter of sensitivity to the form of stressed state. The results of prediction of both strain hardening and durability of materials on the basis of the proposed phenomenological models reveal their fairly high efficiency. [ABSTRACT FROM AUTHOR]

The results of full-scale tests of unitizing multiple-leg sling assemblies loaded with a trapezoidal waveform are considered. Mechanical behavior of the sling webbing material was evaluated by comparing its strength in the initial state and after overloading. A parameter to estimate the damage accumulated in the polymer material held under load is proposed, which makes it possible to determine the residual strength of the webbing at a known loading history. Under cyclic loading of slings the webbing strength markedly deteriorates, which leads to a decrease in the actual value of the sling safety factor. Design calculations of lifting capacity for unitizing sling assemblies should be performed with regard for the breaking force of their webbing tested after overloading. [ABSTRACT FROM AUTHOR]

The effectiveness of predicting the strain hardening of metallic materials at low-cycle fatigue under nonproportional biaxial loading conditions has been analyzed. The cycle nonproportionality coefficient, which characterizes the geometry of a deformation cycle path, is used as the determining parameter. A comparative analysis of the effectiveness of using the most known nonproportionality coefficients to determine the levels of additional hardening of the material in the state of stabilization of its cyclic properties has been carried out. [ABSTRACT FROM AUTHOR]

An engineering approach to fatigue-life prediction is proposed on the basis of an analysis of experimental data on low-cycle fatigue in the case of uniaxial and biaxial (nonproportional) deformation of a wide range of metallic materials. The approach is based on the linear correlation between the reduced strain range and the number of cycles to fracture initiation. The above strain parameter takes into account both the orientation of the cycle path in the strain space and the degree of its nonproportionality. The use of the strain parameter as a criterion made it possible to obtain acceptable results in fatigue-life prediction. [ABSTRACT FROM AUTHOR]

We have analyzed experimental data on low-cycle fatigue in biaxial nonproportional deformation of various materials. To establish a correlation with the fatigue limit, we have proposed a strain parameter-the reduced range of strain. For most of the materials under consideration this correlation dependence can be obtained from uniaxial low-cycle fatigue experiments and can be represented by a linear function. [ABSTRACT FROM AUTHOR]

We develop a version of the endochronic theory of plasticity for the description of the stress-strain state under stress-controlled asymmetric loading. We propose a new rule of kinematic hardening aimed at taking into account the unilateral accumulation of plastic strains. The reliability of the model is confirmed by good agreement between the numerical results and the well-known experimental dependence of the maximum level of strains on the number of cycles under conditions of uniaxial cyclic creep. [ABSTRACT FROM AUTHOR]

We propose a new method for the approximate description of the crack opening displacement caused by the action of two symmetric forces applied to the opposite faces of a crack originating from a concentrator. The solution is sought in the form of a sum of asymptotic and correction terms. The asymptotic term is obtained by applying the operation of inversion to the well-known solution for a half plane with edge crack. The correction term contains unknown coefficients and is negligibly small as compared with the first term if either the point of application of a force or a current point of the crack surface is located near the crack tip. It is shown that the realization of this method reduces to several arithmetic procudures. The high accuracy of the proposed method is demonstrated by analyzing various examples. [ABSTRACT FROM AUTHOR]

A new version of the engineering method of weight functions that combines the advantages of the existing versions is suggested. It is based on finding the dependence of the opening of the crack lips on the concentrated force applied at an arbitrary point of the crack lips and makes essential use of an approximate fundamental solution for a half plane with an edge crack in a combined method of weight functions. The high accuracy and simplicity of the method are illustrated by several examples. [ABSTRACT FROM AUTHOR]

On the basis of theoretical analysis and numerical experiments, we study the efficiency of the endochronic theory of plasticity with regular kernel and isotropically kinematic law of hardening for the description of block loading. It is shown that the theory adequately predicts the kinetics of the stress-strain state in the case of strain-controlled loading. In the case of asymmetric stress-controlled loading, the model qualitatively predicts only the nonstationary stage of the process of cyclic creep of the material. To describe this effect more accurately, we propose a simple model of plasticity taking into account the anisotropy of cyclic properties of the material induced by the asymmetry of loading cycles. [ABSTRACT FROM AUTHOR]

A model is constructed to describe the ductile fracture of a pipe with a rectangular three-dimensional defect. Upper and lower bounds are obtained for the burst pressure. The first is calculated for a rectangular crack, while the second is calculated for an axisymmetric three-dimensional defect. It is shown that if the half-width of the defect b is equal to 2(Rt) - where R is the radius of the pipe and t is the net thickness of the pipe wall-then the defect can be regarded as axisymmetric. Calculated values of burst pressure are compared with literature data from full-scale tests of pipes. The results can be used to analyze actual defects in pipelines (pitting, etc.), as well as in conducting full- scale tests of pipes and pressure vessels. [ABSTRACT FROM AUTHOR]

We propose a cycle nonproportionality coefficient for a broad class of complex cyclic paths, allowing us to establish a clear relationship between the geometry of the strain path and the maximum hardening level attainable in the material in the steady state for a fixed value of the maximum plastic or total strain range. We discuss the effectiveness of using the proposed cycle nonproportionality coefficient compared with analogous parameters familiar from the literature. Based on a previously developed version of the endochronic theory of plasticity, we have numerically modeled the behavior of a material for cyclic loading along different planar nonproportional paths. [ABSTRACT FROM AUTHOR]

To evaluate the effectiveness of the previously proposed constitutive equations of the endochronic theory of plasticity, we examine the deformation of thin-walled tubular specimens along square paths in deviatoric strain space with the combined action of axial force and torque. We obtain the resolving system of equations in application to strain-controlled nonproportional cyclic loading. Good agreement of the modeling results with the experimental data is noted. [ABSTRACT FROM AUTHOR]

A procedure for approximate determination of a point weight function for partly elliptical type-I cracks is refined. It is proposed that the effect of the free surface of the body be considered by introducing a hypothetical load which is symmetric relative to the boundary of the body. Results of calculations of stress-intensity factors obtained by the above procedure and by other authors using the finite-element method are compared for semi- and quarter-elliptical cracks. [ABSTRACT FROM AUTHOR]

Weighing-function expressions are given for an elliptic normal-detachment crack in an infinite body, which have been used to calculate K for uniform and linear load distributions in the range in α (ratio of the minor and major axes of the ellipse) from 0.2 to 1. These K are compared with theoretical values to judge the accuracy of the weighing functions. Good accuracy (about 6%) is provided by the weighing functions constructed with elliptic coordinates and on the bases of the Oore- Burns general treatment in combination with Rice's formula, apart from points on the major axis. An integral condition is derived for the weighing function, and this is used in refinement of the unknown weighing function. [ABSTRACT FROM AUTHOR]

Equations of state for the endochronic theory of plasticity are suggested for describing processes of nonproportional cyclic deformation of materials. New rules are introduced for characteristics of isotropic and kinematic strengthening with regard to the nonproportionality parameter of the plastic deformation cycle path. Basic experiments are considered with the aim of specifying characteristic relationships. As applied to different complex plastic deformation paths, stress distribution is determined in single-crystal tubular specimens with the combined action of an axial force and torsional moment in relation to the number of loading cycles. Calculated and experimental data are compared. [ABSTRACT FROM AUTHOR]

A model is presented for strengthening of a body in order to describe processes of material nonproportional cyclic deformation on the basis of the approach of endochronous plasticity theory. New rules are introduced for isotropic and kinematic strengthening processes taking account of a nonproportionality factor for the plastic deformation cycle trajectory. Basic experiments are described for defining the equations of state. The distribution of stress fields in thin-walled tube specimens in relation to the number of the loading cycle is determined as applied to different plastic deformation trajectories. The adequacy of calculated and test data is confirmed. The values sought are compared with similar results obtained by the two-surface theory of flow. [ABSTRACT FROM AUTHOR]

On the basis of an analysis of the known solutions for a circular and for a semiinfinite normal-separation crack with straight front in an infinite body we constructed, in analogy to the method of Burns and Oore, a weight function for elliptical cracks. Its use for finding the stress intensity factors K at the point of minimal curvature under conditions of uniform loading leads to a maximal error of 10%; at the point of minimal curvature of the crack front the error increases with decreasing ratio of the semiaxes of the ellipse. With the aid of this solution the weight function is found in bounded bodies with a quarter-elliptical, semielliptical, and elliptical crack. A comparison of the data obtained by this method with the values of K calculated by the finite-element method under nonuniform loading showed that the suggested method is very accurate. [ABSTRACT FROM AUTHOR]

The effectiveness of the application of equations of state with a weak peculiarity of the endochronic theory of plasticity in describing complex processes is investigated. The influence exerted by the magnitude of the bend angle of the stress path and various plastic-strain paths on prediction of the material's mechanical behavior is analyzed. The results obtained are compared with the solution for a model employing a regular kernel. [ABSTRACT FROM AUTHOR]

Effectiveness of using the endochronic theory of plasticity for describing unidimensional deformation processes under cyclic loading is investigated. A method is developed for determining parameters of the equations of state which depend on the amplitude of plastic deformation and length of strain path. A sufficiently good agreement is shown between calculated and experimental data in describing the kinetics of the stress-strain state of specimens of steels 316 and 45 subjected to variable loads. [ABSTRACT FROM AUTHOR]