Search

In this paper, optimum test plans of step-stress partially accelerated life tests are developed using failure-censored data from Burr type-XII distribution. The maximum likelihood approach is applied to obtain the estimates of the acceleration factor and the parameters of the distribution. The minimization of the generalized asymptotic variance of the maximum likelihood estimators of the model parameters is used as an optimality criterion to develop optimum plans of the step-stress partially accelerated life tests. For illustrative purposes, simulation studies are presented. [ABSTRACT FROM AUTHOR]

For planning of thermal fatigue tests of materials and comprehensive analysis of their results, it is proposed to use the thermal cyclic test maps (TC-maps) that allow tracing the strain development in a thermal cycle. TC-maps illustrate the diagrams of the temperature-dependent elastic strain, which corresponds to the yield strength of the material, and the free strain variation under constraint conditions. This paper presents the procedure of thermal cycle test map compiling for some materials and their physical as well as mechanical properties. The useful information obtained from the TC-maps is exemplified by some heat-resistant alloys employed for the production of gas-turbine engine blades. [ABSTRACT FROM AUTHOR]

We describe the application of the dual boundary element method for the determination of stress intensity factors in plate bending problems. The loadings considered include internal pressure, and also combined bending and tension. Mixed mode stress intensity factors are evaluated by a crack surface displacement extrapolation technique and the J-integral technique. The boundary element results for the case studies considered in the paper have been compared with either analytical or finite element results and in all cases good agreement has been achieved. [ABSTRACT FROM AUTHOR]

A computational method based on radial basis functions has been applied to the linear solution of thin plates. This meshless numerical method gives high flexibility in the analysis of irregular geometries, due to its insensivity to spatial dimension. The multiquadrics approach is used in this paper. The numerical solution is compared with Kirchhoff theory for plates. [ABSTRACT FROM AUTHOR]

This paper considers the basic principles of the most widespread methods for determining typical states of a mode I through crack in specimens and structures of sheet metals that are fractured under conditions close to the plane stress state. The results of experimental investigations are presented which make it possible to judge of the relative advantages and disadvantages of these methods from the standpoint of their possible use in engineering assessments of the integrity and remaining strength of thin-wall structures. [ABSTRACT FROM AUTHOR]

The paper addresses the well-known (from available publications) phenomenological strength criteria for anisotropic materials, which are most widely used in application software. [ABSTRACT FROM AUTHOR]

The paper provides a review of the known (from available publications) methods and findings of experimental studies of stress-strain state and strength of multilayer thick-walled anisotropic cylinders under dynamic loading. [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]

A three-dimensional stress-strain state calculation is performed by the finite-element method for precracked Charpy specimens side grooves of different depths. Based on the calculated results, the paper addresses the suitability of various brittle fracture criteria to describe experimental data on fracture toughness of these specimens. [ABSTRACT FROM AUTHOR]

The paper addresses the influence of plastic prestraining in tension and combined tension at 150 and 350°C on the rupture stress in 15Kh2MFA steel upon heat treatment that simulates irradiation embrittlement of materials in a WWER-440 type reactor towards the end of its lifetime. The dependence of rupture stress on the dislocation density in the material upon plastic prestraining is studied. It is found out that as the plastic prestraining in tension and combined tension grows, the dislocation density within small-angle boundaries increases and thus results in a larger rupture stress. An increase in rupture stress in the case of prestraining at 350°C is more intensive than that at 150°C. We analyze the microcrack nucleation and growth micromechanisms in 15Kh2MFA steel during tensile plastic deformation and discuss the effect of the material substructure on the microcrack arrest. [ABSTRACT FROM AUTHOR]

The paper presents a procedure whereby the damage accumulation kinetics in structural materials, such as steel 45, stainless steel 12Kh18N10T, aluminum alloy D16T, and titanium alloy VT22, under elastoplastic deformation is studied based on variation parameters of elastic modulus and resistivity. For complex stress conditions, a continuum model for damage accumulation is proposed which relates the damage parameter to the intensity of accumulated plastic strains. The data calculated by the proposed continuum model are compared to the experimental findings of the investigation of the damage accumulation kinetics for some structural metals. [ABSTRACT FROM AUTHOR]

The paper gives the research findings which make it possible to construct an algorithm for the residual life assessment within the framework of a tensor approach to describing damage in a material under inelastic deformation. The authors propose explicit finite criterial relationships for the residual life assessment, which extend the known relationships to cover a wider range of loading processes. [ABSTRACT FROM AUTHOR]

The paper considers a plane stability problem of thin-walled bar of angular profile made of an isotropic linearly elastic material. The critical parameters were determined with the aid of the second version of three-dimensional linearized stability theory and the ground state from equations of the linear theory of elasticity. The approximate solution of the stability problem was sought by the net method. The dependence of the critical parameters on the thin-walledness parameter of structure has been examined, and the errors of the applied stability theory, which is used for the design of frameworks, have been determined. [ABSTRACT FROM AUTHOR]

The paper presents findings of the study of viscoplastic properties of several titanium alloys subjected to cyclic loading under complex stress state conditions. The applicability of elastic viscoplastic models to structural materials has been experimentally verified. [ABSTRACT FROM AUTHOR]

The paper discusses the results of analytical determination of vibrodiagnostic parameters of fatigue damage in the form of Mode I straight cracks of constant depth in a rectangular homogeneous uniform-thickness plate with various plate fastening conditions and vibration modes. It is demonstrated that, in the range of the relative crack depth up to 0.3, the parameter of distortion of vibration monoharmonicity in the 2nd-order superharmonic resonant mode is one-two orders of magnitude higher than the value of a relative change of the plate’s natural frequency. [ABSTRACT FROM AUTHOR]

The effect of the creep of materials of a structure on the nature of the contact interaction between its elements during long-term operation is investigated. The problem of the stress-strain state of the rotor blade interlock for the medium-pressure cylinder of a steam turbine for ultrasupercritical steam parameters (over 240 at and 565°C) is solved using a finite element software package. As a model of creep, a modified model of creep with hardening is considered. The methodology for solving the thermal contact problem of interaction between the interlock elements under creep conditions is based on the application of a contact layer model. The calculation is made for the case of the total loads of the centrifugal forces of blades and the temperature field in which the structural element is operated. The results on the distribution of total strains, equivalent stresse,s and contact stresses during long-term operation are analyzed. The points of time, upon reaching which the permissible strength characteristics of the materials used were exceeded, were determined. A change in the nature and level of contact stresses on the bearing surfaces of interlock elements due to the redistribution of the stress-strain state of the structure as a whole is shown. The moments of the appearance of detachment and absence of contact stresses on individual contact surfaces were determined. The results obtained are compared with the data from the operation, during which the structure was damaged. [ABSTRACT FROM AUTHOR]

The paper considers interrelation between the regularities in the fatigue fracture and inelastic deformation taking into account the material properties, stress gradient and concentration, specimen size, load cycle asymmetry, complex stress state, temperature, loading frequency and conditions, scatter of the test results. Analysis of the data from the literature and results of original investigations has shown that the characteristics of inelasticity, which define nonlocalized fatigue damage, can be used in studies of general regularities of the high-cycle fatigue fracture. Specific results of such analysis are presented. [ABSTRACT FROM AUTHOR]

A finite-element solution for rubber composite shells is presented. Sandwich laminates with a rubber core have also been studied. Incompressibility of a rubber matrix and complexity of composite shells bring forth the need for a sound numerical model to describe the behavior of such engineering materials. The developed model was applied to a degenerate shell element within the limits of the first- and third-order shear deformation theories. The model allows to predict with a sufficient accuracy the nonlinear behavior of sandwich shells with composite skins and rubber cores and composite shells with a rubber matrix. [ABSTRACT FROM AUTHOR]

Depending on boundary conditions, welding parameters and plate thickness, high residual tresses may arise near the weld toe of a welded joint. Compressive or tensile residual stresses significantly influence the fatigue strength of the joints in the high-cycle regime. If the weld toe is modelled as a sharp, zero-radius V-shaped notch, the residual stress fields can be expressed in terms of residual notch stress intensity factors (R-NSIFs) calculated in the elastic or elastic-plastic fields. The possibility to quantify the intensity of the residual singular stress field by means of the R-NSIFs allows the designer to estimate the influence of residual stresses on the fatigue life of welded joints. In this work, the influence of residual stresses on the fatigue resistance of Al-alloy butt-welded joints is estimated by using the local stain energy density approach. Values predicted by the proposed method show a good agreement with experimental data taken from literature. [ABSTRACT FROM AUTHOR]

In this study, a mathematical approach was presented to theoretically determine the change of the yield strength of bcc metals and alloys with temperature and strain rate, which can be determined for any desired strain rate by conducting experiments at two different strain rates. [ABSTRACT FROM AUTHOR]

With consideration of the intermediate principal stress and different strength values in tension and compression, the plastic analysis of metallic elbows under internal pressure is conducted based on the unified strength theory. The unified solutions for elbows with constant and variable wall thickness values, and those with a local thinning are derived. Parametric studies are carried out to investigate the dependence between the unified strength theory parameter, tension-compression ratio, bending coefficient, and wall thickness. Comparing the theoretical results and the experimental data, a good agreement was observed. The results obtained show that the unified solutions are versatile in the theoretical analysis of metallic elbows and can be reduced to many existing results. The latent potentialities of metallic elbows are fully achieved due to considering the intermediate principal stress. The unified solutions have an important practical value for the optimum design and engineering application of metallic elbows. [ABSTRACT FROM AUTHOR]

The authors present a method for predicting the long-term deformation of materials based on the base diagram concept. The long-term strength calculation of steels 18Cr–12Ni–Mo and 21Cr–32Ni–Ti–Al for different specified stress and temperature values showed the efficiency of its defining relations. The advantage this approach has over the most common parametric methods such as the Larson–Miller, Orr–Sherby–Dorn, Manson–Sakkop, and Trunin parameters is shown. [ABSTRACT FROM AUTHOR]

The energy method of calculating the strength of steels and alloys is proposed to be used on the basis of the complex of their mechanical properties. The analytical mechanism provides the differentiated approach to determining the safety factors of materials, which in its turn offers the reduction in the metal intensity of structures. [ABSTRACT FROM AUTHOR]

The ultimate hardening model of materials, described by the authors earlier, which is based on allowance for hardening in the zone of localization of inelastic strains, is applied to the estimation of the high-cycle life of materials. A method for the determination of local inelastic strain for the simple case of loading has been developed. The prospects for the use of the model for various complex cases of loading are shown. [ABSTRACT FROM AUTHOR]

The methods developed at the Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine for determining the allowable stresses for structural elements operating at cryogenic temperatures are considered. The methods make it possible to take into account the effect of the low-temperature hardening and the softening electromagnetic action on the value of allowable stresses. [ABSTRACT FROM AUTHOR]

Based on the review of the available publications and the findings of the original studies, the author demonstrates that with cyclic stresses in the maximum-stressed surface layer being equal the inelastic strains in the presence of stress gradients are smaller than those in the material under uniform stressed state. As a result, there is a difference in the cyclic stress-strain diagrams between these cases. An equation for the cyclic stress-strain diagram is put forward, which allows for the stress gradient effect. A model is substantiated, which provides an explanation for the difference between fatigue strength characteristics under uniform and nonuniform stressed states. [ABSTRACT FROM AUTHOR]

We review the currently available methods that are used to explain the difference in fatigue strength of metals and alloys under conditions of uniform and nonuniform stressed states. We discuss (i) the empirical methods, (ii) the methods allowing for the difference between nominal and effective cyclic stresses, (iii) the methods based on the notions of statistical theories of strength, and (iv) the methods assuming that responsible for fatigue failure are the averaged stresses in the surface layer. The calculated results are verified against experimental findings. [ABSTRACT FROM AUTHOR]

The investigation results are presented for the kinetics of damage accumulation and the process of deformation in a heat-resistant steel 10GN2MFA for different loading conditions under complex stress state. [ABSTRACT FROM AUTHOR]

We consider different methods of modeling impact in vibroimpact systems by contact interaction force which can be regarded as elastic force as well as force corresponding to the Hertz law and with the help of the boundary condition method by using the coefficient of restitution. Comparison of the results of the modeling by means of these methods is performed, and recommendations for their application are proposed. [ABSTRACT FROM AUTHOR]

Holding periods of 300 and 3600 s in a trapezoidal load cycle are shown to increase the crack growth rate dozens of times for alloy ÉP962 and several-fold for alloy ÉP742 at a temperature of 973 K. It is demonstrated that in the presence of the first portion on the creep crack growth diagram, whereon the crack growth rate decreases, the crack growth kinetics for a trapezoidal load cycle can be predicted using the hypothesis of the linear summation of fatigue and creep crack growth rates provided that the peculiarities of the first portion of the creep crack growth diagram are taken into account. Empirical approaches are proposed for determining the mean crack velocity in the first portion of the creep crack growth diagram. [ABSTRACT FROM AUTHOR]

An approach has been developed that allows assessing inelastic phenomena in a material based on the parameter of the phase shift angle distribution between the stress and strain, which is measured in local zones on the surface of the investigated material. The distribution of the phase shift angle variance in the service life range investigated allows tracing the kinetics of the discrete phenomena of inelasticity in the material studied. [ABSTRACT FROM AUTHOR]

The results of special experiments on the size effect in contact fatigue are presented. It is established that under constant contact loading conditions the durability is higher, the larger is the diameter of a tested cylindrical element. The methods for estimation of contact fatigue resistance of gear wheels, which is based on the statistical model for a deformable solid body having a critical volume, are proposed. The limiting stresses of a gear wheel are estimated using a regulated base for this machine parts. [ABSTRACT FROM AUTHOR]

We address the effects of the actual flight cycle on durability of gas turbine engine disks under low-cycle fatigue. An approach is proposed which improves reliability of life cycle prediction owing to schematization of flight cycle with a criterion for reaching the maximum intensity of total strain range. Contribution of subcycles to the cumulative damage is demonstrated. [ABSTRACT FROM AUTHOR]

We developed analytical models of the buckling failure of initially rectilinear pipeline in the environment. We specify the drawbacks of available analytical methods, which take no account of the possible longitudinal displacement of the pipeline points or the limited length of the pipeline portion, as well as, in the limiting case, fail to describe the experimental results of buckling failure in the air. The proposed models make it possible to provide analytical evaluation of the critical load, length and shape of the deformed pipeline in cases of its horizontal and vertical transversal deformations with the account of the above factors. The results obtained are corroborated by the numerical simulation of the deformation process, based on the procedure described by the authors in Part 1, as well as by the full-scale test results. [ABSTRACT FROM AUTHOR]

A mathematical theory is proposed rigorous construction and specialization of constitutive relations for simple (in the Noll’s sense) strain-hardening elastoplastic materials with an initial loading surface and a fading path shape memory at the active deformation segment. Strains and symmetry type of the material are taken to be arbitrary. Physical equations are derived for materials with no path shape memory, with a weak fading memory, and with a fading memory of the nth order. Based on the proposed constitutive relations, physical equations are constructed for isotropic materials. In the context of the fading path shape memory, a definition of an elastic-perfectly plastic material is given. Assuming the condition of smallness of measures of strains throughout the entire “past” history, a theory has been developed for rigorous construction and specialization of constitutive relations for materials with a first-order fading path shape memory for infinitesimal strains. Special emphasis is placed on isotropic materials. [ABSTRACT FROM AUTHOR]

The regularities of the scatter in the characteristics of fatigue and inelasticity of steels 45 and 1Kh13 have been studied under steady-state, step, and block loading. It is shown that the use of the inelastic strain per cycle as a measure of the fatigue damage accumulation rate makes it possible to take into account the scatter in the number of cycles to fracture and in the fatigue limits and to substantiate the cumulative fatigue damage rule. [ABSTRACT FROM AUTHOR]

A mixed projection-mesh scheme for solving a boundary-value problem of thermal plasticity is formulated in a quasi-static statement when the process of non-isothermal elastoplastic deformation of a body is a sequence of equilibrium states. In this case, the stress-strain state depends on the loading history, and the process of inelastic deformation is to be observed over the whole time interval under study. The correctness and convergence of the mixed approximations for stresses, strains and displacements are investigated as applied to the solution of nonlinear boundary-value problems that describe the non-isothermal processes of active loading taking into account the initial strains dependent on the history of deformation and heating. The properties of the projecting operators are studied in detail, and on this basis, the condition that ensures the existence, uniqueness and stability of solution is formulated. The results of the analysis of special formulas of the interpolation-type numerical integration are presented, the use of which considerably simplifies the computation procedure for solving equations of the mixed method. The convergence and accuracy estimations are based on the results of the theory of the generalized boundary-value problems and methods of the functional analysis. According to the estimations obtained, the accuracy of solution of a finite-dimensional problem at the initial stages of loading should be sufficient to avoid the effect of increase of the first coefficients in the expansion of the total error on the accuracy of solution of the elastoplastic problem at the subsequent stages of loading. [ABSTRACT FROM AUTHOR]

Based on equations of linear three-dimensional elasticity theory, a problem is stated for the design calculation for rotating disks with hardening coatings, allowing for boundary layers formed. A numerical integration method is proposed for the stated problem, and some particular calculations are performed. Discussion of the calculated stress fields in the disk base material and coatings provides an explanation of some effects revealed, such as a significant improvement of load-carrying capacity of disks when face-coated with high-modulus layers of about 0.1–0.6 mm in thickness and a deterioration of this parameter (in some cases) when thicker coatings are applied. The authors study the applicability range of the conventional theory of disk strength calculation which assumes a two-dimensional stressed state and no boundary layers. [ABSTRACT FROM AUTHOR]

We present results of finite-element analysis of stressed state in a percussive-rotary drilling bit fitted with hardmetal inserts during its contact interaction with rock, allowing for brazing-induced stresses. The most likely fracture zones in inserts have been identified. Based on the proposed criterion for fatigue strength of structurally inhomogeneous materials, the binder composition of the inserts has been optimized for the longest service life. It is demonstrated that the formation of a gradient structure in a hardmetal, whereby the material preserves its initial mechanical properties in the main volume but has an increased ultimate tensile strength in zones where tensile stresses are involved, results in a longer service life of the hardmetal products and a noticeably diminished dependence of their cyclic strength on the scale factor. [ABSTRACT FROM AUTHOR]

New methods to formulate fracture mechanics parameters are presented. These include the common format that relates the deformation behavior of a cracked structure or specimen to the deformation behavior of a tensile test and the concise format that suggests a shortened form of common fracture mechanics parameters to replace the usual polynomial formulations. These forms may give an easier way to use the fracture mechanics parameters in analytical calculations. [ABSTRACT FROM AUTHOR]

Analytical algorithms for the construction of tensile and compression stress-strain diagrams of two-phase cermets are proposed, which are based on the concept of mean stresses in the phase volume and physical equations of the theory of small elastoplastic strains. The deformation properties and strength of cermet are supposed to depend on such parameters of its microstructure as mean size and coefficient of variation of solid phase grain-size distribution, contiguity coefficient of solid phase grains, and mean thickness of the metal phase inter-layers. A numerical analysis of the characteristic parameters of deformation curves for WC-Co hard metals has been carried out over a wide range of cobalt concentration and carbide grain size. A good agreement between the theoretical values of ultimate tensile and compression strength and known experimental values has been established. The constructed model deformation curves for hard metals may be regarded as alternatives to the corresponding experimental curves of stress against relative change in specimen length. [ABSTRACT FROM AUTHOR]

We perform the optimization of the barrel nozzle of a powder water cannon. The nozzle profile is specified, which makes it possible to obtain impulsive water jets with the required velocities for the minimum pressure inside the installation. The stress-strain state is analyzed both for the actual experimental device and in the case of its optimal geometry. The numerical calculations are carried out according to the maximum distribution of the pressure of water upon the barrel obtained by using the Godunov method, which enables us to construct a lighter mobile and more powerful device. [ABSTRACT FROM AUTHOR]

For an arbitrary body (a structural member) subjected to external steady temperature and force actions, a limiting state condition which ensures the uniform strength of the body until its load-carrying capacity is exhausted has been derived using Rabotnov's kinetic theory of creep. A procedure of strength analysis of structural members by the limiting state is put forward. For the case where the limiting state condition is fulfilled only on some individual portions of the body's surface, a procedure of calculating its stress-strain state with the nearest approach to the uniform-strength state is outlined. [ABSTRACT FROM AUTHOR]

Hold times of 300 and 3600 s under trapezoidal cyclic loading are shown to result in an increase in crack growth rates by many factors of ten for EP962 alloy and several times for EP742 alloy at a temperature 973 K. It has been found that in case the creep crack growth diagram has a first segment where the crack growth rate decreases, the crack growth kinetics under trapezoidal cyclic loading can be predicted by means of the hypothesis of linear summation of fatigue and creep crack growth rates considering special features of the first segment of the creep crack growth diagram. The authors put forward empirical approaches to determining the mean rate on the first segment of the creep crack growth diagram. [ABSTRACT FROM AUTHOR]

Based on the theory of simple hardening materials with elastoplastic behavior, the general constitutive relationships of the deformation theory of plasticity are mathematically strictly constructed for arbitrary continuous, piece-wise continuously differentiable deformation trajectories, any strains and symmetry types of the material properties. Two conditions under which this is possible are considered. The approaches to a strict specialization of general constitutive relationships of the deformation theory of plasticity have been developed by imposing restrictions on the material strains, deformation processes and properties. In this case, the restrictions on the properties of materials formalize the data obtained in the experimental investigations. A series of both new and known constitutive relationships have been constructed that are arranged into a hierarchy according to the level of complexity of the response to deformation. The area of applicability of the derived physical equations has been defined. Special attention has been given to the modeling of finite and infinitesimal strains of isotropic materials. [ABSTRACT FROM AUTHOR]

Bending of elastic circular sandwich plate with light filler, which is on an elastic foundation, is considered. To describe the kinematics of plate bending that is nonsymmetric in sandwich thickness, broken normal hypotheses are accepted. The foundation reaction is described by the Winkler model. The load is local symmetrical. A set of equilibrium equations and their exact solution in displacements have been obtained. Numerical results for a metal-polymeric sandwich plate are given. [ABSTRACT FROM AUTHOR]

It has been shown that the stress concentration at the joint edges increases with rigidity of adhesive joints of pyroceramics. In view of the fact that the traditional methods are unsuitable for the reduction of stresses at the edges of joints of pyroceramics, we propose a new approach, which ensures reduction of the stress concentration in the edge region. The approach is based on the choice of a law of adhesion interlayer elastic modulus variation which would be a “mirror reflection” of the stress distribution function. The variation of elastic modulus makes it possible to maintain the joint rigidity at the level that is ensured by the constant elastic modulus of the interlayer. A model experiment performed confirms the proposed approach to be effective. [ABSTRACT FROM AUTHOR]

Using the Unified Curve method as proposed by the authors earlier and a probabilistic model based on the local approach and physical background of the influence of impurities and carbide size on the local strength of material, a method has been elaborated for predicting a variation of the temperature dependence of fracture toughness with neutron fluence. The results calculated by this method are compared to the available experimental data. [ABSTRACT FROM AUTHOR]

A framework for damage mechanics of concrete is applied to simulate the nonlinear elastic deformation behavior of concrete using finite element method (FEM). A rather simple isotropic damage model containing essentially no adjustable parameters is shown to produce results in remarkably good agreement with sample experimental data: the damage law requires only the fracture energy to be defined completely. The model is achieved by introducing a damage surface that is similar to the yield function in the conventional theory of plasticity. A special form of damage surfaces is constructed to illustrate the application of the model. A new damage criterion, defined as an equivalent strain norm, is proposed, in order to take into consideration the asymmetric behavior of concrete. For verifying the FEM program including the model, deformations predicted by this model are compared with both the experimental ones for the concrete structural model and the ones calculated without application of the continuum damage mechanics. [ABSTRACT FROM AUTHOR]

A procedure has been developed for cyclic crack growth resistance tests of material samples under nonisothermal conditions with a controlled phase shift between cyclic heating and mechanical low-cycle loading. The efficiency of the procedure has been verified by the results obtained. [ABSTRACT FROM AUTHOR]