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The paper presents the results of experimental studies of the possibility to assess the damage process of steels of different classes by using the LM-hardness method under cyclic loading at different cycle characteristics (stress amplitude, mean stress, loading frequency). Fatigue damage of metal under cyclic loading has been assessed by the method outlined in the PNAE standard. It is shown that the discrepancy between the recommended tabulated values and experimentally obtained values of mechanical characteristics of the material has a significant influence on the character of the calculated fatigue curve of the material, which increases with the number of cycles. Analysis of the dependence of stress ratio on the number of operation cycles shows that reduction of stress ratio decreases the durability of the studied material due to increase in the range of stresses in the cycle. It is obtained that increase of constant strain amplitude in the cycle relative to the established level by 10–15% corresponds to initial section of plastic deformation durability loss under cyclic loading. The homogeneity parameter determined in this case will characterize the limiting state of the material, which precedes its failure. Based on the analysis of experimental results, it is found that under conditions of realized loading, the damage kinetics curves for all investigated materials has a linear character. In different materials, the linear sections have different slopes. It can be assumed that the indicated slope of this curve characterizes the conditions of equilibrium between the processes of strain-hardening and softening of metal, achieved at the stage of steady creep. The presented results give grounds for using the homogeneity coefficient according to the LM-hardness method to assess the material condition of structural elements during the current monitoring of their condition. [ABSTRACT FROM AUTHOR]

The statistical analysis performed on the defectiveness of rails in the Kiev subway railway shows that the main cause of the occurrence of most defects is the contact fatigue of the rail material. The paper presents modern concepts of crack growth in the railhead. It has been found that one of the most promising directions of studying the stress-strain state of rails and the crack growth process is a numerical simulation, which results on the contact interaction between rails and subway rolling stock wheels are presented. During the simulation, the problem was solved in an elastic formulation. We considered R50, R65, and UIC60 rail profiles, standard railway wheels, and wheels whose tire profile was developed by the Minetek closed joint-stock company. The conditions for the rail–wheel interaction corresponded to train movement on the straight section of the track without slipping. The effect of the wheel tire profile on the contact stress level in rails of different types has been assessed. Comparison of the maximum equivalent stress levels for a different combination of rail and wheel types shows that the best combination (in terms of reducing the contact stress level) for the straight sections of the subway track is an R50 rail with a gradient of 1 in 20 and 1 in 10 taper wheel, whose profile corresponds to GOST 9036-88. When wheels with the profile developed by the Minetek company are used, the maximum equivalent stress level in the contact zone is over 1000 MPa for all rail types. It is higher than the yield strength of rail steel, which will accelerate wear and give rise to low-cycle fatigue cracks with subsequent formation of contact fatigue defects. [ABSTRACT FROM AUTHOR]

The paper presents a mathematical model of the non-localized fatigue damage accumulation process in rubberlike materials. A relation for the determination of the constants of the corresponding equation with allowance for known fatigue curve characteristics, in particular for their random spread, is proposed. Computational relationships for the determination of the probabilistic characteristics of damage have been derived. Experimental data for the determination of realization of the fatigue accumulation process in rubber specimens, which are in good agreement with theoretical results, are considered. [ABSTRACT FROM AUTHOR]

The paper presents some findings of numerical investigations of torsional vibrations in steam turbine shafting in the turbogenerator abnormal operation modes (short circuit, random paralleling start, etc.). The shafting vibrations are shown to be essentially dependant on the reaction torque surge parameters (magnitude, duration, and form). For three types of the reaction torque surge a comparative hazard analysis is performed from the standpoint of initiation of cyclic damage in the rotor material and the risk of shafting failure. [ABSTRACT FROM AUTHOR]

The paper considers a procedure for the determination of the parameters of a limit state model for structural materials using a transcendental power function. The model is used to construct limiting stress amplitude diagrams for asymmetric high-cycle loading. The procedure has been tested by comparing the values of limit state model parameters obtained as a result of the basic experiment in pulsating loading cycle with the values calculated by the method of minimization of the sum-ofsquares functional of deviations of experimental diagrams from calculated ones. [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 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 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]

The coaxing effect in fatigue refers to an increase in the fatigue strength of material imduced by prior cycling at a stress level below the fatigue limit. In this study, the coaxing effect of multi-level stresses in the fatigue process in the automotive drive shaft material 40Cr was investigated by applying three levels of stress amplitudes below the fatigue limit in different sequences prior to conducting the conventional constant amplitude fatigue test with standard round bar specimens. The existence of coaxing effect under multi-level stress with a load sequence effect was observed and confirmed by the results. The longest fatigue life is achieved through three levels of coaxing load with a step increase sequence, while a decrease sequence will result in the shortest fatigue life. The mechanisms of the coaxing effect which increased the fatigue life at the crack initiation and propagation stages were observed, through fracture morphology analysis, as the regulation of micropores, decreasing of pore density and the narrowed striation spacing. This finding is instrumental for extending the service life of mechanical components. [ABSTRACT FROM AUTHOR]

The conditions of combined deformation for fatigue damage indicator and well-drilling tool joint are studied. The contact stresses on the surfaces of the coupling parts and the relationship between operational load and dimensions of sliding zone at frictional contact have been determined depending on the size of the technological gap. [ABSTRACT FROM AUTHOR]

Detection of subcritical fatigue cracks in low-carbon steel welds with the strain-gauge nullindicator method based on the amplitude difference and phase difference procedures was investigated. It was established that a marked growth of the difference in strain amplitudes pointed to the origin of a main crack 1 mm long near the weld. An initial increase in strain amplitude differences reveals fatigue damage much earlier in comparison with the visual crack detection. The phase difference procedure cuts down the time of fatigue damage detection in low-carbon steel welds tens of times from the visual method. [ABSTRACT FROM AUTHOR]

A computational and experimental assessment of the sensitivity of structural materials to stress concentration under high-cycle asymmetrical loading by tension-compression, bending, and torsion has been performed. A proposed limiting state model is used in the solution. A satisfactory agreement of the calculation results with the experimental data has been obtained. [ABSTRACT FROM AUTHOR]

The strain-measuring null-indicator method for the detection of fatigue cracks in electric locomotive truck frames, which are difficult to reach for visual inspection, was developed and in-service tried. The sensitivity of the method is established to increase after high-frequency mechanical hammering of St. 3sp low-carbon steel butt welds. The method provides detecting the nucleation of fatigue cracks in the subsurface steel layers. [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]

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]

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]

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]

Methods for designing structural elements made of structurally heterogeneous materials by developing vectorial models and studying the scale effect of elastic properties have been developed. Results of calculation of stress-strain states with allowance for structural and geometric factors of stress and strain concentration are presented. [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]

A procedure for calculating the lifetime of metals under biharmonic loading is proposed, which is based on the energy-based criterion of fracture and the use of metal cyclic deformation and fatigue characteristics. The data obtained are compared with the known experimental ones. [ABSTRACT FROM AUTHOR]

The authors consider the influence of temperature, load cycle asymmetry, specimen size, plastic prestraining, and material damage in the course of low-cycle loading on the magnitude of fatigue fracture toughness of heat resistant, high-strength chrome-molybdenum and other steels. They analyze the degree of this influence depending on various factors varied in the process of investigation. [ABSTRACT FROM AUTHOR]

Within an experimental approach we describe the mechanical behavior of different resin-epoxy laminates reinforced with cross-ply Kevlar and glass fibers under conditions of static and cyclic three-point bending. In static tests, we consider the effect of stacking sequence, the thickness of 90°-oriented layers, reinforcement type on the mechanical behavior of laminates under loading and on realization of various damage modes leading to rupture. Cyclic loading studies have been performed in two steps. In the first stage, we inquire into the dependence of the behavior and durability of four glass fiber-reinforced laminate-types on the stacking sequence; the second stage is devoted to studying the dependence of cyclic strength and fatigue behavior of laminates on the reinforcement type. Fatigue tests are carried out in load-control regime for glass and hybrid (Kevlar + glass) fiber laminates. Fatigue curves are constructed in coordinates “stress – number of cycles until fracture” from the criteria corresponding to a drop in stiffness by 5 and 10%. Analysis of the results obtained permits evaluation of the effect of the stacking sequence and the reinforcement type on the behavior of cross-ply laminates in cyclic loading. The presence of Kevlar fibers accounts for nonlinear behavior of laminates in static tests and for low cyclic strength in fatigue tests under three-point bending. [ABSTRACT FROM AUTHOR]