Loading Cycle Asymmetry Research Articles

Evaluation of prevailing draft loads in the couplings of the electric trains

Introduction. Until now, the requirements for manual and automatic couplers for locomotive-hauled passenger carriages and electric trains have been completely identical, despite the significantly lower level of longitudinal forces during the movement of multi-unit rolling stock with distributed traction. The introduction of specialised designs of coupling devices for electric trains led to the development of strength requirements that enable the creation of new models with smaller dimensions and weight. Operating experience has shown that parts of couplings may be destroyed, leading to the uncoupling of trains. Fatigue damage may cause accidents. However, there were studies of the draft loads of electric trains couplers in operation, on the basis of which it was possible to develop regulatory requirements for specialised coupling devices.Materials and methods. The authors measured the forces acting in the couplings of electric trains of various categories and types during normal operation, as well as the frequencies of the occurrence of forces of various levels. The article includes histograms based on the results of processing the received data showing the distribution of longitudinal forces acting on the inter-car couplers and the parameter characterising the accumulation of fatigue damage per 100,000 km. Results. According to the results of processing of the experimental data, the calculation of the accumulation of fatigue damage to the couplers was carried out and the requirements for their expected service life were established. Test procedures have also been developed to evaluate strength and fatigue resistance under high-cycle and low-cycle loading conditions. The article provides ground for introducing a correction factor into the procedure for processing test results, which makes it possible to perform life tests on equipment with different values of the load cycle asymmetry factor.Discussion and conclusion. The authors have evaluated the draft loads in coupling devices during the operation of electric trains, on the basis of which technical requirements and test methods were developed and approved by JSC Russian Railways. Development and acceptance of all new models of coupling devices for electric trains will be carried out in accordance with these regulatory documents.

Study of the static and endurance strength of reinforced concrete specimens loaded with the account of the characteristics arising in the process of aircraft takeoff and landing operations on airfield pavements

The article shows that the impact of aircraft with multi-wheel bearings on airfield pavements is a cyclical pattern, as a result, constant sign and alternating stresses arise in the material. The results of cyclic loading parameters calculations of coatings under the influence of aircraft with multi-wheeled bearings (load cycle asymmetry coefficient, values of the maximum and minimum stresses of the loading cycle), performed using the SCAD software-computer complex, which implements the finite element method in the form of the displacement method, are presented. The necessity of carrying out tests to determine the endurance strength of reinforced concrete under various loading conditions for more complete consideration in determining the operating modes of airfield pavements is shown, and the loading parameters of reinforced concrete specimens during their endurance tests are determined. A general view and a layout diagram of an experimental setup for determining the static and endurance strength of large-sized cement concrete samples are presented. A technique for determining the static strength of reinforced concrete samples is presented, including the determination of the criterion for their destruction (appearance of fluidity in the reinforcement of the sample), the applied strain-measuring, ultrasonic and acoustic methods for determining the stresses magnitude and strains in cement concrete and structural reinforcement, the dynamics of changes in the measured parameters on one of the samples is shown. The results of the reinforced concrete samples fatigue tests with dimensions of 1500x400x200 mm under various loading conditions are presented, with reference to which regression equations were obtained to determine the number of loading cycles to failure, and endurance coefficients were obtained based on 106 loading cycles. Based on the carried out studies, conclusions are drawn, and the need to develop practical recommendations for taking into account the cyclic nature of the load impact from aircraft and the fatigue strength of cement concrete when assigning operating modes for airfield pavements is indicated.

The Influence of 20Kh13 Steel Specimen Surface Pitting Damage on the Growth of Fatigue Cracks

The damage to the surface of steam turbine rotor blades by pitting reduces significantly their fatigue resistance and can lead to fracture resulted from the development of fatigue cracks. To determine the service life of blades or the probability of their breakage, it is necessary to have data that allow one to assess how the degree and nature of damage to their surface affects their fatigue resistance. If the rotor blade has passed, in an off-design operation mode, the zone of increased stress amplitudes, then, given the known number of cycles, the measure of fatigue damage to the blade material can be estimated. If the stress amplitudes did not exceed the endurance limit (taking into account the safety factor and the loading cycle asymmetry value), the fatigue cracks which could lead to blade failure did not appear in the blade material. If the stress amplitudes and the corresponding number of cycles are in the zone to the left of the French line, irreversible fatigue damage did not occur in the blade material. Determination of the lengths of fatigue cracks appearing at the pit bottom as a function of stress amplitude and number of operating cycles is a topical issue. This will make it possible to evaluate the size of the damaged layer and to restore the blade fatigue resistance by removing this layer. The results from studies of crack resistance under fatigue loading of specimens made of 20Kh13 blade steel with artificially created pitting damage to the surface are presented. The possibility of using the calculated dependences for evaluating the French line for samples with initial pitting damage has been confirmed, and an assessment of the damaged metal layer at the pit bottom depending on the stress amplitude and number of operating cycles has been obtained. The length with which the crack must be removed to restore the blade system service life has been determined.

Deformation of a pipeline element under cyclic loading by internal pressure

An experimental study of the strain of a rectilinear segment of a pipeline Du 200 made of steel 20 under cyclic loading by internal pressure was carry out. The tests were performed on a pneumohydraulic stand under conditions of flat stress, normal temperatures, and load cycle asymmetry with registration of circular and axial elastic-plastic strain. The aim of the work was to test an experimental technique for studying the effect of ratcheting (unilateral accumulation of plastic deformations) on real structural elements when changing the modes of cyclic loading. According to the results of the study, curves of cyclic deformation (ratcheting) of the pipe in the circular and axial direction on the basis 100 cycles were obtained.

Structures based on elliptical arcs in the problems of designing trajectories and creating mechanical systems

Abstract. An experimental study of the strain of a rectilinear segment of a pipeline Du 200 made of steel 20 under cyclic loading by internal pressure was carry out. The tests were performed on a pneumohydraulic stand under conditions of flat stress, normal temperatures, and load cycle asymmetry with registration of circular and axial elastic-plastic strain. The aim of the work was to test an experimental technique for studying the effect of ratcheting (unilateral accumulation of plastic deformations) on real structural elements when changing the modes of cyclic loading. According to the results of the study, curves of cyclic deformation (ratcheting) of the pipe in the circular and axial direction on the basis 100 cycles were obtained.

Влияние асимметрии цикла нагружения на усталостную прочность трубопроводов в условиях коррозионного повреждения

Влияние асимметрии цикла нагружения на усталостную прочность трубопроводов в условиях коррозионного повреждения

Application of rod mechanics fundamentals for analysis of stress-strain state of the tubing

A method for analysis of stress-strain state of the tubing column in the spatially curved well section with an arbitrary intensity of changes as the zenith and azimuth angle is developed in the article.Stresses and strains are calculated using the rod mechanics fundamentals. The system of vector differential equilibrium equations in the projections on the axes of the moving coordinate system is used for the analysis of elastic equilibrium of the tubing axis.The analysis is carried out by solving the direct problem of the rod bending by means of iterative numerical method implemented in software-oriented mathematical environment.A method to account for the limiting effect of the well walls to move of the elastic tubing axis is provided based on the processing of its geometrical parameters, as well as data and directional survey and profilometric research of the well bore.Using the designed calculation schemes, it is found that local bend of the well axis substantially increases the bending stress, which can reach the yield strength of the tubing material. For example that discussed in the article, the bending stress is approximately 300 MPa, while the minimum yield strength of steel, which are made tubing, is equal to 379 MPa.Moreover, the axial force change caused by the reciprocating motion of a rod suspension may initiate the occurrence of cyclic bending moments and stresses in the tubing. So, for the considered operating conditions, we received the following: the minimum stress of the cycle σmin = 303 MPa; the maximum stress of the cycle σmax = 346 MPa; stress amplitude of the cycle σa = 43 MPa; loading cycle asymmetry factor Rσ = 0,88. This load pattern allows to speak about the cyclic fatigue of the tubing.

The Influence of Mean Stress on Fatigue Properties of ADI

The paper presents the results of research focused on assessment of the influence of loading cycle asymmetry on fatigue limit values. For tests two heats of unalloyed nodular cast iron were used. Test bars made of keel blocks were heat treated in salt bathes (austenitization at 900 °C during 1 hour, isothermal transformation at 380 and 400 °C) and loaded at symmetrical, repeating and pulsating loading cycles at room temperature. Evaluation of fatigue properties was based on the determination of S-N curves in high-cycle region including the fatigue limit assessment for 107 cycles to fracture. Fatigue and static tests were completed by metallographic and quantitative phase analysis. Most important result obtained from the presented study is that the dependence of stress amplitude on mean stress cannot be approached by the linear relation but by general power law with exponent lower than 1 (i.e. the Haigh diagram has convex shape).

Determination of the threshold for the development of fatigue cracks in steels used in railway structures

Results of experimental studies of the cyclic resistance to cracks of 20GFL steel used in cast truck bolsters of freight cars are presented. Experiments were conducted for two values of the loading cycle asymmetry factor. Regularities in the growth of cracks near the threshold of their development are analyzed. The conditional threshold level of the stress intensity factor is determined for the studied steel. Different methods for determining this factor are considered.

Fatigue Behaviour of Nodular Cast Iron at Various Loading Cycle Asymmetry

Fatigue behaviour of nodular cast irons is described in specialized literature quite well but the majority of fatigue tests in high cycle region has been made at symmetrical bending or at symmetrical tension-compression loading. The role of loading cycle asymmetry is very important because the fatigue limit of nodular cast irons is substantially influenced by mean static stress. Unfortunately, the number of papers describing the influence of mean static stress is quite low. The aim of the presented paper consists in presenting the results of long-term research of nodular cast irons at high cycle region at different loading cycle asymmetry. Several heats with various heat treatments leading to various structures of matrix were used for fatigue tests performed mainly at symmetrical tension-compression loading and at repeating tensile loading. From S-N curves the fatigue limits for 107 cycles were determined and then used for the calculation and construction of the Haigh and the Smith diagrams. Nodular cast irons seem to be more sensitive to mean static stress than structural steels. It was found that, in contrast to many papers, the dependence of upper stress on mean stress is not linear but parabolic with the exponent lower than 1 whose value is linear function of UTS.

Comparison of a New Equation Describing Fatigue Crack Growth Curves with the Nasgro Equation

The so-called NASGRO equation allows a very good description of fatigue crack growth curves, i.e. the dependence of crack length increase per one fatigue cycle da/dN on the stress intensity factor range K. In 1999 the first author of this paper published quite similar equation, which written for given loading cycle asymmetry with positive stress ratio R contains the same parameters having similar meaning as in the NASGRO equation. In most cases studied by the authors this equation leads to a better fit than the NASGRO equation, above all when the experimental curve contains relatively long Paris straight line and/or relatively sharp bend from this line to the threshold stress intensity factor range. The generalization of the NASGRO equation for various values of stress ratio R was made in a quite complicated way. The new equation was generalized using the Walker model based on the relation K(R) K(0)(1 R)m which is valid also for threshold value Kth but not for critical stress intensity Kc being a constant independent of R. Then the shift of the growth curves with the change of R ratio is described only by one parameter m (0 m 1) both for positive and for negative values of R. It means that the crack closure models are very important for explanation and deep study of fatigue crack growth mainly in negative R region but play no crucial role in a phenomenological description of the growth in the case of short-term tests in non-aggressive media.

Nonlocalized Fatigue Damage of Metals and Alloys. Part 2. Interrelation between Fatigue and Inelasticity

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.

Fatigue of NPP Components and Piping under Service Conditions

Empirical equations are proposed for description of fatigue curves (up to 1012 cycles) of steels employed in NPP equipment and pipelines. Parameters of the obtained equations allow one to take into account the loading cycle asymmetry, corrosive and mechanical interaction between the coolant and pipeline metal, and the metal ductility reduction under service conditions.

Fatigue Fracture Toughness of Metals and Alloys. Part 2. The Influence of Service and Manufacturing Factors

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.

Prevention of fatigue failures in elastic coupling discs and in fan blades

It is proven by fatigue tests that the generalized S N curve is an invariant in relation to a load cycle asymmetry. It is shown by tests on a large batch of coupling elastic discs and fan blades that the S N curve, represented in relative stresses (relative to the endurance limit S ar), i.e. S/ S ar-N, can be effectively applied to studies of fatigue strength. As an example, the tolerances of mounting dimensions are determined using the ( S/ S ar)-N curve based on the data of rig tests for representative models. These tolerances provide the required quality and endurance of a locomotive elastic coupling.

Cyclic corrosion cracking in steel at some nuclear power stations at high water temperatures

Cyclic corrosion cracking is considered for low-carbon low-alloy nuclear power station steels as influenced by the high-parameter coolant. Numerous results are examined on the effects of temperature, frequency, asymmetry coefficient, and form of loading cycle on the fatigue crack growth rates in nuclear power station steels. The mechanics are defined and parameters are derived that define the effects of high-temperature water. The magnitude and character of the threshold range in the stress-intensity coefficient for cyclic corrosion cracking differ from those in static corrosion cracking. The threshold range in the stress-intensity coefficient for cyclic corrosion cracking can be used in engineering as an independent characteristic in the cracking of constructional materials. An autoclave test method has been proposed that enables one to determine the effects of the loading cycle amplitude, frequency, asymmetry coefficient, and form on the rate of fatigue crack growth in a single specimen. A mathematical technique has been developed for calculating the fatigue crack growth rates for constructional elements when the cyclic corrosion cracking mechanism applies for a wide range of cyclic loading parameters.

Determination of the coefficients of reduction of fatigue strength of structural elements with stress raisers in the range of limited life

The article considers the possibility of using in practice the Socie-Morrow-Chen model for calculating the life of structural elements with stress raisers. It shows that there is good agreement of the theoretical estimates with the results of the experimental determination of the number of cycles corresponding to the initial stage of development of damage, NL. To develop the model the article suggests a procedure of taking into account the influence of the local mean stress of the strain cycle on NL, based on the use of a single dependence of the strain amplitude on the number of cycles up to the appearance of a crack with any coefficients of load cycle asymmetry R that takes into account the influence of R solely on the crack growth rate in the zone of stress concentration. The numerical calculations make it possible to establish the “effective” coefficients of reduction of fatigue strength which increase with an increase of the cyclic load base.

Computation and experimental investigation of the fatigue strength of a material in the plane stress state with allowance for stress concentration

A computational-experimental method of determining the longevity of structural components with stress risers, which is based on the finite-element analysis of the stress-strain state under a variable load, is proposed. The longevity is determined using the δ-model. Fatigue curves for smooth specimens with different coefficients of load-cycle asymmetry and the results of tests of smooth specimens with holes are used as experimental data. The results of the computation are compared with data on the cyclic failure of plates with holes.

A method of testing for crack resistance under a cyclic shearing load

An experimental method is proposed for determination of the characteristics of both mode I and mode II cyclic crack resistance of flat specimens for different values of the coefficient of load-cycle asymmetry. For this scheme, which is realized on an apparatus for fatigue testing in tension-compression, the K-calibration was obtained by the finite-element method. The scheme was tested on aluminum alloy AMg6N specimens.

Structural hysteresis in a shell-type vibration damper with distributed friction

Results of a study of nonmonotonic loading of the elastic filler in a thin cylindrical shell by accounting for Coulomb friction are presented. An analytical description of the structural damping loop is obtained based on the solution of contact problems using applied models of shell and filler. The effect of loading cycle asymmetry on histeris is properteis of the shell-type damper is is studied.