The work is devoted to the study of the effect of regular cyclic loading on fatigue crack growth kinetics. Duration of fatigue crack growth of titanium alloys VT6 (α + β structure) and PT-3V (pseudo-α structure) under external cyclic loading at room temperature was investigated. Flat compact samples of type C(T) with a thickness of 5 mm were used according to the ASTM-E647 standard. The estimation results of the fatigue crack growth at a maximum load value of 3–5 kN and different asymmetries R = 0.1–0.7 are shown. Fractographic studies of crack growth were carried out. It was found that the kinetics of crack growth largely depends on the force parameters of variable loading and loading asymmetry. The experimental data on fatigue crack growth kinetics of a wide range of external loading were obtained. The analysis of the crack growth kinetics, depending on the properties of materials and the nature of variable external exposures, was carried out based on the study of the obtained fatigue fracture kinetics diagrams (henceforth FFKD) and fractographic features of crack development. The mechanisms of such crack behavior and modeling of the duration of its development are discussed. The effect of the asymmetry on crack growth is estimated. Modeling of the kinetics and duration of crack growth using the effective stress intensity factor (henceforth SIF), taking into account the method of “cycle-by-cycle” calculation of a crack length, is proposed. The comparative analysis of the calculated duration of crack growth and the experimental data obtained for these titanium alloys was carried out.

The fatigue crack growth kinetics were studied under regular and irregular cyclic loading with different asymmetry ratios, as well as spectral loading typical for the operation of various structural objects, in the middle section of the fatigue crack growth diagram on C(T) type specimens cut from low-alloy steel used in the automotive industry. The R-ratio of different levels influence and irregular loading character on the fatigue crack growth duration at different values of maximum loading is shown. A model of the crack growth duration is proposed on the basis of considering the “crack closure,” and for irregular cyclic loading, in addition, the character of variable amplitude loading with its reduction to an equivalent regular loading. This made it possible to reduce the fatigue crack growth diagrams group with different asymmetry ratios to one equivalent curve. The forecasting of the fatigue crack growth duration was carried out by the cyclic calculation method (“cycle-by-cycle”) and by the proposed empirical dependence.

Fatigue crack growth kinetics of various titanium alloys BT6 (α+β structure) and 3M (α-structure) under constant amplitude loading with different types of cyclic disturbances in the form of over- and underloading at a loading frequency of 25 Hz was investigated.The tests were conducted on a servohydraulic testing machine under controlled load with simultaneous automatic recording of loading parameters and readings from a crack opening gauge. Tests were carried out on compact specimens with an edge crack which were subjected to off-centre loading. The crack opening data was converted into crack length using a yield strength measurement method. Experimental results, namely fatigue crack growth curves and fatigue fracture kinetic diagrams, were analyzed. It is shown that crack growth kinetics depends mainly on the type of alternating loading amplitude sequences. Reliability estimation of crack growth on the basis of crack opening gauge and study of fractographic features of materials fracture has been carried out.

The kinetics of fatigue crack growth has been studied in tensile testing of compact steel tensile specimens (S(T)-type) in the middle section of the kinetic diagram of fatigue fracture (fatigue crack growth diagram) under regular and irregular loading with different asymmetry and maximum load values. The samples were tested on a BISS Nano-25kN servo-hydraulic machine. Standard loading spectra typical for different technical objects exposed to alternating loading during operation were used. The values of the crack growth rate per cycle in the loading block were obtained. Parameters for assessing the character of irregular loading and crack closure, namely, the irregularity factor and crack closure coefficient were proposed. When calculating the effective value of the range of the stress intensity factor (SIF) at the crack mouth, we propose also to take into account the loading irregularity in addition to the closure coefficient. With this approach, the obtained fatigue crack growth diagrams can be grouped into one equivalent curve, which is characteristic of regular loading with R = 0. Moreover, grouping of the fatigue crack growth diagrams provided the use of unified parameters when calculating the crack growth kinetics, regardless of the type and parameters of loading, which rather simplified the crack growth determination. The fatigue crack growth life was predicted taking into account the crack «closure» and the nature of loading according both to the approach developed by the authors and by cyclic calculation method (cycle-by-cycle). All the data obtained are tabulated and classed according to the type of loading. The calculation results and experimental data showed good convergence, which was confirmed by the high values of the correlation coefficient.

The fatigue crack growth kinetics in a 2024-T3 aluminum alloy during constant- and variable-amplitude loading a various stress ratios in the medium-amplitude segment of a fatigue crack growth rate curve is studied. Variable-amplitude loading is represented by quasi-random samples of standard random loading spectra, which were preliminarily schematized using a rainflow algorithm. Parameters are proposed to estimate the character of random loading. The influence of the character of random loading using various standard spectra on the fatigue crack growth rate is analyzed. The fatigue crack growth time is estimated with allowance for fatigue crack closure and the character of random loading using a proposed formula and a cycle-by-cycle calculation method.

Currently, among other models for crack growth prediction, crack closure models that consider the decrease in stress intensity factor (SIF) range associated with cyclic loading asymmetry are more popular. One of drawbacks of these models is impossibility of considering the loading history sequence. The theory related threshold SIF range and Delta K-th and local near-tip stresses, induced by overloads, and postulated that the overload effect in the near-threshold region of growth rates is caused by residual local stresses. The proposed model applies the local stress and strain approach to estimate stress sigma* in the stress concentration region for fatigue analysis. This region is characterized by local near-tip stress sigma*, whose amplitude is determined by cyclic inelastic reaction at crack tip. Further development of the model is due to varying nature of the threshold SIF range Delta K-th. As a basis, the Forman-Mettu formula was adopted, which describes the fatigue crack growth curve in all three regions of fatigue crack growth rate. The range Delta K was determined by the peak load Delta P of the load history. The crack closure was considered by the Schijve equation, considering the asymmetry of the half-cycle U = f(R), and effective SIF was estimated by Delta K-eff = Delta K*U. Given known SIF range Delta K, the value of the local stress sigma* at distance from the crack tip r* was determined for each half cycle by Neuber and Ramberg-Osgood equations, and threshold SIF was estimated from the analytical formula of K-th-f(sigma). Thus, known loading history made it possible to determine Delta K-eff, K-max, and K-th on each cycle for fatigue life estimation. Mathematical modeling of fatigue crack growth life, especially in near-threshold region of its growth, according to the Sunder's scheme, showed that investigated aluminum alloy 2024-T3 exhibited crack growth sensitivity to various types of force action, including various types of random loading. (C) 2019 The Authors. Published by Elsevier B.V.

Near threshold fatigue crack response is extremely sensitive to load history. It is associated with the influence of near-tip stress on instantaneous resistance of crack tip surface layers to failure under atmospheric conditions. The paper deals with computational aspects of stress and strain distributions at the crack tip. A new approach is proposed to obtain near-tip stress under variable amplitude loading. The method considers a combination of standard rules for determining local strains (Linear rule, Neuber rule, ESED method, etc.) and well-known in Finite Element Analysis (FEA) return mapping algorithm in context of nonlinear mixed hardening material model. Iterative procedure based on Newton-Raphson method is proposed to calculate plastic variables and near-tip stress at each load step. The calculated results obtained by new model are validated with the traditional approach in case of Al2024-T3 alloy for constant amplitude loading after tensile/comprehensive overloads.

Most of steering problems were related with a pitman arm and a steering joint. For these parts, using the example of two buses the bending stress and safety margin were determined with the help of virtual 3D models. A dangerous section and maximum loads were determined, the calculation of bending under static loading was carried out. The proposals were drafted that aimed at improving the operational reliability of the steering. The problem of road safety was one of the most significant in modern conditions. An important contribution in safety was made by the reliability of vehicles, which, in turn, was associated with the development of their design, and also the technical condition. An especially important thing was to ensure the reliability of vehicles that carry passengers.

Currently, among other models for crack growth prediction, crack closure models that consider the decrease in stress intensity factor (SIF) range associated with cyclic loading asymmetry are more popular. One of the drawbacks of these models is that they do not take into account the loading history sequence. The crack closure was made allowance by the Schijve equation, considering the asymmetry of the half-cycle U = 0), and effective SIF was estimated by AKeff = AK'U. Given known SIF range AK, the value of the local stress ci* at distance from the crack tip r* was determined for each half cycle by Neuber and Ramberg Osgood equations, and threshold SIF was estimated from the analytical formula of Kin =f(6). Thus, known loading history made it possible to determine AKeir, Km., and Kth on each cycle for fatigue life estimation. Mathematical modeling of fatigue crack growth life, especially in near-threshold region of its growth, according to the Sunder's scheme, showed that investigated aluminum alloy 2024-T3 exhibited crack growth sensitivity to various types of force action, including various types of random loading.

The kinetics of fatigue crack growth in 2024-T3 Al-alloy specimens under constant and variable amplitude loading with different cycle ratios in the intermediate (Paris) region of the fatigue crack growth diagrams is studied. Variable amplitude loading is represented by quasi-random samples of standard random loading spectra. Crack growth curves are obtained and the crack growth rate per cycle in the loading block at various values of its length is estimated. The crack growth life is predicted view of crack closure and the nature of the random loading according to the proposed formula and cycle-by-cycle calculation.

Исследована кинетика роста усталостной трещины на образцах из четырёх видов сталей при регулярном и нерегулярном циклическом нагружении с различной асимметрией.Параметры нагружения были подобраны таким образом, чтобы получаемые кривые скорости роста трещины укладывались на средний участок кинетической диаграммы усталостного разрушения (КДУР).Для испытаний применялись стандартные компактные образцы с краевой трещиной.Нагружение образцов осуществлялось на современном сервогидравлическом испытательном оборудовании, которое позволяет задать различные законы перемещения штока, записывать все параметры

Рассматривается проблема учета последовательности нагружения, возникающая при расчете долговечности деталей с дефектом в виде трещины.Приведен краткий обзор существующих моделей, в частности, большая часть из них объясняет исследуемый феномен эффектом закрытия трещины.В то же время чувствительность к последовательности нагружения наблюдается при высоких значениях асимметрии цикла, когда закрытие минимально, что ставит под сомнение адекватность данных подходов.С целью построения физически обоснованной модели авторами исследуются механизмы роста трещины при различных скоростях (участок Пэриса, околопороговая область), предлагается теория хрупкого разрушения в околопороговой области роста трещины.Новый подход основан на связи напряжения раскрытия в окрестности вершины трещины с пороговым значением размаха коэффициента интенсивности напряжений.Разработана численно-аналитическая методика расчета напряжений в окрестности вершины трещины при произвольной последовательности нагружения.В основе данной методики лежит вариант теории пластичности, рассматривающий комбинированное изотропно-трансляционное упрочнение и линейное правило определения деформации в окрестности трещины.Трансляционное упрочнение реализовано в рамках закона Фредерика-Армстронга и правила суммирования смещений Шабоши.Интегрирование скоростных соотношений осуществлялось на основе процедуры проецирования напряжений на поверхность нагружения (неявный метод Эйлера).Приведена методика экспериментальных исследований, необходимых для настройки модели и ее верификации.Представлены результаты сравнения экспериментальных и расчетных данных по долговечности, полученные при регулярном нагружении с перегрузками

Duration of growth of the fatigue crack of various steels such as AISI 4030, 40X, 40 at external excitation of various character is investigated. It should be noted that kinetics of the crack growth depends on the nature of variable loading in many respects. Experimental data of kinetics of fatigue cracks’ growth of the wide spectrum of block and random loadings of investigated steels are received and an approach for cracks’ growth estimate depending on material properties and the nature of variable external input is offered. It is shown that the duration of growth of the fatigue crack on the middle-amplitude segment of its growth depends on the completeness (irregularity) of block or accidental loading. It is offered the dependence of the longevity evaluation of fatigue crack durability from a given target size to its critical value corresponding to the moment of material destruction. For this calculation it was used the value of the fatigue crack growth duration was used at a zero regular loading with the maximum load corresponding to the maximum load of the later used block or accidental loading, also the fatigue properties of the material determined from the slope of the endurance curve under soft symmetrical loading were taken into account. The second approach is related to the application of averaged parameters of the fatigue crack growth kinetics — the slope of curve of the kinetic fatigue fracture diagram (KFFD) at regular cyclic loading with the same force parameters. The linear principle of summation of crack growth without interaction of amplitudes was used in both cases. For the study the accepted assumptions it was made a calculation of the duration growth of cracks in the program of summation the crack growth for each loading cycle (cycle-by-cycle) without taking into account the interaction of load amplitudes.

The present paper considers a topical problem of a fuel feed system installed on a bus powered by methane gas, the problem c the consisting in a condensed substance in the cylinders. The main difficulty in solving this problem is that the condensed substance accumulated in the cylinders cannot be removed since the cylinders are mainly horizontally installed. Hence, the accumulated condensed substance cannot outflow from the cylinder. To remove the condensed substance from the cylinders one should release the compressed natural gas from the feed system, disconnect the cylinders, to clean them and then to connect and to fill them with gas. This is not a very efficient and cost-effective method to solve this problem. The comparison of the cylinder structure for the compressed natural gas with cylinders with a liquefied gas helps to find the way to eliminate the problem. It should be noted that the tube should fit a particular type of cylinder, located in parallel with the wall at the bottom to fully take the condensed substance from the cylinder. Along with that, after an already used cylinder or a new cylinder is upgraded, the marks should be made at the valve of the upper part and lower part of the cylinder. In case the cylinder has a wrong position, no condensed substance will be removed from it. The upgraded feed system allows one to prolong the period of regular technical maintenance of the cylinders, as well as to reduce the labor contribution in repair works and maintenance of the feed system.

Near threshold fatigue crack response is extremely sensitive to load history. It is associated with the influence of near-tip stress on instantaneous resistance of crack tip surface layers to failure under atmospheric conditions. The paper deals with computational aspects of stress and strain distributions at the crack tip. A new approach is proposed to obtain near-tip stress under variable amplitude loading. The method considers a combination of standard rules for determining local strains (linear rule, Neuber rule, ESED method, etc.) and well-known in finite element analysis (FEA) return mapping algorithm in context of nonlinear mixed hardening material model. Iterative procedure based on Newton Raphson method is proposed to calculate plastic variables and near-tip stress at each load step. The calculated results obtained by new model are validated with the traditional approach in case of A12024-T3 alloy for constant amplitude loading after tensile/compressive overloads.

At present, there are two approaches to fatigue life prediction. There is an approach of continuous damaging popular among the Russian scientists and engineers that is based on the crack initiation implicit modelling and the damage determination contributed by each cycle. In this approach, fatigue damage is determined as a fatigue crack occurring and a structural element failure. Another approach is presented with failure mechanics allowing the small crack existence and describing its growth up to the size limit. The present work aims at studying the problem of applicability of the fatigue damage models, which are efficient for standard samples and the simplest structural elements, for complex transport structures subjected to variable amplitude loading, and proposing the joint use of approaches to model fatigue damage, while preliminary describing advantages, disadvantages and assumptions of this approach.

The results of testing for overloads and underloads of aluminum alloy specimens at near-threshold crack growth rates aimed at identifying the most suitable model for the fatigue life estimation are analyzed. The paper describes the analysis of the influence of the character and value of overloads and underloads on the fatigue crack growth. The obtained curves for crack growth and fatigue life could not be explained by classical crack growth models. This necessitates the development of a new model.

Possible variability of endpoints on the da/dN-ΔK curve is analyzed, and its influence on crack growth life is shown. Basics of the fatigue crack growth model of material with a single crack are presented. The proposed model calculates local stress at a remoted point close to the crack tip. The model establishes a relationship between the stresses and the near-threshold range of stress intensity factor. Tests were carried out for the low crack growth rates on aluminum alloy 2024-T3 and low carbon steel with rare small overloads of different sequence, and as a result, a-N and da/dN-ΔK plots were received. Fatigue predictions were done using existing Elber, Barsom, Wheeler, Willenborg models, NASGRO and FASTRAN software and the proposed model. It is shown that the proposed model of local stress calculation at the distance from crack tip is able to predict fatigue life considering load sequence of no less accurate than the best of existing models. However, application area is restricted to small overloads, high cycle ratio, and low crack growth rate at the moment.