Hypothesis Of Linear Summation Research Articles

AVOID DAMAGE TO LOADER CRANE COMPONENTS A STUDY OF CONCENTRATION

Crane parts work with non-stationary variable loads. The hypothesis of linear summation of fatigue damage during non-stationary cyclic tests is not confirmed. Under the influence of alternating stresses, a process of gradual accumulation of damage occurs, which leads to the formation of cracks of various nature, which gradually increase, forming macroscopic values. The work is devoted to the study of stress concentratıons leading to damage to crane parts. The results of the research revealed that the set of possible methods for non-stationary modes is not considered valid and was not fully justified from a physical point of view. From a physical point of view, the direction of the ongoing processes is determined by the method of lowering the curtain. However, this method itself requires further development and accuracy. The application of the Dahlgren method is a manner that inaccurately describes the process and its application is incorrect. When choosing materials that work for fatigue in non-stationary modes, along with other characteristics, it is necessary to take into account the criterion of sensitivity to loads. Keywords: step loading, hypothesis, damage, secondary fatigue curves, experiment.

Comprehensive analysis of existing endurance testing methods of flexible dowel crutches of composite reinforced concrete road bridges according to the standards of a number of countries

In the article, the author considers the verification issue of the endurance of the venture elements of steel-reinforced concrete spans of bridges, in particular, flexible dowel crutches («Nelson’s anchors»). According to the existing domestic regulatory framework, the endurance test of the flexible dowel crutches for road bridges is not required; at the same time, the survey’s results of the operated superstructures indicate the presence of problems associated with the insufficient endurance of these elements. The article presents comprehensive (qualitative and quantitative) comparative analysis results of the world’s existing approaches to testing the endurance of the venture elements. At the stage of qualitative analysis, the author considered methods of checking the endurance of elements in general according to the norms of 8 countries and regions (Russia, the European Union, the USA, Ukraine, Australia, Canada, Japan, China) and highlighted a basic set of methods for checking the endurance of the venture elements (Russia, the European Union, USA, Ukraine). The models of the basic set were subjected to a detailed qualitative analysis according to the criteria of the models used and the parameters included in them. The quantitative analysis of the selected basic set techniques, carried out in relation to the existing bridge, included an analysis of the effects of moving loads on the venture elements, direct elements verification using the considered methods, as well as the design damage comparison of the elements obtained by the Palmgren-Miner linear summation hypothesis. Based on the obtained values of design damage, the values of the use of endurance reserves were determined and analyzed for each of the considered methods and the conditional load model. Based on the results of the qualitative and quantitative analysis, the author draws conclusions based on the comparison of various methods and highlights the rational directions for improving the existing domestic methodology.

Метод расчета усталостного повреждения регулярных зон крыла самолета при случайном нагружении на этапах типового полета

In accordance with the airworthiness standards, the aircraft structure must be operationally survivable, it means that structure must be able to remain efficiency in the presence of admissible damage. But, accumulating above than a certain level, damages cause fatigue failure of the structure, in the form of micro- and submicrocracks, thus reducing its strength characteristics. Currently, several approaches have been formed to ensure the safety of an aircraft structures in terms of strength. One of them is ensuring a safe resource (safe durability). This principle implies that during the specified service life of the product, no damage will occur in it, reducing the strength below the permissible level. The aircraft resource is limited “from above” by the durability of the regular zones of airframe. Therefore, predicting the durability of an aircraft wing structure at the design stage is a fundamental engineering problem to ensure its safety and economic efficiency. At the same time, the first step in dealing with aircraft fatigue damage at the design stage is the collection and assessment of the operational loads of the analog aircraft. However, at the design stage of a new aircraft model, obtaining such data is not always possible. Therefore, the purpose of this article is to develop a method for calculating fatigue damage at the stage of cracking and assessing the durability of regular zones of a transport aircraft wing, taking into account the conditions of its operation. The tasks to be solved are: to isolate the factors that determine the durability of the aircraft at flying in turbulent air; to take into account the asymmetry of loads and accumulated damage that occurs at each stage during the entire flight of the aircraft; to determine the aircraft's resource depending on the profile of a typical flight. The method is based on a standardized atmospheric turbulence model, typical flight profiles, fatigue characteristics of materials, the hypothesis of linear summation of damages and calculation based on nominal stresses. As result, comparison between the calculated integral repeatability of overloads and equivalent bending moments with the results of processing flight test data showed good agreements. Conclusions. The scientific novelty of the work lies in the fact that a method for calculating the fatigue damage of the regular wing zones, taking into account the expected flight profile of the aircraft was developed. This means that the proposed method makes it possible to carry out a preliminary assessment of the resource when designing an airplane without using data on the operational loads of an analogue airplane, and also estimate the residual resource of the airplane during its operation.

Analysis of Fatigue Damage Accumulation in Structural Materials under Quasi-Random Load

The spectrum of stresses at a hazardous point on bearing metal structures of mining, road-building and transporting machines, as a rule, is random by nature. Objective complexity of the fatigue process and variety of fatigue crack nucleation mechanisms are the main causes of errors in prediction of their lifespan, along with an experimentally untestable hypothesis of linear summation of damages. The method of complete stress-strain diagrams uses a representative parameter of the fatigue process, selected in accordance with the results of testing, carried out on trained thin specimens of a specific material. The summation of fatigue damages is based on experimental kinematic curves, whose intersection with a load vector is used as a criterion of fatigue failure. It is shown that the approach proposed to calculate the lifespan gives certain advantages, as far as individual properties of the material are concerned, and when it comes to describing the kinetics of the fatigue process and determining the limiting state of the material in a structure.

Investigation of the influence of low amplitude loads on the fatigue life of CFRP specimens with impact damage under irregular loading

The research was made to study the effect of low amplitude loads on the fatigue life of CFRP samples with impact damage in a cyclogram of irregular loading. The evaluation of the applicability of the linear summation hypothesis of damage (Palmgren’s Miner’s rule) at this level of loading was also carried out. The results have shown that in some cases correction factors need to be introduced.

Fatigue strength of asphalt concrete pavement on orthotropic steel deck considering temperature factor

Fatigue of any material is a process when micro-damage accumulates because of multiple load applications. The result is the appearance of defects after a while. Asphalt concrete pavement on orthotropic steel deck (OSD) of roadway bridges is prone to fatigue failure. There are several reasons for this. Firstly, there is a large number of vehicle impacts for pavement service life. Secondly, the pavement lies on the flexible load-bearing elements of the roadway deck. This leads to high stresses in the asphalt concrete. In general, stress limitation prevents fatigue defects. The value of this limit is set according to the results of fatigue calculation. This calculation is performed for main load-bearing elements but not for the pavement on the roadway. In this article the necessary of fatigue strength calculation of pavement on orthotropic steel deck is justified. Also, disadvantages of the existing fatigue calculation method of pavement on the ground are noted. This design method is not applied for fatigue strength calculation for OSD pavement because of these disadvantages. Available experimental results of fatigue strength of asphalt concrete are analyzed. As a result, the need to take into account the temperature factor is established. The hypothesis of linear summation of damages allows to take into account the change of fatigue strength of asphalt concrete at different temperatures. In the article the endurance of asphalt concrete as part of the typical pavement system on OSD is evaluated in accordance with the described approach. In addition, the tasks for the future are set. Solution of these tasks will allow to developing the method of pavement calculation on different bridges.

Evaluation Method of the Fatigue Life of Power Plant Pipelines

The method for estimating the fatigue life of power plant pipelines based on the results of one or two tests without destruction is proposed. The proposed method uses the classical model of fatigue resistance. Weller curves for power plant pipelines are determined experimentally, the experimental data are presented. Three curves are constructed for the probability of destruction of 2%, 50%, and 98%. The proposed method uses the hypothesis of linear summation of Palmgren-Miner injuries. When developing the method, much attention is paid to the ways of schematizing the actual process of loading the pipelines. Calculation of the distribution of the total number of cycles was carried out by the “Rainflow” method and the Gusev method. The application of this method with a set of design measures provides more than fivefold margin for fatigue resistance of all pipelines of the power plant.

Simulation of the Propagation of Circumferential Crack in the Cylindrical Element Structure Exposed to Cyclic Loadings

The purpose of these studies was to work out the methods for the computation of crack development in the plate and axisymmetric elements of structures at cyclic loading for the plasto-elastic arrangement using the conception of the accumulation of scattered damages in the material. In the domain in question, we simulated the processes of alternating-sign plasto-elastic deformation and the material crack resistance was estimated using the fatigue test data for smooth specimens. Thermal and elastically-deformed states of the structure were determined for different loading modes using the finite elements method. The material damageability was assessed for the curves of low-cycle fatigue and for the range of plasto-elastic deformations on the route of probable crack propagation using the hypothesis of linear summation. As soon as a critical value of the damageability is achieved the crack is propagated stepwise for the specified step. As an example, consideration was given to the kinetics of circumferential crack in the cylindrical element exposed to the axial-symmetric cyclic loading. The contact of crack edges was taken into consideration for compressing loading semicycles. The obtained data agree with the data obtained using the Newman method. The suggested technique can be used for the estimation of survivability of the elements of steam turbines (heat rotor grooves at start-up-stop modes, blade and disk interlocks, etc.).

Evaluation of the Crack Development at a Cyclic Loading of Plate Elements Using the Parameters of Scattered Damages of the Material

The purpose of these studies was to work out the methods for the computation of crack development in the plate and axisymmetric elements of structures at cyclic loading for the plasto-elastic arrangement using the conception of the accumulation of scattered damages in the material. The processes of sign-changing plasto-elastic deformation and crack resistance of the material were simulated at the crack tip using the smooth specimen fatigue test data. Thermal and stress-strain behaviors of the construction were determined in different loading modes using the method of final elements for several fixed crack depths. The material damage was estimated using the curves of low-cycle fatigue and those of the range of plasto-elastic deformations on the would-be way of the crack development resorting to the hypothesis of linear summation. When the damageability reaches a critical value at the crack tip its depth is increased nonuniformly by a value of the specified step. This scientific paper gives consideration to the kinetics of the surface crack in the region exposed to the action of zero-repetition cyclic loading. The results were compared with the data obtained using the Paris equations. A satisfactory agreement of the data is observed. This technique can be used for the evaluation of the service life of the elements of steam turbines.

Study of cycle life of a material in conditions of low-cycle fatigue and long-term strength

Experimental studies shows that the main parts - the shaft and the disks of gas turbine engines, working in the conditions of low-cycle fatigue and durability, have a durability far exceeding the resource identified by calculation based on the hypothesis of linear summation of damage. Therefore, further clarification is needed calculation methods, allowing greater use of the reserves of strength parts. The performed research studies the influence on the durability of the increasing duration of zero cycle of loading samples with a period of T=200 s. and the restraint at the maximum and minimum levels: Dtmax=120 s. and Dtmin=60 s. The studies were performed on a standard round specimens with the diameter of the working area of 7.5mm. Shown that when you restrain a loading there is in the material proceeds relaxation process, combined with a deforming cyclic hardening and increasing of the plastic deformation. It was found that the hypothesis of a linear summation of static and cyclic components of the damage in the conditions of low-cycle fatigue and long-term strength is not confirmed. The increasing of the restraint in the loading cycle under the load to Dtmax=20 s. or more leads to a significant increase - up to 94 % of the static component of damage. The research results obtained during cyclic and repeated static loading, helped to confirm and substantiate the existence of a previously unknown Effect of cyclic hardening materials, due to the duration of the maximum load in the cycle of loading.

Comments on Linear Summation Hypothesis of Fatigue Failures

Abstract This paper presents a comparative analysis of results of fatigue life calculations with the u se of the linear summation hypothesis of fatigue failures (LHSUZ), confronted with experimental test results. Te calculations and fatigue tests were performed for variable amplitude(VA), two-step and ten-step loading conditions, both in the low-cycle fatigue (LCF) and high-cycle fatigue (HCF) range, for the case of C45 steel as an example. Experimental verification of the hypothesis LHSUZ did not revealed any significant influence of load level and form of load spectrum on conformity of results of the calculation by using the LHSUZ, to results of fatigue tests on C45 steel. However, it enabled to assess magnitude of a correction factor which appears in the considered linear hypothesis.

Features method of determination tireless longevity gondola taking into account operating damages

The article represents the features and the results of the proposed method of gondolas operating life calculation considering the accumulated damage in general. The methodology is based on the use of linear hypothesis counting the total number of damages. The current method of determining the reliability values is based on data of operational or dynamic performance trials. Thereby it doesn’t contain the algorithm of approximate calculation of the operating life on the project phase. That fact complicates its applying in engineering practice. Destruction of structures is mainly related to the accumulation of irreversible damage of its elements. Calculations of cyclical strength in that case are carried out using semi-empirical (phenomenological) models of damage accumulation. Considering the increased requirements for safety, it is advisable to evaluate the limit state in assessing the quality of design on the criterion of fatigue cracks. Given the process of accumulation of fatigue cracks used corrected linear summation hypothesis damages. Assessment of durability performed for the most loaded node. As a result of the modal and spectral analysis identified the main causes of fatigue damage gondola body elements - the coincidence frequency of natural oscillations with a frequency of exciting effects with maximum amplitude. Based on the analysis of the spectral power density we obtained the calculation of operating life considering the accumulated damage. The article proposes a specific evaluation of operational reliability and dynamic loading, which can improve the accuracy of resource estimates at the design stage by the criterion of fatigue durability

Estimation of the strength and lifetime of technical objects using the acoustic-emission method

Estimation of the lifetime of a construction consists of forecasting the time remaining before failure and is based on extrapolation of the time dependences of the state parameters of a technical object to their critical values. The use for this purpose of simplified damageability models (e.g., a hypothesis of linear summation of flaws) and the absolute values of acoustic-emission (AE) parameters (e.g., activity, counting rate, duration of pauses) leads to a dependence of the estimate results on a large number of destabilizing factors and, as a result, to low accuracy or laboriousness of these techniques. An earlier suggested micromechanical model of AE parameters makes it possible to find a trade-off between simplicity and accuracy.

Theoretical-Experimental Model for Predicting Crack Growth Rate in Structural Alloys of Aircraft Gas Turbine Engine Disks with Regard to Fatigue and Creep

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.

Effect of prior plastic deformation and creep strain on fatigue damage for a high-temperature alloy

A procedure is developed, an experimental study is provided, and a comparative evaluation is given for the effect of prior plastic deformation and creep strain on fatigue damage for high-temperature metallic materials on the example of alloy eI827. A different effect is demonstrated for prior plastic deformation and creep strain on fatigue damage of this alloy at 1073 K. In the case of prior creep a marked difference is established from the hypothesis of linear summation for damage in the direction of weakening during evaluation of the overall fatigue and static damage both in the form of relative endurance and relative strain.

低合金鋼の高温高圧水中腐食疲れ破面の解析とき裂進展機構

Fractographical observations of low alloy steels in high temperature water were carried out, and the responsible mechanisms for corrosion fatigue crack growth were discussed. The fracture surface of a low sulfur content steel was wholly occupied by transgranular ductile cracking characterized by striation patterns. In a medium sulfur content steel, on the other hand, quasi-cleavage cracking characterized by both striation and river patterns was observed. The crack growth in the medium sulfur content steel was abruptly accelerated at ΔK≅19MPa·m1/2. This acceleration was caused by hydrogen embrittlement associated with dissolution of MnS. Fourier power spectrum analysis was found to be a very powerful tool for the striation spacing measurement, especially when the striation patterns were made indistinct by derusting treatment. Based on the linear summation hypothesis with consideration for fracture area fractions, the quasi-cleavage crack growth rate in the medium sulfur content steel was derived.

Mechanical Effects of Corrosion Products in Corrosion Fatigue Crack Growth of a Steel

Corrosion fatigue crack growth test of a high-tension steel HT55 was conducted in 1% NaCl, and the wedge effect by corrosion products and crack extension by corrosive dissolution have been estimated in terms of ΔKeff. da/dN against ΔK in 1% NaCl decreases in the order of 0.01 Hz, 0.1 Hz, in air (having no frequency effect) and 5 Hz under R=0.1. da/dN in NaCl at 5 Hz is smaller than that in air on account of the wedge effect by corrosion products. There is no wedge effect under R=0.5. When the wedge effect is eliminated by re-plotting the results in terms of ΔKeff, da/dN under R=0.1 decreases in the order of 0.01 Hz, 0.1 Hz, 5 Hz and in air as is under R=0.5. Supposing the linear summation hypothesis in terms of ΔKeff, the crack extension by corrosive dissolution (da/dt)D is reduced.

Stress Corrosion Cracking and Corrosion Fatigue in Cr-Mo Low Alloy Steel

Abstract Corrosion fatigue crack propagation tests were performed below and above KISCC in AISI 4145 steel heat treated to 1230 MN/m2 and 0.1 N H2SO4 system with particular reference to effects of frequency and stress intensity parameters. The crack growth results above KISCC could not be described by simple linear summation hypothesis, and this fact suggested the importance of the synergestic effect term in crack growth rate. This synergestic effect denoted hereafter as the dynamic SCC rate was strongly influenced by frequency and showed the maximum at a particular frequency. This frequency effect on the dynamic SCC rate suggested that crack growth enhancement under cyclic loading would be induced by dissolution mechanism in spite of the fact that the material had high sensitivity to hydrogen stress cracking under constant load.

Programmed fatigue testing of welded specimens

1. When the loading process is schematized by the method of enlarged amplitudes the stress amplitude distributions can be taken as homologous. This greatly simplifies the methods of evolving fatigue testing programmes. 2. When programmed testing of welded specimens is conducted under symmetrical cyclic stresses higher than the endurance limit the accumulation of defects follows the hypothesis of linear summation of defects. It can be assumed, with sufficient accuracy, that the average limiting value of cumulative defectsa is equal to unity. (This assumption is valid for the stages of crack development and fracture.) The distribution fora follows the logarithmic-normal law quite closely. 3. The considerable concentration of stresses in welded specimens causes the early development of fatigue cracks. (Cracks 2–3 mm long have been noticed after an average period corresponding to 30% of the life before fracture.) This fact increases the destructive effect of stresses lower than the initial endurance limit. When fracture is the criterion of fatigue life the lower stresses should be taken into account during calculations. This is done by extending the summation of defects to a stress of 0.7 σ−1 after extrapolating the fatigue curve. When crack development is the criterion the effect of stresses lower than σ−1 should be neglected.