Papers by Keyword: Fatigue Damage

Abstract: This study is aimed to estimate the fatigue damage ratio and to analyze the structural integrity for the monopile foundation of NREL 5-MW wind turbine. For simulating the uncertainty of environmental conditions, various random seeds, tide heights, and wave orientations are considered in the analyses. The Design Load Case 1.2 of IEC 61400-3 is applied in this study. A sequential approach is adopted to calculate the fatigue damage ratio. Firstly, the environmental conditions are implemented into the BLADED code. Secondly, ANSYS is employed to compute the stress/deformation using the output loading data from BLADED. Finally, the fatigue damage ratio is calculated by MATLAB using relevant fatigue theories. The results show that an increase in tide height causes a greater fatigue damage in the foundation. The maximum fatigue damage ratio is found in the case with a middle wind speed which is closer to the rated wind speed and is associated with the occurrence probability. The overall results demonstrate that the methodology developed in this study is applicable to the assessment of fatigue damage in the monopile foundation of OWT.

Abstract: Diffraction contrast tomography using ultrabright synchrotron radiation X-rays was performed on an austenitic stainless steel with a bimodal harmonic structure in which a network structure of fine grains (Shell) surrounds a coarse grain structure (Core). Then, not only were the shape and position of each grain reconstructed, but the change in excess dislocation density during the fatigue process, Δρ, was also measured. The results show that Δρ depends on the diffraction plane, Schmidt factor, and grain size, and that the change in Δρ during the fatigue process of a harmonic structured material is less than that of a conventional material. This result indicates that the network of fine grains in the harmonic structure supports microdeformation and suppresses the deformation of coarse grains. Furthermore, it was found that Δρ of grains unrelated to crack initiation increased continuously with the number of cycles, whereas that around the crack initiation site decreased with crack initiation.

Abstract: The paper summarizes results of failure root cause analysis of the driven part of the screw conveyor shaft for transfer of combustion residues (ash and slag) of a power plant steam boiler. The paper presents also the results of material analysis of selected components of this conveyor to be evaluated their material quality.

Abstract: The work carried out complex and systematic studies based on of which the causes of accidental destruction of reinforcement of reinforced concrete structures of hydraulic structures is established, which is that corrosion damage to metal reinforcement is accompanied by its active flooding and embrittlement, resulting in reduced material properties against corrosion and mechanical destruction of reinforced concrete structures, especially after 20-40 years of operation. Based on existing approaches to the assessment of ageing equipment, developed in various branches of mechanical engineering, including the construction industry, identified trends and shortcomings in determining the efficiency of metal structures and their residual life, in connection with which the calculation and experimental methods of using the criteria of crack resistance, sensitive to changes in the structure of the metal during long-term operation. It is established that the decrease in crack resistance characteristics in the surface layers of reinforcement of reinforced concrete structures in contact with the aggressive environment is caused by the degrading effect of hydrogen on the crystal structure of the metal. The calculated-experimental method is offered, allowing to carry out engineering forecast estimation of residual (working) resource of the reinforced concrete designs operated in corrosive-aggressive environments of hydraulic engineering constructions, which adequately reflects physical and mechanical processes in metal which occur during its long operation.

Abstract: It is a widely known fact that the stiffness of polymer composite materials decreases with the accumulation of fatigue damage under cyclic loading. The purpose of this article is to develop a method and obtain experimental data on decrease of the elastic characteristics of a fiber-reinforced laminate, as a result of progressive fatigue damage. The developed technique consists of two stages. At the first one, the natural frequencies and eigenmodes of the samples during their fatigue testing are experimentally obtained. The dependences of the natural frequencies of the samples on the number of loading cycles are found. At the second stage, the four elasticity parameters of the laminate monolayer (two Young modules, the shear module and Poisson's ratio) are identified via the natural frequencies. The inverse numerical/experimental technique for material properties identification is applied. The dependences of the natural frequencies and mentioned elastic characteristics on the relative fatigue life are obtained as experimental results of both modal and fatigue tests. The results can be useful to study the fatigue behavior of the investigated materials and to create methods for calculating fatigue life.

Abstract: Challenging structural applications such as customized jet engine parts are increasingly fabricated by Selective Laser Melting (SLM) of Inconel 718 powder. The as-built surface quality of SLM parts is however inferior of the machined version and the fatigue behavior is negatively affected. The as-built fatigue response of SLM Inconel 718 was quantified here using three sets of directional specimens. Since the surface quality is influenced by powder characteristics, process parameters and layer-wise fabrication, fatigue results showed a directional contribution that was interpreted using metallography and fractography.

Abstract: 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.

Abstract: The metal foils at micrometer scale are applied in micro-electromechanical systems (MEMS) and devices widely, which the mechanical behaviors of them are significantly different from that of bulk materials and thin films constrained by a substrate. In this paper the annealed polycrystalline Cu foil with two thickness (t =100, 150 μm) was applied on the cantilever beam bending fatigue testing as a model material. The fatigue properties and the damage behaviors of the annealed polycrystalline Cu foil at the total strain control was investigated. The results showed that the bending fatigue life of the polycrystalline Cu foil with the grain size (d =9.2 μm) was significantly larger than that of the Cu bulk and thin Cu films with t =3 μm under the same strain range. The fatigue damage formation of the extrusions/intrusions and cracks along grain boundaries on the Cu foil surface caused fatigue fracture and final failure.

Abstract: Direct Metal Laser Sintering (DMLS) is a complex process where a part is build-up by localized melting of gas atomized powder layers by a concentrated laser beam followed rapid solidification. The microstructure of DMLS produced material is substantially different from that of conventionally manufactured materials, although the ultimate strength is similar. However, yield strength and elongation and especially fatigue behavior may vary considerably according to the process parameters and post fabrication heat treatment because they affect structural heterogeneity, porosity content, residual stresses, and surface conditions. Fatigue tests of DMLS Ti6Al4V alloy are interpreted in the light of a thorough metallographic and fractographic investigation. The fatigue crack initiation for three different cyclic stress directions with respect to build direction is determined by fractography.

Abstract: In view of the significant role of material science in industrial production, the analysis and research on mechanical properties of materials and parts are of great significance to equipment production and machining. This paper, based on the finite element analysis of ANSYS software, will carry out the exploration and research on characteristics of material fatigue damage within mechanical materials and parts in a degree. Then, the general laws of material fatigue properties will be concluded from theoretical analysis, drawing of curve and equal-life curve as well as some comparisons between materials and parts. Finally, the correctness will be tested with the help of ANSYS software, which is an excellent finite element analysis software with abilities of modeling and simulation. Meanwhile, the whole process can provide somewhat valuable references and citations for practical applications, teachings, researches and academic conferences on material fatigue damage theory.