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The high-temperature oxidation behaviors of Co- and CoNi-base alloys with different Ni and Cr additions were investigated by isothermal oxidation in air at 900 and 1000 °C. The structure, composition, and element distribution in the three layers within the oxide scales in different alloys have been explored in detail by electron microscopy, especially with regard to the fine oxide particles in the intermediate layer. Based on the microstructure and phase constitution of the oxide scales, the synergetic effects of alloying elements on the oxidation mechanisms and resistance have been elucidated in detail with the aid of CALPHAD calculation.
Influence of alloy composition and thermal history on carbide precipitation in γ-based TiAl alloys
(2017)
Die Ergebnisse aus drei Traglastversuchen und einer umfangreichen numerischen Parameterstudie (FE-Berechnungen) zum Tragverhalten von Wellstegträgern mit quadratischen Stegöffnungen werden vorgestellt. Es werden versteifte Stegöffnungen mit einem umlaufenden Versteifungskranz betrachtet. In der Parameterstudie werden die Einflüsse der Stegöffnungsgeometrie, der Trägergeometrie und der Position der Stegöffnung auf die Querkrafttragfähigkeit der Wellstegträger untersucht.
Atomic site occupancy of alloying elements and Laves phase stability in γ-γ′ Co-base superalloys
(2022)
The atomic site occupancy of alloying elements in the ZrCo2 and HfCo2 based Laves phases formed in the Co-9Al-9W-2Zr and Co-9Al-9W-2Hf alloys was experimentally determined by the combined techniques of energy-dispersive X-ray spectroscopy mapping with atomic resolution and the atom location by channeling enhanced microanalysis (ALCHEMI) method. The results show that both Al and W occupy the Zr site in the C15 and the Hf site in the C36 crystal structure. The atomic locations of Al and W were further confirmed by comparing calculated inelastic cross-sections with experimental ALCHEMI results. Using the determined atomic site occupancies of alloying elements, special quasirandom structure solid solution models with 192 atoms were constructed and employed in first-principles calculations. It is found theoretically that the energy of formation of the C15 structure is always lower than that of the C36 structure at 0 K in both ZrCo2 and HfCo2 phases, no matter whether Al and W are incorporated or not. However, further ab initio molecular dynamics simulations suggest that lattice vibration at finite temperature contributes significantly to the phase stability, stabilizing the C36 structure compared to the C15 type for the (Hf, Al, W)Co2 phase at finite temperature, which fits well with the experimental findings.
The lateral-torsional buckling resistance of welded steel girders is affected by their residual stresses and geo-metric imperfections. Generally, both influences are mainly controlled by the manufacturing process, in partic-ular the welding of the girder. This paper presents experimental investigations on four laterally unbraced welded steel girders with thin-walled I-sections, whose flange-to-web junctions are welded from one or two sides to evaluate the fabrication influence. The investigations include lateral-torsional buckling tests under combined bending and torsion loading caused by three-point bending due to an eccentric vertical single force. The geometric imperfections of the welded test girders are determined by 3D laser scanning. The test results show that the influence of the geometric imperfections and the residual stresses on the lateral-torsional buck-ling resistance is limited for the tested girders.
Developed numerical Analysis of Residual Stress caused by Welding and Cutting in Steel Structures
(2023)
The welding and flame‐cutting process in the steel construction are commonly used, especially for the bridge structure. Some experiments about welding and cutting have been done separately by some researchers. The results from the experiments were also compared with the results from the numerical simulation under 3D or 2D models. The agreements between those results are as good as they predicted. Therefore, the numerical simulation is a better way to analyze the quality of the steel construction after welding and cutting without time‐consuming and a large amount of cost. But there are only a few research about the numerical simulation and the some of them are aimed to verify the experiments. In this study, the calculation about the residual stresses after welding and thermo‐cutting are considered from 3D‐model into 2D‐Model. The heat source model is simply regarded as curves of time and welding energy with the algorithm methods. And no matter the change of the parameters of the model and the heat source, the heat transfer and the residual stresses can be calculated with the 2D models as fast as possible. By the means of the finite element software Abaqus, the I‐profiles are easy to receive the job results with the input parameters. The influence of the welding and cutting process are visual and direct.
Corrugated web beams are welded I‐sections with thin‐walled sinusoidal corrugated webs and flanges made of wide flat steel that have been developed as an alternative to steel girders with flat webs. In comparison with hot‐rolled or welded I‐beams with flat webs, corrugated web beams are connected with considerable material savings and fully automated production. Assuming the same load‐bearing capacity, corrugated web beams are lighter than those with a flat web, since the corrugated web beams can often be loaded up to their plastic resistance without reinforcement of the web. The research on the load‐bearing behavior of corrugated web beams without web openings is relatively mature. However, the resistance of corrugated web beams with reinforced web openings, as they are often used in buildings for installations, has not well experimentally been investigated, yet. This paper deals with the shear resistance of corrugated web beams with reinforced square web openings. Three specimens of different geometry with two square web openings were tested. The experiments are compared with a Finite Element Analysis by Abaqus® and with an analytical model allowing simplified calculation. Both models can relatively accurately calculate the shear resistance. The experimental and numerical results show that the increase of the shear resistance of perforated corrugated web beams depends on the wall‐thickness and the steel grade of the web opening reinforcement.
Biological soil crusts serve as a vanguard for improving the ecological environment in arid, semi-arid desertification areas.It is a good indicator of the level of improvement which the local ecological evironment is undertaking. In desert areas, water condition is a key factor of improving the ecological environment. As a first layer protection, biological crusts play an important role in local vegetation succession due to their abilities to conserve and maintain moisture. Using Maowusu desert in Yanchi of Ningxia province as an example, after three years of research, this paper chooses three kinds of biological crusts including lichen, moss and cyanobacterial which are under the cover of Artemisia ordosicaas research objects. The results of this study indicate that, the closer biological crusts are to Artemisia ordosica vegetation, the thicker they become. In the same position of Artemisia ordosica vegetation, the
thickness of moss crusts is the highest, followed by lichen crusts, and the thickness of cyanobacterial crusts is the lowest. Biological soil crusts coverage protects the natural water content of soil layers from 0 to 5cm. Also, it effects falling water to infiltrate deeper, and cannot prevent the surfacewater content from evaporating effectively. The effect of biological crusts blocking water infiltration decreases with the increase of rainfall. At the same rainfall level, moss crusts provide the strongest water infiltration blockage, followed by lichen crusts and cyanobacterial crusts. With the increase of rainfall, the depth of water infiltration increases.At the same rainfall level, the relationship of water infiltration depth is as follows: cyanobacterial crusts > lichen crusts > moss crusts. With the increase of biological crusts thickness, they blocking water infiltration capacity is stronger, and the depth of water infiltration is smaller. Analysis on the characteristic of simulated rainfall process on biological crusts shows that sandy land can be fixed by applying appropriate artificial biological crusts to build a sustainable forest protection system and to create a stable ecosystem in desertification area.
Geometric Properties of Steel Components with Stability and Fatigue Risks Using 3D-Laser-Scanning
(2024)
Nowadays, 3D laser scanning technology is extensively employed in laboratory investigations of steel structural components, providing accurate geometric dimensions to reduce uncertainties caused by indeterminate geometry in experimental results. It is often used in conjunction with the Finite Element (FE) Method and analytical solutions, which are more accurate deterministic operators in the research on steel structures. However, establishing a common methodological framework for transferring or mapping 3D-scanned information into finite element models for complex steel structures with stability and fatigue risks remains an ongoing task. In light of this, this study has developed a 3D scanning platform capable of obtaining accurate geometric dimensions for various types of steel components. Different coordinate systems and point cloud mapping algorithms have been established for different types of components to construct actual finite element models with initial imperfections. The feasibility of the self-developed 3D scanning platform and finite element modelling has been validated through three experimental cases: weld details, steel girders, and cylindrical shells. The research findings demonstrate that the captured point cloud can be automatically processed and corrected using the developed algorithm. The scanned data can then be input into the numerical model using various mapping algorithms tailored to the specific geometric properties of the specimens. The differences between the experimental test results and the simulated results obtained from the 3D-scanned finite element models remain within a small range. The self-developed 3D scanning platform and finite element modelling technique effectively capture the actual dimensions of different steel components, enabling the prediction of their stability and fatigue risks through numerical simulations.
Stacking fault formation in perovskite Ti3AlC carbides in a TiAl based alloy during creep at 800°C
(2023)
Corrugated web beams are an alternative to steel girders with flat webs. They consist of a welded Isection with a thin-walled trapezoidally corrugated web and flanges made of wide flat steel. In comparison with hot-rolled or welded I-beams with flat webs, corrugated web beams offer significant material savings and are produced fully automated. Assuming the same load-bearing capacity, corrugated web beams are lighter than those with a flat web, since the corrugated web beams can often be loaded up to their plastic resistance without reinforcement of the web. Research on the loadbearing behaviour of corrugated web beams without web openings is well-established. However, the resistance of corrugated web beams with reinforced web openings, which are necessary for installations in buildings, has not been well investigated experimentally yet. This paper examines the shear force resistance of trapezoidally corrugated web beams with reinforced square web openings. Two beam-like specimens with the same geometry but differently reinforced web openings were tested. The experimental results are compared with a Finite Element Analysis using Abaqus® and with an analytical model that allows a simplified calculation. The comparison identifies how the wall thickness of the web opening reinforcement affects the shear force resistance of perforated corrugated web beams.