查看更多>>摘要：Shield tail sealing is a critical problem that influences work safety during shield tunneling processes. In recent years, the number of accidents caused by the failure of shield grease during shield tunnel construction has gradually increased. Taking Foshan Metro Line 2 accident as the research background, this study adopts the indoor model test and Ansys computational fluid dynamics (CFD) to estimate the grease sealing performance under the influence of environmental factors. The results indicate that, temperature increase, seawater, and sand mixing will greatly weaken the sealing performance of grease. In fact, the grease viscosity has been greatly weakened due to environmental factors. Using the numerical model, a simulation study was carried out on the sealing performance of the shield tail under the conditions of different grease viscosity and length of grease cavity. Grease viscosity and length of grease cavity have a linear positive relationship with the time of initial water leakage. Finally, a grease sealing performance evaluation model is proposed. Based on the above research results, the environmental impact coefficient k can be determined. The results of this study can be beneficial in both the selection and evaluation of the shield tail grease.
查看更多>>摘要：Infrared thermography is defined as the collection, display, storage, and evaluation of electromagnetic radiation in the infrared bandwidth of the electromagnetic spectrum using an electronic display device. Heat transfer is important in this case. The way heat is distributed can be by conduction, convection, or radiation. Infrared thermography is also used in the field of nondestructive testing (NDT) of technical elements. The article describes the methodology of measuring steel ropes. The dependence between temperature increase and increase in force was recorded by means of a thermal imaging camera. Individual results were displayed in form of thermograms, histograms and graphs of the temperature dependance on the load force of a sample of steel ropes. Based on these measurements, conclusions were drawn about the use of this method. Within the research it was identified an average limit of 43.01% of the minimum load capacity of the steel rope, when the temperature begins to rise, which can be monitored by a thermo-camera. It has been confirmed that the mounting of the thermo-camera affects the accuracy of the measurement. Based on the research results, it is possible to state the suitability of the infrared thermography method for the field of failure analysis of steel ropes.
查看更多>>摘要：Electrochemical tests were employed to study the chloride, sulfate and carbonate ions synergistically effects on pitting. The results showed that SO42- increased the pitting potential of 316L at first and then decreased, peaking at 0.2 M. In this case, the highest critical pitting temperature (12 degrees C), the breakdown potential (0.45 V-SCE), and the maximum impedance (257 k Omega cm(2)) were obtained. The existing CO32- raised the pitting potential but accelerated the general corrosion of 316L surface. The effective adsorption area based on the competitive adsorption mechanism of the anions was proposed to interpret the pitting and the passivation behaviors of 316L, and the properties of the passive film were comprehensive analyzed.
查看更多>>摘要：Creep deformation and rupture behavior of a martensitic heat resistant steel (10Cr-3Co-2W) welded joints prepared by vacuum electron beam welding for supercritical power generation unit are investigated employing traditional uniaxial tensile creep, 898 K (625 degrees C) to 948 K (675 degrees C) and 105 MPa to 250 MPa, and room temperature nanoindentation on different welding micro zones. The rupture location shifts from base metal (ductile rupture) for the shortest creep time of 74 h, to heat affected zone (brittle Type IV rupture) for the rest crept specimens. Accordingly, a softening phenomenon is observed in fine grain heat affected zone in crept specimens for longer creep times, where voids, precipitates and reduced dislocation density are found and further discussed. The extrapolating creep rupture time under various creep temperature and stress combinations and the 100,000 h creep rupture strength for different temperatures are predicted employing the Larson-Miller parameter method, which satisfies the industry service condition. Compared with tensile creep the nanoindentation creep yields faster creep rates and higher stress exponents; and the largest strain rate sensitivity value (m) in weld metal implies that weld metal has the best creep resistance while heat affected zone and base metal are more likely to appear creep rupture.
查看更多>>摘要：High-strength steel pipeline technology has been developing in the past decade in response to rapidly increasing energy demand. The unique metallurgical microstructure at pipeline welds, which is combined with a synergism of hydrogen and stress, makes the area prone to hydrogeninduced degradation, resulting in pipeline failure. This work developed a comprehensive review on hydrogen permeation, diffusion and trapping, as well as the resulting hydrogen degradation at pipeline welds. While major efforts have been made to study the hydrogen degradation behavior, there is still a big space on further investigation and understanding of the problem, especially for the high-strength steel welds. Conventional testing techniques are difficult to obtain convincing results to understand the mechanism for hydrogen degradation, especially at an atomic scale. Modern computational and modeling techniques provide promising alternatives to define the hydrogen diffusion and trapping and the pipeline degradation at the welds. It is expected that a combination of computational modeling and material characterization with an improved spatial resolution can advance our understanding about the hydrogen degradation of the high-strength steel pipeline welds.
查看更多>>摘要：The main goal of the article is to study, analyze and explain the most apparent cause of transient steam leakage from a high pressure turbine stage body. The analyses focus on transient heat transfer on the metal-metal flange connection of an external turbine casing and start-up tests. The coupled effect of thermal expansions and their impact on the residual bolt pretension force was analyzed in detail using FE models. Series of experiments were performed to confirm the results obtained by numerical calculation. The conclusion provided in this article can be used to improve the design of the flange connection working in variable temperature conditions.
查看更多>>摘要：The main goal of the article is to study, analyze and explain the most apparent cause of transient steam leakage from a high pressure turbine stage body. The analyses focus on transient heat transfer on the metal-metal flange connection of an external turbine casing and start-up tests. The coupled effect of thermal expansions and their impact on the residual bolt pretension force was analyzed in detail using FE models. Series of experiments were performed to confirm the results obtained by numerical calculation. The conclusion provided in this article can be used to improve the design of the flange connection working in variable temperature conditions.
查看更多>>摘要：The 3Cr steel used as the tubing has serious corrosion damage in a flowing CO2 environment with silty sand. Hence, the investigation for the influence of flow velocity on the corrosion behavior of 3Cr steel has an important and practical significance. It could not only unveil the corrosion damage mechanism of the steel under dynamic conditions, but also provide a theoretical basis for the corrosion protection of the tubing. In this paper, the effect of flow velocity on the corrosion film of 3Cr steel in a CO2 environment with silty sand was investigated by adjusting rotation speed. The results revealed that the corrosion resistance of 3Cr steel was first reduced, followed by enhancing and then reduced as the rotation speed increased. There was a critical rotation speed for the transformation of corrosion mechanism of 3Cr steel, i.e., 600 r/min. When the rotation speed was lower than the critical speed, the mass transfer process of corrosive medium dominated the corrosion resistance of 3Cr steel, and silty sand participated in the formation of corrosion film. The increasing rotation speed accelerated the mass transfer process, which first reduced and then improved the corrosion resistance. When the rotation speed was higher than the critical speed, the shear stress of silty sand played a predominant role. At the same time, silty sand impinged on the corrosion film, and then destroyed the stability of the film, leading to a significant decrease of the corrosion resistance. Silty sand changed from participating in the corrosion film to destroying the corrosion film with increasing rotation speed.
查看更多>>摘要：Historical steel bridges represent an important construction typology integrating the constructive heritage of the past century that needs to be preserved. Exposure to fatigue phenomenon, aging, improper design or execution, extreme events, and other aggressive environmental agents can seriously compromise the conservation and performance of these structures as some recent catastrophic collapses have shown (Mississippi River bridge, Minneapolis, Minnesota 2007; Kinzua Bridge State Park, Pennsylvania 2003). In this context, the diagnostic of historical steel bridges becomes of basic importance to identify and implement maintenance, monitoring and conservation strategies. Fault Trees are useful tools for practitioners that provide a complete overview of possible failures. In the related literature, this instrument is mainly applied with approaches that are based on previous experience or failures detected in similar structures, and so the Fault Tree can only provide qualitative support. This paper proposes a new method to achieve an "Analytical" Bridge Fault Tree linking the technician's experience and the numerical simulations of different fault scenarios. The latter are achieved by a numerical model able to consider fatigue failure and different concurring causes in the analysis (e.g., and aggressive phenomena, corrosion, defects, lack of maintenance). The proposed approach was applied to a historical railway steel truss bridge located in the East of Spain (Valencia Region) in order to show its applicability and potential.
查看更多>>摘要：One of the rivets connecting the frame and cover plate was broken unexpected in use. To analyze the fracture mode of this Al-Mg alloy pop rivet and prevent the similar events, scanning electron microscopy observation and chemical composition tests were carried out. Fatigue tests were performed to further find out the cause of failure of the Al-Mg alloy pop rivets. The failed rivets showed obvious fatigue fracture characteristics, whereas no corrosive medium could be tracked. The fatigue limit of Al-Mg alloy pop rivets was about 400 N. The stress concentration generated during riveting process and the maximum external forces beyond fatigue limit of aluminum rivets led to the initiation of plastic deformation zone. The low content of Mn and the possible existence of crevice between rivet core and rivet body may promote the initiation of plastic deformation zone. Under the action of continuous external load, the fatigue crack continued to expand, and finally caused the fracture of pop rivet. The effect of corrosive medium on cracking can be eliminated.
NSTL
Elsevier