查看更多>>摘要：Many industries utilize metals and alloys because of their exceptional properties, including high strength, conductivity, load-bearing capability, ductility, creep, and fatigue resistance. Among the metals and alloys, aluminum and its alloys are mostly subject to corrosion when encountering under severely adverse conditions that result in oxidation, failure of metallurgic luster, modifications in the sizes, strength, and changes in other physical and chemical properties. In this study, we produce superhydrophobic coated (SHC) aluminum 2024 alloy (AA2024) substrates for corrosion mitigation using a combination of physical and chemical modification processes. Plasma surface and heat treatment have been utilized for physical modification by forming nanoscaled roughness on the AA2024 substrates. To improve the surface hydrophobicity, chemical modification was achieved using low surface energy coatings. The corrosion behavior of plasma surface and heat-treated superhydrophobic coated (PSH-SHC) AA2024 substrates were evaluated by immersing into a 3.0% sodium chloride (NaCl) solution. The domination of plasma surface and heat treatment on the surface roughness, wettability, and corrosion resistance of the prepared AA2024 substrates was examined by applying water contact angle (WCA) measurements, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and salt soaking tests. The test results confirm that the PSH-SHC AA2024 substrates remain superhydrophobic with a WCA <= 168 degrees for an extended period of time with superior corrosion resistance in harsh environments. The WCA measurements slowly reduced from 168 degrees to 157 degrees after immersion in the 3.0% NaCl solution for 30 days. It demonstrates that the plasma surface and heat treatment mechanisms drastically enhanced the adhesive strength between the AA2024 substrate and the superhydrophobic coatings. The PDP and EIS results also showed that the corrosion rates of the 8H-PSH-SHC AA2024 substrate were undesirably low and raised with expending immersion time in the 3.0% NaCl solution. It is concluded that techniques applied in this study are found to be promising and critically important for a longer service time of the metals and alloys for broader industrial applications to mitigate the corrosion problems.
查看更多>>摘要：The design of double shear bolted connections in structural steel is governed by four different failure modes; tear out, splitting, net-section, and bearing. Ten machine learning (ML) approaches were explored on a comprehensive database of 455 experimental results for identifying the failure modes of double shear bolted connections. Among them, Random Forest (RF), CatBoost, XGBoost, and Gradient Boosting (GB) attained 90-92% accuracy on the testing dataset for classifying the failure modes. The best-performing models revealed that the ratio of the edge distance-to-bolt diameter (e2/d0) is the most important feature with an influence of nearly 30% on the failure mode of the connections. Interestingly, the number of bolt rows in a connection also influences the failure mode, which was not captured by existing equations and design codes. Finally, a user interface capturing all proposed ML models was developed to identify the failure modes of double shear bolted connections.
查看更多>>摘要：The mechanical behavior in liquid lead of the 15-15Ti steel coated by an alumina layer and an Alumina Forming Austenitic (AFA) steel, with a chemical composition allowing for the formation of alumina at the surface of the steel, has been studied. To investigate the liquid metal embrittlement (LME) sensitivity by liquid lead, tensile tests in air and in liquid lead have been carried out at 400 ? and 500 ?. Then analyses of the cracking and of the fracture surfaces have been performed. No LME sensitivity at the tested conditions was observed for the Al2O3 coated 15-15Ti steel. The AFA steel is not sensitive to LME at 400 ? but suffers from LME at 500 ?. For the AFA steel, the liquid lead promotes at 500 ? an intergranular propagation of surface cracks which suggests a grain boundary wetting-dominated LME mechanism of this steel at 500 ?.
查看更多>>摘要：In recent years, there have been frequent gas explosion accidents in China. On October 21, 2021, a gas explosion occurred in Northeast China, resulting in 5 deaths and 52 injuries. To further improve accident investigation theories and methods, with the help of key accident investigation technologies, this study carried out traceability, investigation, and failure analyses regarding the above-mentioned typical natural gas leakage and explosion. The investigation showed that natural gas leaked from a gas pipeline almost filled the entire accident building within 3 h and 25 min and the gas concentration was between 7 and 9 vol% before the accident. An electric spark generated by the action button of a refrigerator was the key ignition factor. The peak explosion overpressure near the ignition was 57 kPa and peak temperature reached 1620 K. The outdoor explosion venting overpressure less than 30 kPa, but the venting airflow velocity exceeded 90 m/s, with many items and fragments thrown outside, resulting in three deaths and many injuries. In addition, based on calculations of gas concentration and diffusion volume, the total amount of natural gas leakage in the accident reached 150 m(3). Based on the TNT equivalent method, the TNT explosion equivalent of the accident was 80 kg. These investigation results ideally restored the entire explosion accident process and the research methods and failure analysis ideas developed here will provide scientific guidance for the investigation of such accidents and evaluation of explosion effects.
查看更多>>摘要：Reinforced concrete cover plates are the main engineering protection measure for oil and gas pipelines in densely populated areas with high consequences. However, they have disadvantages such as high cost, difficult construction, long construction time, and inability to promptly and effectively warn mechanical operators. On-site excavation tests were carried out in order to verify the warning performance and impact resistance of a high-strength flexible cover. In the warning performance verification test, when an excavator hits the high-strength flexible cover plate set above the oil and gas pipeline during operation, the excavator driver should immediately receive obvious warnings that there are obstacles underground and that the digging should be stopped immediately. In the impact resistance verification test, the high-strength flexible cover plate should provide sufficient mechanical protection to withstand the penetration of the cover plate by an excavator weighing at least 30 tons and to avoid the damage of a pipeline below. At the same time, it is necessary to determine the one-time impact depth of the excavator bucket by designing the working condition under which the high-strength flexible cover is not buried in the soil, and is affected by the vertical action of the excavator bucket. This depth value serves as an important data support for the minimum safety distance between the high-strength flexible cover and the oil and gas pipeline. This paper proposes the application of a new type of the high-strength flexible cover plates instead of reinforced concrete cover plates above pipelines. Field tests verified the important role of the high-strength flexible cover plates in the protection of oil and gas pipelines. When long-distance oil and gas pipelines pass through densely populated high-consequence areas, they can replace traditional reinforced concrete cover plates to achieve an effective pipeline protection. At the same time, they have a simple structure, their construction is convenient, they are reliable and stable, and economical and rational. By using the high-strength flexible cover plates, savings that exceed 70% of the project investment may be achieved.
查看更多>>摘要：Highly stressed aerospace structures, such as the wing attachment points and load carry-through bulkheads of fighter aircraft, often grow fatigue cracks from micro-scale discontinuities. These cracks spend much of their lives close to these discontinuities in the small crack regime. The accurate prediction of fatigue crack growth in this small crack regime leads to setting realistic inspection thresholds for the aircraft and assisting in accurate risk-based assessment of the lives of such cracks. Fractography can greatly aid the assessment of the growth of small cracks. It has traditionally been carried out using optical and Scanning Electron Microscopes to gather the necessary small crack growth rate data, however, limitations on the resolution and the surface sensitivity of these instruments restricts the gathering of crack growth rate data at small crack lengths. As an extension to the current capabilities, the newly introduced Helium Ion Microscope offers improved nano-scale resolution and topographical sensitivity. As a result, the Helium Ion Microscope offers the potential for both quantitative and qualitative analysis of small fatigue crack growth about nucleating discontinuities at resolutions previously unobtainable. This paper presents an overview of the technology and gives three different fractographic examples of its possible applications to failure analysis. These examples include extension of small crack fractography, extension of near initiation fractographic feature analysis, and extension of nano-scale feature analysis. To demonstrate these capabilities, fatigue fractures produced by experimental loading spectra that were applied to the aluminium alloy AA7050-T7451 have been investigated.
查看更多>>摘要：In an oil field, numerous tube bundles in the reboiler at the bottom of a regeneration tower cracked and leaked after only 2.8 years of service. Analysis was conducted on the material properties, failure characteristics, and corrosion products of the cracked 316L stainless steel tube bundles, and the causes of the cracking of the tube bundles of the heat exchanger were clarified. The results indicated that the failure of the tube bundles was caused by typical stress corrosion cracking (SCC), and the cracking originated from the pitting corrosion that occurred on the exterior surface of the tube bundles. The chemical composition, metallography, and mechanical properties of the failed tube bundles met standard requirements. The synergistic effect of chloride ions and oxygen in the solution was the main cause of the SCC of the heat exchanger tube bundles.
查看更多>>摘要：This paper improves the method of high-efficiency calibrating the meso-parameters considering the concrete aggregate gradation, and carries out numerical simulations of four-point bending strength test and direct shear strength test on concrete beams. Firstly, unconfined uniaxial compression test carried out to calibrate the meso-parameters of C40 concrete based on DEM. Secondly, the accuracy of the numerical model is verified according to the stress-strain curve and failure morphology of concrete prism. Finally, the crack propagation and load-displacement curves of concrete beams under four-point bending strength test and direct shear strength test were analyzed. The result show as follow: & x2460; The DEM numerical model which take into account the concrete as a three-phase composite material can well realize the mechanical property of concrete. The display of force chains and micro-cracks greatly contributes to the concrete failure mechanism. & x2461; The numerical model is generated by the random distribution of particles, which shows the characteristics of concrete heterogeneity, and defines the contact model between particles with parallel bond model (PBM). Compared with the numerical simulation and experiment results, the error is within 10.4%; & x2462; In four-point bending strength test, the crack initiates from the bottom edge of the beam and propagates upward. With the increase and coalescence of micro-cracks, only one concentrated, wide and long macro-crack is formed; In direct shear strength test, however, the high shear stress of two shear surfaces resulted in the generation of cracks. With the one side crack coalescence, the other side of crack initiate follow, and finally two coalescence macro-cracks are formed.
查看更多>>摘要：Interfacial wear damage is one of the primary causes of joint failure, especially in the marine environment since material aging greatly aggravates the wear damage between the contact interfaces of joint. In this paper, interfacial wear damage mechanism of CFRP/Ti-alloy single-lap bolted joint after long-term seawater aging was researched through the wear experiments and material microstructure analysis. The results show that CFRP demonstrates strong tribological anisotropy where 0 degrees wear shows more serious wear damage than 90 degrees wear. The wear damage of contact interface can be inhibited by surface treatment of Ti-alloy through reducing the hard contact. Long-term seawater aging has significant impact on the wear damage evolution of CFRP. The aging time and temperature exaggerate the hygrothermal effect, weakening fiber-matrix interface and intensifying the wear damage. The influence of salt concentration depends more on the presence of salt ions rather than their concentration.
查看更多>>摘要：The present paper investigates crack growth in specially designed specimens of 6061 aluminium alloy sheets subjected to dynamic shear loads. The specimens were tested with a low-frequency alternating shear load until complete fracture. The main objective of this work was to observe the crack initiation and growth under alternating shear load using the Digital Image Correlation (DIC) method. Each time the peak amplitude was reached, a photograph of the shear area on the specimen surface was taken to investigate crack grow. The length of the crack in every load cycle was then estimated by an algorithm using the DIC strain fields as input data. The DIC results were compared to FE. The observation of the shear strain component is presented as the most appropriate method to predict the location of the crack even before its initiation.
NSTL
Elsevier