查看更多>>摘要：? 2022 Elsevier LtdIn this study, the effects of hydrogen on the fracture toughness and crack propagation of X80 steel base metal and girth weld were assessed. The crack tip opening displacement (CTOD) tests were conducted under in-situ strong cathodic current hydrogen charging in simulated soil solutions and compared to the ones in air. Fracture surfaces and cracks were observed using scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD). Under in-situ cathodic hydrogen charging conditions, pre-hydrogen charging did not substantially affect the test results of fracture toughness, and hydrogen could quickly achieve the crack tips and exert its effect on crack propagation. In hydrogen-containing environments, X80 steel base metal and girth welds both experienced a decline in fracture toughness. The reduction of the fracture toughness in the base metal increased with increasing current density, reaching 33.2% at a current density of 50 mA/cm2. In turn, that of the girth welds remained almost unchanged, exceeding 45% at the current densities of 10 and 50 mA/cm2, indicating that the girth weld was more susceptible to hydrogen. The difference in the effects of hydrogen on the fracture toughness of the base metal and the weld was attributed to specific microstructural features of both materials. Compared with the weld, the base metal exhibited more refined microstructure, the higher HAGB fraction and the larger grain boundary density, which was conducive to crack arrest performance in hydrogen-containing environments.
查看更多>>摘要：? 2022 Elsevier LtdThe present work aims to explore the potential significance of microalloying with Nb in enhancing the resistance to hydrogen-induced delayed fracture (HIDF) of a novel 1300-MPa-grade V-microalloyed high-strength bolt steel 42CrNiMoV using constant load tensile test. The results show that ～ 82% of the added 0.03 wt% Nb is consumed to form NbC carbides which causes significant grain refinement and thus an overall strengthening of ～ 60 MPa. The effective hydrogen diffusion coefficient is reduced by ～ 39% and the corrosion resistance is significantly improved through the Nb addition. The Nb-added steel has enhanced tolerance of diffusible hydrogen content and its resistance to HIDF is enhanced by ～ 13%. The mechanism of Nb in enhancing the resistance to HIDF was discussed. It is suggested that microalloying with Nb is an effective approach to further improve the HIDF resistance for high-strength bolt steels.
查看更多>>摘要：? 2022 Elsevier LtdVariations in gear tooth meshing stiffness and tribological characteristics caused by tooth root cracks seriously affect the dynamic performance of gears, limiting the development of durable high strength gears. Therefore, the potential energy method was applied to obtain the time-varying meshing stiffness of the spur gear pair, and the finite element method was applied to verify the calculation results. Secondly, the load distribution of the gear with the cracked tooth root was derived using coordinated deformation conditions, and the effect of the tooth root crack on the tribological characteristics of the tooth surface was obtained based on the elastohydrodynamic lubrication (EHL) model. Finally, the tribo-dynamic behaviors of the gear under the action of the root cracks were investigated. The results indicated that root cracks change the load distribution ratio in the double-tooth meshing zone, which leads to corresponding changes in the tooth surface film thickness and friction coefficient. In addition, when the cracked teeth are engaged in meshing, the reduction in meshing stiffness causes vibration and shock, which causes the appearance of sidebands near the main meshing frequency. The amplitude of the dynamic meshing force increased with the increase in the degree of root cracks.
查看更多>>摘要：? 2022 Elsevier LtdThe mesh stiffness characteristics and tooth root crack propagation of planetary gear system not only have an important impact on its transmission error, but further affect its vibration and noise level and the bearing capacity of gearbox. It is of great theoretical and engineering significance to study them comprehensively. First, the stiffness-damping model of the planetary gear mechanism is obtained by simplification, and the meshing phase relationships between the studied gear pairs are gotten by theoretical calculation. Then, through the FE modeling, the time-varying mesh stiffness of the sun-planet substructure, ring-planet substructure, and single-branch sun-planet-ring substructure are calculated respectively, and the influence of carrier and bearing flexibility on mesh stiffness is further studied. The mesh stiffness prediction method of the single-branch substructure is proposed and verified, and the overall mesh stiffness of the planetary gear system is predicted and analyzed. Finally, a sensitivity analysis model is established to study the effect of crack propagation at the root of sun and ring on mesh stiffness, capacity, and NVH features. The results show that the sun tooth root crack has a more significant impact on the mean stiffness and bearing capacity, and the ring tooth root crack has a more dramatic influence on the peak-to-peak stiffness and the vibration and noise characteristics of the gear system.
查看更多>>摘要：? 2022 Elsevier LtdIn this paper, a new framework for accurate reliability analysis is proposed based on improving the directional simulation by using metaheuristic algorithms. Usually for highly nonlinear and complex performance functions, finding the unit vector direction requires very high calculations or impossible practically. Hence, the novel improved version incorporates the Harris Hawks Optimization algorithm, where the unit vector of direction is formulated as a constrained optimization problem and estimated using metaheuristic algorithms. Given that metaheuristic algorithms have been introduced to solve unconstrained problems, the penalty function method is used to convert the constrained problem into an unconstrained problem. The applicability of the proposed framework is firstly tested on five highly nonlinear benchmark functions and then applied to solve four high-dimensional engineering problems. The performance of six simulations-based reliability analysis methods and the first-order reliability method were compared with the proposed method. Besides the feasibility of other metaheuristic algorithms were investigated. The results show high-performance abilities of the improved version of the directional simulation for solving highly nonlinear engineering problems.
查看更多>>摘要：? 2022 Elsevier LtdThis study investigated creep deformation behavior of a porous polymer membrane under biaxial stress by using the small-punch test. We first conducted a mechanical test for creep deformation, using uniaxial tension to examine creep compliance, which revealed that creep compliance was strongly dependent on applied stress in a nonlinear creep-deformation relationship. Using the time–temperature–stress superposition principle (TTSSP), we obtained a master curve that unified the compliance curves for various applied stresses. Next, a small-punch test was conducted to apply biaxial stress to the membrane. This method was also able to apply a stress gradient to the membrane during the test. Creep deformation due to the small-punch test was found to be dependent on the applied load. Additionally, we created a homogenous model based on the finite element method (FEM) and the TTSSP of nonlinear creep theory. The FEM model predicted creep deformation behavior during a small-punch test for various load levels with good accuracy.
查看更多>>摘要：? 2022 Elsevier LtdThis paper presents the results of an in-situ test and experimental study on a reinforced concrete wet quenching tower that had been exposed to high temperature and dry-wet cycles for 34 years. Fourteen concrete cylinder samples were drilled along the height. The variations in the carbonation depth, compressive strength, pH, phase compositions, substance contents and morphologies of concrete along the height and depth of the tower were tested. The carbonation depth of concrete at the top of the tower was 2.3 times greater than the bottom, and the compressive strength of concrete at the top was only about 30% of the bottom. Variations of pH with the content of CaCO3 in different depths have been quantified. The pH at different depths of concrete in the severe dry-wet cycle zone fluctuated around 9.0, the content of CaCO3 was around 8%. Calcium carbonate coexisted with gypsum in the transition zone of carbonated concrete. A modified stochastic carbonation depth model is proposed for concrete exposed to high temperatures with dry-wet cycles and was in acceptable agreement with natural carbonation test results.
查看更多>>摘要：? 2022 Elsevier LtdFixed bollards are very significant elements in providing urban roadside safety. These structures, called passive systems, allow pedestrians and disabled people to use sidewalks effectively and safely, as well as they have a crucial role in protecting the above-ground assets of infrastructure facilities against run-off-road (ROR) crashes. In this study, structural and safety performance analyses of fixed bollards that protect natural gas above ground Reducing and Metering Station-B (RMS-B) type regulating stations used in many points in Istanbul were made. In this regard, finite element (FE) models of the steel pipe of the fixed bollard, the concrete of the foundation and the soil where the bollard was mounted were created. Existing fixed bollards have been verified with accident data. In the light of the calibration and validation, numerical models were created and combined with the vehicle models specified in PAS 68:2013 standard to simulate dynamic behaviours. The combined FE models were analysed applying LS-DYNA software. As a result of the numerical analysis, the fixed bollard system cannot safely stop the 30000 kg N3, 18000 kg N3 and 7500 kg N2 class vehicles, and cannot prevent them from damaging the RMS-B. Besides, the fixed bollard system cannot provide safety at the speed level of 48 km/h, considering the urban speed limit for 3500 kg N1 and 2500 kg N1G class vehicles. However, the fixed bollard system can safely stop 1500 kg and 900 kg M1 class vehicles up to 64 km/h speed limit. These analyses revealed that considering the regional traffic characteristics of Istanbul, the existing fixed bollard systems are not sufficient for the protection of hazardous roadside facilities and need to be improved. Full-scale crash tests should be carried out in future studies to validate the FE simulation models. Furthermore, various bollard designs should be developed, especially urban road requirements.
查看更多>>摘要：? 2022 Elsevier LtdFatigue fracture took place on a double-row tapered roller bearing outer rings applied in coal loaded train. The fracture is started from one side of the outer ring representing the possible overload condition of the axial or thrust force. The metallurgical investigation includes the hardness test, visual observation, microstructure investigation, and composition. Detailed metallurgical analysis discovered that the defects mainly occurred on the outer ring: (1) the color-changing phenomena might be occurred because of the temperature increase caused by the continuous friction load on the one side of the outer ring raceway, (2) The thermal softening resulting from the continuous load causing decrease in hardness as well as the decrease of the tensile stress. The crack origin is from one side of the bearing and propagate to another side. The thermal softening resulting from possibly continuous applied fatigue and a sudden impact is probably the leading responsible cause of the initial crack. The thermal softening occurrence consequently leads to a decrease in the surface hardness producing low tensile strength. Then, the sudden impact with considerable high force or acceleration caused by the track topology or other factors can cause the initial crack followed by the fracture of the outer ring. In sum, the failure of the bearing is investigated, and the original crack probably occurs because of the high temperature and the axial force overload.
查看更多>>摘要：? 2022 Elsevier LtdHistorical buildings play a monumental role in religious communities around the world. Masonry structures such as minarets built in areas prone to dynamic effects such as earthquakes are vulnerable as a consequence of their brittle materials and slenderness. Structural health monitoring (SHM) has not been carried out in the regular control of minarets’ health for early warning of collapse. Meanwhile, the protection and preservation of such a cultural heritage are urgent. Effective techniques to recognize damage occurrence and location in minarets are vital. In this study, a health monitoring solution on damage detection of minarets is presented. In particular, damage detection of the minaret (24.25 m high) of Hac?lar mosque that was built using stone and brick in 1467 in Bursa city (Turkey) is aimed. Only four accelerometers are erected along with the height of the minaret to collecte lateral displacements, making an incomplete number of measured degrees of freedom (DOFs). To circumvent the complicated problem, the minaret is transferred into a lumped mass system correspondingly to the erected sensor network considering the similarities in terms of bending behavior at low modes between slender minarets and beam-like structures. Due to the changes in modal data at two bending modes, an eigenvalue problem-based inverse solution is employed to localize damage in the minaret based on the reduction in terms of stiffness of the simplified system. As a result, reliable damage detection is achieved as long as the modal data is noise-free or low-level noise-contaminated (about 0.5%-1% random noise level).
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