查看更多>>摘要：In order to effectively control the large deformation of the upper structure during the excavation of the shallow-buried underpass tunnel, it is necessary to find out the reasons for the failure of the upper structure. In this study, the large deformation and failure of the box-frame channel subjected to the disturbance of an under-construction shallow underpass tunnel are taken as the research object. Through on-site deformation monitoring, defect detection and numerical simulation, the main reasons for the structural failure of the upper channel are discussed. In addition, the feasibility prediction of the channel reinforcement scheme is carried out through numerical simulations. The results show that: (1) failing to implement the ground beam reinforcement that can strengthen the channel foundation stiffness and control the differential settlement is one of the causes of structural disaster; (2) the broken soil mass at the bottom of the channel contains a lot of pore water that can take away a lot of fine particles in the soil during tunnel excavation, resulting in the decline of the bearing capacity of the foundation; (3) the defects of the channel are mainly distributed above the central axis of the underpass tunnel, and the deformation of the channel structure is accelerated when the tunnel is excavated beneath the channel, indicating that the tunnel excavation is the most direct cause of the large deformation and failure of the channel; (4) the distance between the tunnel and the channel is extremely close in this project, which was one of the causes of the channel failure; (5) the temporary support of the steel pipe pier can effectively control the vertical displacement of the channel and offset the additional internal force generated by the uneven displacement of the foundation. The proposed permanent reinforcement measures have good feasibility and can provide guidance for similar projects.
查看更多>>摘要：This study investigates the dynamic response, damage mechanisms, and residual capacity of precast hollow reinforced concrete beams with prestress tendons (HRCBPT) under impact loads by using High-Fidelity Physics-Based (HFPB) finite element (FE) models. The accuracy of the numerical model developed in this study has been fully calibrated against two different drop-weight impact tests available in open literature, including a scaled hollow reinforced concrete beam (HRCB) and a full-scale reinforced concrete (RC) beam. The results in this study show that under low to moderate impact loads, the flexural cracks and global deformation of the HRCBPT are similar to the hollow beam with normal reinforcement. Under high impact loads, although these beams exhibit similar diagonal shear cracks near the impact location, the HRCBPT experiences more local concrete spalling damage on the bottom flange, higher residual capacity index, and smaller reaction force at supports owing to the contribution of the initial prestress force. The results also suggest that the maximum compressive stress at the bottom flange of the section should be smaller than 17.6% of its compressive strength in order to maximise the beam's impact-resistant capacity, while the prestress level in the tendon should be lower than 65% of the tendon yield strength to prevent premature failure under moderate impact conditions. Furthermore, different combinations of the impact velocity and impact mass would generate different impact force profiles on the beam resulting in dissimilar impact responses and residual capacity. In the design analysis, together with the critical impact location at the mid-span, the impact location close to the supports which may cause critical shear response of the beam, also needs be considered. In this study, the impact in the vicinity of the support yields the most severe damage to the beam resulting in the lowest residual capacity.
查看更多>>摘要：The armor rod (AR) is used to repair the damaged overhead ground wire (OGW) and prevent the wear of the OGW in the suspension clamp in engineering. Once the OGW at the AR terminal breaks, it may lead to temporary blackout, which also causes the severe economic losses, or even threatening public safety. However, there are few literature or report about this type of accident at present. Therefore, based on a rupture accident of OGW at the AR terminal, the reason for this accident was investigated in this paper. Firstly, the description of the accident background and the material analysis for the accident sample were presented to preliminarily determine the damage characteristics of OGW and the cause of the accident. Secondly, simulation experiments for the transient temperature rise of the AR terminal were conducted. The macroscopic morphology characteristics of the experimental samples were also analyzed. Finally, based on the electromagnetic thermal coupling, a finite element model of the AR terminal of OGW was established. The overheating mechanism at the AR terminal was also discussed according to the model. The discharge occurrs at the AR terminal under the short-circuit current (SCC), and it is also the prime factor that causes the high temperature of OGW. The research methods and conclusions in this paper can provide theoretical guidance for the analysis of similar accidents and the optimization design of structures.
查看更多>>摘要：To evaluate the service life of spur gears and design high-performance gears, fracture characteristics must be examined. Since, asymmetric spur gears are not standardised, the majority of the existing literature studies of spur gear fracture behaviour are based on symmetric spur gears. The research of spur gear with asymmetric teeth is attracting attention as the industry seeks to improve load transmission capabilities and service life. This paper investigates the combined effect of pressure angle and other gear parameters suchs as module, no.of teeth and gear ratio on the fracture characteristics of asymmetric spur gears subjected to mix mode fracture. The stress intensity factor (SIF) at the fillet crack tip along the face width for each gear pair is determined based on strain energy release rate approach. An increase in pressure angle reduces the contact ratio and top land thickness which is not recommended for smooth operation of spur gears. Hence, combined effect of pressure angle with the other gear parameters such as module, number of teeth and gear ratio on SIF to improve resistance towards fracture is addressed. Further, the effective SIF and influence of mode I, mode II and mode III on effective SIF for each gear pair are explored. This study aims to provides valuable input to design asymmetric spur gears and to select the gear parameters.
查看更多>>摘要：Buried high-density polyethylene (HDPE) pipelines have been extremely vulnerable to terrorist attacks and third-party damage in recent years, so it is important to study the vibration response and safety criterion of buried HDPE pipelines under the action of surface blast loads. In this paper, based on the full-scale field tests, the damage characteristics of HDPE pipes under the action of surface blast loads are studied. Next, a 3D numerical calculation model was established to analyze the response characteristics of the buried pipes subjected to surface blast loads and its reliability was verified using the field monitoring data. Then, the safety of the pipes under different blast load conditions was studied in combination with numerical simulation software, and different damage models of the pipe were established based on damage theory. The results show that the peak particle velocity (PPV) of both the pipe and the surface increases as the distance from the explosive decreases, and the vibration velocity of all measurement points is dominated by the vertical direction (Y). Meanwhile, as the surface explosive is triggered, the pipe is prone to overall bending deformation in the axial direction, resulting in the destruction of the pipe. Finally, based on the comprehensive numerical calculations, numerical simulation results were obtained for different damage levels of the pipes under surface blast loading. What's more, the damage prediction model of the pipes is also proposed, and the critical curves corresponding to different damage levels are derived.
查看更多>>摘要：The fatigue phenomenon has been historically related to the railway industry. Nowadays, thanks to the high quality of the materials used, most of the recorded problems have been overcome although there are still degradation processes which are associated to fatigue and need to be considered for the definition of the maintenance campaigns and the general progress of the sector. In this paper, the latest improvements in the isotropic damage based high-cycle fatigue constitutive law proposed by Oiler et al. (2005) are presented and the approach is used for the study of two regions of the railway path where fatigue mechanisms are experimentally detected: a straight section and a crossing element. The analysis of the affected areas is performed through a finite element simulation identifying the critical regions liable to the fatigue degradation when the structure interacts with high speed vehicles and predicting the initiation of the degradation at the rail head while capturing the physics of the problem. The potential of the methodology is shown through the case studies and the current shortcomings and the future lines of research are clearly stated.
查看更多>>摘要：Dew point corrosion is one of the most common corrosion forms in an atmospheric tower. In this paper, process analysis and numerical simulation were performed to reveal the corrosion phenomenon at the top of atmospheric tower and the volatilization lines in a petrochemical enterprise. Through the analysis of corrosion morphology and corrosion products, it shows that the corrosive area is caused by the dew point corrosion under the H2O-HCl-H_2S environment, and the dew point temperature is 97.8 °C. Moreover, the Eulerian Wall Films (EWF) model is established to simulate the formation process of liquid film at the initial stage of dew point condensation. The high-risk area of dew point corrosion is the aggregation position of the liquid phase which is located at the interface at the top of the tower, the inner elbow of the volatilization line, and the lower wall of the horizontal tube of the volatilization line. It is found that low-temperature and low-velocity are two main reasons that cause dew point condensation. The predicted corrosion areas are in line with the actual failure location, which verifies that the CFD simulation can be used to predict the thinning area of the atmospheric tower.
查看更多>>摘要：The defects on rail surface may have negative effects on the rolling contact fatigue (RCF) performance of the rail materials. This investigation researched on the RCF behaviors of the defected rail materials under various slip ratios and contact stresses. The results show that both the wear loss and the average crack length increase if the slip ratio or the contact stress increases. Moreover, the surface damage characteristics change from simple adhesion spots to significant fatigue crack grouping when slip ratio and contact stress rise. Finally, the experimental results also highlight that the slip ratio provides the deformation area for crack initiation whereas the contact stress accelerates the crack propagation by emphasizing the geometric effects.
查看更多>>摘要：This paper investigated the relationship between the morphology of corrosion pits and the stress concentration factor. Several images of corroded wire with different corrosion durations were obtained based on corrosion-accelerated test under the coupling effect of alternating load and salt-spray environment, then the morphology and size of pits were identified by the maximum entropy threshold segmentation method. A finite-element model of high-strength wire with corrosion pits was constructed and analyzed to explore the stress concentration factor close to the corrosion pits. Subsequently, a Bayesian prediction model of the stress concentration effect on wires with corrosion pits was proposed. In conclusion, the morphology of corrosion pits was described as "ellipsoidal", with its length and width obeying the lognormal distribution and its depth obeying the Weibull distribution. Bayesian analysis indicated that the stress concentration factors around a corrosion pit were affected by the length-width and width-depth ratios of the pit. The stress concentration effect close to corrosion pits with narrow or deep appearance was more dramatic than that of other pits.
查看更多>>摘要：The reliability of a disc cutter is critical to shield tunneling performance in compound strata. In this regard, a series of three-dimensional models were developed to evaluate the interaction between the disc cutter and the soil-rock interface. The influences of the penetration depth and the tip width were also addressed. Then, this study presented a rudimentary identification method of disc cutter's potential failure modes in compound strata. The results convincingly indicated that the cutting coefficient could be separated into two stages at the interface of soil and rock: linear reduction stage and constant value stage. For a disc cutter with a tip width of 20 mm and 22 mm, about 1.5% and 1% increments were added to C_(c,r) as the disc cutter penetrated every millimeter deeper, respectively. It showed that a cutter ring was apt to fracture at the soil-rock interface. The risk of cutter ring fracture was mounting dramatically as the penetration depth raised. The fracture failure possibility at the soil-rock interface for a narrow cutter ring was higher under a great thrust. And it also revealed that the cutter ring of a disc cutter might be led to nonuniform wear due to the insufficient starting torque to rotate in soft layers. Overall, the conducted research could offer helpful guidance for TBM operation in the compound stratum.
NSTL
Elsevier