查看更多>>摘要：The modal characteristics are important issues of a Francis turbine runner. It is prone to cause excessive vibration even resonance of turbine if the natural frequency is close to the excitation frequency. Thus, in the runner design and optimization processes, not only hydraulic characteristics but also dynamic characteristics should be considered to ensure the safe and stable operation of the unit. The modal analysis of hydrofoil NACA0009 is calculated by means of coupled acoustic fluid-structure model to verify the feasibility of the method and the results show good agreement with the experimental results. Then the effect of gap and structure parameters are discussed. Results show that the effect of fluid in the crown gap and seal on natural frequency and added mass is small, but the effect of fluid in the band gap and seal is considerable. The mode shapes are similar under different structural parameters, but the natural frequency and added mass factor are different. The effect of root rounding radius is more obvious than that of blade thickness and band thickness. Besides, the effect mechanism of band thickness on modal characteristics is quite complex, the lower-order frequency decreases while the higher-order frequency increases with the increase of the band thickness. The results in this paper are helpful to avoid the typical resonance frequency such as rotor-stator interaction frequency in the design and optimization processes.
查看更多>>摘要：The relays used in aviation electronic equipment are required to have extremely high reliability. Due to strong shock and vibration in the service process, premature failure of reeds in aviation relays is a common problem. Nonetheless, limited published investigations focus on reed failures. In this study, a reed in a force-balanced aviation relay was investigated, which comprised of internally oxidized Ag-Mg-Ni alloy. The fracture and microstructural investigation of the failed reed were conducted systematically, and the microhardness of the reed was also measured. The results indicated that the intergranular fracture occurred in the failed reed, and the failure of the reed was caused by severe Ni segregation. Abnormally large grains and remarkable difference in grain size for the core regions were observed in the failed reed, and the overall hardness of the failed reed was lower than the technical specification. The results indicated that Ni segregation led to inhomogeneous microstructure. In addition, secondary particles in the fracture surface were identified as Ni particles and oxides of Ni. Numerous nanoparticles were distributed in the grain boundaries, which led to intergranular brittle fracture of the reed. This study provides guidance on the evaluation of microstructure and failure analysis for Ag-Mg-Ni reeds.
查看更多>>摘要：The failure of superheater tubes in an air quenching cooler waste heat boiler from a cement plant is presented in this study. The tubes were composed of material specified to ASTM SA210C. After three months in service, one leak was detected in one of the tubes over its bent section. Subsequently, several leaks also were found in other tubes of the same boiler. In order to find out the root cause of the leaks, the failed tubes were investigated by visual inspection, optical emission spectroscopy, metallographic analysis, tensile test, micro-hardness measurements, and SEM/EDS analysis. Visual inspection revealed that no obvious wall thinning, swelling and mechanical damage were observed in the failure zone. The material met the chemical composition, micro-structure, and tensile property requirements, while the micro-hardness far exceeded the requirements as specified by ASTM. It has been found that the fracture surfaces in general exhibited intergranular brittle fracture. Furthermore, the cracks with corrosion products originating on the water side of tubes were branched. According to finite element calculations, the residual stress in the failure zone due to bending confirmed that cracks propagated in a general direction perpendicular to the stresses. Therefore, the failure mechanism of the tubes was identified as stress corrosion cracking.
查看更多>>摘要：The pantograph commonly suffers from two types of defects in metro-rail transit: abnormal wear and crack defect. It very often suffers both, thereby increasing the risks posed to train-operation safety. Previous research has not considered the influence of pantograph cracks on contact strip wear. This study establishes a pantograph-catenary dynamics model with crack defect to investigate the influencing pattern of crack defect, as well as the catenary arrangement form under cracked condition on graphite contact strip wear. Results show that the presence of pantograph cracks accelerates the wear rate and induces the eccentric wear and marginal wear problem. An 8 m span and a splayed arrangement form of the pull-out value are preferred.
查看更多>>摘要：Abrasive belt grinding technology is widely used in the processing of aero-engine blades. Residual stress is used as an indicator of grinding gauge integrity, which has a significant impact on the fatigue performance of aero-engine blades. In fatigue life research, experiments are mainly used to explore the influence of residual stress on fatigue life. Restricted by factors such as experimental efficiency and cost, it is necessary to simulate the influence of residual stress in the process of fatigue failure. Based on the theory of crack initiation and propagation, this paper proposes a life prediction algorithm considering residual stress; carries out a bending vibration fatigue experiment and simulation fatigue life analysis of aero-engine blade grinding with abrasive belt; compares the results of experiments and simulations and proposes a residual stress equivalent calibration method. Used theory, simulation, and experiment to verify the effectiveness of the method, and uses this equivalent calibration method to analyze the effect of residual stress on the fatigue life of aero-engine blades ground by abrasive belts. The experimental results show that the surface residual compressive stress will optimize the size and distribution of the surface stress during the service process of the aero-engine blade. Under the same external load and surface roughness conditions, a larger surface residual compressive stress will increase the aero-engine blade stiffness, reduce the aero-engine blade's vibration amplitude, and reduce Surface stress, which in turn, has a beneficial effect on fatigue life. The proposed calibration method considering residual stress can accurately predict the fatigue life of aero-engine blades with a prediction accuracy up to 90%.
查看更多>>摘要：Non-metallic inclusions are often considered to be detrimental to the toughness of steels. Here, we found that, in a cleavage-dominated fracture mode, the toughness, particularly the room temperature fracture toughness of peariitic wheel steel can be improved by refining oxide inclusions or by increasing sulfur content. The roles of these inclusions in crack initiation and propagation were carefully investigated. Intriguingly, no spherical oxides or elongated sulfides were found to be the direct sources of cracks in the cleavage fracture within both Charpy V-notch and compact tension specimens. Cleavage cracks were found to be originated only from the steel matrix. Based on the theory of cleavage fracture and direct microscopic observations, the mechanisms were proposed for understanding the inclusion-toughness relations: (i) the oxide inclusions could indirectiy induce crack initiation by enhancing local stress concentration, therefore promoting cleavage fracture and being detrimental to the toughness; (ii) in addition to enveloping the oxides to reduce the local stress concentration, the sulfide inclusions could also break the continuous propagation of cleavage cracks in the longitudinal direction by sulfide cracking or debonding, which released the stress concentration at the crack tip and enhanced the plastic work of fracture, thereby being beneficial to the toughness improvement.
查看更多>>摘要：Finger-type expansion joints (FJs) have been extensively used in highway bridges and are exposed to the repeated impacts of traffic loading and corrosive environments. In this study, the corrosion and cracking behaviors of FJs serving in bridges for more than 25 years were investigated. The microstructure, electrochemical properties, and dynamic response of the FJs were investigated to analyze the fracture mechanism. The results showed that the crack-induced mechanisms of the two main failure modes were different because the synergistic effect of corrosion with residual stress changed depending on the degree of corrosion. First, considerable residual tensile stress, which enhanced the effective stress range, was the critical cause of fatigue crack initiation in the less corroded heat-affected zone (HAZ). Second, the dominant mechanism for cracking was the applied stress to the severely eroded FJ bottom, which was accelerated by galvanic corrosion. Crack-inducing factors included a combination of stress concentration and dynamic springback behavior.
查看更多>>摘要：Dynamic pressure roadway affected by the mining activity (DPRAMA), which is aimed at promoting the efficiency of panel replacement, is a new mode of roadway arrangement in neighboring panels. However, the failure characteristics and stability control of DPRAMA surrounding rock are rarely reported. In this study, the fragment grade of two-side coal mass of DPRAMA in Gaojiapu Coal Mine was evaluated using the basic measurement scale-fragment grade evaluation method. Then, the evolution of rock mass failure was analyzed through Flac~(3D) software. The results show that (1) under static loading, DPRAMA is highly stable. (2) Under the second dynamic loading, the stress concentration coefficient and deformation value of surrounding rock both surges. (3) Under dynamic-static loadings, the roof falling value is far larger than the floor heave value, and the deformation of pillar-side coal mass is more serious than that of panel-side coal mass. Surrounding rock deformation is divided into three phases, and the specific support strategy is selected in accordance with the characteristics of each stage. (4) The plastic region shows a tendency of butterfly-shaped failure. Based on these findings, the surrounding rock control strategy of "slow yield and long-short control" was proposed to alleviate roof and pillar-side coal mass deformation. Furthermore, the coupling support scheme of "high-strength yield bolt (cable) & steel band" was designed for DPRAMA.
查看更多>>摘要：A stress analysis model was established to analyze the thermal and structural stress of aluminum brazing structure with fin-plate-side bar (ABS) in the LNG plate-fin heat exchanger. The fatigue life of ABS was evaluated based on the above stress analysis results by the different standard. The result shows that the thermal and structural stress concentration is generated at the brazed joint of ABS. Under the common action of thermal and structural stress, the equivalent stress is influenced mainly by the equivalent thermal stress. The equivalent alternating stress was calculated to be 153.5 MPa based on thermal and structural stress. The permissible cycle numbers at the brazed joint of ABS, which is calculated by the fatigue assessment procedure of ASME and JB4732, are 3311 and 2953, respectively. The fatigue assessment results based on the third strength theory is more conservative for the ABS. The influence of operation parameters is also analyzed for the fatigue life of ABS. The temperature difference between NG and MR, MR temperature and NG pressure will be the main factors to impact the fatigue life of that. And a method is proposed to predict the fatigue life of ABS according to the main factors. The error will be less than 10% between the prediction results and the simulation results.
查看更多>>摘要：A M_w 6.4 earthquake hit the NW region of Albania on November 26, 2019, resulting in extensive damage to the civil structures in the broader area of Durres city and its surroundings. According to the official statistics, it caused 51 death toll and ~ 1.2 billion US dollars economic loses. This study investigates the seismic response of masonry buildings damaged during the earthquake. Four important typologies are selected to draw general conclusions from the performance analysis and damage assessment. A detailed field survey after the earthquake was conducted in Albanian territory to observe the impact on low and mid-rise buildings. Seven masonry buildings with 3 to 5 floors were chosen to represent the commonly constructed template designs. Mathematical models of each building were prepared in the light of experimental test results on the quality of structural wall components. Seismic displacement capacities of each building were estimated by pushover analysis. The significance of the findings was further explored by considering the inelastic response of a group of representative buildings during the 2019 Albania earthquake sequences. According to the analyses made, Near Collapse damage state may be reached at a PGA of 0.16 g for some type of buildings. At 0.26 g all considered buildings are expected to be either collapsed or at a near collapse stage. The expected PGA with a return period of 475 years is around 0.3 g and even reaches to 0.4 g in the region. Considering the analysis results, it can be said that decision makers should be aware of the catastrophic consequences of similar future events.
NSTL
Elsevier