查看更多>>摘要：A casing leaked in a shale gas well during pressure testing. A series of experiments including macroscopic observation, non-destructive inspection, physical and chemical inspection, optical microscope, scanning electron microscope (SEM) and energy dispersive spectrometry techniques (EDS) were carried out to analyze the failure accident. The finite simulation analysis (FEA) method was used to analyze the effect of internal pressure on the defective parts of the casing. Based on the experiment result and manufacturing process investigation, the root cause was obtained. During the casing pipe rolling process, the bolts of size Ml 2 x 60 mm on the balance cylinder of the continuous pipe mill were loosened and dropped onto the pipe. Under the dual action of the roller and the mandrel, the bolts are 'inlaid' inside the casing as a dissimilar metai substance. The bolt head almost occupies the entire wall thickness of the casing, resulting in the remaining wall thickness of the casing being less than 1 mm. The dissimilar metal inside the casing destroys the continuity of the pipe and severely reduces the pressure bearing capacity of the casing. The FEA calculation results show that the equivalent stress at the defect is lower than the yield strength of the pipe body during the factory hydraulic test. While, in the field pressure test, the equivalent stress at the defect is far greater than the yield strength of the casing under the action of axial tension and internal pressure. Therefore, the crack at the defect quickly propagated and penetrated the entire wall thickness of the casing, resulting in leakage. In order to avoid similar accidents, suggestions to prevent such failures are proposed.
查看更多>>摘要：We studied samples of 17-4 PH stainless steel with stress concentrators fabricated via direct metal laser sintering (DMLS). Tests were carried out both on these samples and on smooth specimens. The mechanical properties of the samples under cyclic and static loading were also determined. Moreover, the surface morphology of the destroyed samples was studied by optical and electron microscopy. It was found that the resistance to cyclic loading in samples with grooves of radius R = 0.5, 1.0, and 2.5 mm is higher than that of a smooth sample. The decrease in cyclic and static strength characteristics was influenced by the metal structure of the samples. It was found that the sample with a coarse-grained structure favors crack development. Lastly, the specimen with a sharp concentrator of radius R = 0-5 mm was characterized to have a highly dispersed structure, which reduces crack development under cyclic loading. The novelty of this work lies in the fact that for the first time the ambiguous influence of stress concentrators was established depending on the type of loading. All these factors have an impact on the strength of the samples in cyclic and static tests.
查看更多>>摘要：Friction stir welding (FSW) is a solid-state joining process that is still effectively utilized for joining Mg alloys, Ti alloys, and many other similar and dissimilar materials. Application and dissimilar joining of titanium to magnesium are very attractive in the automobile industry due to the reduction in automobile weight and fuel consumption. In this study, parameter optimization of friction welding on the shear strength of pure commercial titanium to AZ31 magnesium alloy joint by response surface methodology (RSM) was carried out. Rotation speed, friction pressure, and forging pressure on three levels were considered as the variable parameters in welding for a friction time of 4 s and a fixed forge time of 8 s. The torsion test results showed that the best shear strength (103.285 MPa) is related to the sample with optimum parameters, namely, a friction pressure of 50 MPa, forging pressure of 40 MPa, and rotation speed of 1300 rpm. Analysis of variance (ANOVA) was used to investigate the capability of the expanded model. The ANOVA results also showed that among the parameters, rotation speed and friction pressure significantly affect the shear strength. The reason is that high rotation speed and friction pressure can result in rapid transformation for achieving axial displacement stability and escalation of temperature.
查看更多>>摘要：In this paper, a dynamics model of a bending-torsion coupled gear-bearing transmission system is developed, and the vibration features of a pitting fault gear system considering eccentricity and friction are analyzed. Firstly, the time-varying meshing stiffness of pitting faulty propagation is calculated by the parsing method. The vibrational differential equations for this model are obtained using an energy approach based on Lagrange equations. Then, the equations are solved using the Runge-Kutta method to obtain the vibration response of the system. Finally, the effect of the pitting area ratio on the vibration features of the pitting faulty gear is analyzed by the time, frequency, and time-frequency domains. The sideband rating factor is presented to evaluate the influences of pitting faults. The effects of the eccentricity and friction on the impact caused by the pitting is analyzed.
查看更多>>摘要：Thermally growth oxide (TGO) is formed between ceramic coat and bond coat during the service of thermal barrier coatings (TBCs). Characterization of the mechanical properties of the TGO layer is important for simulating the stress or predicting the failure of TBCs. Based on nanoindentation and tensile methods, the elastic modulus, hardness and fracture strength of TGO are obtained. Due to the influence of the indentation size effect (ISE), the measured indentation hardness and elastic modulus of TGO decrease with increasing indentation depth. These values are much smaller than those of bulk alumina. By analyzing the ISE, the intrinsic hardness and elastic modulus of TGO are obtained as 13.58 GPa and 326.46 GPa, respectively. The residual stress in TGO with the value of 133.1 MPa is determined by XRD method. Combining the tensile tests with a digital image correlation (DIC) method, the tensile fracture strain and fracture strength of TGO are determined to be 0.25% and 683 MPa, respectively.
查看更多>>摘要：A surface notch will be produced in crack grinding process of cracked blade, and fatigue life of surface notch is a critical measure to assess its remanufacturability. In this paper, a total fatigue life separation model based on surface notch is studied, which is defined as the sum of crack initiation life and crack propagation life. Tensile fatigue experiments were performed on surface notched plate specimens. At a given notch depth, there is a negative correlation between the secondary crack density and the notch width. Combined with Neuber's rule and elastic-plastic finite element results, a residual thickness correction coefficient S_ε is proposed to reflect the effect of finite thickness on local strain. Depending on simulation results, it is found that Neuber's rule underestimates the notch-root strains of surface notch, which is contrary to the current conclusions. Crack initiation life is estimated by modifying Morrow method with the proposed S_ε. The initial crack size a_o of surface notch is qualified by equivalent initial flaw size (EIFS) modified by fatigue strength calculated by Neuber approach. Crack initiation life is estimated by Labg equation with the improved a_o. The proposed model is verified by fatigue experiment results with a prediction accuracy falling into the scatter band of factor 1.5. This study enriches the current notch fatigue analysis, which can provide reference suggestions by the model for crack grinding design in remanufacturing pre-treatment process.
查看更多>>摘要：Pipelines used in the oil and gas transport usually contain some sulfur dioxide, which absorbs hydrogen from the pipe wall and accelerates hydrogen damage of steels. Also, the welding process and the type of manufacturing process can facilitate the entry of hydrogen into the steel. In this research, API 5L X60 pipes used in the refinery of Tang-e-Bijar region of Kermanshah, Iran, were studied. Microstructural studies showed that the primary microstructure of the steel consisted of 55% ferrite and 45% pearlite. The microstructure in the welding zone could include Widman-statten ferrite, based on its morphology shown in the optical etched microstructure. The observed cracks in the microstructure were also of step-wise cracking and represent the possibility of hydrogen induced cracking. Pre-etching observations using light microscopy shows MnS impurities in the structure. Also, SEM/EDS analysis revealed that the cracks were formed from the accumulation of MnS impurities and these impurities were preferred sites for nucleation and growth of the cracks. The results of the mechanical testing including hardness, impact and tensile test showed that the steel had a ductile behavior for all the sections and no hydrogen embrit-tlement has been probable.
查看更多>>摘要：To ascertain the anchor cable failure caused by the overturning and slipping of the station structure under unilateral excavation with zero distance from the station, this study takes the balance of the station-cable system under zero-distance unilateral excavation as the research object, and derives the total anti-slip force and anti-overturning moment of the cables based on the Rankine's earth pressure theory and the force balance of the metro station. In addition, a novel failure prediction method is proposed to judge the failure of the station-cable system under various design conditions by taking the supporting capacity of the multi-layer cables as the evaluation index and multiple main influencing factors as the control indexes, so as to predict the structural risk. This method comprehensively considers multiple independent variables such as excavation depth, groundwater level, interval spacing and anchor cable design angle. Furthermore, based on Pearson correlation analysis, the sensitivity of the cable supporting force to various factors decreases in the following order: excavation depth > groundwater level > interval spacing > station width > cable design angle. Therefore, reducing the excavation depth is the most effective way to reduce the risk of the station-cable system caused by unilateral excavation. If it is necessary to increase the excavation depth, the cover-excavation reverse method can be used to indirectly reduce the effective excavation depth, thereby ensuring the safety of the structural system. This method can predict the risks of similar projects and reduce the occurrence of accidents.
查看更多>>摘要：The prestressed concrete sleeper is a primary type of sleeper for railway systems around the world. Compared with wooden and composite sleepers, concrete sleepers are of good elasticity and sufficient longitudinal and lateral resistance to ensure the stability of the track. As the railway system is developing to a heavier load and higher speed system, the cracking of concrete sleepers under dynamic load and long-term service is becoming more and more critical. It is essential to evaluate the crack behaviour of prestressed concrete sleepers. In this paper, three-dimensional FEM models have been established using Abaqus based on the brittle cracking model. After the validation by experimental results, the numerical research is performed to evaluate the crack propagation behaviours of prestressed concrete sleepers with different concrete compressive strength, different prestressing force magnitudes, and different loading rates. The results reveal that the cracking resistance of a concrete sleeper turns enhanced by improving the compressive strength of concrete and the initial prestressing force of tendons. It is also observed that the cracking becomes more progressive as the loading rate increases.
查看更多>>摘要：This paper presents a new gear dynamic model for the transmission system under the non-stationary condition with the consideration of the coupling between the external load and the internal gear excitation. The gear mesh stiffness variation is found to be the main internal excitation and therefore, the effect of the varying external load on the gear mesh stiffness is evaluated firstly, which can then be incorporated into the gear dynamic model. When the gearbox is subjected to a wide range of external load, the mesh stiffness varies correspondingly and an iterative process is proposed to ensure the convergence of the gear dynamic simulation at each step. The dynamic responses from the model with and without the iterative process have been compared. It indicates that the proposed model can successfully incorporate the external-internal coupling effect in gearbox and further shows that the operating condition has a significant influence on the gear response. The proposed model can provide an effective tool to improve the accuracy of gear dynamic modelling and improve the accuracy of gear fault diagnostics under non-stationary conditions.
NSTL
Elsevier