查看更多>>摘要：This paper studies the structural integrity of fastening systems used in fiber-reinforced foamed urethane (FFU) composite sleepers. 24 FFU specimens are studied with screw pull-out tests and digital image correlation (DIC) to investigate the effects of parameters such as wet conditions, the sleeper drilled hole diameter and the screw active length inside the sleeper. The pull-out test results reveal that a moisture content of 0.3% in FFU specimens can reduce the pull-out strength between the dry and water-absorbed specimens with 18 mm FFU hole and 80 mm screw active length in dry and wet states (18-AD &18-AW), 18 mm FFU hole and 90 mm screw active length in dry and wet states (18-BD & 18-BW), 20 mm FFU hole and 110 mm screw active length in dry and wet states (20-CD & 20-CW), and 20 mm FFU hole and 90 mm screw active length in dry and wet states (20-BD & 20-BW) by approximately 18%, 19%, 6% and 13%, respectively. Specimens with 20 cm holes have the highest pull-out loads of 71.9 kN and 68 kN in dry and water-absorbed states, respectively. When the active length of the screw decreases from 110 mm to 90 mm, the pulling strength decreases by approximately 14% and 16% in dry and water-absorbed states, respectively. Therefore, the experimental and FEM results show that a 20 mm FFU hole with a 110 mm active length and a 24 mm thick screw is the best option for an FFU sleeper fastening system, especially in wet conditions.
查看更多>>摘要：A tin-based Babbitt self-lubricating composite coating was fabricated on 20 steels substrate by laser cladding through the mixed powders of nickel-plated graphite (Ni-Gr) composite powders and Babbitt alloy powders, aiming at improving wear resistance of Babbitt alloy coating. The results showed that the self-lubricating phase (graphite) coated by the nickel still remained elemental in the composite coating after laser cladding. Both Babbitt coating and Ni-Gr/Babbitt composite coating were uniform and density, without pore, cracks or any other defects, and were mainly composed of alpha-Sn solid solution phase, Cu6Sn5 and SnSb precipitated phases. The grains were obviously refined for the composite coating due to the heterogeneous nucleation. The nano-hardness (H), nano-hardness/elastic modulus (H/E) and (H-3/E-2) for the Ni-Gr/Babbitt composite coating were 1.15 GPa, 0.0137 and 1.9 x 10(-4), which was much higher than that of the Babbitt coating with the value of 0.57 GPa, 0.0068 and 2.3 x 10(-5). With respect to the increase in the nano-hardness, Ni-Gr/Babbitt composite coating also exhibited a low friction coefficient and wear rate with the value of 0.301 and 6.31 x 10(-6) mm(3)/N.;m. The addition of Ni-Gr can effectively improve the self-lubricating and wear behavior of Babbitt alloys.
查看更多>>摘要：During contiguous coal seams mining, the upper goaf residual pillars often cause strong ground stress in underlying longwall mining face and stress accumulation can result in severe mining roadway failure and deformation. To address this issue, the strata stress behavior of working face and the failure and deformation process of an affected roadway at Tashan coal mine were studied in this work. The failure and deformation process of the mining roadway surrounding rock were monitored using the borehole endoscope and the laser range finder. The distribution characteristics and evolution process of vertical stress and failure zone around the affected mining roadway were simulated by FLAC(3D). The mechanism of the large deformation and failure of the roadway were revealed based on the numerical simulation. The results indicate that: (1) the supports working resistance below the residual pillar are generally high with an average value of 9726.6 kN, while that below the goaf are generally low with an average value of 7655.5 kN; (2) the deformation and failure degree are relatively moderate for the affected mining roadway beyond 30 m away from the mining face, and it is relatively severe within 30 m away from P30515 working face; (3) the fundamental causes of large deformation and failure of the affected mining roadway are the combined concentrated stress of the residual pillar and the advanced abutment pressure of the working face; (4) hydraulic fracturing was adopted to degrade the residual pillar's mechanical properties based on its concentrated stress distribution characteristics, which cuts off the stress transfer path of the overburden strata and improves the stress environment of the mining roadway.
查看更多>>摘要：Comprehensive laboratory experiments were first conducted to investigate the deterioration and failure process of Tasmanian sandstones subjected to various numbers of freeze-thaw cycles (FTCs). A theoretical damage model was then derived to quantify the degradation of the physical-mechanical properties of the sandstones. After that, a series of three-dimensional (3D) numerical modelling was conducted using a unique 3D hybrid finite-discrete element software developed by the authors previously to clarify the damage mechanisms and reproduce the failure process. The results from the laboratory experiment, theoretical model and numerical simulation were then compared with each other to validate the theoretical and numerical models for the deterioration of the sandstones subjected up to 60 FTCs. Beyond 60 FTCs, the deterioration of the sandstones predicted by the theoretical model is validated against the numerical simulation with input parameters extrapolated from those up to 60 FTCs. It is concluded that the theoretical model is able to capture the deterioration while the numerical model is not only able to quantify the deterioration rate but also reproduce the failure process and mechanism of the Tasmanian sandstones subjected to various number of FTCs.
查看更多>>摘要：The intrinsic correlation between the flow characteristics at the core of the bar and the crankshaft surface cracks for an S43CMnV non-quenched and tempered steel three-cylinder crankshaft is systematically investigated by a combination of material nonlinearity numerical simulations, microstructural characterizations, and bar streamline experiments. The results indicate that the source of the jagged crack is located inside the transition fillet of the rod journal (parting surface) and that the failure mode is fatigue fracture. An analysis of the failure defects based on numerical simulations indicates that the volume of material flowing from the core of the bar to the inside and surface of the rod journals is larger, and that the average grain sizes at the cracks in the core material area are larger. Among them, the maximum displacement at the 1st and 3rd rod journals are 15.9 mm and 19.6 mm, and the average grain sizes in the core material area are approxi-mately 151 mu m and 148 mu m, respectively, while the 2nd rod journal is approximately 147 mu m. A comparison of the three-cylinder and four-cylinder crankshafts of the S43CMnV material under the same process reveals that the volume of the outflowing core material contained in the rod journals of the three-cylinder crankshaft is larger than that of the four-cylinder crankshaft. Moreover, the analysis results for the microstructures and bar streamline experiments indicate the presence of numerous riverine sulfides and other impurities in the crack source area of the rod journals. These impurities mainly originate from the core of the bar, consistent with the simu-lation results.
查看更多>>摘要：In long-term operation, buried pipelines are prone to corrosion, deformation, and fracture, resulting in leakage. It will cause serious environmental pollution and economic loss. Distributed temperature sensing (DTS) and location method based on temperature signals have been gradually applied to the pipe network. However, the DTS is difficult to comprehensively monitor pipeline leakage and has blind monitoring areas. In addition, fiber optics are difficult to capture continuous low-temperature changes because of the heat dissipation of soil porous media. Thus, distributed acoustic sensing (DAS) is needed to assist the comprehensive monitoring. The acquisition of acoustic signals caused by the leakage field is of great significance to the study of pipeline leakage monitoring methods. In this paper, the attenuation model of leakage acoustic amplitude is established, and the law of vibration acoustic signal in porous soil is studied. Based on the real natural gas leakage test, the characteristic parameters of the time-domain acoustic wave signal are studied. It is proved that the vibration acoustic signal is a kind of acoustic wave caused by the transient mass loss at the beginning of leakage. The main conclusions are as follows: the time-domain signal amplitude is large and the vibration is obvious when pipeline leakage occurs. Acoustic wave leakage occurs in a very short time and acoustic vibration amplitude reaches its peak. Subsequently, the pipeline is in a stable leakage state, and the waveform continues to expand. When there is no natural gas leakage, the vibration signal intensity is less than 3000, and the amplitude changes little. The difference is that when the pipeline leaks, the vibration test curve signal has obvious bulge and fluctuation, and the vibration signal intensity is basically above 4000. The amplitude of the vibration signal increases with the increase of the leakage hole. The research results provide a theoretical basis for the field application of distributed optical fiber acoustic detection.
Ramirez, Edgar I.Figueroa, Carlos G.Romero, Jorge L.Ramos, E....
10页
查看更多>>摘要：The failure of the connectors between the slip ring and rotor of a 2 MW wind turbine was investigated. Visual inspection, low magnification stereomicroscopy and scanning electron microscopy showed the presence of highly branched transgranular cracks. The cracks are in zones of increased stress, as demonstrated by finite elements. The observations show that the failure is caused by stress corrosion cracking (SCC). Fourier transform infrared spectroscopy (FTIR) was used to characterise the insulating varnish of the slip ring. Tropical climate conditions cause the wetting of the components and with a close-by livestock plant is a source of NH3 contamination. Corrections to the design and manufacturing process of the slip rings were suggested to increase the life expectancy of the generator, although the selection of an insulating varnish with higher resistance to environmental exposure is probably the most efficient manner to reduce the incidence of this phenomenon in the future.
查看更多>>摘要：In the paper presented a study of the fracture process of explosive welded layered material AA2519-AA1050-Ti6Al4V (Al-Al-Ti laminate) at ambient (+20 degrees C) and reduced (-50 degrees C) temperatures. The tensile tests and fracture toughness tests were conducted for both types specimens made of base materials plates and of laminated plate. During tensile tests performed on flat specimens the signals of loading force (P) and specimen extension (uext) were recorded. The signals of loading force (P), specimen deflection (udef) and the crack mouth opening displacement (COD) were recorded while performing the tests on fracture toughness of materials on SENB type specimens. The breakthroughs of the tested specimens were observed by scanning electron microscope (SEM).The results obtained during the experimental tests indicate the complicated nature of the cracking process of specimens from the Al-Al-Ti laminate. The deviation from linearity was observed during loading on the sections where it should be linear. There were also slightly decreases in the force value during the load. During macro observation of the fractured surfaces of SENB specimens, delamination cracks between the Al and Ti layers are clearly visible. The size of these cracks increases as the test temperature decreases. The SEM observations clearly indicate that the delamination crack is formed in the AA1050 layer joining the Ti6Al4V and AA2519 base materials. Precise observation by SEM allowed to establish that the development of a delamination crack begins with different type of particles fracture and material discontinuous in narrow strip (20-40 mm) near at Ti6Al4V material. Then the delamination crack develops in the layer of AA1050 material according to a shear mechanism.In order determine the stress and strain distributions in tested specimens the they numerical models were developed and simulation of loading were performed by ABAQUS program. The results of stress and strain distributions obtained clearly shows on large differences they values in the different layers of the laminate. Maximal differences of stress levels occurs between layers Ti6Al4V and AA1050 materials. Due to the fact that the material of the connecting layer (AA1050) has a low level of yield strength, a very high level of plastic strain occurs. Based on the results obtained during the experimental tests, SEM observation of fracture surfaces and numerical calculations, it can be concluded that the weakest area in the tested laminate is the connecting layer AA1050. Namely is it, a thin strip between AA1050 and Ti6Al4V, which consists of metal base particles, intermetallic compounds, their oxides and discontinuities of the material in form of voids.When using this type of laminate, special attention should be paid to the strength of the AA1050 bonding layer.
Guimaraes, F. A.Guimaraes, V. A.Goedel, F.Batista, N. L....
16页
查看更多>>摘要：A method for improving the fracture resistance and mechanical strength behavior of polymer composites by incorporating short carbon fibers into continuous fiber/poly(phenylene sulfide) laminate was explored in this manuscript. This method involves the incorporation of short carbon fibers into continuous carbon fiber thermoplastic laminates, by using the hot compression molding technique. Characterization of a continuous and discontinuous carbon fiber composite material with poly(phenylene sulfide) matrix (S-CCF/PPS laminate) was performed by using tensile, V-Notched Iosipescu, combined load compression and impulse excitation tests. According to the found results for this composite material, it was observed that the tensile strength and the elastic modulus values were 41% and 48% lower than a composite reinforced only by continuous fiber, respectively. The shear strength of the mixed material was found to be 32% lower than a continuous carbon fiber composite. These results were expected since it was used a combination of 50% of continuous and 50% of short carbon fiber, in volume. However, the compression strength of the mixed composite was found to be only 11.9% lower than a laminate reinforced with solely continuous carbon fiber, showing a synergic gain in this case. In addition, the impulse excitation results show that the material mechanical properties are within the expected range, but a high dispersion of the values was observed due probably to the random nature of the discontinuous fiber. Furthermore, it was observed that the failure modes for the composite evaluated in this work are similar to those found for composites processed only with continuous reinforcements and that the models used during the simulations presented similar results to what was found experimentally.
查看更多>>摘要：A pipe rack was subjected to deterioration being the result of many years of use. The structure required assessment of damages and rehabilitation. During evaluation of the pipe rack structure several factors had to be taken into consideration, such as lack of technical documentation, increased loading, and changing design code requirements. A site inspection allowed for identi-fication of numerous damages and their origin. It was concluded that an inadequate coating of paint, an aggressive environment, and leakage from installations in the pipe rack resulted in advanced corrosion, especially at the connection zones. Increased loading without adequate strengthening of the steel truss led to deformation of some structural members. The rehabilitation was supposed to bring the pipe rack back to a suitable condition for service, although it was required to limit the intervention to the structure and maintain the heating pipelines under service. Since the structure was originally designed according to the Polish design code from the 1960's using the permissible stress method and nowadays it should fulfil requirements of the Eurocodes, the state of the pipe rack was assessed based on static strength analyses performed for both codes. The results of calculations are shown, compared, and analyzed. The selected solution for strengthening in the form of steel profiles welded to the most stressed members of the existing structure are presented.
NSTL
Elsevier