查看更多>>摘要：To study the self-loosening mechanism of fastening bolts, the triaxial random vibration environment test for a print circuit board is carried out with the triaxial vibrostand, and the simulation of the self-loosening process of bolts under the same vibration environment is made by the finite element method analysis, which obtains the curves of root mean square value of strain of monitored points varying with tightening torques. The strain curves obtained from the simulation have good consistency with both the test results and the fitted curves of the mode frequency differences, which effectively reveals the self-loosening mechanism of fastening bolts. Further the curves of the root mean square of strains respectively under the triaxial and uniaxial random vibrations are compared, and the difference between the curves reflecting their bolt self-loosening extents is found. Thus, a new method for determining the bolt self-loosening in the triaxial random vibration test by the root mean square of strains is proposed.
查看更多>>摘要：The torsion bar used in the suspension system of the heavy off-road tracked vehicle was broken during the pre-strained manufacturing. A series of investigations were conducted including chemical composition analysis, metallographic examination, residual stress detection, hardness evaluation, and fractographic analysis. The anomalous phenomena were analyzed comprehensively under the condition that the fracture features could not conclude directly. The results showed that the low tempering temperature of the torsion bar was the root cause of fracture, which resulted in excessive residual stress. The hardness is higher than the standard 14 HBW. The maximum difference of residual stress was 278 MPa. They reduced the torsional resistance. Consequently, the fracture occurred during pre-strained. The excessive residual stress triggered the advent of cracks and intergranular fracture. The possibility of failure caused by chemical composition and inclusions was excluded. Due to the results and the related research, changing the tempering temperature to 290-310celcius (holding for 2 h) may avoid failure. The analysis procedure in this work provides a means to determine failure cause under the difficulty of fracture analysis, and it offers possible routes to improve the performance of the torsion bar.
查看更多>>摘要：Galvanised pipes are used extensively for borewell application to tap underground water at various geographical and geological conditions. The depth of borewell varies from place to place depending on many factors such as, (i) the quantity of water need to be tapped out per unit time, (ii) the surrounding geological conditions and (iii) the depth of underground water level. Early failure of galvanised borewell pipe is very common phenomena and purpose of this work is to identify the probable causes for such failures. The present work details on the analysis of such corroded borewell pipes from south eastern part of India. The steels substrate, coating and the corrosion products from different depth of the borewell assembly as well as the soil and water samples were investigated under SEM/EDS, XRD. Two major parameters were identified for early failure of borewell galvanised pipe, such as, (i) higher level of chloride content (1065 ppm) & high hardness (1802 ppm) in the underground water and (ii) wrong design of the borewell pipe setting including dissimilar grades of steel used in the pipes. Primary borewell steel pipes are more electronegative than the sleeve steels. This resulted in corrosion at the surface of primary pipe near the sleeve joint. The ground water with elements like magnesium, calcium and sodium leads to formation of hard scale inside of the pipe. Such scale further enhances the deposition of chloride ions on the steel surface and triggers the pitting corrosion. The most susceptible region for early failure of galvanised borewell pipe is the sleeve joint region. Many bare (without zinc coating) and exposed joint threads are visible near the sleeve joint area with lower steel thickness. Crevice corrosion at the edge of the sleeve joint also plays important role in failure due to absence of any insulating materials at the joint.
查看更多>>摘要：This work aims to evaluate the degradation mechanisms of selected composite bars subjected to exposure to various chemical environments. Sets of three different groups of bars, such as glass fiber reinforced epoxy pultruded at Wroclaw University of Science and Technology (WUST), commercial glass fiber reinforced epoxy overwrapped with carbon fiber, and hybrid bars (H-bar) manufactured by a combination of pultrusion and filament winding methods. The hybrid bar is a composition of a pultruded carbon fiber reinforced epoxy core and the filament-wound glass overwrap. The research included a macro and microscopic evaluation of bars before and after 50 days of degradation in three types of chemical solutions, that is, 5% NaCl, 5% H2SO4, NaOH with pH = 12 and a comparative analysis of bar damage mechanisms under the influence of compressive and tensile forces. These loading conditions are provided by an out-of-plane compression test, which allows one to assess the quality of the matrix, especially in unidirectional bars, where the applied force is carried only by the matrix. Furthermore, the four-point bending test is performed to assess the flexural behavior of the investigated bars. Research has shown that the most degrading solution is 5% H2SO4 causing the greatest drop in the maximum force concerning the exemplified specimens. Moreover, the most resistant to aggressive environments and mechanical degradation appears to be the "H" hybrid bar; its greater resistance may be affected by the additional filament-wound glass layer, which protects the carbon fiber core.
查看更多>>摘要：Excellent properties of railway wheels are of vital significance for adapting to complex and changeable operating environments to reduce maintenance costs and extend service life. This paper presents the results suggesting that the hardness distribution in wheel rim is of obvious inhomogeneity, especially in areas near and away from the flange as 'on two rim sides', which undoubtedly affects the subsequent use of the wheel, such as eccentric abrasion and peeling. Product properties (hardness of wheels) are determined, to a certain extent, by temperature and microstructure changes caused by relevant parameters during water-air alternate spraying process. It leads to differences in heat transfer between the surface and the medium or internal heat conduction. To fully investigate such a complicated quenching, a thermo-phase modeling of wheel quenching was used with transient heat transfer, phase change and hardness calculation taken into account. It was found that unsynchronized cooling on both sides of the rim alters the phase behavior of bainite and martensite, especially in stage III of spraying. However, as the depth from the tread increases, the hardness difference becomes almost less obvious. In this condition, the angle of spraying becomes the key parameter influencing temperature-time process during cooling. The calculated results reveal that the hardness is gradually unified on two rim sides by adjusting the deflection angle of spraying that was simultaneously applied to the tread and the rim. The effect of the adjustment was verified experimentally through subsequent actual wheel production. The test analysis mode combined with transient thermal simulation model can be used to effectively and rapidly optimize parameters for intelligent heat treatment process.
查看更多>>摘要：Crack detection of turbine disks is of great significance in aero-engine health monitoring. The traditional non-destructive testing (NDT) methods are not applicable for in-situ or online monitoring of crack propagation. In order to monitor the crack propagation via measured signals, it is necessary to investigate the dynamic characteristics of cracked turbine disks first. In this paper, finite element analysis (FEA) is carried out to study the blade tip characteristics of turbine disks with different types and size of cracks. Three observation parameters, i.e., blade tip clearance change, blade spacing and blade twist angle, are proposed as crack indicators. The effects of different crack types (e.g., circumferential crack, radial crack, crack at the center hole and crack at the bottom of the mortise) on the observation parameters are discussed qualitatively. Then the circumferential crack is selected as an example for the quantitative analysis. The circumferential cracks with different length, width, location and number are studied and compared systematically. The qualitative analysis results show that the three observation parameters can be used to detect different crack types except the crack at the center hole. The quantitative analysis results show that blade spacing is most sensitive to size parameters without crack width. The conclusions can provide proofs for the blade tip timing (BTT) of cracked turbine disks.
查看更多>>摘要：Axle is easy to be hit by ballast and other foreign object rolled up due to aerodynamic effect to form the typical foreign object damage (FOD) on the surface, leading to deteriorate the axle structural integrity and reduction of fatigue performance. In order to evaluate the effects of surface damage and strengthening treatment on the fatigue performance, the EA4T axle steel specimens with or without shot peening (SP) were induced FOD by a compressed-gas gun. Meanwhile, the strengthening mechanism of shot peening were investigated by both nanoindentation technique and X-ray diffraction. Then the probabilistic fatigue S-N curves (P-S-N) were acquired for unSPed and SPed specimens subjected to FOD or not. In addition, the fatigue limit of FODed axle estimated by Murakami equation was corrected by size effect, surface quality, loading mode factors. Finally, the fatigue limits with 97.5% reliability were adopted to construct the modified Kitagawa-Takahashi (K-T) diagrams for FODed axle with or without shot peening treatment, which can be used in the service performance evaluation of foreign object damaged axle.
查看更多>>摘要：NiCoCrAlY coatings were prepared using laser cladding technology and subjected to a continuous hot corrosion test in a eutectic Na2SO4/NaCl (75:25, wt. %/wt. %) salt at 800 degrees C in air. The corrosion kinetics curves of the NiCoCrAlY coating and the untreated substrate over 100 h of hot corrosion were compared and analyzed. The results show that in the early stage of hot corrosion, a chemical reaction that produces alkaline fusion near the oxide scale takes place at the molten salt/coating interface, causing Al2O3/Cr2O3 to form and dissolve continuously. In the late stage, a new continuous Al2O3 film is formed.
查看更多>>摘要：The heat treatment design and optimization of Re-free low-cost single crystal (SX) superalloys, which are commonly regarded as the most practical and economic alloys to manufacture guide vanes and high-pressure turbine blades of aero-engines, are in great demand. In this study, significant adjustments to the primary ageing temperature (PAT) were made, and the synthetic impact of PAT variations from 1050 degrees C to 1100 degrees C on the microstructure and creep behaviours in novel Re-free SX alloys was investigated in detail. The results showed that the gamma' size and gamma/gamma' lattice misfit increased with increasing PAT, resulting in a reduction in the gamma/gamma' interface area and a shift in the morphology of gamma' from spherical to cubical. Notably, element diffusion and the resultant coarsening of gamma/gamma' structures were facilitated by increasing the PAT. The alloy with 1080 degrees C PAT showed the longest creep life at 1100 degrees C/137 MPa, which was approximately 1.5 and 1.2 times that of alloys with PATs of 1050 degrees C and 1100 degrees C, respectively. Moreover, as PAT increased, the number of microvoids decreased due to increasing the diffusion rate of elements and hindering the formation of vacancies. When the PAT was too low or too high, the "inland shapes" of the gamma' phase and topological inversion phenomenon occurred during creep, which were detrimental to the creep properties. Furthermore, increasing the PAT resulted in the morphology of gamma/gamma' interfacial dislocation networks shifting from sparse waves to fine hexagons and then to coarse hexagons. The dense interfacial dislocation networks in the sample with 1080 degrees C PAT remarkably impeded superdislocations cutting into gamma' phases. Ultimately, the thermodynamics and kinetics of gamma' formation and growth were also discussed, and the optimum PAT was ascertained to provide further guidance to design and develop Re-free low-cost SX superalloys.
查看更多>>摘要：Boiler tube alloys in waste-to-energy (WTE) facilities suffer from significant corrosion, induced by metal chlorides and sulfates at high temperatures. This work aims to explore the effects of alloying elements (Ni and Cr) on the hot corrosion of boiler tube alloys, and clarify the role of elevated temperature and service time on their protective and failure mechanisms. Long-term hot corrosion experiments were performed on three typical corrosion resistance alloys (CRAs, including Inconel 625, 310S, and 00Cr30Mo2) in simulated WTE salts at 850 degrees C. Results confirmed that Cr2O3 is the main phase for corrosion protection of these alloys at 850 degrees C. The greatest hot corrosion resistance is demonstrated by 00Cr30Mo2, due to its highest Cr content among these three CRAs. Although the measured corrosion rates are all lower than the conventional "acceptable" corrosion rate of 1 mm/yr., the estimated penetration rates of these alloys range from 1.58 to 8.50 mm/yr. Therefore, none of the selected CRAs can be recommended for the WTE environment at 850 degrees C. Operating temperature and exposure time are also confirmed as two critical factors for the formed oxides, influencing the protective mechanism and corrosion behaviour. In addition, hot corrosion mechanisms of all alloys were revealed with the support of cross-sectional SEM and EDS analysis.
NSTL
Elsevier